Title of Invention	WRITE-ONCE READ-MANY OPTICAL RECORDING MEDIUM, SPUTTERING TARGET AND THE PRODUCTION METHOD THEROF
Abstract	A write-once-read-many optical recording medium enabling excellent recording and reproducing properties at a wavelength of blue-laser wavelengths or shorter, i.e. 500nm or less, particularly at wavelengths of near 405nm and high density recording. To this end, a write-once-read-many optical recording medium of the invention comprises a recording layer using a material represented by BiOx (0 < x < 1.5), in which a recorded mark comprises crystal of Bi and/or crystal of a Bi oxide. Another write-once-read-many optical recording medium comprises a recording layer which comprises Bi, oxygen, and M (M represents at least one element selected from Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and Ta), in which a recorded mark comprises crystal of the elements contained in the recording layer and crystal of an oxide of the elements.

Title of Invention

WRITE-ONCE READ-MANY OPTICAL RECORDING MEDIUM, SPUTTERING TARGET AND THE PRODUCTION METHOD THEROF

Abstract

A write-once-read-many optical recording medium enabling excellent recording and reproducing properties at a wavelength of blue-laser wavelengths or shorter, i.e. 500nm or less, particularly at wavelengths of near 405nm and high density recording. To this end, a write-once-read-many optical recording medium of the invention comprises a recording layer using a material represented by BiOx (0 < x < 1.5), in which a recorded mark comprises crystal of Bi and/or crystal of a Bi oxide. Another write-once-read-many optical recording medium comprises a recording layer which comprises Bi, oxygen, and M (M represents at least one element selected from Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and Ta), in which a recorded mark comprises crystal of the elements contained in the recording layer and crystal of an oxide of the elements.

Full Text	DESCRIPTION WRITE-ONCE-READ-MANY OPTICAL RECORDING MEDIUM, SPUTTERING TARGET AND THE PRODUCTION METHOD THEREOF Technical Field The present invention relates to a write-once-read-many (WORM) optical recording medium. More specifically, the present invention relates to a write- once-read-many optical recording medium enabling high density recording particularly at blue-laser wavelengths. The present invention further relates to a sputtering target which can be used for forming an oxide-layer which is a layer constituting the write-once-read- many optical recording medium. Background Art Relating to a write-once-read-many optical recording medium enabling recording and reproducing at laser wavelengths of blue or shorter, a blue laser allowing super high density recording is rapidly developed, and the development of write-once-read-many optical recoding media sensitive to blue-laser wavelengths is promoted. In conventional write-once-read-many optical recording media, a laser beam is irradiated to a recording layer which comprises an organic material to change the refractive index typically due to the decomposition and degeneration of the organic material, and thus recording pjts areformed. The optical constant and decomposition behavior of the organic material used in the recording layer play an important role to form satisfactory recording pits. For use in a recording layer of write-once-read-many optical recording media sensitive to blue-laser wavelengths, an organic material must have suitable optical properties and decomposition behavior with respect to light at blue-laser wavelengths. More specifically, the wavelengths at which recording is performed are set at a tail on the longer-wavelength side of a major absorption band to increase the reflectance in unrecorded portions and to substantially increase difference in refractive index invited by the decomposition of the organic material upon irradiation of laser to thereby yield a higher modulated amplitude. This is because wavelengths at the tail on the longer-wavelength side of a major absorption band of such an organic material yield an appropriate absorption coefficient and a high refractive index. However, an organic material having optical properties with respect to light at blue-laser wavelengths equivalent to those of conventional materials has not yet been found. To produce such an organic material having an absorption band in the vicinity of blue-laser wavelengths, the molecular skeleton must be downsized or the conjugate system must be shortened. However, this invites a lowered absorption coefficient and a lowered refractive index. More specifically, there are many organic material having an absorption band in the vicinity of blue-laser wavelengths and it is possible to control their absorption coefficients, however they do not have a sufficiently high refractive index and fail to yield a higher degree of modulated amplitude. Then the one using inorganic materials and organic materials, and the one using only inorganic materials are presently being studied for a material having optical properties sensitive to light at blue-laser wavelengths. As the one using oxides, Patent Literature 1 discloses a recording layer which comprises Bi, rare-earth, Ga, Fe, and O, and the invention describes a composition capable of forming garnet, Patent Literature 2 discloses an optical recording medium using organic oxides. However, these conventional techniques do not study in what shape recording marks should be formed to effectively form excellent recording marks and yield excellent properties. As a matter of course, these techniques do not allow for the shape of recording marks to yield higher modulated amplitudes when used with light at blue-wavelengths, which are brought up herein as an issue. Besides, for write-once-read-many optical recording media using oxides of metals or semimetals as a recording layer, TeOx-Pd recording layers having high-reliabilities have been proposed in Patent Literatures 3 and 4. In Patent Literatures 3 and 4, the composition ratio of the TeOx-Pd recording layer is varied in a direction of the thickness of the layer to enhance reliabilities such as storage stability. Besides, recording layers which comprise TeOx-Pd are also disclosed in Non-Patent Literatures 1 and 2, however, they have only descriptions on controlling of degrees of oxidation therein as a method for improving reliabilities. As to materials containing a bismuth oxide, which are similar to the present invention though, they are disclosed respectively in the following Patent Literatures- Patent Literature 5 discloses an amorphous and ferromagnetic oxide expressed by the formula Ax(MmOn)y(Fe2O3)z, in which individual ratios of various oxides of A, various elements of M, and x, y, and z are defined; Patent Literature 6 discloses a metallic oxide which comprises a 50% or more amorphous phase expressed by the formula (Bi2O3)x(MmOn)y(Fe2O3)z, in which individual ratios of m and n of MmOn, individual ratios of x, y, and z are defined, and the production method thereof; Patent Literature 7 discloses an amorphous compound having a composition expressed by the formula (B2O3)x(Bi2O3)1-x, the range of the composition x, and the quenching method; and Patent Literature 8 discloses bismuth-iron amorphous compound material having a composition of (Bi2O3)1-x(Fe2O3)x, however, x is represented by 0.90 ≥ x > 0. However, these techniques respectively relate to amorphous oxide materials which are optically transmissive and ferromagnetic, and they are . typically used for photoelectromagnetic optical recording media, functional devices for controlling light by means of actions of magnetism, photoelectromagnetic sensors, transparent and electrically conductive films, piezo-electric films, and the like. In addition, these techniques provided by other companies basically aim for patents relating to materials and/or production methods of the materials and have no descriptions on applications to writeonce-read-many optical recording media. On the other hand, as one of the methods for producing a recording layer for an optical recording medium, there has been a sputtering method. The sputtering has been widely known in the art as—one of the methods for forming thin-layers under vapor-phase and utilized in producing thin-layers for industrial purposes. In the sputtering method, a target material having the same component as that of a layer to be formed is prepared, typically, argon gas ions generated by glow-discharge is crashed with the target material to beat the constituent atoms of the target material, and the atoms are deposited on a substrate to thereby form a layer. An oxide typically has high-melting point, and thus it is not suitably formed by an evaporation method or the like, and the high-frequency-wave sputtering method, in which high-frequency waves are applied, is usually used. There are many achievements of the sputtering method in its production process, and sputtering is also advantageous in terms of throughput. However, when a layer containing mixed materials from two or more elements is formed, there may be cases where the composition of a target is not same as that of the layer, and therefore, the composition of the target must be considered. Further, there are many cases where the structure and characteristics of a layer vary depending on the configuration of a compound constituting the target, and thus this point must be considered. As a technique known in the art, for example, Patent Literature 9 discloses a target which comprises a Bi oxide as a sputtering target for forming a dielectric film. However, Patent Literature 9 does not mention a target which comprises Fe. Since when the type of constituent elements varies, the relation between the composition and constituting compound of the target as well as the relation between the structure and composition of a layer vary. Thus, the structure of target must be changed, and the findings disclosed in Patent Literature 9 do not serve as a reference to the sputtering target proposed in the present invention. In addition, Patent Literature 10 discloses a target for producing a thin layer made from Bi3Fe5O12, however, the invention is for producing a thin layer having so-called a garnet structure, in which high-degree of magneto-optical effect is obtained, and the invention employs the ratio of Bi to Fe being 35 to 3.5:4.5. Accordingly it differs from the sputtering target proposed in the present invention. Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 10-92027 Patent Literature 2 Japanese Patent Application Laid-Open (JP-A) No. 2003-48375 Patent Literature 3 Japanese Patent Application Laid-Open (JPA) No. 06-150366 Patent Literature 4 Japanese Patent Application Laid-Open (JPA) No. 06-93300 Patent Literature 5 Japanese Patent Application Laid-Open (JPA) No. 61101450 Patent Literature 6 Japanese Patent Application Laid-Open (JP-A) No. 61101448 Patent Literature 7 Japanese Patent Application Laid-Open (JP-A) No. 598618 Patent Literature 8 Japanese Patent Application Laid-Open (JP-A) No. 59-73438 Patent Literature 9 Japanese Patent Application Laid-Open (JP-A) No. 11-92922 Patent Literature 10 Japanese Patent Application Laid-Open (JP-A) No. 02-42899 Non-Patent Literature 1 pp. 23-28, Proceedings of The 14th Symposium on PCOS2002 Non-Patent Literature 2 pp. 5-8, Vol. 28, Eijogaku Giho Disclosure of Invention It is therefore an object of the present invention to provide a write-once-read-many optical recording medium capable of presenting excellent recording-reproducing properties at wavelengths of 500nm or less and enabling recording and reproducing particularly at wavelengths of near 405nm and high density recording. Further, another object of the present invention is to provide a sputtering target which can be used for producing an oxide-layer which is a layer constituting the optical recording medium and is suitably used for arbitrarily forming a layer having a stable composition and a stable structure, and the production method thereof. A first aspect of the present invention is a write-once-read-many optical recording medium which comprises a substrate, a recording layer, and a reflective layer, wherein the recording layer comprises a material represented by BiOx (0 therein comprises crystal of Bi and/or crystal of a Bi oxide. A second aspect of the present invention is a write-once-read-many optical recording medium according to the first aspect, wherein the recording mark comprises tetravalent Bi. A third aspect of the present invention is a write-once-read-many optical recording medium which comprises a substrate, a recording layer, and a reflective layer, wherein the recording layer comprises Bi, O, and M as constituent elements, M represents at least one element selected from the group consisting of Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and Ta, and a recording mark with information recorded therein comprises one or more crystals from crystal of the one or more elements contained in the recording layer and crystal of an oxide of the one or more elements. A fourth aspect of the present invention is a write-once-read-many optical recording medium according to the third aspect, wherein the recording mark comprises tetravalent Bi. A fifth aspect of the present invention is a write-once-read-many optical recording medium according to the third aspect, wherein the atomic number ratio of the total amount of the element M to bismuth is 1.25 or less. A sixth aspect of the present invention is a write-once-read-many optical recording medium according to the third aspect, wherein the write-once-read-many optical recording medium has any one of laminar structures of a laminar structure in which at least the recording layer an upper coating layer, and the reflective layer are disposed on the substrate in this order, and a laminar structure in which at least the reflective layer, an upper coating layer, the recording layer, and a cover layer are disposed on the substrate in this order. A seventh aspect of the present invention is a write-once-read-many optical recording medium according to the third aspect, wherein the write-once-read-many optical recording medium is produced using a sputtering target which comprises one or more selected from BiFeO3, Bi25FeO40, and Bi36Fe2O57. An eighth aspect of the present invention is a write-once-read-many optical recording medium which comprises a substrate, a recording layer, and a reflective layer, wherein the recording layer comprises Bi, 0, and L as constituent elements, and the recording layer comprises a Bi oxide, and L represents at least one element selected from the group consisting of B, P, Ga, As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At, and Cd. A ninth aspect of the present invention is a write-once-read-many optical recording medium according to the eighth aspect, wherein the element L represents at least one element selected from the group consisting of B, P, Ga, Se, Pd, Ag, Sb, Te, W, Pt, and Au. A tenth aspect of the present invention is a write-once-read-many optical recording medium according to the eighth aspect, wherein the atomic number ratio of the total amount of the element Ltobismuth is 1.25 or less. An eleventh aspect of the present invention is a write-once-read-many optical recording medium according to the eighth aspect, wherein the write-once-read-many optical recording medium further comprises an upper coating layer and has a laminar structure in which the recording layer, the upper coating layer, and the reflective layer are disposed on the substrate in this order. A twelfth aspect of the present invention is a write-once-read-many optical recording medium according to the eleventh aspect, wherein the write-once-read-many optical recording medium further comprises an under coating layer and has a laminar structure in which the under coating layer, the recording layer, the upper coating layer, and the reflective layer are disposed on the substrate in this order. A thirteenth aspect of the present invention is a write-once-read-many optical recording medium according to the eighth aspect, wherein the write-once-read-many optical recording medium further comprises an upper coating layer and a cover layer and has a laminar structure in which the reflective layer, the upper coating layer, the recording layer, and the cover layer are disposed on the substrate in this order. A fourteenth aspect of the present invention is a write-once-read-many optical recording medium according to the thirteenth aspect, wherein the write-once-read-many optical recording medium further comprises an under coating layer and has a laminar structure in which the reflective layer, the upper coating layer, the recording layer, the under coating layer, and the cover layer are disposed on the substrate in this order. A fifteenth aspect of the present invention is a write-once-read-many optical recording medium according to the eighth aspect, wherein the write-once-read-many optical recording medium further comprises at least any one of an under coating layer and an upper coating layer, and at least any one of the under coating layer and the upper coating layer comprises ZnS and/or SiO2. A sixteenth aspect of the present invention is a write-once-read-many optical recording medium according to the eighth aspect, wherein the write-once-read-many optical recording medium further comprises an under coating layer and an upper coating layer, and at least any one of the under coating layer and the upper coating layer comprises an organic material. A seventeenth aspect of the present invention is a write-once-read-many optical recording medium according to the eighth aspect, wherein recording and reproducing are enabled with a laser beam having a wavelength of 680nm or less. An eighteenth aspect of the present invention is a sputtering target which comprise Bi, Fe, and Q. A nineteenth aspect of the present invention is a sputtering target according to the eighteenth aspect, wherein the sputtering target consists of Bi, Fe, and oxygen. A twentieth aspect of the present invention is a sputtering target according to any one of aspects of the eighteenth aspect to the nineteenth aspect, wherein the sputtenng target is used for forming a recording layer for an optical recording medium in which recording and reproducing are performed with a laser beam at a wavelength of _550nm or less. A twenty-first aspect of the present invention is a sputtering target according to any one of aspects of the eighteenth aspect to the twentieth aspect, wherein the sputtering target comprises a Bi oxide and a Fe oxide, or comprises a complex oxide of Bi and Fe. A twenty-second aspect of the present invention is a sputtering target according to the twenty-first aspect, wherein the sputtering target comprises the complex oxide of Bi and Fe and further comprises one or more selected from the Bi oxide and the Fe oxide. A twenty-third aspect of the present invention is a sputtering target according to the eighteenth aspect, wherein the sputtering target comprises one or more selected from a Bi oxide, a Fe oxide, and a complex oxide of Bi and Fe, and the oxide is an oxide having a smaller amount of oxygen compared to the stoichiometric composition. A twenty-fourth aspect of the present invention is a sputtering target according to any one of aspects of the eighteenth aspect to the twenty-third aspect, wherein the sputtering target comprises one or more selected from BiFeO3, Bi25FeO40, and Bi36Fe2O57. A twenty-fifth aspect of the present invention is a sputtering target according to any one of aspects of the eighteenth aspect to the twenty-fourth aspect, wherein the sputtering target comprises Bi2O3 and/or Fe2O3. A twenty-sixth aspect of the present invention is a sputtering target according to any one of aspects of the eighteenth aspect to the twenty-fifth aspect, wherein the sputtering target does not comprise Bi2Fe4O9. A twenty-seventh aspect of the present invention is a sputtering target according to any one of aspects of the eighteenth aspect to the twenty-sixth aspect, wherein the content of Co, Ca, and Cr is less than the detection limit of the inductively coupled plasma emission spectrometry. A twenty-eighth aspect of the present invention is a sputtering target according to any one of aspects of the eighteenth aspect to the twenty-seventh aspect, wherein the sputtering target has a packing density of 65% to 96%. A twenty-ninth aspect of the present invention is a sputtering target according to any one of aspects of the eighteenth aspect to the twenty-eighth aspect, wherein the atomic ratio of Bi and Fe satisfies the requirement of Bi/Fe ≥ 0.8. A thirtieth aspect of the present invention is a sputtering target production method in which powders of Bi2O3 and Fe2O3 are calcined to produce a sputtering target according to any one of aspects of the eighteenth aspect to the twenty-ninth aspect. A thirty-first aspect of the present invention is an optical recording medium which comprises a substrate, a recording layer, and a reflective layer, wherein the recording layer is formed using a sputtering target which comprises one or more Brief Description of the Accompanying Drawings FIG. 1 is a diagram showing radial distribution functions in the vicinity of O (oxygen) atoms in an unrecorded portion and a recorded portion. FIG. 2 is a diagram showing radial distribution functions in the vicinity of O (oxygen) atoms of a Bi oxide measured according to the FEFF calculation. FIG. 3 is a diagram showing the measurement results of Example B-26. FIG. 4 is a diagram showing an X-ray diffraction pattern of a sputtering target 1. FIG. 5 is a diagram showing recording properties of an optical recording medium produced by using the sputtering target 1. FIG. 6 is a diagram showing an X-ray diffraction pattern of a sputtering target 2. FIG. 7 is a diagram showing recording properties of an optical recording medium produced by using the sputtering target 2. FIG. 8 is a diagram showing an X-ray diffraction pattern of a sputtering target 3. FIG. 9 is a diagram showing an X-ray diffraction pattern of a sputtering target 4. FIG. 10 is a diagram showing recording properties of an optical recording medium produced by using a sputtering target which has a different ratio of Bi/Fe. FIG. 11 is a diagram showing an X-ray diffraction pattern of a sputtering target 5. FIG. 12 is a diagram showing an X-ray diffraction pattern of a sputtering target 7. Best Mode for Carrying Out the Invention Hereinafter, the present invention will be described in detail. To achieve a write-once-read-many optical recording medium capable of performing excellent recording at a wavelength of blue-laser wavelengths or shorter, the following items (1) to (3) are taken up as important issues: (1) Small recording marks can be formed. (2) Less interference between recording marks (3) High-stability in recoding marks When a blue-laser is used, a material by which recording can be excellently performed at blue-wavelengths must be selected, unlike in the case where a laser is used at near-infrared wavelengths and red-wavelengths used for CD and DVD. Since a Bi oxide easily absorbs light at blue-wavelengths, excellent recording can be expected. The first aspect of a write-once-read-many optical recording medium according to the present invention comprises a substrate, a recording layer, and a reflective layer, in which a material represented by BiOx (e recording layer, and recording marks with information recorded therein comprise crystal of Bi and/or crystal of a Bi oxide. To yield a higher modulated amplitude, it is required that the (difference in refractive index between a recording mark an unrecorded portion be large. When a material represented by BiOx (0 crystal of Bi and/or crystal of a Bi oxide in a recording mark, the variance in refractive index is larger, and higher modulated amplitude can be realized. For example, when an unrecorded portion is amorphous and a recoding mark comprises a crystallized portion, higher modulated amplitude can be yielded. When an unrecorded portion comprises a Bi oxide, by forming precipitation of a simple substance of Bi-metal, not oxide, in a recording mark, the difference in refractive index is much larger, and much higher modulated amplitude can be yielded. The method for forming a recording mark by crystallizing amorphous portions has been used so far, however, in the present invention, when recording is performed in an oxide, much greater effect can be expected by utilizing the phenomenon that the recorded portion is changed to a substance other than oxides and the recorded portion is crystallized. A recording mark which comprises crystals having two or more different crystal-structures can restrain the crystals from growing and spreading and enables forming a small recording mark, because mixed crystals each having a different crystal structure can restrain the growth of crystals. The changes in a recording layer induced by recording will be described more. The material represented by BiOx (0 metastable state which is hard to exist with normal conditions, however, such a state can be realized in an optical recording medium by forming a recording layer by sputtering. When a laser beam is irradiated to a recording layer in a metastable state of BiOx (0 easily to separate into Bi and a Bi oxide, because BiOx attempts to go back to a more stable state. At this point in time, it is believed that some Bi oxides isolate from oxygen to become no longer an oxide to be in a state of Bi. Since a more stable state is a crystalline state, crystal of Bi and crystal of a Bi oxide are formed, and thus a recording mark becomes in a state where crystal of Bi and/or crystal of a Bi oxide are included. An aspect of the write-once-read-any optical recording medium of the present invention comprises a substrate, a recording layer, and a reflective layer, wherein the recording layer comprises Bi, O, and M as constituent elements, wherein M represents at least one element selected from the group consisting of Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and Ta, and a recording mark with information recorded therein comprises one or more crystals from crystal of the one or more elements contained in the recording layer and crystal of an oxide of the one or more elements. An another aspect of the write-once-read-many optical recording medium comprises a substrate, a recording layer, and a reflective layer, wherein the recording layer comprises Bi, O, and L as constituent elements, and the recording layer comprises a Bi oxide, and L represents at least one element selected from the group consisting of B, P, Ga, As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At, and Cd. Bismuth may be contained in any of the states such as metallic bismuths, bismuth alloys, bismuth oxides, bismuth sulfides, bismuth nitrides, bismuth fluorides, however, the recording layer preferably comprises one bismuth oxide. Since a recording layer containing a bismuth oxide enables lowering thermal conductivity of the recording layer, achieving high-sensitivities and lower jitter values, and making an imaginary part of complex index of refraction smaller, the recording layer has an excellent transparency, and a multi-layered recording layer is easily formed. Preferably, bismuth and one or more elements selected from the group consisting Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, Ta, B, P, Ga, As, Se, Te, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At, and Cd reside in the recording layer in an oxidized state from the perspective of improvements in stability and thermal conductivity, however, they are not necessarily oxidized completely. In other words, when the recording layer of the present invention comprises three elements of bismuth, oxygen, and one element such as Mg, it may comprise bismuth, a bismuth oxide, an element such as Mg, and an oxide of the element such as Mg. Examples of the method for making bismuth or metallic bismuth and a Bi oxide mixed in the recording layer, i.e. the method for forming a recording layer in which bismuth elements reside in different states include the following methods (A) to (C). (A) Sputtering the recording layer using a bismuth-oxide target (B) Sputtering the recording layer using a bismuth target and a bismuth-oxide target (C) Sputtering the recording layer using a bismuth target while introducing oxygen into the recording layer or co-sputtering method When the method (A) is employed, bismuth in the target is completely oxidized, and this method utilizes the phenomenon that oxygen is likely to be deficient depending on the sputtering conditions such as degree of vacuum and sputtering power. First, an aspect of the write-once-read-many recording medium in which the recording layer comprises Bi, M and oxygen as constituent elements will be described below. It is noted that the element M represents at least one element selected from the group consisting of Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and Ta. Recording can be excellently performed to light at blue-wavelengths by using a material which comprises Bi, M, and oxygen for a recording layer. In an aspect of the write-once-read-many optical recording medium of the present invention, in which a recording layer comprises M, by forming a recording mark in a state where crystals of two or more types of oxides are mixed, the difference in refractive index between a recording mark and an unrecorded portion is larger, and higher modulated amplitudes can be yielded. Further, greater effects can be obtained by making not only crystal of respective oxides but also crystal of a simple substance of element reside in the recording layer. Since growth of crystals can be restrained by making crystals of different elements or crystals each having a different crystal structure mixed, a recording mark which comprises crystals of two or more different elements and/or crystal-structures can restrain the crystals from growing and spreading and makes it possible to form a small recording mark. A recording mark preferably comprises tetravalent Bi. Typically, as the valence of Bi, trivalent Bi is in the most stable state, however to yield a much higher modulated amplitude, a tetravalent Bi is used. It is possible to make the valence of Bi into tetravalence depending on the conditions of oxygen surrounding a Bi atom. Since physical properties are changed by changing the valence of Bi, higher modulated amplitude can be yielded. Examples of the tetravalent Bi compound include BiO2. Typically, a Bioxide having a structure of Bi2O3 is in a stable state. However, a Bi-oxide can take a structure like BiO2 depending on the conditions. By making a recording layer have a crystalline structure that is not typically employed, higher modulated amplitudes can be yielded. A still another aspect of the write-once-read-many optical recording medium of the present invention will be described below. In the write-once-read-many optical recording medium, a recording layer comprises Bi, L, oxygen, and a Bi oxide as constituent elements. It is noted that the element L represents at least one element selected from the group consisting of B, P, Ga, As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At, and Cd. The recording layer comprises bismuth as the principal constituent. As an element to be added to the recording layer which comprises Bi and O as constituent elements, the element M can be named. However, the study about basic properties of the constituent elements that can be added to the constituent elements shows that the element L can also be named. One of the reasons why the element L can be added as a constituent element to a recording layer which comprises bismuth as the principal constituent and comprises a Bi oxide is to reduce thermal conductivity to facilitate forming a minute recording mark. Since thermal conductivity is a value attributable to the presence of photon scattering, thermal conductivity can be reduced when the particle size and crystal size are downsized, when there are a number of atoms constituting a material, and when the difference in atomic mass of constituent atoms is large, and the like. Thus, by adding the element L as a constituent element to a recording layer which comprises bismuth as the principal constituent and comprises a Bi oxide, it is possible to control thermal conductivity and improve high density recording properties. Further, in a recording layer which comprises bismuth as the principal constituent and further comprises a bismuth oxide, the bismuth oxide and the bismuth are crystallized by recording of information, however the size of the crystal and crystallized particles can be controlled by the element L. Thus, the element L enables controlling the size of crystal and crystallized particles in recorded portions and greatly improving recording and reproducing properties such as jitter. This is another reason for adding the element L as a constituent element to a recording layer. From the perspective of thermal conductivity, there are little requirements that are assumed by the element L which can be added as a constituent element to the recording layer except for simple requirements such as the stability of raw materials and the difficulty level of production. However, the inventors of the present invention found that reliabilities of a recording layer, i.e. reproducing stability and storage stability substantially vary depending on the selected element L, and definitely there are requirements assumed by the element L with respect to reliabilities. Namely, as a result of keen examinations on the requirements of the element L to be added as a constituent element to the recording layer, the inventors of the present invention found that the following requirements (I) or (II) are effective: (I) An element having a Pauling's electronegativity value of 1,80 or more. (II) An element having a Pauling's electronegativity value of 1.65 or more and the standard enthalpy of formation AHf° of oxides of-1,000 (kJ/mol) or more, excluding transition metals. By using an element L satisfying (I) or (II), it is possible to achieve a write-once-read-many optical recording medium having excellent recording and reproducing properties such as jitter, and high-reliabilities. Hereinafter, the requirements (I) and (II) will be explained in detail. Degradation in reliabilities of the recording layer which comprises bismuth as the principal constituent of constituent elements and comprises a bismuth oxide is primarily caused by progression of oxidation, or changes in oxidization state such as changes in the valence, and the like. Pauling's electronegativity values and the standard enthalpy of formation "AHf0" of oxides are really important as physical property values of the element L, because the progression of oxidation and changes in oxidization state are liable to invite degraded reliabilities. To sufficiently enhance reliabilities of an optical recording medium, first, it is preferred to select an element having a Pauling's electronegativity value of 1.80 or more as an element L. This is because oxidation is unlikely to make progress with an element having a high Pauling's electronegativity value, and to secure satisfactory reliabilities, an element having a Pauling's electronegativity value of 1.80 or more is effective. The standard enthalpy of formation of "∆Hf0" of oxides may take any values, provided that the Pauling's electronegativity value is 1.80 or more. Examples of the element L having an electronegativity of 1.80 or more include B, Si, P, Fe, Co, Ni, Cu, Ga, Ge, As, Se, Mo, Tc, Ru, Rh, Pd, Ag, Sn, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Po, and At. Here, the electronegativity will be described briefly. The electronegativity is a scale representing how strongly the existing atom in a molecule can attract electrons thereto. The ways to determine a value of electronegativity include Pauling's electronegativity, Mulliken's electronegativity, and Allred-Rochow's electronegativity. In the present invention, the adequacy of an element L is determined by using Pauling's electronegativity. The Pauling's electronegativity defines that the binding energy E (AB) of molecules A and B is greater than the average of binding energy of molecules AA and molecules BB [E (AA) and E (BB), respectively], and the difference therebetween is a square of the difference between the electronegativity of the respective atoms (XA, XB). Namely, it is represented by the following formula: E (AB) - [E (AA) + E (BB)] / 2 = 96.48 (XA- XB)2 ....(1) In Pauling's electronegativity, the equation includes a conversion coefficient of 96.48 (leV = 96.48kJmol-1), because a value of electronegativity is determined by using electron volt. Since the electronegativity varies depending on with what valence an intended element takes in a molecule, the following definitions are applied when the electronegativity is determined in the present invention. Namely, as shown below, the value when the following each group element respectively takes the following valence is defined as the Pauling's electronegativity value of the element: Elements in 1 group take monovalence; elements in 2 group take divalence," elements in 3 group take trivalence; elements in 4 group to 10 group take divalence; elements in 11 group take monovalence' elements in 12 group take divalence,' elements in 13 group take trivalence; elements in 14 group take tetravalence; elements in 15 group take trivalence; elements in 16 group take divalence; elements in 17 group take monovalence; and elements in 18 group take zero valence. For the above-noted elements each having electronegativity of 1.80 or more, the respective Pauling's electronegativity values defined in the present invention are B (2.04), Si (1.90), P (2.19), Fe (1.83), Co (1.88), Ni (1.91), Cu (1.90), Ga (1.81), Ge (2.01), As (2.18), Se (2.55), Mo (2.16), Tc (1.90), Ru (2.20), Rh (2.28), Pd (2.20), Ag (1.93), Sn (1.96), Sb (2.05), Te (2.10), W (2.36), Re (1.90), Os (2.20), Ir (2.20), Pt (2.28), Au (2.54), Hg (2.00), Tl (2.04), Pb (2.33), Po (2.00), and At (2.20). These elements can be added in combination with two or more as constituent elements to a recording layer. Further, the inventors of the present invention found that even if the Pauling's electronegativity value is less than 1,80, an element having a Pauling's electronegativity value of 1.65 or more and the standard enthalpy of formation AHf° of oxides of the element being -1,000 (kJ/mol) or more enables ensuring satisfactory reliabilities. The reason why the requirement is effective is because when an element having a great value of standard enthalpy of formation ∆Hf0 of oxides, the element is hard to form oxides thereof even if the value of Pauling's electronegativity value is small in some degree. In the present invention, when determining a Pauling's electronegativity value, it was determined with each valence fixed to every group of elements, and the same definitions are applied when determining the standard enthalpy of formation ∆Hf0. Namely, the value when an oxide is constituted with the following each valence in each group element is defined as the standard enthalpy AHf° of formation of the element. Namely, elements in group 1 take monovalence," elements in 2 group take divalence; elements in 3 group take trivalence; elements in 4 group to 10 group take divalence; elements in 11 group take monovalence; elements in 12 group take divalence; elements in 13 group take trivalence! elements in 14 group take tetravalence; elements in 15 group take trivalence; elements in 16 group take divalence; and elements in 17 group take monovalence; and elements in 18 group take zero valence. However, in the case of a transition metal, it is impossible to easily determine the standard enthalpy of formation AHf0 of oxide thereof, because it forms an oxide with various valences. Typically, the higher the valence of the element which forms an oxide thereof is, the lower the standard enthalpy of formation ∆Hf0 of the oxide is. In other words, it is considered that in the case of a transition metal, an oxide or oxides thereof are easily formed because there are oxides having various valences to be formed, and thus in the present invention transition metals are not preferably used as an element L. For example, since V (vanadium) takes divalence, the standard enthalpy of formation ∆Hf0 of a V (vanadium) oxide takes the value of the standard enthalpy ∆Hf0 of formation of VO = -431 (kJmol-1), and it falls under the requirements for the element L (II) in the present invention. However, V (vanadium) also easily forms oxides such as V2O3 (trivalence), V2O4 (tetravalence), V2O5 (pentavalence). The ∆Hf0 values of these oxides are respectively V2O3 (-1,218kJmol-1), V2O4 (-l,424kJmol-1), V2O5 (-1,550kJmol-1), and these values do not fall under the requirements for the element L (II) in the present invention. Namely, assuming that V forms oxides with almost only divalence, V (vanadium) falls under the requirements of (I) and (II) of the present invention, however V (vanadium) easily forms oxides other than divalent oxides, and the oxides are easily oxidized, i.e. are in a more stable state. Thus, these oxides are excluded from the preferred element L of the present invention. The description of the exclusion is clearly described in the requirement (II) for the element L in the present invention as "excluding transition metals." Here, the standard enthalpy of formation ∆Hf° will be described briefly. Typically, a chemical reaction is represented by the following chemical equation (2): H2 (gas) + (1/2) O2 (gas) = H2O (liquid) (2) The left side of the chemical equation is referred to as original system, and the right side thereof is referred to as product. The coefficient attached to a molecule is called a stoichiometric coefficient. Heat moves in and out associated with a chemical reaction of the system at a constant temperature is called heat of reaction, and heat of reaction at a constant pressure is called heat of reaction at constant pressure. In most cases, under typical laboratory conditions, heat of reaction is measured at a constant pressure, therefore, heat of reaction at constant pressure is typically used. Heat of reaction at constant pressure is equal to the difference in enthalpy "∆H" between product and original system. A reaction expressed as ∆H > 0 is referred to as endothermic reaction, and a reaction expressed as ∆H referred to as exothermic reaction. Heat of reaction generated from a reaction forming a compound from a chemical element of the compound is called heat of formation or enthalpy of formation. Heat of reaction generated from a reaction forming one mole compound in standard condition from a chemical element of the compound is called standard enthalpy of formation. In the standard condition, the standard enthalpy of formation is marked with a designated temperature, typically 298K, under a pressure of 0.1MPa or nearly equal to 1 atmospheric pressure, and the standard enthalpy of formation is represented by the symbol of ∆Hf°. In the standard condition, it is ruled that enthalpies of respective chemical elements are zero. Thus, it can be said that the smaller the value of standard enthalpy of formation of oxide of an element is or the greater in negative value of the standard enthalpy of formation is, the oxide is more stable and the element is easily oxidized. The specific values of the standard enthalpy of formation are written, for example, in "Electrochemistry Handbook Vol. 5" (Denkikagaku Binran) edited by The Electrochemistry Society of Japan, Maruzen). Since the standard enthalpy of formation ∆Hf° varies depending on with what valence an intended element takes in a molecule to form an oxide thereof, the above-noted requirements are applied when the standard enthalpy of formation of oxides of respective elements is determined in the present invention. Examples of elements having a Pauling's electronegativity value of 1.65 or more and the standard enthalpy of formation of oxides thereof ∆Hf0 of -1,000 (kJ/mol) or more include Zn, Cd, and In. For the Pauling's electronegativity value according to the definitions of the present invention, these elements respectively are determined as follows: Zn (1.65), Cd (1.69), and In (1.78). For the standard enthalpy of formation ∆Hf0 according to the definitions of the present invention, they are respectively determined as follows: Zn (-348kJmon), Cd (-258kJmol-1), and In (-925kJmorl-1). The atomic number ratio of the total amount of the element L to bismuth is preferably 1.25 or less. Since a recording layer of the present invention is based on the assumption that it comprises bismuth as the principal constituent of the recording layer and also comprises a Bi oxide, when the atomic number ratio of the total amount of the element L to bismuth is more than 1.25, intrinsic recording and reproducing properties may not be obtained. Further, adding B, P, Ga, Se, Pd, Ag, Sb, Te, W, Pt, and Au as an element L to the recording layer is preferable from the perspective of improving storage stability of the recording layer. The improvement of storage stability is very likely to be caused by the fact that it is hard to break a once-formed crystalline structure by strengthening binding force between the atoms or when atoms in different size reside side-byside, it makes the crystalline structure stable because smaller atoms in size can reside in lattices of greater atoms in size. In particular, elements such as B and Pd are bound to oxygen, and when making Bi and O exist together is effective in stabilizing an amorphous structure. It may consequently stabilize the structure of the crystal, which leads to improvements in storage stability of the write-once-read-many recording medium. In a write-once-read-many optical recording medium of the present invention, it is preferred that recording and reproducing of information be performed through the use of a laser beam at a wavelength of 680nm or less. Unlike dyes, the recording layer of the present invention has an appropriate absorption coefficient at a wide range of wavelengths and has a high refractive index, and therefore it is possible to perform recording and reproducing with a laser beam at a red-laser wavelength of 680nm or less as well as to achieve excellent recording and reproducing properties and high-reliabilities. Among them, the most preferable advantage is to perform recording and reproducing with a laser beam at a wavelength of 450nm or less. This is because the recording layer having bismuth as the principal constituent and having a Bi oxide has a complex refractive index particularly suitable for a write-once-read-many optical recording medium used with a laser beam at a wavelength of 450nm or less. A write-once-read-many optical recording medium according to the present invention preferably has the following configurations, however they are not particularly limited to the configurations. (A) Substrate, recording layer, upper coating layer, and reflective layer (B) Substrate, under coating layer, recording layer, upper coating layer, and reflective layer (C) Substrate, reflective layer, upper coating layer, recording layer, and cover layer (D) Substrate, reflective layer, upper coating layer, recording layer, under coating layer, and cover layer. Further, on the basis of the above configurations, a structure of layers may be formed in a multi-layered structure. For example, when formed in two-layered based on the configuration of (A), it may has a configuration as follows: Substrate, recording layer, upper coating layer, reflective layer or semi-transparent layer, binder layer, recording layer, upper coating layer, reflective layer, and substrate. For the under coating layer and the upper coating layer, the following oxides and nonoxides are available' examples of the oxides include simple oxide such as B2O5, Sm2O3, Ce2O3, Al2O3, MgO, BeO, ZrO2, UO2, and ThO2; silicate such as SiO2, 2MgOSiO2, MgO.SiO2, CaO.SiO3, ZrO2SiO2, 3Al2O3.2SiO2, 2MgO.2Al2O3-5SiO2, Li2O.Al2O3-4SiO2; double oxide such as Al2TiO5, MgAl2O4, Ca10 (PO4)6(OH)2, BaTiO3, LiNbO3, PZT [Pb (Zr, Ti) O3], PLZT [(Pb, La) (Zr, Ti) O3],and ferrite. Examples of the nonoxides include nitrides such as Si3N4, A1N, BN, and TiN' carbides such as SiC, B4C, TiC, and WC; borides such as LaB6, TiB2, and ZrB2; sulfides such as ZnS, CdS, and MoS2; silicides such as MoSi2; and carbons such as amorphous carbon, graphite, and diamond. Organic materials such as dyes and resins can also be used for the under coating layer and the upper coating layer. Examples of the dyes include polymethine dyes, naphthalocyanine dyes, phthalocyanine dyes, squarylium dyes, chroconium dyes, pyrylium dyes, naphthoquinone dyes, anthraquinone (indanthrene) dyes, xanthene dyes, triphenylmethane dyes, azulene dyes, tetrahydrocholine dyes, phenanthrene dyes, triphenothiazine dyes, azo dyes, formazan dyes, and metal complexes of these compounds. Examples of the resins include polyvinyl alcohols, polyvinyl pyrrolidones, cellulose nitrates, cellulose acetates, ketone resins, acrylic resins, polystyrene resins, urethane resins, polyvinyl butyrals, polycarbonates, and polyolefins. Each of these resins may be used alone or in combination with two or more. A layer which comprises organic materials can be formed by means of vapor depositions, sputtering, CVD, i.e. Chemical Vapor Deposition, coating of a solvent or the like, which are typically used. When a coating method is used, the above-noted organic materials and the like are dissolved in an organic solvent and the solvent is coated by a commonly used coating method such as spraying, roller-coating, dipping, and spin-coating. Examples of typical organic solvents to be used include alcohols such as methanol, ethanol, and isopropanol; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; amides such as N, N-dimethylacetoamide, and N, N-dimethylformamide; sulfoxides such as dimethylsulfoxide; ethers such as tetrahydrofuran, dioxane, diethyl ether, and ethylene glycol monomethyl ether; esters such as methyl acetate, and ethyl acetate; aliphatic halocarbons such as chloroform, methylenechloride, dichloroethane, carbon tetrachloride, and trichloroethane; aromatic series such as benzene, xylene, monochlorobenzene, and dichlorobenzene; cellosolves such as methoxyethanol, and ethoxyethanol; and hydrocarbons such as hexane, pentane, cyclohexane, and methylcyclohexane. For the reflective layer, light reflection materials having high reflectance against laser beams are used. Examples of the light reflection materials include metals such as Al, Al-Ti, Al-In, Al-Nb, Au, Ag, and Cu, semimetals, and alloys thereof. Each of these materials may be used alone and in combination with two or more. When a reflective layer is formed with an alloy, it is possible to prepare it by using an alloy as a target material and by sputtering. Besides, it is also possible to form a reflective layer by tip-on-target method (for example, a Cu tip is placed on an Ag target material to form a reflective layer), and by cosputtering (for example, an Ag target and a Cu target are used). It is also possible to alternately stack low-refractive index layers and high-refractive index layers and form a multi-layered structure to use it as a reflective layer. A reflective layer may be formed, for example, by sputtering, ion-plating, chemical vapor deposition, and vacuum deposition. The reflective layer preferably has a thickness of 5nm to 300nm. Materials for a substrate are not particularly limited, as long as they have excellent thermal and machine properties, and when recording and reproducing is performed from the side of a substrate and through the substrate, they have also excellent light transmission properties. Specifically, examples thereof include polycarbonates, polymethyl methacryiates, amorphous polyolefins, cellulose acetates, polyethylene terephthalate, of which polycarbonates and amorphous polyolefins are preferable. The thickness of the substrate varies depending on the application and is not particularly limited. Materials for a protective layer to be formed on a reflective layer, an optically transparent layer or the like are not particularly limited, provided that the material can protect reflective layers, optically transparent layers or the like from external forces. Examples of organic materials include thermoplastic resins, thermosetting resins, electron beam curable resins, and ultraviolet curable resins. Examples of inorganic materials include SiO2, Si3N4, MgF2, and SnO2. On a reflective layer and/or an optically transparent layer and the like, a protective layer can be formed using a thermoplastic resin and/or thermosetting resin. First, a thermoplastic resin and/or a thermosetting resin are dissolved in a suitable solvent to prepare a coating solution. Then, the coating solution is coated to a reflective layer and/or an optically transparent layer and dried to thereby form a protective layer. A protective layer using an ultraviolet curable resin can be formed by directly coating an ultraviolet curable resin to a reflective layer and/or an optically transparent layer or dissolving an ultraviolet curable resin in a suitable solvent to prepare a coating solution and coating the coating solution to a reflective layer and/or an optically transparent layer, and then irradiating ultraviolet ray to the coating solution to harden it. For ultraviolet curable resins, for example, acrylate resins such as urethane acrylates, epoxy acrylates, and polyester acrylates can be used. Each of these materials may be used alone and in combination with two or more and may be formed in not only a single layer but also in a multi-layered structure. For a method for forming a protective layer, coating methods such as spin-coating and casting, sputtering, chemical vapor deposition, and the like are used, of which spin-coating is preferable. The thickness of the protective layer is typically 0.1µm to 100µm, however it is preferably 3pm to 30pm in the present invention. Further, a substrate may be disposed on the surface of a reflective layer or an optically transparent layer. The reflective layer and the optically transparent layer may be arranged so as to face each other. Two sheets of optical recording media may be laminated after arranging a reflective layer and an optically transparent layer so as to face each other. In addition, an ultraviolet curable resin layer, an inorganic resin layer or the like may be formed on a mirror surface side of a substrate to protect the surface and/or to prevent dust or the like from attaching thereto. An optically transparent layer or a cover layer should be formed when a lens with a high numerical aperture is used for higher recording density. With increasing numeral aperture, the portion through which the reproducing light passes must have a reduced thickness. This is because the allowance in the aberration occurs when the perpendicular direction to the surface of the medium deviates from the optical axis of an optical pickup, the deviation angle so called a tilt angle is reduced with increasing numerical aperture. The tilt angle is in proportional to the square of the product of reciprocal of the wavelength of an optical source multiplied by the numerical aperture of the objective lens and is susceptible to the aberration due to the thickness of the substrate. To reduce the aberration due to the thickness of the substrate, the thickness of the substrate is reduced. For this purpose, some optical recording media are presented, for example, an optical recording medium in which a substrate, a recording layer with concave convex or irregularities formed thereon, a reflective layer, and an optically transparent layer or a cover layer which is a layer transmitting light are disposed in this order in a laminar structure, in which the reproducing light is irradiated from the optically transparent layer to reproduce information in the recording layer. Another example is an optical recording medium in which a reflective layer, a recording layer, and an optically transparent layer or a cover layer having optical transparency are disposed in this order on a substrate, in which the reproducing light is irradiated from the optically transparent layer to reproduce information in the recording layer. These optical recording media can allow the use of an objective lens with a high numerical aperture by reducing the thickness of the optically transparent layer. Namely, higher-density recording can be performed by recording and/or reproducing information on a medium having a thin optically transparent layer, in which the reproducing light is irradiated from the optically transparent layer. The cover layer may typically comprise a polycarbonate sheet or an ultraviolet curable resin. The cover layer used herein may include a binder layer for binding the cover layer to an adjacent layer. Hereinafter, a sputtering target suitably used for the optical recording medium of this invention having a recording layer which comprises Bi, Fe, and oxygen, and the production method will be disclosed in detail. A sputtering target according to the present invention comprises Bi, Fe, and 0. The sputtering target is suitably used for forming a recording layer of an optical recording medium which performs recording and reproducing with a laser beam at wavelengths of 550nm or less. The relationship between extinction coefficient k which is one of optical constants and is a coefficient representing a light absorption degree and wavelength of light was examined. When the value of k is zero, it represents that there is no absorption of light, and with increasing value of k, absorption light increases. A recording layer formed using the sputtering target of the present invention which comprises Bi, Fe, and O has absorption light of nearly zero at a wavelength of 600nm or more, and has approx. zero value of k, therefore, it is hard to record information. However, the value of k abruptly becomes greater at wavelengths shorter than 550nm and excellent recording is enabled. Particularly, in the vicinity of a wavelength of 400nm, absorption of light becomes substantially greater, fairly excellent properties are exhibited as a recording layer of an optical recording medium. The sputtering target of the present invention preferably comprises a Bi oxide and a Fe oxide or a complex oxide of Bi and Fe. The sputtering target may comprise a complex oxide of Bi and Fe and further comprises one or more selected from a Bi oxide and a Fe oxide. There may be cases where a Bi oxide or a Fe oxide remains in a sputtering target without making complex of Bi and Fe in a well-balanced condition, depending on the composition. An aspect of the sputtering target comprises a Bi oxide and a Fe oxide, or another aspect of the sputtering target comprises a complex oxide of Bi and Fe, or still another aspect of a sputtering target comprises a complex oxide of Bi and Fe and further comprises one or more selected from a Bi oxide and a Fe oxide. In any of the aspects stated above, an optical recording medium produced using such a sputtering target as stated above exhibits excellent properties. In particular, by including a Bi oxide as a constituent of a recording layer, the produced recording layer can include the Bi oxide. Since a recorded mark can be excellently recorded by precipitating Bi metal, it is remarkably effective to use a sputtering target which comprises a Bi oxide. The sputtering target of the present invention preferably comprises one or more selected from a Bi oxide, a Fe oxide, and a complex oxide of Bi and Fe, and the oxide is preferably an oxide having a smaller amount of oxygen compared to the stoichiometric composition. Examples of the oxide which is a Bi oxide, a Fe oxide, or a complex oxide of Bi and Fe and has a smaller amount of oxygen compared to the stoichiometric composition include BiOx (x 3), Bi25FeOx (x amount of oxygen to enhance metallic properties, sputtering is enabled through the use of a direct-current power (DC power). DC sputtering has advantages in that apparatuses used for the manufacturing are cheaper than those used in RF sputtering and the apparatuses used in DC sputtering are to be more downsized than those used in RF sputtering. When a layer is formed by DC sputtering, the amount of oxygen in the layer can be controlled by externally introducing oxygen. By employing DC sputtering, recording properties are easily controlled. A sputtering target having a smaller amount of oxygen compared to the stoichiometric composition is calcined after a Bi metal and Fe2O3 being mixed. It is also possible to produce a target having a small amount of oxygen by the methods such as by reducing the amount of oxygen remaining in the target by controlling the temperature. The sputtering target of the present invention preferably comprises one or more selected from BiFeO3, Bi25FeO40, and Bi36Fe2O57 as the principal constituent. As mentioned above, an optical recording medium using a recording layer which comprises Bi, Fe, and oxygen exhibits excellent recording properties. There have been a sputtering target containing Bi, Fe, and oxygen for magnetic optical recording and having a garnet structure, however, such sputtering target do not comprise the compounds used in the present invention as the principal constituent from the perspective of the composition for forming a garnet structure. A sputtering target which comprises three elements of Bi, Fe, and oxygen has not been used so far, because it is impossible to form a garnet structure only with these three elements. In addition, the present invention does not relate to magnetic optical recording media, and the present invention relates a sputtering target used for forming a layer which primarily comprises Bi, Fe, and oxygen, which is hereafter referred to as a BiFeO layer, for an optical recording medium without using magnetism, particularly for a write-once-read-many optical recording medium capable of high-density recording. The sputtering target preferably consists of three elements of Bi, Fe, and oxygen. In this case, the sputtering target may comprise impure elements other than the three elements, however, it is not preferred to include the microelements that would impair properties of layers. A sputtering target of the present invention is produced by mixing and calcining powder of oxides of raw materials. It is possible to produce the sputtering target using calcine powder of a Bi oxide and powder of a Fe oxide. It is also possible to produce a sputtering target by producing powder of a compound which primarily comprises three elements of Bi, Fe, and oxygen and then calcining the powder. As the calcining method, press-heat calcination processes such as hot-pressing, hot isostatic pressuring or HIP may be used. For calcination temperatures, higher temperatures are preferred to reinforce the strength of a sputtering target, however, in the case of a compound which comprises three elements of Bi, Fe, and oxygen, separation of the phase and/or fusion occur at a temperature of approx. 800°C or more, and it is difficult to uniformly calcine it. Then, the calcination temperature must be controlled not so as to be more than approx. 750°C. In particular, an optical recording medium in which a BiFeO layer is formed using a sputtering target which comprises one or more selected from BiFeO3, Bi25FeO40, and Bi36Fe2O57 as the principal constituent for the recording layer exhibits excellent properties. The presence of BiFeO3, Bi25FeO40, and Bi36Fe2O57 can be checked by means of X-ray diffraction. For the radiation source, Cu is used to measure it with the angle of 29 of an X-ray diffractometer from 5° to 60°. The term the principal constituent means that the highest content or the highest % by mass is contained in a compound. Typically, a material exhibiting the highest diffraction peak as a result of an X-ray diffraction analysis can be determined as the principal constituent, however, the content may not be in proportion to the scale of diffraction peak. The case of a sputtering target which comprises two or more elements selected from BiFe03, Bi25FeO40, and Bi36Fe2O57 as the principal constituent means that each content of these two or more compounds is same and the content of these two or more compounds is higher than that of other components. The sputtering target preferably comprises Bi2O3 and/or Fe2O3. An optical recording medium with a BiFeO layer formed using a sputtering target which comprises these compounds for the recording layer exhibits excellent properties. The presence of Bi2O3 and/or Fe2O3 can be checked by means of X-ray diffraction. For the radiation source, Cu is used to measure it at an angle of 29 from 5° to 60°. In the. X-ray diffraction analysis for identifying constituents of BiFeO3, Bi25FeO40, Bi36Fe2O57, and Bi2O3, and Fe2O3, when the angle of 2θ where a diffraction peak is expected to be detected is defined as θ1, a substance actually having a diffraction peak within the range of θ1 ± 1 degree is determined to be included in the above-noted compounds. In X-ray diffraction analysis, the lattice constant varies, and misalignments are developed in angles at which diffraction peaks appear depending on causes such as measurement temperature, internal stress of the layer, X-ray wavelength error of measurement, and shift of composition. For well-known substances, at what angles diffraction peaks are detected can be known through the use of ASTM (American Society for Testing and Material) cards and JCPDS search. When a sample is analyzed to identify the component, ASTM cards and JCPDS card charts are widely used. The term, JCPDS stands for Joint Committee on Powder Diffraction Standards, and it is a chart of X-ray diffraction patterns distributed by the organization called International Center for Diffraction Data, and a number of charts of diffraction patterns of standard substances are stored for search. An X-ray diffraction pattern chart of a sample having unknown components is compared to the charts of standard substances, and to what chart of standard substance the X-ray diffraction pattern chart corresponds or is closely related is determined. With this comparison, the substance of the sample is identified. The identification method using JCPDS card charts is a method widely used in the world, as shown in X-ray Diffraction Analysis Rules, and descriptions of X-ray Diffraction Analysis Rules in Japanese Industrial Standard (JIS) as well as in X-ray Diffraction Analysis - Ceramics Basic Structures 3 edited by Tokyo Institute of Technology. In the measurement, to what chart of standard substance the X-ray diffraction pattern of the substance having unknown components corresponds or is closely related is examined, and then the substance is identified. nλ = 2d sinθ In the above equation, n represents a positive integral number, λ, represents a wavelength, d represents distance of lattice plane, and θ represents a glancing angle or a supplementary angle of incident angle. Since the Bragg principle is approved, a peak of diffraction also appears at the positions where n represents an integral multiple at the side of higher-angle of 2θ. The identification of substances is enabled by analyzing peaks of positions represented by integral multiple at the same time of the analysis of diffraction peaks based on 2θ. As described above, in X-ray diffraction analysis, lattice constant varies, and misalignments are developed in angles at which diffraction peaks appear due to measurement temperature, internal stress of the layer, X-ray wavelength error of measurement, shift of composition, and the like, therefore, when a substance having a peak near at the angle where a diffraction peak appeared at a known substance, it can be determined that the substance corresponds to the known substance having that diffraction peak. For the misalignment of diffraction peak, for example, peaks having 29 = 22.491 degree sought from the standard chart of BiFeO3 (reference code 20-0169) was studied. The results of measurement on four sputtering targets produced in the same manner and under same conditions were respectively observed. The degree 2θ of the standard data of BiFeO3 is 22.491°. On the other hand, these targets showed very slight misalignments of 22.380°, 22.500°, 22.420°, and 22.420°, respectively. This study showed that the misalignments of approx. ± 1 degree are within the error range of measurement, when repeatedly measured. Therefore, even though such misalignments in angle of ± 1 degree diffraction peak are observed, it can be identified as a known peak. For example, when a substance having a peak with a misalignment of ± 1 degree from 22.491°, it can be determined that the BiFeO3 described in the present invention are included in the substance. The sputtering target which comprises Bi, Fe, and O and has a diffraction peak at 22.380 ≤ 29 ≤ 22.500 is particularly effective in recording properties. Similarly, according to the standard data of Bi36Fe2O57, Bi36Fe2O57 has a peak at 2θ = 27.681 degree. The above-noted four targets were measured, and it was found that they had a peak at 27.65°, 27.64°, 27.76°, and 27.67°. According to the standard data of Bi25FeO40, it has a peak at 2θ = 27.683 degree. These four targets were measured, and it was found that they had a peak at 27.66°, 27.64°, 27.76°, and 27.68°, respectively. Since misalignments of approx. ± 1 degree are caused in the repeated measurements, it is considered that the misalignments of ± 1 degree in 2θ of any of these substances fall under the error range of measurement. Preferably, the sputtering target does not further comprise Bi2Fe4O9. With increasing content of Bi, the content of Bi2Fe4O9 tends to lower. The presence of Bi2Fe4O9 can be checked by means of X-ray diffraction. For the radiation source, Cu is used to measure it with the angle of 29 of an X-ray diffractometer from 5° to 60°. It is found that an optical recording medium with a BiFeO layer formed for a recording layer using a sputtering target in which the presence of Bi2Fe4O9 is recognized by means of X-ray diffraction does not satisfactorily exhibit recording properties and does not suit high-density recording. The sputtering target preferably has a content of Co, Ca and Cr less than the detection limit. When a sputtering target which comprises impurities is used, a formed layer also comprises elements of impurities. In optical recording, recording is performed by absorbing laser beam irradiation into a recording layer to elevate the temperature at the recording layer and to induce physical or chemical changes. The term physical or chemical changes means changes such as crystallization or the like. A layer used for a recording layer containing impurities is not preferably used because crystallization temperature varies, and the size of crystal differs at the time of crystallization. For detection of impurities, the composition is quantitatively analyzed by means of the inductively coupled plasma emission spectrometry. This analysis is suitable when analyzing element in minute amount, and even when using this analysis, the sputtering target preferably has a content of Co, Ca and Cr less than the detection limit. The sputtering target preferably has a packing density of 65% to 96%. The higher the packing density of the sputtering target is, the higher the strength of the target is, and the time for forming a layer tends to be shortened because of its high-density elements, at the same time, the difference in composition between the target and the formed layer tends to increase. There may be cases where the composition of the target can be close to that of the layer by reducing the density of the target, however, a density-reduced target causes problems with not only delayed layer-forming speed but also causes a problem that the target itself goes brittle to be broken when forming a layer. Herein, the term packing density represents a value obtained as density by comparing the weight of a target actually produced to the weight of the target calculated when the weight of the target is dominated by an intended substance at 100%. Table 1 shows packing densities of a target, conditions of the target during the time for forming a layer or after the forming, and recording properties of a recording medium with a layer formed therein when using a Bi10Fe5Ox target having a diameter of 76.2mm and a thickness of 4mm. When the packing density was 50% or less, the target was not able to be formed as a sputtering target even after calcinations. With a packing density of 61%, the target was able to be formed, however, it was easily broken immediately after applying an electric power of 100W. With a packing density of 98%, the density was so high and the target was so hard that it was easily broken. With a packing density of 65% to 96%, the target was able to be formed with no problem, and excellent recording properties were exhibited. Even with a packing density of 61% and 98%, excellent recording properties were obtained by forming a layer under the conditions ensuring no occurrence of fracture of target in forming a layer. As can be seen from the above results, the packing density of the sputtering target is preferably 65% to 96%. The sputtering target preferably has an atomic ratio of Bi to Fe satisfying the relation of Bi/Fe ≥ 0.8. An optical recording medium having a BiFeO-layer formed using the target exhibits excellent properties and is particularly suitable for high-density recording. Theoretically, the upper limit of the ratio Bi/Fe is 25 based on the assumption that a sputtering target which comprises Bi25FeO40 as the principal constituent is used, however, virtually, it is 15 or so. A sputtering target production method of the present invention relates to a method for producing the BiFeO target of the present invention by calcining powder of Bi2O3 and Fe2O3. Bi2O3 naturally exists as an oxide of Bi, and Fe2O3 exists as an oxide of Fe. These powers are crushed by a dry process or a wet process and then classifying into a uniformly sized particle diameter. Next, the powders are mixed, heated, and pressed to put into shape and then calcined at a maintained temperature of 750°C in the atmosphere. The strength of a sputtering target can be improved by repeatedly performing the process of re-crushing a calcined target and forming the shape by heating and pressing. A sputtering target can be obtained by bonding the target calcined as above to a backing plate made from oxygen-free copper by means of metal bonding or resin bonding. The present invention also proposes an optical recording medium which comprises a BiFeO layer formed using the sputtering target of the present invention. In the optical recording medium, necessary layers are formed on a resin substrate which comprises a polycarbonate or the like. Grooves and pits may be formed on the resin substrate for controlling tracking or the like. A BiFeO-layer is formed by applying radiofrequency power while introducing an argon gas in vacuo. Besides, a metallic layer and a protective layer for improving properties may be disposed as a reflective layer. The above paragraphs describe the sputtering target of the present invention with a focus on optical recording media, and the application of the sputtering target of the present invention is not limited to optical recording media and may be used for other purposes, as long as performance of the layer meets requirements. For example, the sputtering target can be used for forming a thin layer made from magnetic materials, for forming a thin-layer for producing an isolator for optical controlling, and for forming a thin layer for an optical switch. For a rough production line of forming the sputtering target, it is possible to take procedure steps of weighing of raw materials, mixing by a dry-process ball mill, hot-pressing, shape forming, and bonding can be used. It is also usable to take the procedure steps of weighing of raw materials, mixing by a dry-process ball mill, spray-drying, hot-pressing, shape forming, and bonding. According to the present invention, it is possible to present a write-once-read-many recording medium capable of recording small recording marks at higher degrees of modulated amplitudes with stability. With the use of the additional elements that have not been found so far, it is possible to provide a write-once-read-many optical recording medium having excellent recording and reproducing properties and reliabilities. Further, the present invention can provide a sputtering target suitable for arbitrarily forming a layer having a stable composition and structure, the production method thereof, and can also provide a high-density optical recording medium using the target. Hereinafter, the present invention will be described in detail on an optical recording medium which comprises a recording layer in which a material of the present invention represented by BiOx (0 write-once-read-many optical recording medium which comprises Bi, M and oxygen as constituent elements of a recording layer, referring to specific examples, however, the present invention is not limited to the disclosed examples. Example A-1 On a polycarbonate-substrate with a guide groove having a groove depth of 21nm formed thereon, a layer having the composition represented by BiOx (0 10nm was formed by sputtering to yield a write-once-read-many optical recording medium of the present invention. This layer was formed at a radiofrequency power of 100W and an Ar gas flow rate of 40sccm using a sputtering target having the composition of Bi2O3 and a diameter of 76.2mm. Recording was performed to the optical recording medium under the following conditions using an optical disk evaluation system DDU-1000 manufactured by PULSTEC INDUSTRIAL CO., LTD. having a lens numerical aperture of 0.65 at a wavelength of 405nm. • Modulation Mode: 1-7 modulation • Recording linear density: The shortest mark length (2T) = 0.231µm • Recording linear velocity: 6.0m/s • Waveform equalization: Normal equalizer As a result, an excellent jitter value of 9.9% was obtained in a consecutively recorded portion at a recording power of 5.2mW, and excellent binary recording properties having a modulated amplitude of 55% were realized. Example A-2 A write-once-read-many recording medium of the present invention was produced by sputtering a layer which comprises Fe and O, having a thickness of 10nm on a polycarbonate-substrate with a guide groove having a groove depth of 21nm formed thereon. This layer was formed at a radiofrequency power of 100W and an Ar gas flow rate of 40sccm using a sputtering target having the composition of Bi10Fe5Ox and a diameter of 76.2mm. The target was prepared by calcining a mixture of Bi2O3 and Fe2O3 at a ratio of 2:1. Theoretically it should have been Bi10Fe5O22.5, however, the amount of oxygen was unable to be measured with accuracy because of leaked oxygen in a calcination process, and therefore the oxygen is represented by Ox. The optical recording medium was evaluated under the following conditions using an optical disk evaluation system DDU-1000 manufactured by PULSTEC INDUSTRIAL CO., LTD. having a lens numerical aperture of 0.65 at a wavelength of 405nm. • Modulation Mode- 1-7 modulation • Recording linear density: The shortest mark length (2T) = 0.231pm • Recording linear velocity: 6.0m/s • Waveform equalization: Normal equalizer As a result, an excellent jitter value of 8.9% was obtained in a consecutively recorded portion at a recording power of 5.8mW, and excellent binary recording properties having a modulated amplitude of 52% were realized. Example A-3 Reflection EELS measurements were performed using a write-once-read-many optical recording medium produced and used for recording in Example A-1. A scanning Auger electron spectrometer PHI4300 manufactured by Perkin-Elmer was modified for the measurements. EELS stands for Electron Energy Loss Spectroscopy and is a spectroscopy system in which electrons are irradiated to a sample to measure an energy distribution of electrons scattered by interaction with the outer surface of the sample. When a primary electron of certain energy excites the inner shell of an atom to be measured, an electron of certain energy is discharged, resulting in the scattering of the primary electron. During this process, some energy is lost by the interaction with the neighboring atoms. Therefore, by examining the way the electrons are scattered, information such as radial distribution function of neighboring atoms can be obtained. The radial distribution function in the vicinity of an oxygen atom was measured based on the EELS spectrum obtained by EELS measurements. A radial distribution function represents a probability of existence of electron in the vicinity of an atom and enables calculations and presumptions of the valence and structure of the atom. Among analysis software packages based on the photoelectron multiple scattering theory, the FEFF software published by Washington University are widely used. The number of valences and structure thereof can be presumed by using this analysis software and referring the calculated values against actually measured values. FIG. 1 illustrates values of radial distribution function measured by the method as stated above. FIG. 2 shows radial distribution functions calculated using an FEFF. The diagram shows radial distribution functions respectively in the case of Bitrivalent which could be taken by Bi; the case of Bi-trivalent however taking a structure of β-Bi2O3; and the case of BiO2 of Bi-tetravalence. Comparing these radial distribution functions, peaks 1011 and 1012 in the vicinity of 6 angstrom in the recorded portion are distinctively shown. When comparing the two diagrams, the peaks 1011 and 1012 match each other. It proves that BiO2, i.e. BiO2 of tetravalent Bi exists in the recorded portion. A write-once-read-many optical recording medium having such a recording mark enables excellent recording with higher modulated amplitudes and realizing high-density recording. Hereinafter, the present invention will be further described in detail on a write-once-read-many optical recording medium using the element L used in the present invention as an additional element, referring to Examples and Comparative Examples, however, the present invention is not limited to the disclosed examples. (Examples B-1 to B18) A write-once-read-many optical recording medium was produced by taking a structure in which a polyolefin-substrate (ZEONOR manufactured by NIPPON ZEON CO., LTD.); a recording layer which comprises bismuth as the principal constituent of the elements constituting the recording layer and a Bi oxide; a heat-insulating layer; and a reflective layer were formed in a laminar structure, and using the following materials for these respective layers. A sputtering target was produced using raw materials in which Bi2O3 and oxides of the element shown in Table 2 were mixed at a ratio from 2:1 to 5:1, and then a recording layer was formed using this sputtering target so as to have a thickness approx. 7nm. Table 2 shows resulting respective Pauling's electronegativity values of each element L added to the respective recording layer, and the standard enthalpy values of formation ∆Hf0 of oxides of the element L, however, these values are based on the definitions of the present invention. When the Pauling's electronegativity value is 1.80 or more, the standard enthalpy of formation ∆Hf° of oxides of the element L is not important, and thus some elements in Table 2 have not their values of ∆Hf0. As described above, in the present invention, Pauling's electronegativity values and standard enthalpy values of oxide-formation of an element L were obtained with each valence fixed to each element group. The individual valences of the individual elements based on the definitions of the present invention are also written in Table 2. In Table 2, the term type A represents an element L which falls under the definition (I) of the present invention, and the term type B represents an element L which falls under the definition (II) of the present invention. A material in which ZnS-SiO2 was used for a heat-insulating layer at a ratio of 85:15 (mol%) and formed so as to have a thickness of 15nm. For a reflective layer, an Ag alloy was used and formed so as to have a thickness of 100nm. The track pitch of a polyolefin substrate was 0.437µm and the thickness was 0.6mm. Recording was performed to these optical recording media under the following conditions using an optical disk evaluation system DDU1000 manufactured by PULSTEC INDUSTRIAL CO., LTD. having a lens numerical aperture of 0.65 at a wavelength of 405nm. As a result, extremely excellent recording and reproducing properties, i.e. jitter values shown in Table 2 were realized. • Modulation Mode: 1-7 modulation • Recording linear density: The shortest mark length (2T) = 0.204pm • Recording linear velocity: 6.6m/s • Waveform equalization: Limit equalizer • Reproducing power: 0.5mW Next, these write-once-read-many optical recording media were left under the conditions at a temperature of 80°C and a relative humidity of 85% for 100 hours to measure the amount of change in jitter value. The amount of change in jitter value is calculated as follows: (Jitter value after a storage test) - (Initial jitter value) Table 2 shows the results. As is clear from Table 2, the elements satisfying the definition (I) for the element L of the present invention respectively demonstrated an excellent initial jitter value and a small degree of degradation in jitter value, suffering from the storage test. It was also demonstrated that the standard enthalpy of formation of oxides "∆Hf0" may take any values, as long as the definition (I) is satisfied. Also, the elements satisfying the definition (II) for the element L of the present invention respectively demonstrated an excellent initial jitter value and a small degree of degradation in jitter value suffering from the storage test. In the Examples above, the wavelength for recording and reproducing was set at 405nm, however, recording was also excellently performed with a jitter value of 9% or less for a laser beam at a wavelength of 660nm by adjusting the thickness of a heat-insulation layer from 15nm to 120nm. In the test, the track pitch of the substrate was 0.74µm, and the recording and reproducing conditions were based on those of DVD+R. The amount of increased jitter value obtained in the same storage test as the Examples above gave results substantially similar to those shown in Table 2. (Comparative Example B-1) A write-once-read-many optical recording medium was produced in the same manner as Example B-1, provided that the principal constituent is bismuth, and a recording layer formed by sputtering through the use of a metallic bismuth target was used instead of the recording layer containing a Bi oxide. The optical recording medium was then evaluated. The analysis by X-ray photoelectron spectroscopy showed that no bismuth oxide was detected in the recording layer except in the interface between the substrate and the recording layer and the interface between the recording layer and ZnS-SiO2. Thus, it proved that the recording layer produced in Comparative Example B-1 did not include a Bi oxide. As a result of the measurements of recording and reproducing properties, the initial jitter value exceeded 15%, and it was impossible to measure jitter after the storage test. From the results, the importance of a recording layer which comprises not only bismuth as the principal constituent but also a Bi oxide is confirmed. (Example B-19) On a polycarbonate substrate with a guide groove having a groove depth of 50nm and a track pitch of 0.40µm formed thereon, a ZnS-SiO2 layer, i.e. an under coating layer having a thickness of 65nm and a BiPdO layer, i.e. a recording layer having a thickness of 15nm were disposed in this order in a laminar structure by sputtering. The atomic number ratio of Bi to Pd, or Bi:Pd was approx. 3:1. Next, on the recording layer, an organic-material layer, i.e. an upper coating layer which comprises a pigment or a dye represented by the following Chemical Formula 1 was formed by spin-coating so as to have the average thickness of approx. 30nm. On the organic-material layer, a reflective layer which comprises Ag, having a thickness of 150nm was disposed by sputtering, and then a protective layer made from an ultraviolet curable resin having a thickness of 5pm was further disposed on the reflective layer by spin-coating so as to thereby yield a write-once-read-many recording medium of the present invention. A pigment or a dye represented by Chemical Formula 1 is typically used for materials of conventional DVD±R, and such materials respectively have little absorption in blue-laser wavelengths. Recording and reproducing were performed to the optical recording medium under the recording and reproducing conditions in accordance with HD and DVD-R to evaluate it using an optical disk evaluation system DDU-10 having a numerical aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm.. The measurement of the optical recording medium resulted in an excellent value of PRSNR 22 at a recording power of 5.8mW, and excellent recording and reproducing properties were achieved. (Example B-20) On a polycarbonate substrate with a guide groove having a groove depth of 20nm and a track pitch of 0.32µm formed thereon, a reflective layer made from Ag having a thickness of 100nm, a ZnS-SiO2 layer, i.e. an upper coating layer having a thickness of 16nm and a BiPdO layer or a recording layer having a thickness of 7nm were disposed in this order in a laminar structure by sputtering. The atomic number ratio of Bi to Pd, or Bi:Pd was approx. 3:1. Next, a cover layer made from a resin having a thickness of 0.1mm was laminated on the recording layer to yield a write-once-read-many recording medium of the present invention. Recording and reproducing were performed to the optical recording medium under the recording and reproducing conditions in accordance with BD-R to evaluate it using an optical disk evaluation system having a numerical aperture of 0.85 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm.. The measurement of the optical recording medium resulted in an excellent jitter value of 6.0% at a recording power of 7.0mW, and excellent recording and reproducing properties were achieved. (Example B-21) A write-once-read-many optical recording medium of the present invention was produced in the same manner as Example B-19, provided that a BiBO layer was used for the recording layer. The write-once-read-many optical recording medium was then evaluated. The atomic number ratio of Bi to B, or Bi:B was approx. 2:1. The measurement of the optical recording medium resulted in an excellent value of PRSNR 23 at a recording power of 5.6mW, and it exemplified that excellent recording and reproducing properties are realizable with the optical recording medium. (Example B-22) A write-once-read-many optical recording medium of the present invention was produced in the same manner as Example B-20, provided that a BiBO layer was used for the recording layer. The write-once-read-many optical recording medium was then evaluated. The atomic number ratio of Bi to B, or Bi:B was approx.. 2:1. The measurement of the optical recording medium resulted in an excellent jitter value of 5.9% at a recording power of 6.7mW, and excellent recording and reproducing properties were achieved. (Example B-23) On a polycarbonate substrate with a guide groove having a groove depth of 50nm and a track pitch of 0.32pm formed thereon, a reflective layer made from Ag having a thickness of 100nm was formed by sputtering, an organic-material layer, i.e. an upper coating layer which comprises a pigment or a dye represented by Chemical Formula 1 was formed by spin-coating so as to have the average thickness of approx. 30nm, then a BiBO layer, i.e. a recording layer having a thickness of 15nm, and a ZnS-SiO2 layer, i.e. an under coating layer were disposed in this order in a laminar structure by sputtering. The atomic number ratio of Bi to B, or Bi:B was approx. 2:1. Next, a cover layer made from a transparent resin having a thickness of 100nm was laminated on the recording layer to thereby yield a write-once-read-many recording medium of the present invention. Recording and reproducing were performed to the optical recording medium under the recording and reproducing conditions in accordance with BD-R to evaluate it using an optical disk evaluation system having a numerical aperture of 0.85 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm.. The measurement of the optical recording medium resulted in an excellent jitter value of 6.5% at a recording power of 4.8mW, and excellent recording and reproducing properties were achieved. (Example B24) On a polycarbonate substrate with a guide groove having a groove depth of 40nm and a track pitch of 0.74pm formed thereon, a BiBO layer, i.e. a recording layer having a thickness of 15nm and a ZnS-SiO2 layer, i.e. an upper coating layer having a thickness of 40nm were disposed in this order in a laminar structure by sputtering. The atomic number ratio of Bi to B, or Bi:B was approx.. 2:1. Next, on the recording layer, a reflective layer made from Ag having a thickness of 100nm and a protective layer made from an ultraviolet curable resin having a thickness of approx. 5pm were disposed to thereby yield a write-once-read-many recording medium of the present invention. Recording and reproducing were performed to the optical recording medium under the recording and reproducing conditions in accordance with DVD+R to evaluate it using an optical disk evaluation system DDU-1000 having a numerical aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 660nm. The measurement of the optical recording medium resulted in a jitter value of 7.2% at a recording power of 12.0mW, and excellent recording and reproducing properties were achieved. (Example B-25) A write-once-read-many optical recording medium of the present invention was produced in the same manner as Example B-24, provided that Sb was added to the materials of the recording layer. The write-once-read-many optical recording medium was then evaluated. The atomic number ratio of Bi to Sb, or Bi:Sb was approx.. 4:1. The measurement of the optical recording medium resulted in a jitter value of 7.6% at a recording power of 10.0mW, and excellent recording and reproducing properties were achieved. (Example B-26) On a polycarbonate substrate with a guide groove having a groove depth of 20nm and a track pitch of 0.437µm formed thereon, a BiPdO layer, i.e. a recording layer having a thickness of 5nm and a ZnS-SiO2 layer, i.e. an upper coating layer having a thickness of 15nm were disposed in this order in a laminar structure by sputtering. On the ZnS_SiO2 layer, a reflective layer which comprises Ag, having a thickness of 100nm was disposed by sputtering, and a protective layer which comprises an ultraviolet curable resin, having a thickness of approx. 5pm was further disposed by spin-coating to thereby yield a write-once-read-many optical recording medium of the present invention. In Example B-26, the atomic number ratio of the total amount of Pd to bismuth was changed in the recording layer to evaluate jitter value. The recording and reproducing conditions were same as those in Examples B-l to B-18. The measurement of the optical recording medium, as shown in FIG. 3, exemplified that an excellent jitter value can be obtained in the range where the atomic number ratio of the total amount of Pd relative to bismuth is 1.25 or less. The value 1.25 is represented by the dotted-line in FIG. 3. In addition, it was exemplified that the elements defined in the present invention except for Pd respectively have a tendency similar to the above. Next, the present invention relating to the sputtering target will be specifically described referring to examples and comparative examples, however, the present invention is not limited to the disclosed examples. Example C-1 Powders of Bi2O3 and Fe2O3 were mixed so that the atomic ratio of Bi to Fe was 6:5 and then mixed in a ball mill by dry-process for 1 hour. The mixed powder was formed by pressing at 100MPa to 200MPa and then calcined at 750°C for 5 hours in the atmosphere to yield a sputtering target. The target had a diameter of 152.4φ and a thickness of 4mm. The target was bounded to a backing plate made from oxygen-free copper by metal bonding to yield a sputtering target 1. The sputtering target had a packing density of 75%. X-ray diffraction pattern of the sputtering target was measured. The measurement conditions are as shown in Table 3. FIG. 4 shows the measurement results. To identify diffraction peak positions obtained in the measurement, the diffraction peak positions were searched and checked against those of known substances. The diagram marked with (a) at the top of FIG. 4 represents the diffraction pattern of target 1. The diagram marked with (b) represents diffraction peak positions of BiFeO3 which are based on known data. In X-ray diffraction analysis, data on the position of diffraction lines and on the intensities of substances have been compiled in a database from the source of data on X-ray diffraction measured in the past. Therefore, the measured substance can be identified by comparing the measured diffraction result with the previous data. As a result of a search after comparing BiFeO3 data marked with (b) with the measured data marked with (a), it was found that diffraction peaks with a mark "°" were those of BiFeO3. Similarly, the diagram marked with (c) is the known data of Fe2O3, and the diagram marked with (d) is the known data of Bi2O3. Similarly, diffraction peaks of Bi2O3 and Fe2O3 were identified. The greatest peak was the one corresponding to BiFe2O3, and this demonstrated that this compound is the principal component. Further, the sputtering target was subjected to the ICP analysis, i.e. the inductively coupled plasma emission spectrometry. Part of the sputtering target was dissolved in an aqua regia as a sample and then diluted with superpure water for the analysis. For the solution, elements of Co, Ca, and Cr were respectively analyzed. The result of analysis was that the content of the respective elements was less than the detection limit. Example 02 An optical recording medium was produced using the sputtering target prepared in Example C-1. On a polycarbonate substrate with a guide groove having a groove depth of 50nm and a track pitch of 0.44pm formed thereon, a BiFeO layer having a thickness of 15nm was formed by sputtering, an organic-material layer containing a pigment or a dye represented by the following Chemical Formula 1 was formed on the BiFeO layer by spin-coating so as to have the average thickness of approx. 30nm, a reflective layer made from Ag having a thickness of 150nm was disposed on the organic-material layer by sputtering, and then a protective layer made from an ultraviolet curable resin having a thickness of approx. 5pm was further disposed on the reflective layer by spin-coating to thereby yield a write-once-read-many recording medium of the present invention. A pigment or a dye represented by Chemical Formula 1 is typically used for materials of conventional DVD±R, and such materials respectively have little absorption in blue-laser wavelengths. Binary recording was performed to the optical recording medium under the following conditions using an optical disk evaluation system DDU-1000 having a numerical aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm. • Modulation Mode: 8-16 modulation • Recording linear density: 1T = 0.0917pm The shortest mark length (3T) = 0.275pm • Recording linear velocity: 6.0m/s • Waveform equalization: Normal equalizer As a result, as shown in FIG. 5, an excellent jitter value of 10.2% was obtained at a recording power of 6.1mW, and excellent recording properties were realized. Example C-3 Sputtering target 2 was prepared in the same manner as Example C-1, provided that powders of Bi2O3 and Fe2O3 were mixed so that the atomic ratio of Bi to Fe was 35:5. The sputtering target had a packing density of 67%. X-ray diffraction pattern of the sputtering target was measured. The measurement conditions were as shown in Table 3. FIG. 6 shows the measurement result. To identify diffraction peak positions obtained in the measurement, the diffraction peak positions were referred to those of known substances. As shown in FIG. 6, most if not all peaks of the diffraction pattern matched to those of Bi25FeO40- As a matter of course, the highest peak was that of Bi25FeO40, and it proves that this compound was the principal constituent. Example C-4 An optical recording medium was produced using the sputtering target prepared in Example C-3. On a polycarbonate substrate with a guide groove having a groove depth of 50nm and a track pitch of 0.44pm formed thereon, a ZnSSiO2 layer having a thickness of 50nm and a BiFeO layer having a thickness of 15nm were formed in this order in a laminar structure by sputtering, an organic-material layer containing a pigment or a dye represented by the following Chemical Formula 1 was formed on the BiFeO layer by spin-coating so as to have the average thickness of approx. 30nm, a reflective layer which comprises Ag, having a thickness of 150nm was disposed on the organic-material layer by sputtering, and then a protective layer made from an ultraviolet curable resin having a thickness of approx. 5pm was further disposed on the reflective layer by spin-coating to thereby yield a write-once-read-many recording medium. A pigment or a dye represented by Chemical Formula 1 is typically used for materials of conventional DVD±R, and such materials respectively have little absorption in blue-laser wavelengths. Binary recording was performed to the optical recording medium under the following conditions, using an optical disk evaluation system DDU-1000 having a numerical aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm. • Modulation Mode: 8-16 modulation • Recording linear density: IT = 0.0917pm The shortest mark length (3T) = 0.275pm • Recording linear velocity: 6.0m/s • Waveform equalization: Normal equalizer As a result, as shown in FIG. 7, an excellent jitter value of 8.6% was obtained at a recording power of 7.0mW, and excellent recording properties were realized. Even when the recording power exceeded the optimum recording power, a write-once-read-many optical recording medium having a high modulated amplitude and a wide range of recording power margin was enabled without a great change in reproducing signal level or RF level of the recorded portions. Example 05 Sputtering target 3 was obtained in the same manner as Example C-1, provided that powders of Bi2O3 and Fe2O3 were mixed so that the atomic ratio of Bi to Fe was 1-5. The target had a packing density of 67%. X-ray diffraction pattern of the sputtering target was measured. The measurement conditions are as shown in Table 3. FIG. 8 shows the measurement result. To identify diffraction peak positions obtained in the measurement, the diffraction peak positions were searched and checked against those of known substances. As a result, it was found that the sputtering target had diffraction peaks corresponding to those of Bi2Fe4O9, Bi2O3, and Fe2O3, however, the other diffraction peaks of the sputtering target did not match to those of known substances. The greatest peak was the one corresponding to Fe2O3, and this demonstrated that this compound was the principal constituent. Example 06 An optical recording medium was produced using the sputtering target prepared in Example C-5. On a polycarbonate substrate with a guide groove having a groove depth of 50nm and a track pitch of 0.44µm formed thereon, a ZnS-SiO2 layer having a thickness of 50nm and a BiFeO layer having a thickness of 10nm were disposed in this order in a laminar structure by sputtering, an organic-material layer containing a pigment or a dye represented by Chemical Formula 1 was formed on the BiFeO layer by spin-coating so as to have the average thickness of approx. 30nm, a reflective layer made from Ag having a thickness of 150nm was disposed on the organic-material layer by sputtering, and then a protective layer which comprises an ultraviolet curable resin, having a thickness of approx. 5pm was further disposed on the reflective layer by spin-coating to thereby yield a write-once-read-many recording medium. A pigment or a dye represented by Chemical Formula 1 is typically used for materials of conventional DVD±R, and such materials respectively have little absorption in blue-laser wavelengths. Binary recording was performed to the optical recording medium under the following conditions using an optical disk evaluation system DDU-1000 having a numerical aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm. • Modulation Mode: 8-16 modulation • Recording linear density: 1T = 0.0917µm The shortest mark length (3T) = 0.275µm • Recording linear velocity: 6.0m/s • Waveform equalization: Normal equalizer The recording resulted in a degraded jitter value of 22.6% at a recording power of 8.1mW. Further, the thickness of the BiFeO layer was changed to 15nm and 20nm respectively, however, the resulting jitter values were further worsened, and the measurement was impossible. Example C-7 Sputtering target 4 was obtained in the same manner as Example C-1, provided that the powders of Bi2O3 and Fe2O3 were mixed so that the atomic ratio of Bi to Fe was 4:5. Sputtering target 4 has a packing density of 77%. X-ray diffraction pattern of the sputtering target was measured. The measurement conditions are as shown in Table 3. FIG. 9 shows the measurement results. The diffraction pattern obtained in the measurement is shown at (a) in FIG. 9. Diffraction peak positions of the sputtering target were referred to those of known substances of (b) BiFeO3 and (c) Bi2Fe4O9. The search result showed that the sputtering target had only diffraction peaks corresponding to those of BiFeO3 and Bi2Fe4O9. The greatest peak was the one corresponding to BiFe2O3, and this demonstrated that this compound was the principal constituent. Example C-8 An optical recording medium was produced using the sputtering target prepared in Example C-7. On a polycarbonate substrate with a guide groove having a groove depth of 50nm and a track pitch of 0.44pm formed thereon, a ZnSSiO2 layer having a thickness of 50nm and a BiFeO layer having a thickness of 10nm were disposed in this order in a laminar structure by sputtering, an organic-material layer containing a pigment or a dye represented by Chemical Formula 1 was formed on the BiFeO layer by spin-coating so as to have the average thickness of approx. 30nm, a reflective layer made from Ag having a thickness of 150nm was disposed on the organic-material layer by sputtering, and then a protective layer made from an ultraviolet curable resin having a thickness of approx. 5pm was further disposed on the reflective layer by spin-coating to thereby yield a write-once-read-many recording medium. A pigment or a dye represented by Chemical Formula 1 is typically used for materials of conventional DVD±R, and such materials respectively have little absorption in blue-laser wavelengths. Binary recording was performed to the optical recording medium under the following conditions using an optical disk evaluation system DDU-1000 having a numerical aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm. The shortest mark length was set at 0.205µm to examine the capability of high density recording. • Modulation Mode: 1-7 modulation • Recording linear density: The shortest mark length (2T) = 0.205µm • Recording linear velocity: 6.0m/s • Waveform equalization: Normal equalizer FIG. 10 shows the result. Example C-9 An optical recording medium was produced in the same manner as Example C-8 using a sputtering target prepared in the same manner as Example C-7, provided the atomic ratio of Bi to Fe was changed to prepare the sputtering target. The optical recording was measured under the same recording conditions as Examples. FIG. 10 shows the measurement result. As shown in FIG. 10, an optical recording medium using a sputtering target having an atomic ratio represented by Bi/Fe > 0.8, i.e. the sputtering target having an atomic ratio of Bi/(Bi + Fe) ≥ 4/9 in FIG. 10, demonstrated a jitter value of approx. 14%, and it was demonstrated that an excellent jitter value can be obtained even at high density recording. The jitter value was substantially improved even with an optical recording medium using a sputtering target having an atomic ratio represented by Bi/(Bi + Fe) ≥ 3/8, and it turned out to be effective. Example C-10 Sputtering target 5 was prepared in the same manner as Example 01, provided that powders of Bi2O3 and Fe2O3 were mixed so that the atomic ratio of Bi to Fe was 10:5. The target had a packing density of 85%. X-ray diffraction pattern of the sputtering target was measured. The measurement conditions are as shown in Table 3. FIG. 11 shows the measurement result. To identify diffraction peak positions of the diffraction pattern (a) obtained in the measurement, the diffraction peak positions were referred to those of known substances (b) to (e). As a result, there were diffraction peaks corresponding to those of Bi25FeO40, BiFeO3, Bi2O3, and Fe2O3, however, the other diffraction peaks of the sputtering target did not match to those of other substance. The greatest peak was the one corresponding to Bi25FeO40, and this demonstrated that this was the principal constituent. The sputtering target was subjected to the ICP analysis, i.e. inductively coupled plasma-atomic emission spectroscopy. Part of the sputtering target was dissolved in an aqua regia as a sample and then diluted with superpure water for the analysis. Elements of Co, Ca, and Cr were respectively analyzed for the solution. The result of analysis was that the content of the respective elements was less than the detection limit. In addition, sputtering target 6 in which 0.003% by mass of Al and 0.001% by mass of Co had been detected as impurity was prepared in the same manner as above. Example C-11 Optical recording media were prepared using the sputtering targets 5 and 6 prepared in Example C-10, respectively. On a polycarbonate substrate with a guide groove having a groove depth of 50nm and a track pitch of 0.44pm formed thereon, a ZnS-Si02 layer having a thickness of 50nm and a BiFeO layer having a thickness of 15nm were disposed in this order in a laminar structure by sputtering, an organic-material layer containing a pigment or a dye represented by Chemical Formula 1 was formed on the BiFeO layer by spin-coating so as to have the average thickness of approx. 30nm, a reflective layer made from Ag having a thickness of 150nm was disposed on the organic-material layer by sputtering, and then a protective layer made from an ultraviolet curable resin having a thickness of approx. 5µm was further disposed on the reflective layer by spin-coating to thereby yield a write-once-read-many recording medium. A pigment or a dye represented by Chemical Formula 1 is typically used for materials of conventional DVD±R, and such materials respectively have little absorption in blue-laser wavelengths. Binary recording was performed to the optical recording media under the following conditions using an optical disk evaluation system DDU-1000 having a numerical aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm. • Modulation Mode: 8-16 modulation • Recording linear density: IT = 0.0917pm The shortest mark length 3T = 0.275pm • Recording linear velocity: 6.0m/s • Waveform equalization: Normal equalizer As a result, excellent jitter values were obtained. Specifically, the optical recording medium using target 5 had a jitter value of 8.4% at a recording power of 5.0mW, and the optical recording medium using target 6 had a jitter value of 8.2% at a recording power of 5.0mW, and both optical recording media achieved excellent recording properties. Even when the recording power exceeded the optimum recording power, a write-once-read-many optical recording medium having a high modulated amplitude and a wide range of recording power margin was enabled without a great change in reproducing signal level or RF level of the recorded portions. Example C-12 Sputtering target 7 was obtained in the same manner as Example C-1, provided that powders of Bi2O3 and Fe2O3 were mixed so that the atomic ratio of Bi to Fe was 10:5. The sputtering target had a packing density of 71%. X-ray diffraction pattern of the sputtering target was measured. The measurement conditions are as shown in table 3. FIG. 12 shows the measurement result. To identify diffraction peak positions obtained in the measurements the diffraction peak positions were referred to those of known substances. The diagram marked with (a) represents the diffraction pattern of target 7, and the diagram marked with (b) represents the diffraction pattern with known data of Bi36Fe2O57. The diagram marked with (c) shows known data of BiFeO3, and the diagram marked with (d) shows known data of Fe2O3. The greatest peak was the one corresponding to Bi36Fe2O57, and this demonstrated that this compound was the principal constituent. Example C-13 An optical recording medium was produced using the sputtering target prepared in Example C-12. On a polycarbonate substrate with a guide groove having a groove depth of 21nm and a track pitch of 0.44µm formed thereon, a BiFeO layer having a thickness of 5nm, a ZnS-SiO2 layer having a thickness of 14nm, and a reflective layer made from Ag having a thickness of 100nm were disposed in this order in a laminar structure by sputtering. On the reflective layer, a protective layer made from an ultraviolet curable resin having a thickness of approx. 5pm was disposed by spin-coating to yield a write-once-read-many optical recording medium. Binary recording was performed to the optical recording medium under the following conditions using an optical disk evaluation system DDU-1000 having a numerical aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm. • Modulation Mode: 8-16 modulation • Recording linear density: IT = 0.0917µm The shortest mark length 3T = 0.275µm • Recording linear velocity: 6.0m/s • Waveform equalization: Normal equalizer As a result, an excellent jitter value of 6.2% was obtained at a recording power of 10.1mW, and excellent recording properties were enabled by the write-once-read-many optical recording medium. Example C-14 Sputtering target 8 was obtained in the same manner as Example C-1, provided that powders of Bi2O9 and Fe were mixed so that the atomic ratio of Bi to Fe was 35:5. The sputtering target had a packing density of 69%. X-ray diffraction pattern of the sputtering target was measured, and very slight peaks of Bi and Fe were observed, and it was clarified that a compound corresponding to Bi25FeO40 or a compound corresponding to Bi36Fe2O57 was the principal constituent. Example C-15 An optical recording medium was produced using the sputtering target prepared in Example C-14. On a polycarbonate substrate with a guide groove having a groove depth of 50nm and a track pitch of 0.44pm formed thereon, a ZnS-SiO2 layer having a thickness of 20nm and a BiFeO layer having a thickness of 13nm were disposed in this order in a laminar structure by sputtering. The BiFeO layer was formed with introducing oxygen thereto at a flow rate of 2% relative to Ar at the same time. On the BiFeO layer, a ZnS-SiO2 layer having a thickness of 20nm was formed, and an Alalloy reflective layer having a thickness of 100nm was formed on the ZnS-SiO2 layer by sputtering, and a protective layer made from an ultraviolet curable resin having a thickness of approx. 5pm was further disposed on the Al-alloy reflective layer by spin-coating to yield a write-once-read-many optical recording medium. Binary recording was performed to the write-once-read-many optical recording medium under the following conditions using an optical disk evaluation system DDU-1000 having a numerical aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm. • Modulation Mode: 8-16 modulation • Recording linear density- IT = 0.0917µm The shortest mark length 3T = 0.275µm • Recording linear velocity- 6.0m/s • Waveform equalization: Normal equalizer As a result, an excellent jitter value of 7.6% was obtained at a recording power of 9.0mW, and excellent recording properties were enabled by the write-once-read-many optical recording medium. It was possible to achieve a write-once-read-many optical recording medium having a high modulated amplitude and a wider recording power margin without great changes in reproducing signal level or RF level even when the recording power exceeded the optimum recording power. Example C-16 For a sputtering target of Bi10Fe5Ox, four targets thereof were produced in the same manner. X-ray diffraction patterns of these sputtering targets were measured. Table 4 shows the values of 2θ of the diffraction peaks of these Bi10Fe5Ox targets detected as a result of the measurement. Targets having such a peak at those angles of 2θ shown in Table 4 can be presented as examples of the sputtering target of the present invention. WE CLAIM: 1. A write-once-read-many optical recording medium comprising: a substrate, a recording layer, and a reflective layer, wherein the recording layer comprises a material represented by BiOx (0 1.5), and a recording mark with information recorded therein comprises crystal of Bi and/ or crystal of a Bi oxide. 2. The write-once-read-many optical recording medium as claimed in claim 1, wherein the recording layer comprises a material represented by BiOx (0 recording mark with information recorded therein comprises tetravalent Bi in form of crystal of Bi oxide. 3. The write-once-read-many optical recording medium as claimed in claim 1, wherein the recording layer comprises Bi, O, and M as constituent elements, wherein M represents at least one element selected from the group consisting of Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and Ta, and a recording mark with information recorded therein comprises crystal of the one or more elements contained in the recording layer, tetravalent Bi in form of a crystal of Bi oxide and crystal of an oxide of M. 4. The write-once-read-many optical recording medium as claimed in claim 3, wherein the atomic number ratio of the total amount of the element M to bismuth is 1.25 or less. 5. The write-once-read-many optical recording medium as claimed in claim 3, wherein the write-once-read-many optical recording medium has any one of laminar structures of a laminar structure in which at least the recording layer, an upper coating layer, and the reflective layer are disposed on the substrate in this order, and a laminar structure in which at least the reflective layer, an upper coating layer, the recording layer, and a cover layer are disposed on the substrate in this order. 6. The write-once-read-many optical recording medium as claimed in claim 3, wherein the write-once-read-many optical recording medium is produced using a sputtering target which comprises one or more selected from BiFeO3, Bi25FeO40, and Bi36Fe2O57. 7. The write-once-read-many optical recording medium as claimed in claim 1, wherein the recording layer comprises Bi, O, and L as constituent elements, and the recording layer comprises a Bi oxide, and wherein L represents at least one element selected from the group consisting of B, P, Ga, As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At, and Cd, and a recording mark with information recorded therein comprises tetravalent Bi in form of crystal of Bi oxide. 8. The write-once-read-many optical recording medium as claimed in claim 7, wherein the element L represents at least one element selected from the group consisting of B, P, Ga, Se, Pd, Ag,.Sb, Te, W, Pt, and Au. 9. The write-once-read-many optical recording medium as claimed in claim 7, wherein the atomic number ratio of the total amount of the element L to bismuth is 1.25 or less. 10. The write-once-read-many optical recording medium as claimed in claim 7, wherein the write-once-read-many optical recording medium comprises an upper coating layer and has a laminar structure in which the recording layer, the upper coating layer, and the reflective layer are disposed on the substrate in this order. 11. The write-once-read-many optical recording medium as claimed in claim 10, wherein the write-once-read-many optical recording medium comprises an under coating layer and has a laminar structure in which the under coating layer, the recording layer, the upper coating layer, and the reflective layer are disposed on the substrate in this order. 12. The write-once-read-many optical recording medium as claimed in claim 7, wherein the write-once-read-many optical recording medium comprises an upper coating layer and a cover layer and has a laminar structure in which the reflective layer, the upper coating layer, the recording layer, and the cover layer are disposed on the substrate in this order. 13. The write-once-read-many optical recording medium as claimed in claim 12, wherein the write-once-read-many optical recording medium comprises an under coating layer and has a laminar structure in which the reflective layer, the upper coating layer, the recording layer, the under coating layer, and the cover layer are disposed on the substrate in this order. 14. The write-once-read-many optical recording medium as claimed in claim 7, wherein the write-once-read-many optical recording medium comprises at least one of an under coating layer and an upper coating layer, and at least any one of the under coating layer and the upper coating layer comprises ZnS and/or SiO2. 15. The write-once-read-many optical recording medium as claimed in claim 7, wherein the write-once-read-many optical recording medium comprises at least one of an under coating layer and an upper coating layer, and at least any one of the under coating layer and the upper coating layer comprises an organic material. 16. The write-once-read-many optical recording medium as claimed in claim 7, wherein recording and reproducing are enabled with a laser beam having a wavelength of 680 nm or less. 17. A process for the production of write-once-read-many optical recording medium as claimed in claim 3, wherein a sputtering target comprises: Bi, Fe, and O, wherein the said sputtering target comprises one or more selected from BiFeO3, Bi25FeO40, and Bi36Fe2O57. 18. The process as claimed in claim 17, wherein the sputtering target consists of Bi, Fe, and O. 19. The process as claimed in any one of claims 17 to 18, wherein the sputtering target is used for forming a recording layer for an optical recording medium in which recording and reproducing are performed with a laser beam at a wavelength of 550 nm or less. 20. The process as claimed in any one of claims 17 to 19, wherein the sputtering target comprises a Bi oxide and a Fe oxide, or comprises a complex oxide of Bi and Fe. 21. The process as claimed in claim 20, wherein the sputtering target comprises the complex oxide of Bi and Fe and comprises one or more selected from the Bi oxide and the Fe oxide. 22. The process as claimed in claim 17, wherein the sputtering target comprises one or more selected from a Bi oxide, a Fe oxide, and a complex oxide of Bi and Fe, and the oxide is an oxide having a smaller amount of oxygen compared to the stoichiometric composition. 23. The process as claimed in any one of claims 17 to 22, wherein the sputtering target comprises Bi2O3 and/or Fe2O3. 24. The process as claimed in any one of claims 17 to 23, wherein the sputtering target does not comprise Bi2Fe4O9. 25. The process as claimed in any one of claims 17 to 24, wherein the content of Co, Ca, and Cr is less than the detection limit of the inductively coupled plasma emission spectrometry. 26. The process as claimed in any one of claims 17 to 25, wherein the sputtering target has a packing density of 65% to 96%. 27. The process as claimed in any one of claims 17 to 26, wherein the atomic ratio of Bi and Fe satisfies the requirement of Bi/Fe ≥ 0.8. 28. A method of producing a sputtering target comprising: calcining powders of Bi2O3 and Fe2O3 in the process as claimed in any one of claims 17 to 27. 29. An optical recording medium as claimed in claim 1 comprising: a substrate, a recording layer, and a reflective layer, wherein the recording layer is formed using a sputtering target which comprises one or more selected from BiFeO3, Bi25FeO40, and Bi36Fe2O57. ABSTRACT WRITE-ONCE-READ-MANY OPTICAL RECORDING MEDIUM, SPUTTERING TARGET AND THE PRODUCTION METHOD THEREOF A write-once-read-many optical recording medium enabling excellent recording and reproducing properties at a wavelength of blue-laser wavelengths or shorter, i.e. 500nm or less, particularly at wavelengths of near 405nm and high density recording. To this end, a write-once-read-many optical recording medium of the invention comprises a recording layer using a material represented by BiOx (0 Another write-once-read-many optical recording medium comprises a recording layer which comprises Bi, oxygen, and M (M represents at least one element selected from Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and Ta), in which a recorded mark comprises crystal of the elements contained in the recording layer and crystal of an oxide of the elements.

Full Text

DESCRIPTION
WRITE-ONCE-READ-MANY OPTICAL RECORDING MEDIUM,
SPUTTERING TARGET AND THE PRODUCTION METHOD THEREOF
Technical Field
The present invention relates to a write-once-read-many (WORM) optical
recording medium. More specifically, the present invention relates to a write-
once-read-many optical recording medium enabling high density recording
particularly at blue-laser wavelengths. The present invention further relates to a
sputtering target which can be used for forming an oxide-layer which is a layer
constituting the write-once-read- many optical recording medium.
Background Art
Relating to a write-once-read-many optical recording medium enabling
recording and reproducing at laser wavelengths of blue or shorter, a blue laser
allowing super high density recording is rapidly developed, and the development
of write-once-read-many optical recoding media sensitive to blue-laser
wavelengths is promoted.
In conventional write-once-read-many optical recording media, a laser
beam is irradiated to a recording layer which comprises an organic material to
change the refractive index

typically due to the decomposition and degeneration of the
organic material, and thus recording pjts areformed. The
optical constant and decomposition behavior of the organic
material used in the recording layer play an important role to
form satisfactory recording pits.
For use in a recording layer of write-once-read-many
optical recording media sensitive to blue-laser wavelengths, an
organic material must have suitable optical properties and
decomposition behavior with respect to light at blue-laser
wavelengths. More specifically, the wavelengths at which
recording is performed are set at a tail on the longer-wavelength
side of a major absorption band to increase the reflectance in
unrecorded portions and to substantially increase difference in
refractive index invited by the decomposition of the organic
material upon irradiation of laser to thereby yield a higher
modulated amplitude. This is because wavelengths at the tail
on the longer-wavelength side of a major absorption band of
such an organic material yield an appropriate absorption
coefficient and a high refractive index.
However, an organic material having optical properties
with respect to light at blue-laser wavelengths equivalent to
those of conventional materials has not yet been found. To
produce such an organic material having an absorption band in
the vicinity of blue-laser wavelengths, the molecular skeleton

must be downsized or the conjugate system must be shortened.
However, this invites a lowered absorption coefficient and a
lowered refractive index. More specifically, there are many
organic material having an absorption band in the vicinity of
blue-laser wavelengths and it is possible to control their
absorption coefficients, however they do not have a sufficiently
high refractive index and fail to yield a higher degree of
modulated amplitude.
Then the one using inorganic materials and organic
materials, and the one using only inorganic materials are
presently being studied for a material having optical properties
sensitive to light at blue-laser wavelengths. As the one using
oxides, Patent Literature 1 discloses a recording layer which
comprises Bi, rare-earth, Ga, Fe, and O, and the invention
describes a composition capable of forming garnet, Patent
Literature 2 discloses an optical recording medium using organic
oxides.
However, these conventional techniques do not study in
what shape recording marks should be formed to effectively form
excellent recording marks and yield excellent properties. As a
matter of course, these techniques do not allow for the shape of
recording marks to yield higher modulated amplitudes when
used with light at blue-wavelengths, which are brought up
herein as an issue.

Besides, for write-once-read-many optical recording media
using oxides of metals or semimetals as a recording layer,
TeOx-Pd recording layers having high-reliabilities have been
proposed in Patent Literatures 3 and 4. In Patent Literatures
3 and 4, the composition ratio of the TeOx-Pd recording layer is
varied in a direction of the thickness of the layer to enhance
reliabilities such as storage stability. Besides, recording layers
which comprise TeOx-Pd are also disclosed in Non-Patent
Literatures 1 and 2, however, they have only descriptions on
controlling of degrees of oxidation therein as a method for
improving reliabilities.
As to materials containing a bismuth oxide, which are
similar to the present invention though, they are disclosed
respectively in the following Patent Literatures- Patent
Literature 5 discloses an amorphous and ferromagnetic oxide
expressed by the formula Ax(MmOn)y(Fe2O3)z, in which individual
ratios of various oxides of A, various elements of M, and x, y,
and z are defined; Patent Literature 6 discloses a metallic oxide
which comprises a 50% or more amorphous phase expressed by
the formula (Bi2O3)x(MmOn)y(Fe2O3)z, in which individual ratios
of m and n of MmOn, individual ratios of x, y, and z are defined,
and the production method thereof; Patent Literature 7 discloses
an amorphous compound having a composition expressed by the
formula (B2O3)x(Bi2O3)1-x, the range of the composition x, and

the quenching method; and Patent Literature 8 discloses
bismuth-iron amorphous compound material having a
composition of (Bi2O3)1-x(Fe2O3)x, however, x is represented by
0.90 ≥ x > 0.
However, these techniques respectively relate to
amorphous oxide materials which are optically transmissive and
ferromagnetic, and they are . typically used for
photoelectromagnetic optical recording media, functional devices
for controlling light by means of actions of magnetism,
photoelectromagnetic sensors, transparent and electrically
conductive films, piezo-electric films, and the like. In addition,
these techniques provided by other companies basically aim for
patents relating to materials and/or production methods of the
materials and have no descriptions on applications to
writeonce-read-many optical recording media.
On the other hand, as one of the methods for producing a
recording layer for an optical recording medium, there has been
a sputtering method. The sputtering has been widely known in
the art as—one of the methods for forming thin-layers under
vapor-phase and utilized in producing thin-layers for industrial
purposes. In the sputtering method, a target material having
the same component as that of a layer to be formed is prepared,
typically, argon gas ions generated by glow-discharge is crashed
with the target material to beat the constituent atoms of the

target material, and the atoms are deposited on a substrate to
thereby form a layer. An oxide typically has high-melting point,
and thus it is not suitably formed by an evaporation method or
the like, and the high-frequency-wave sputtering method, in
which high-frequency waves are applied, is usually used.
There are many achievements of the sputtering method in
its production process, and sputtering is also advantageous in
terms of throughput. However, when a layer containing mixed
materials from two or more elements is formed, there may be
cases where the composition of a target is not same as that of
the layer, and therefore, the composition of the target must be
considered. Further, there are many cases where the structure
and characteristics of a layer vary depending on the
configuration of a compound constituting the target, and thus
this point must be considered.
As a technique known in the art, for example, Patent
Literature 9 discloses a target which comprises a Bi oxide as a
sputtering target for forming a dielectric film. However, Patent
Literature 9 does not mention a target which comprises Fe.
Since when the type of constituent elements varies, the relation
between the composition and constituting compound of the
target as well as the relation between the structure and
composition of a layer vary. Thus, the structure of target must
be changed, and the findings disclosed in Patent Literature 9 do

not serve as a reference to the sputtering target proposed in the
present invention.
In addition, Patent Literature 10 discloses a target for
producing a thin layer made from Bi3Fe5O12, however, the
invention is for producing a thin layer having so-called a garnet
structure, in which high-degree of magneto-optical effect is
obtained, and the invention employs the ratio of Bi to Fe being
3*5 to 3.5:4.5. Accordingly it differs from the sputtering target
proposed in the present invention.
Patent Literature 1 Japanese Patent Application
Laid-Open (JP-A) No. 10-92027
Patent Literature 2 Japanese Patent Application
Laid-Open (JP-A) No. 2003-48375
Patent Literature 3 Japanese Patent Application
Laid-Open (JPA) No. 06-150366
Patent Literature 4 Japanese Patent Application
Laid-Open (JPA) No. 06-93300
Patent Literature 5 Japanese Patent Application
Laid-Open (JPA) No. 61101450
Patent Literature 6 Japanese Patent Application
Laid-Open (JP-A) No. 61101448
Patent Literature 7 Japanese Patent Application
Laid-Open (JP-A) No. 598618
Patent Literature 8 Japanese Patent Application

Laid-Open (JP-A) No. 59-73438
Patent Literature 9 Japanese Patent Application
Laid-Open (JP-A) No. 11-92922
Patent Literature 10 Japanese Patent Application
Laid-Open (JP-A) No. 02-42899
Non-Patent Literature 1 pp. 23-28, Proceedings of The
14th Symposium on PCOS2002
Non-Patent Literature 2 pp. 5-8, Vol. 28, Eijogaku Giho
Disclosure of Invention
It is therefore an object of the present invention to provide
a write-once-read-many optical recording medium capable of
presenting excellent recording-reproducing properties at
wavelengths of 500nm or less and enabling recording and
reproducing particularly at wavelengths of near 405nm and high
density recording.
Further, another object of the present invention is to
provide a sputtering target which can be used for producing an
oxide-layer which is a layer constituting the optical recording
medium and is suitably used for arbitrarily forming a layer
having a stable composition and a stable structure, and the
production method thereof.
A first aspect of the present invention is a
write-once-read-many optical recording medium which comprises

a substrate, a recording layer, and a reflective layer, wherein
the recording layer comprises a material represented by BiOx (0

therein comprises crystal of Bi and/or crystal of a Bi oxide.
A second aspect of the present invention is a
write-once-read-many optical recording medium according to the
first aspect, wherein the recording mark comprises tetravalent
Bi.
A third aspect of the present invention is a
write-once-read-many optical recording medium which comprises
a substrate, a recording layer, and a reflective layer, wherein
the recording layer comprises Bi, O, and M as constituent
elements, M represents at least one element selected from the
group consisting of Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr,
Ti, Hf, Sn, Mo, V, Nb, Y, and Ta, and a recording mark with
information recorded therein comprises one or more crystals
from crystal of the one or more elements contained in the
recording layer and crystal of an oxide of the one or more
elements.
A fourth aspect of the present invention is a
write-once-read-many optical recording medium according to the
third aspect, wherein the recording mark comprises tetravalent
Bi.
A fifth aspect of the present invention is a

write-once-read-many optical recording medium according to the
third aspect, wherein the atomic number ratio of the total
amount of the element M to bismuth is 1.25 or less.
A sixth aspect of the present invention is a
write-once-read-many optical recording medium according to the
third aspect, wherein the write-once-read-many optical
recording medium has any one of laminar structures of a
laminar structure in which at least the recording layer an upper
coating layer, and the reflective layer are disposed on the
substrate in this order, and a laminar structure in which at
least the reflective layer, an upper coating layer, the recording
layer, and a cover layer are disposed on the substrate in this
order.
A seventh aspect of the present invention is a
write-once-read-many optical recording medium according to the
third aspect, wherein the write-once-read-many optical
recording medium is produced using a sputtering target which
comprises one or more selected from BiFeO3, Bi25FeO40, and
Bi36Fe2O57.
An eighth aspect of the present invention is a
write-once-read-many optical recording medium which comprises
a substrate, a recording layer, and a reflective layer, wherein
the recording layer comprises Bi, 0, and L as constituent
elements, and the recording layer comprises a Bi oxide, and L

represents at least one element selected from the group
consisting of B, P, Ga, As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt,
Au, Hg, Tl, Po, At, and Cd.
A ninth aspect of the present invention is a
write-once-read-many optical recording medium according to the
eighth aspect, wherein the element L represents at least one
element selected from the group consisting of B, P, Ga, Se, Pd,
Ag, Sb, Te, W, Pt, and Au.
A tenth aspect of the present invention is a
write-once-read-many optical recording medium according to the
eighth aspect, wherein the atomic number ratio of the total
amount of the element Ltobismuth is 1.25 or less.
An eleventh aspect of the present invention is a
write-once-read-many optical recording medium according to the
eighth aspect, wherein the write-once-read-many optical
recording medium further comprises an upper coating layer and
has a laminar structure in which the recording layer, the upper
coating layer, and the reflective layer are disposed on the
substrate in this order.
A twelfth aspect of the present invention is a
write-once-read-many optical recording medium according to the
eleventh aspect, wherein the write-once-read-many optical
recording medium further comprises an under coating layer and
has a laminar structure in which the under coating layer, the

recording layer, the upper coating layer, and the reflective layer
are disposed on the substrate in this order.
A thirteenth aspect of the present invention is a
write-once-read-many optical recording medium according to the
eighth aspect, wherein the write-once-read-many optical
recording medium further comprises an upper coating layer and
a cover layer and has a laminar structure in which the reflective
layer, the upper coating layer, the recording layer, and the cover
layer are disposed on the substrate in this order.
A fourteenth aspect of the present invention is a
write-once-read-many optical recording medium according to the
thirteenth aspect, wherein the write-once-read-many optical
recording medium further comprises an under coating layer and
has a laminar structure in which the reflective layer, the upper
coating layer, the recording layer, the under coating layer, and
the cover layer are disposed on the substrate in this order.
A fifteenth aspect of the present invention is a
write-once-read-many optical recording medium according to the
eighth aspect, wherein the write-once-read-many optical
recording medium further comprises at least any one of an under
coating layer and an upper coating layer, and at least any one of
the under coating layer and the upper coating layer comprises
ZnS and/or SiO2.
A sixteenth aspect of the present invention is a

write-once-read-many optical recording medium according to the
eighth aspect, wherein the write-once-read-many optical
recording medium further comprises an under coating layer and
an upper coating layer, and at least any one of the under coating
layer and the upper coating layer comprises an organic material.
A seventeenth aspect of the present invention is a
write-once-read-many optical recording medium according to the
eighth aspect, wherein recording and reproducing are enabled
with a laser beam having a wavelength of 680nm or less.
An eighteenth aspect of the present invention is a
sputtering target which comprise Bi, Fe, and Q.
A nineteenth aspect of the present invention is a

sputtering target according to the eighteenth aspect, wherein
the sputtering target consists of Bi, Fe, and oxygen.
A twentieth aspect of the present invention is a
sputtering target according to any one of aspects of the
eighteenth aspect to the nineteenth aspect, wherein the
sputtenng target is used for forming a recording layer for an
optical recording medium in which recording and reproducing
are performed with a laser beam at a wavelength of _550nm or
less.
A twenty-first aspect of the present invention is a
sputtering target according to any one of aspects of the
eighteenth aspect to the twentieth aspect, wherein the

sputtering target comprises a Bi oxide and a Fe oxide, or
comprises a complex oxide of Bi and Fe.
A twenty-second aspect of the present invention is a
sputtering target according to the twenty-first aspect, wherein
the sputtering target comprises the complex oxide of Bi and Fe
and further comprises one or more selected from the Bi oxide
and the Fe oxide.
A twenty-third aspect of the present invention is a
sputtering target according to the eighteenth aspect, wherein
the sputtering target comprises one or more selected from a Bi
oxide, a Fe oxide, and a complex oxide of Bi and Fe, and the
oxide is an oxide having a smaller amount of oxygen compared to
the stoichiometric composition.
A twenty-fourth aspect of the present invention is a
sputtering target according to any one of aspects of the
eighteenth aspect to the twenty-third aspect, wherein the
sputtering target comprises one or more selected from BiFeO3,
Bi25FeO40, and Bi36Fe2O57.
A twenty-fifth aspect of the present invention is a
sputtering target according to any one of aspects of the
eighteenth aspect to the twenty-fourth aspect, wherein the
sputtering target comprises Bi2O3 and/or Fe2O3.
A twenty-sixth aspect of the present invention is a
sputtering target according to any one of aspects of the

eighteenth aspect to the twenty-fifth aspect, wherein the
sputtering target does not comprise Bi2Fe4O9.
A twenty-seventh aspect of the present invention is a
sputtering target according to any one of aspects of the
eighteenth aspect to the twenty-sixth aspect, wherein the
content of Co, Ca, and Cr is less than the detection limit of the
inductively coupled plasma emission spectrometry.
A twenty-eighth aspect of the present invention is a
sputtering target according to any one of aspects of the
eighteenth aspect to the twenty-seventh aspect, wherein the
sputtering target has a packing density of 65% to 96%.
A twenty-ninth aspect of the present invention is a
sputtering target according to any one of aspects of the
eighteenth aspect to the twenty-eighth aspect, wherein the
atomic ratio of Bi and Fe satisfies the requirement of Bi/Fe ≥
0.8.
A thirtieth aspect of the present invention is a sputtering
target production method in which powders of Bi2O3 and Fe2O3
are calcined to produce a sputtering target according to any one
of aspects of the eighteenth aspect to the twenty-ninth aspect.
A thirty-first aspect of the present invention is an optical
recording medium which comprises a substrate, a recording
layer, and a reflective layer, wherein the recording layer is
formed using a sputtering target which comprises one or more

Brief Description of the Accompanying Drawings
FIG. 1 is a diagram showing radial distribution functions in the vicinity
of O (oxygen) atoms in an unrecorded portion and a recorded portion.
FIG. 2 is a diagram showing radial distribution functions in the vicinity
of O (oxygen) atoms of a Bi oxide measured according to the FEFF calculation.
FIG. 3 is a diagram showing the measurement results of Example B-26.
FIG. 4 is a diagram showing an X-ray diffraction pattern of a sputtering
target 1.
FIG. 5 is a diagram showing recording properties of an optical recording
medium produced by using the sputtering target 1.
FIG. 6 is a diagram showing an X-ray diffraction pattern of a sputtering
target 2.
FIG. 7 is a diagram showing recording properties of an optical recording
medium produced by using the sputtering target 2.
FIG. 8 is a diagram showing an X-ray diffraction pattern of a sputtering
target 3.
FIG. 9 is a diagram showing an X-ray diffraction pattern

of a sputtering target 4.
FIG. 10 is a diagram showing recording properties of an
optical recording medium produced by using a sputtering target
which has a different ratio of Bi/Fe.
FIG. 11 is a diagram showing an X-ray diffraction pattern
of a sputtering target 5.
FIG. 12 is a diagram showing an X-ray diffraction pattern
of a sputtering target 7.
Best Mode for Carrying Out the Invention
Hereinafter, the present invention will be described in
detail.
To achieve a write-once-read-many optical recording
medium capable of performing excellent recording at a
wavelength of blue-laser wavelengths or shorter, the following
items (1) to (3) are taken up as important issues:
(1) Small recording marks can be formed.
(2) Less interference between recording marks
(3) High-stability in recoding marks
When a blue-laser is used, a material by which recording
can be excellently performed at blue-wavelengths must be
selected, unlike in the case where a laser is used at
near-infrared wavelengths and red-wavelengths used for CD and
DVD.

Since a Bi oxide easily absorbs light at blue-wavelengths,
excellent recording can be expected.
The first aspect of a write-once-read-many optical
recording medium according to the present invention comprises
a substrate, a recording layer, and a reflective layer, in which a

material represented by BiOx (e recording layer, and recording marks with information recorded
therein comprise crystal of Bi and/or crystal of a Bi oxide. To
yield a higher modulated amplitude, it is required that the
(difference in refractive index between a recording mark an
unrecorded portion be large. When a material represented by
BiOx (0 crystal of Bi and/or crystal of a Bi oxide in a recording mark, the
variance in refractive index is larger, and higher modulated
amplitude can be realized. For example, when an unrecorded
portion is amorphous and a recoding mark comprises a
crystallized portion, higher modulated amplitude can be yielded.
When an unrecorded portion comprises a Bi oxide, by forming
precipitation of a simple substance of Bi-metal, not oxide, in a
recording mark, the difference in refractive index is much larger,
and much higher modulated amplitude can be yielded. The
method for forming a recording mark by crystallizing amorphous
portions has been used so far, however, in the present invention,
when recording is performed in an oxide, much greater effect

can be expected by utilizing the phenomenon that the recorded
portion is changed to a substance other than oxides and the
recorded portion is crystallized. A recording mark which
comprises crystals having two or more different
crystal-structures can restrain the crystals from growing and
spreading and enables forming a small recording mark, because
mixed crystals each having a different crystal structure can
restrain the growth of crystals.
The changes in a recording layer induced by recording
will be described more.
The material represented by BiOx (0 metastable state which is hard to exist with normal conditions,
however, such a state can be realized in an optical recording
medium by forming a recording layer by sputtering. When a
laser beam is irradiated to a recording layer in a metastable
state of BiOx (0 easily to separate into Bi and a Bi oxide, because BiOx attempts
to go back to a more stable state. At this point in time, it is
believed that some Bi oxides isolate from oxygen to become no
longer an oxide to be in a state of Bi. Since a more stable state
is a crystalline state, crystal of Bi and crystal of a Bi oxide are
formed, and thus a recording mark becomes in a state where
crystal of Bi and/or crystal of a Bi oxide are included.
An aspect of the write-once-read-any optical recording

medium of the present invention comprises a substrate, a
recording layer, and a reflective layer, wherein the recording
layer comprises Bi, O, and M as constituent elements, wherein
M represents at least one element selected from the group
consisting of Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf,
Sn, Mo, V, Nb, Y, and Ta, and a recording mark with information
recorded therein comprises one or more crystals from crystal of
the one or more elements contained in the recording layer and
crystal of an oxide of the one or more elements. An another
aspect of the write-once-read-many optical recording medium
comprises a substrate, a recording layer, and a reflective layer,
wherein the recording layer comprises Bi, O, and L as
constituent elements, and the recording layer comprises a Bi
oxide, and L represents at least one element selected from the
group consisting of B, P, Ga, As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os,
Ir, Pt, Au, Hg, Tl, Po, At, and Cd.
Bismuth may be contained in any of the states such as
metallic bismuths, bismuth alloys, bismuth oxides, bismuth
sulfides, bismuth nitrides, bismuth fluorides, however, the
recording layer preferably comprises one bismuth oxide. Since
a recording layer containing a bismuth oxide enables lowering
thermal conductivity of the recording layer, achieving
high-sensitivities and lower jitter values, and making an
imaginary part of complex index of refraction smaller, the

recording layer has an excellent transparency, and a
multi-layered recording layer is easily formed.
Preferably, bismuth and one or more elements selected
from the group consisting Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si,
Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, Ta, B, P, Ga, As, Se, Te, Pd, Ag,
Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At, and Cd reside in the
recording layer in an oxidized state from the perspective of
improvements in stability and thermal conductivity, however,
they are not necessarily oxidized completely. In other words,
when the recording layer of the present invention comprises
three elements of bismuth, oxygen, and one element such as Mg,
it may comprise bismuth, a bismuth oxide, an element such as
Mg, and an oxide of the element such as Mg.
Examples of the method for making bismuth or metallic
bismuth and a Bi oxide mixed in the recording layer, i.e. the
method for forming a recording layer in which bismuth elements
reside in different states include the following methods (A) to
(C).
(A) Sputtering the recording layer using a bismuth-oxide
target
(B) Sputtering the recording layer using a bismuth target
and a bismuth-oxide target
(C) Sputtering the recording layer using a bismuth target
while introducing oxygen into the recording layer or

co-sputtering method
When the method (A) is employed, bismuth in the target
is completely oxidized, and this method utilizes the phenomenon
that oxygen is likely to be deficient depending on the sputtering
conditions such as degree of vacuum and sputtering power.
First, an aspect of the write-once-read-many recording
medium in which the recording layer comprises Bi, M and
oxygen as constituent elements will be described below. It is
noted that the element M represents at least one element
selected from the group consisting of Mg, Al, Cr, Mn, Co, Fe, Cu,
Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and Ta.
Recording can be excellently performed to light at
blue-wavelengths by using a material which comprises Bi, M,
and oxygen for a recording layer. In an aspect of the
write-once-read-many optical recording medium of the present
invention, in which a recording layer comprises M, by forming a
recording mark in a state where crystals of two or more types of
oxides are mixed, the difference in refractive index between a
recording mark and an unrecorded portion is larger, and higher
modulated amplitudes can be yielded. Further, greater effects
can be obtained by making not only crystal of respective oxides
but also crystal of a simple substance of element reside in the
recording layer. Since growth of crystals can be restrained by
making crystals of different elements or crystals each having a

different crystal structure mixed, a recording mark which
comprises crystals of two or more different elements and/or
crystal-structures can restrain the crystals from growing and
spreading and makes it possible to form a small recording mark.
A recording mark preferably comprises tetravalent Bi.
Typically, as the valence of Bi, trivalent Bi is in the most stable
state, however to yield a much higher modulated amplitude, a
tetravalent Bi is used. It is possible to make the valence of Bi
into tetravalence depending on the conditions of oxygen
surrounding a Bi atom. Since physical properties are changed
by changing the valence of Bi, higher modulated amplitude can
be yielded.
Examples of the tetravalent Bi compound include BiO2.
Typically, a Bioxide having a structure of Bi2O3 is in a stable
state. However, a Bi-oxide can take a structure like BiO2
depending on the conditions. By making a recording layer have
a crystalline structure that is not typically employed, higher
modulated amplitudes can be yielded.
A still another aspect of the write-once-read-many optical
recording medium of the present invention will be described
below. In the write-once-read-many optical recording medium,
a recording layer comprises Bi, L, oxygen, and a Bi oxide as
constituent elements. It is noted that the element L represents
at least one element selected from the group consisting of B, P,

Ga, As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po,
At, and Cd. The recording layer comprises bismuth as the
principal constituent. As an element to be added to the
recording layer which comprises Bi and O as constituent
elements, the element M can be named. However, the study
about basic properties of the constituent elements that can be
added to the constituent elements shows that the element L can
also be named. One of the reasons why the element L can be
added as a constituent element to a recording layer which
comprises bismuth as the principal constituent and comprises a
Bi oxide is to reduce thermal conductivity to facilitate forming a
minute recording mark. Since thermal conductivity is a value
attributable to the presence of photon scattering, thermal
conductivity can be reduced when the particle size and crystal
size are downsized, when there are a number of atoms
constituting a material, and when the difference in atomic mass
of constituent atoms is large, and the like. Thus, by adding the
element L as a constituent element to a recording layer which
comprises bismuth as the principal constituent and comprises a
Bi oxide, it is possible to control thermal conductivity and
improve high density recording properties.
Further, in a recording layer which comprises bismuth as
the principal constituent and further comprises a bismuth oxide,
the bismuth oxide and the bismuth are crystallized by recording

of information, however the size of the crystal and crystallized
particles can be controlled by the element L. Thus, the element
L enables controlling the size of crystal and crystallized
particles in recorded portions and greatly improving recording
and reproducing properties such as jitter. This is another
reason for adding the element L as a constituent element to a
recording layer.
From the perspective of thermal conductivity, there are
little requirements that are assumed by the element L which can
be added as a constituent element to the recording layer except
for simple requirements such as the stability of raw materials
and the difficulty level of production. However, the inventors of
the present invention found that reliabilities of a recording layer,
i.e. reproducing stability and storage stability substantially
vary depending on the selected element L, and definitely there
are requirements assumed by the element L with respect to
reliabilities.
Namely, as a result of keen examinations on the
requirements of the element L to be added as a constituent
element to the recording layer, the inventors of the present
invention found that the following requirements (I) or (II) are
effective:
(I) An element having a Pauling's electronegativity value
of 1,80 or more.

(II) An element having a Pauling's electronegativity value
of 1.65 or more and the standard enthalpy of formation AHf° of
oxides of-1,000 (kJ/mol) or more, excluding transition metals.
By using an element L satisfying (I) or (II), it is possible
to achieve a write-once-read-many optical recording medium
having excellent recording and reproducing properties such as
jitter, and high-reliabilities.
Hereinafter, the requirements (I) and (II) will be
explained in detail.
Degradation in reliabilities of the recording layer which
comprises bismuth as the principal constituent of constituent
elements and comprises a bismuth oxide is primarily caused by
progression of oxidation, or changes in oxidization state such as
changes in the valence, and the like. Pauling's
electronegativity values and the standard enthalpy of formation
"AHf0" of oxides are really important as physical property values
of the element L, because the progression of oxidation and
changes in oxidization state are liable to invite degraded
reliabilities. To sufficiently enhance reliabilities of an optical
recording medium, first, it is preferred to select an element
having a Pauling's electronegativity value of 1.80 or more as an
element L. This is because oxidation is unlikely to make
progress with an element having a high Pauling's
electronegativity value, and to secure satisfactory reliabilities,

an element having a Pauling's electronegativity value of 1.80 or
more is effective. The standard enthalpy of formation of "∆Hf0"
of oxides may take any values, provided that the Pauling's
electronegativity value is 1.80 or more.
Examples of the element L having an electronegativity of
1.80 or more include B, Si, P, Fe, Co, Ni, Cu, Ga, Ge, As, Se, Mo,
Tc, Ru, Rh, Pd, Ag, Sn, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb,
Po, and At.
Here, the electronegativity will be described briefly.
The electronegativity is a scale representing how strongly
the existing atom in a molecule can attract electrons thereto.
The ways to determine a value of electronegativity include
Pauling's electronegativity, Mulliken's electronegativity, and
Allred-Rochow's electronegativity. In the present invention, the
adequacy of an element L is determined by using Pauling's
electronegativity.
The Pauling's electronegativity defines that the binding
energy E (AB) of molecules A and B is greater than the average
of binding energy of molecules AA and molecules BB [E (AA) and
E (BB), respectively], and the difference therebetween is a
square of the difference between the electronegativity of the
respective atoms (XA, XB). Namely, it is represented by the
following formula:
E (AB) - [E (AA) + E (BB)] / 2 = 96.48 (XA- XB)2 ....(1)

In Pauling's electronegativity, the equation includes a
conversion coefficient of 96.48 (leV = 96.48kJmol-1), because a
value of electronegativity is determined by using electron volt.
Since the electronegativity varies depending on with what
valence an intended element takes in a molecule, the following
definitions are applied when the electronegativity is determined
in the present invention.
Namely, as shown below, the value when the following
each group element respectively takes the following valence is
defined as the Pauling's electronegativity value of the element:
Elements in 1 group take monovalence; elements in 2
group take divalence," elements in 3 group take trivalence;
elements in 4 group to 10 group take divalence; elements in 11
group take monovalence*' elements in 12 group take divalence,'
elements in 13 group take trivalence; elements in 14 group take
tetravalence; elements in 15 group take trivalence; elements in
16 group take divalence; elements in 17 group take
monovalence; and elements in 18 group take zero valence.
For the above-noted elements each having
electronegativity of 1.80 or more, the respective Pauling's
electronegativity values defined in the present invention are B
(2.04), Si (1.90), P (2.19), Fe (1.83), Co (1.88), Ni (1.91), Cu
(1.90), Ga (1.81), Ge (2.01), As (2.18), Se (2.55), Mo (2.16), Tc
(1.90), Ru (2.20), Rh (2.28), Pd (2.20), Ag (1.93), Sn (1.96), Sb

(2.05), Te (2.10), W (2.36), Re (1.90), Os (2.20), Ir (2.20), Pt
(2.28), Au (2.54), Hg (2.00), Tl (2.04), Pb (2.33), Po (2.00), and At
(2.20).
These elements can be added in combination with two or
more as constituent elements to a recording layer.
Further, the inventors of the present invention found that
even if the Pauling's electronegativity value is less than 1,80, an
element having a Pauling's electronegativity value of 1.65 or
more and the standard enthalpy of formation AHf° of oxides of
the element being -1,000 (kJ/mol) or more enables ensuring
satisfactory reliabilities. The reason why the requirement is
effective is because when an element having a great value of
standard enthalpy of formation ∆Hf0 of oxides, the element is
hard to form oxides thereof even if the value of Pauling's
electronegativity value is small in some degree.
In the present invention, when determining a Pauling's
electronegativity value, it was determined with each valence
fixed to every group of elements, and the same definitions are
applied when determining the standard enthalpy of formation
∆Hf0.
Namely, the value when an oxide is constituted with the
following each valence in each group element is defined as the
standard enthalpy AHf° of formation of the element.
Namely, elements in group 1 take monovalence," elements

in 2 group take divalence; elements in 3 group take trivalence;
elements in 4 group to 10 group take divalence; elements in 11
group take monovalence; elements in 12 group take divalence;
elements in 13 group take trivalence! elements in 14 group take
tetravalence; elements in 15 group take trivalence; elements in
16 group take divalence; and elements in 17 group take
monovalence; and elements in 18 group take zero valence.
However, in the case of a transition metal, it is impossible to
easily determine the standard enthalpy of formation AHf0 of
oxide thereof, because it forms an oxide with various valences.
Typically, the higher the valence of the element which forms an
oxide thereof is, the lower the standard enthalpy of formation
∆Hf0 of the oxide is.
In other words, it is considered that in the case of a
transition metal, an oxide or oxides thereof are easily formed
because there are oxides having various valences to be formed,
and thus in the present invention transition metals are not
preferably used as an element L.
For example, since V (vanadium) takes divalence, the
standard enthalpy of formation ∆Hf0 of a V (vanadium) oxide
takes the value of the standard enthalpy ∆Hf0 of formation of VO
= -431 (kJmol-1), and it falls under the requirements for the
element L (II) in the present invention. However, V (vanadium)
also easily forms oxides such as V2O3 (trivalence), V2O4

(tetravalence), V2O5 (pentavalence). The ∆Hf0 values of these
oxides are respectively V2O3 (-1,218kJmol-1), V2O4
(-l,424kJmol-1), V2O5 (-1,550kJmol-1), and these values do not
fall under the requirements for the element L (II) in the present
invention. Namely, assuming that V forms oxides with almost
only divalence, V (vanadium) falls under the requirements of (I)
and (II) of the present invention, however V (vanadium) easily
forms oxides other than divalent oxides, and the oxides are
easily oxidized, i.e. are in a more stable state. Thus, these
oxides are excluded from the preferred element L of the present
invention.
The description of the exclusion is clearly described in the
requirement (II) for the element L in the present invention as
"excluding transition metals."
Here, the standard enthalpy of formation ∆Hf° will be
described briefly.
Typically, a chemical reaction is represented by the
following chemical equation (2):
H2 (gas) + (1/2) O2 (gas) = H2O (liquid) (2)
The left side of the chemical equation is referred to as
original system, and the right side thereof is referred to as
product. The coefficient attached to a molecule is called a
stoichiometric coefficient.
Heat moves in and out associated with a chemical

reaction of the system at a constant temperature is called heat
of reaction, and heat of reaction at a constant pressure is called
heat of reaction at constant pressure.
In most cases, under typical laboratory conditions, heat of
reaction is measured at a constant pressure, therefore, heat of
reaction at constant pressure is typically used. Heat of
reaction at constant pressure is equal to the difference in
enthalpy "∆H" between product and original system.
A reaction expressed as ∆H > 0 is referred to as
endothermic reaction, and a reaction expressed as ∆H referred to as exothermic reaction.
Heat of reaction generated from a reaction forming a
compound from a chemical element of the compound is called
heat of formation or enthalpy of formation. Heat of reaction
generated from a reaction forming one mole compound in
standard condition from a chemical element of the compound is
called standard enthalpy of formation. In the standard
condition, the standard enthalpy of formation is marked with a
designated temperature, typically 298K, under a pressure of
0.1MPa or nearly equal to 1 atmospheric pressure, and the
standard enthalpy of formation is represented by the symbol of
∆Hf°. In the standard condition, it is ruled that enthalpies of
respective chemical elements are zero.
Thus, it can be said that the smaller the value of

standard enthalpy of formation of oxide of an element is or the
greater in negative value of the standard enthalpy of formation
is, the oxide is more stable and the element is easily oxidized.
The specific values of the standard enthalpy of formation
are written, for example, in "Electrochemistry Handbook Vol. 5"
(Denkikagaku Binran) edited by The Electrochemistry Society
of Japan, Maruzen).
Since the standard enthalpy of formation ∆Hf° varies
depending on with what valence an intended element takes in a
molecule to form an oxide thereof, the above-noted requirements
are applied when the standard enthalpy of formation of oxides of
respective elements is determined in the present invention.
Examples of elements having a Pauling's
electronegativity value of 1.65 or more and the standard
enthalpy of formation of oxides thereof ∆Hf0 of -1,000 (kJ/mol)
or more include Zn, Cd, and In. For the Pauling's
electronegativity value according to the definitions of the
present invention, these elements respectively are determined
as follows: Zn (1.65), Cd (1.69), and In (1.78). For the
standard enthalpy of formation ∆Hf0 according to the definitions
of the present invention, they are respectively determined as
follows: Zn (-348kJmon), Cd (-258kJmol-1), and In
(-925kJmorl-1).
The atomic number ratio of the total amount of the

element L to bismuth is preferably 1.25 or less.
Since a recording layer of the present invention is based
on the assumption that it comprises bismuth as the principal
constituent of the recording layer and also comprises a Bi oxide,
when the atomic number ratio of the total amount of the element
L to bismuth is more than 1.25, intrinsic recording and
reproducing properties may not be obtained.
Further, adding B, P, Ga, Se, Pd, Ag, Sb, Te, W, Pt, and
Au as an element L to the recording layer is preferable from the
perspective of improving storage stability of the recording layer.
The improvement of storage stability is very likely to be caused
by the fact that it is hard to break a once-formed crystalline
structure by strengthening binding force between the atoms or
when atoms in different size reside side-byside, it makes the
crystalline structure stable because smaller atoms in size can
reside in lattices of greater atoms in size. In particular,
elements such as B and Pd are bound to oxygen, and when
making Bi and O exist together is effective in stabilizing an
amorphous structure. It may consequently stabilize the
structure of the crystal, which leads to improvements in storage
stability of the write-once-read-many recording medium.
In a write-once-read-many optical recording medium of
the present invention, it is preferred that recording and
reproducing of information be performed through the use of a

laser beam at a wavelength of 680nm or less. Unlike dyes, the
recording layer of the present invention has an appropriate
absorption coefficient at a wide range of wavelengths and has a
high refractive index, and therefore it is possible to perform
recording and reproducing with a laser beam at a red-laser
wavelength of 680nm or less as well as to achieve excellent
recording and reproducing properties and high-reliabilities.
Among them, the most preferable advantage is to perform
recording and reproducing with a laser beam at a wavelength of
450nm or less. This is because the recording layer having
bismuth as the principal constituent and having a Bi oxide has a
complex refractive index particularly suitable for a
write-once-read-many optical recording medium used with a
laser beam at a wavelength of 450nm or less.
A write-once-read-many optical recording medium
according to the present invention preferably has the following
configurations, however they are not particularly limited to the
configurations.
(A) Substrate, recording layer, upper coating layer, and
reflective layer
(B) Substrate, under coating layer, recording layer, upper
coating layer, and reflective layer
(C) Substrate, reflective layer, upper coating layer,
recording layer, and cover layer

(D) Substrate, reflective layer, upper coating layer,
recording layer, under coating layer, and cover layer.
Further, on the basis of the above configurations, a
structure of layers may be formed in a multi-layered structure.
For example, when formed in two-layered based on the
configuration of (A), it may has a configuration as follows:
Substrate, recording layer, upper coating layer, reflective layer
or semi-transparent layer, binder layer, recording layer, upper
coating layer, reflective layer, and substrate.
For the under coating layer and the upper coating layer,
the following oxides and nonoxides are available' examples of
the oxides include simple oxide such as B2O5, Sm2O3, Ce2O3,
Al2O3, MgO, BeO, ZrO2, UO2, and ThO2; silicate such as SiO2,
2MgOSiO2, MgO.SiO2, CaO.SiO3, ZrO2SiO2, 3Al2O3.2SiO2,
2MgO.2Al2O3-5SiO2, Li2O.Al2O3-4SiO2; double oxide such as
Al2TiO5, MgAl2O4, Ca10 (PO4)6(OH)2, BaTiO3, LiNbO3, PZT [Pb
(Zr, Ti) O3], PLZT [(Pb, La) (Zr, Ti) O3],and ferrite. Examples of
the nonoxides include nitrides such as Si3N4, A1N, BN, and TiN'
carbides such as SiC, B4C, TiC, and WC; borides such as LaB6,
TiB2, and ZrB2; sulfides such as ZnS, CdS, and MoS2; silicides
such as MoSi2; and carbons such as amorphous carbon, graphite,
and diamond.
Organic materials such as dyes and resins can also be
used for the under coating layer and the upper coating layer.

Examples of the dyes include polymethine dyes,
naphthalocyanine dyes, phthalocyanine dyes, squarylium dyes,
chroconium dyes, pyrylium dyes, naphthoquinone dyes,
anthraquinone (indanthrene) dyes, xanthene dyes,
triphenylmethane dyes, azulene dyes, tetrahydrocholine dyes,
phenanthrene dyes, triphenothiazine dyes, azo dyes, formazan
dyes, and metal complexes of these compounds.
Examples of the resins include polyvinyl alcohols,
polyvinyl pyrrolidones, cellulose nitrates, cellulose acetates,
ketone resins, acrylic resins, polystyrene resins, urethane resins,
polyvinyl butyrals, polycarbonates, and polyolefins. Each of
these resins may be used alone or in combination with two or
more.
A layer which comprises organic materials can be formed
by means of vapor depositions, sputtering, CVD, i.e. Chemical
Vapor Deposition, coating of a solvent or the like, which are
typically used. When a coating method is used, the above-noted
organic materials and the like are dissolved in an organic
solvent and the solvent is coated by a commonly used coating
method such as spraying, roller-coating, dipping, and
spin-coating.
Examples of typical organic solvents to be used include
alcohols such as methanol, ethanol, and isopropanol; ketones
such as acetone, methyl ethyl ketone, and cyclohexanone;

amides such as N, N-dimethylacetoamide, and N,
N-dimethylformamide; sulfoxides such as dimethylsulfoxide;
ethers such as tetrahydrofuran, dioxane, diethyl ether, and
ethylene glycol monomethyl ether; esters such as methyl acetate,
and ethyl acetate; aliphatic halocarbons such as chloroform,
methylenechloride, dichloroethane, carbon tetrachloride, and
trichloroethane; aromatic series such as benzene, xylene,
monochlorobenzene, and dichlorobenzene; cellosolves such as
methoxyethanol, and ethoxyethanol; and hydrocarbons such as
hexane, pentane, cyclohexane, and methylcyclohexane.
For the reflective layer, light reflection materials having
high reflectance against laser beams are used.
Examples of the light reflection materials include metals
such as Al, Al-Ti, Al-In, Al-Nb, Au, Ag, and Cu, semimetals, and
alloys thereof. Each of these materials may be used alone and
in combination with two or more.
When a reflective layer is formed with an alloy, it is
possible to prepare it by using an alloy as a target material and
by sputtering. Besides, it is also possible to form a reflective
layer by tip-on-target method (for example, a Cu tip is placed on
an Ag target material to form a reflective layer), and by
cosputtering (for example, an Ag target and a Cu target are
used).
It is also possible to alternately stack low-refractive

index layers and high-refractive index layers and form a
multi-layered structure to use it as a reflective layer.
A reflective layer may be formed, for example, by
sputtering, ion-plating, chemical vapor deposition, and vacuum
deposition. The reflective layer preferably has a thickness of
5nm to 300nm.
Materials for a substrate are not particularly limited, as
long as they have excellent thermal and machine properties, and
when recording and reproducing is performed from the side of a
substrate and through the substrate, they have also excellent
light transmission properties.
Specifically, examples thereof include polycarbonates,
polymethyl methacryiates, amorphous polyolefins, cellulose
acetates, polyethylene terephthalate, of which polycarbonates
and amorphous polyolefins are preferable.
The thickness of the substrate varies depending on the
application and is not particularly limited.
Materials for a protective layer to be formed on a
reflective layer, an optically transparent layer or the like are
not particularly limited, provided that the material can protect
reflective layers, optically transparent layers or the like from
external forces. Examples of organic materials include
thermoplastic resins, thermosetting resins, electron beam
curable resins, and ultraviolet curable resins. Examples of

inorganic materials include SiO2, Si3N4, MgF2, and SnO2.
On a reflective layer and/or an optically transparent layer
and the like, a protective layer can be formed using a
thermoplastic resin and/or thermosetting resin. First, a
thermoplastic resin and/or a thermosetting resin are dissolved
in a suitable solvent to prepare a coating solution. Then, the
coating solution is coated to a reflective layer and/or an optically
transparent layer and dried to thereby form a protective layer.
A protective layer using an ultraviolet curable resin can
be formed by directly coating an ultraviolet curable resin to a
reflective layer and/or an optically transparent layer or
dissolving an ultraviolet curable resin in a suitable solvent to
prepare a coating solution and coating the coating solution to a
reflective layer and/or an optically transparent layer, and then
irradiating ultraviolet ray to the coating solution to harden it.
For ultraviolet curable resins, for example, acrylate
resins such as urethane acrylates, epoxy acrylates, and
polyester acrylates can be used.
Each of these materials may be used alone and in
combination with two or more and may be formed in not only a
single layer but also in a multi-layered structure.
For a method for forming a protective layer, coating
methods such as spin-coating and casting, sputtering, chemical
vapor deposition, and the like are used, of which spin-coating is

preferable.
The thickness of the protective layer is typically 0.1µm to
100µm, however it is preferably 3pm to 30pm in the present
invention.
Further, a substrate may be disposed on the surface of a
reflective layer or an optically transparent layer. The reflective
layer and the optically transparent layer may be arranged so as
to face each other. Two sheets of optical recording media may
be laminated after arranging a reflective layer and an optically
transparent layer so as to face each other.
In addition, an ultraviolet curable resin layer, an
inorganic resin layer or the like may be formed on a mirror
surface side of a substrate to protect the surface and/or to
prevent dust or the like from attaching thereto.
An optically transparent layer or a cover layer should be
formed when a lens with a high numerical aperture is used for
higher recording density. With increasing numeral aperture,
the portion through which the reproducing light passes must
have a reduced thickness. This is because the allowance in the
aberration occurs when the perpendicular direction to the
surface of the medium deviates from the optical axis of an
optical pickup, the deviation angle so called a tilt angle is
reduced with increasing numerical aperture. The tilt angle is
in proportional to the square of the product of reciprocal of the

wavelength of an optical source multiplied by the numerical
aperture of the objective lens and is susceptible to the
aberration due to the thickness of the substrate. To reduce the
aberration due to the thickness of the substrate, the thickness of
the substrate is reduced.
For this purpose, some optical recording media are
presented, for example, an optical recording medium in which a
substrate, a recording layer with concave convex or
irregularities formed thereon, a reflective layer, and an optically
transparent layer or a cover layer which is a layer transmitting
light are disposed in this order in a laminar structure, in which
the reproducing light is irradiated from the optically
transparent layer to reproduce information in the recording
layer. Another example is an optical recording medium in
which a reflective layer, a recording layer, and an optically
transparent layer or a cover layer having optical transparency
are disposed in this order on a substrate, in which the
reproducing light is irradiated from the optically transparent
layer to reproduce information in the recording layer. These
optical recording media can allow the use of an objective lens
with a high numerical aperture by reducing the thickness of the
optically transparent layer. Namely, higher-density recording
can be performed by recording and/or reproducing information
on a medium having a thin optically transparent layer, in which

the reproducing light is irradiated from the optically
transparent layer.
The cover layer may typically comprise a polycarbonate
sheet or an ultraviolet curable resin. The cover layer used
herein may include a binder layer for binding the cover layer to
an adjacent layer.
Hereinafter, a sputtering target suitably used for the
optical recording medium of this invention having a recording
layer which comprises Bi, Fe, and oxygen, and the production
method will be disclosed in detail.
A sputtering target according to the present invention
comprises Bi, Fe, and 0. The sputtering target is suitably used
for forming a recording layer of an optical recording medium
which performs recording and reproducing with a laser beam at
wavelengths of 550nm or less.
The relationship between extinction coefficient k which is
one of optical constants and is a coefficient representing a light
absorption degree and wavelength of light was examined.
When the value of k is zero, it represents that there is no
absorption of light, and with increasing value of k, absorption
light increases. A recording layer formed using the sputtering
target of the present invention which comprises Bi, Fe, and O
has absorption light of nearly zero at a wavelength of 600nm or
more, and has approx. zero value of k, therefore, it is hard to

record information. However, the value of k abruptly becomes
greater at wavelengths shorter than 550nm and excellent
recording is enabled. Particularly, in the vicinity of a
wavelength of 400nm, absorption of light becomes substantially
greater, fairly excellent properties are exhibited as a recording
layer of an optical recording medium.
The sputtering target of the present invention preferably
comprises a Bi oxide and a Fe oxide or a complex oxide of Bi and
Fe. The sputtering target may comprise a complex oxide of Bi
and Fe and further comprises one or more selected from a Bi
oxide and a Fe oxide. There may be cases where a Bi oxide or a
Fe oxide remains in a sputtering target without making complex
of Bi and Fe in a well-balanced condition, depending on the
composition. An aspect of the sputtering target comprises a Bi
oxide and a Fe oxide, or another aspect of the sputtering target
comprises a complex oxide of Bi and Fe, or still another aspect of
a sputtering target comprises a complex oxide of Bi and Fe and
further comprises one or more selected from a Bi oxide and a Fe
oxide. In any of the aspects stated above, an optical recording
medium produced using such a sputtering target as stated above
exhibits excellent properties. In particular, by including a Bi
oxide as a constituent of a recording layer, the produced
recording layer can include the Bi oxide. Since a recorded mark
can be excellently recorded by precipitating Bi metal, it is

remarkably effective to use a sputtering target which comprises
a Bi oxide.
The sputtering target of the present invention preferably
comprises one or more selected from a Bi oxide, a Fe oxide, and
a complex oxide of Bi and Fe, and the oxide is preferably an
oxide having a smaller amount of oxygen compared to the
stoichiometric composition. Examples of the oxide which is a Bi
oxide, a Fe oxide, or a complex oxide of Bi and Fe and has a
smaller amount of oxygen compared to the stoichiometric
composition include BiOx (x 3), Bi25FeOx (x amount of oxygen to enhance metallic properties, sputtering is
enabled through the use of a direct-current power (DC power).
DC sputtering has advantages in that apparatuses used for the
manufacturing are cheaper than those used in RF sputtering
and the apparatuses used in DC sputtering are to be more
downsized than those used in RF sputtering. When a layer is
formed by DC sputtering, the amount of oxygen in the layer can
be controlled by externally introducing oxygen. By employing
DC sputtering, recording properties are easily controlled.
A sputtering target having a smaller amount of oxygen
compared to the stoichiometric composition is calcined after a Bi
metal and Fe2O3 being mixed. It is also possible to produce a
target having a small amount of oxygen by the methods such as

by reducing the amount of oxygen remaining in the target by
controlling the temperature.
The sputtering target of the present invention preferably
comprises one or more selected from BiFeO3, Bi25FeO40, and
Bi36Fe2O57 as the principal constituent. As mentioned above,
an optical recording medium using a recording layer which
comprises Bi, Fe, and oxygen exhibits excellent recording
properties. There have been a sputtering target containing Bi,
Fe, and oxygen for magnetic optical recording and having a
garnet structure, however, such sputtering target do not
comprise the compounds used in the present invention as the
principal constituent from the perspective of the composition for
forming a garnet structure. A sputtering target which
comprises three elements of Bi, Fe, and oxygen has not been
used so far, because it is impossible to form a garnet structure
only with these three elements. In addition, the present
invention does not relate to magnetic optical recording media,
and the present invention relates a sputtering target used for
forming a layer which primarily comprises Bi, Fe, and oxygen,
which is hereafter referred to as a BiFeO layer, for an optical
recording medium without using magnetism, particularly for a
write-once-read-many optical recording medium capable of
high-density recording.
The sputtering target preferably consists of three

elements of Bi, Fe, and oxygen.
In this case, the sputtering target may comprise impure
elements other than the three elements, however, it is not
preferred to include the microelements that would impair
properties of layers.
A sputtering target of the present invention is produced
by mixing and calcining powder of oxides of raw materials. It is
possible to produce the sputtering target using calcine powder of
a Bi oxide and powder of a Fe oxide. It is also possible to
produce a sputtering target by producing powder of a compound
which primarily comprises three elements of Bi, Fe, and oxygen
and then calcining the powder. As the calcining method,
press-heat calcination processes such as hot-pressing, hot
isostatic pressuring or HIP may be used. For calcination
temperatures, higher temperatures are preferred to reinforce
the strength of a sputtering target, however, in the case of a
compound which comprises three elements of Bi, Fe, and oxygen,
separation of the phase and/or fusion occur at a temperature of
approx. 800°C or more, and it is difficult to uniformly calcine it.
Then, the calcination temperature must be controlled not so as
to be more than approx. 750°C.
In particular, an optical recording medium in which a
BiFeO layer is formed using a sputtering target which comprises
one or more selected from BiFeO3, Bi25FeO40, and Bi36Fe2O57 as

the principal constituent for the recording layer exhibits
excellent properties. The presence of BiFeO3, Bi25FeO40, and
Bi36Fe2O57 can be checked by means of X-ray diffraction. For
the radiation source, Cu is used to measure it with the angle of
29 of an X-ray diffractometer from 5° to 60°. The term the
principal constituent means that the highest content or the
highest % by mass is contained in a compound. Typically, a
material exhibiting the highest diffraction peak as a result of an
X-ray diffraction analysis can be determined as the principal
constituent, however, the content may not be in proportion to
the scale of diffraction peak. The case of a sputtering target
which comprises two or more elements selected from BiFe03,
Bi25FeO40, and Bi36Fe2O57 as the principal constituent means
that each content of these two or more compounds is same and
the content of these two or more compounds is higher than that
of other components.
The sputtering target preferably comprises Bi2O3 and/or
Fe2O3. An optical recording medium with a BiFeO layer formed
using a sputtering target which comprises these compounds for
the recording layer exhibits excellent properties. The presence
of Bi2O3 and/or Fe2O3 can be checked by means of X-ray
diffraction. For the radiation source, Cu is used to measure it
at an angle of 29 from 5° to 60°.
In the. X-ray diffraction analysis for identifying

constituents of BiFeO3, Bi25FeO40, Bi36Fe2O57, and Bi2O3, and
Fe2O3, when the angle of 2θ where a diffraction peak is expected
to be detected is defined as θ1, a substance actually having a
diffraction peak within the range of θ1 ± 1 degree is determined
to be included in the above-noted compounds.
In X-ray diffraction analysis, the lattice constant varies,
and misalignments are developed in angles at which diffraction
peaks appear depending on causes such as measurement
temperature, internal stress of the layer, X-ray wavelength error
of measurement, and shift of composition. For well-known
substances, at what angles diffraction peaks are detected can be
known through the use of ASTM (American Society for Testing
and Material) cards and JCPDS search. When a sample is
analyzed to identify the component, ASTM cards and JCPDS
card charts are widely used. The term, JCPDS stands for Joint
Committee on Powder Diffraction Standards, and it is a chart of
X-ray diffraction patterns distributed by the organization called
International Center for Diffraction Data, and a number of
charts of diffraction patterns of standard substances are stored
for search. An X-ray diffraction pattern chart of a sample
having unknown components is compared to the charts of
standard substances, and to what chart of standard substance
the X-ray diffraction pattern chart corresponds or is closely
related is determined. With this comparison, the substance of

the sample is identified. The identification method using
JCPDS card charts is a method widely used in the world, as
shown in X-ray Diffraction Analysis Rules, and descriptions of
X-ray Diffraction Analysis Rules in Japanese Industrial
Standard (JIS) as well as in X-ray Diffraction Analysis -
Ceramics Basic Structures 3 edited by Tokyo Institute of
Technology. In the measurement, to what chart of standard
substance the X-ray diffraction pattern of the substance having
unknown components corresponds or is closely related is
examined, and then the substance is identified.
nλ = 2d sinθ
In the above equation, n represents a positive integral
number, λ, represents a wavelength, d represents distance of
lattice plane, and θ represents a glancing angle or a
supplementary angle of incident angle.
Since the Bragg principle is approved, a peak of
diffraction also appears at the positions where n represents an
integral multiple at the side of higher-angle of 2θ. The
identification of substances is enabled by analyzing peaks of
positions represented by integral multiple at the same time of
the analysis of diffraction peaks based on 2θ. As described
above, in X-ray diffraction analysis, lattice constant varies, and
misalignments are developed in angles at which diffraction
peaks appear due to measurement temperature, internal stress

of the layer, X-ray wavelength error of measurement, shift of
composition, and the like, therefore, when a substance having a
peak near at the angle where a diffraction peak appeared at a
known substance, it can be determined that the substance
corresponds to the known substance having that diffraction
peak.
For the misalignment of diffraction peak, for example,
peaks having 29 = 22.491 degree sought from the standard chart
of BiFeO3 (reference code 20-0169) was studied. The results of
measurement on four sputtering targets produced in the same
manner and under same conditions were respectively observed.
The degree 2θ of the standard data of BiFeO3 is 22.491°. On
the other hand, these targets showed very slight misalignments
of 22.380°, 22.500°, 22.420°, and 22.420°, respectively. This
study showed that the misalignments of approx. ± 1 degree are
within the error range of measurement, when repeatedly
measured. Therefore, even though such misalignments in angle
of ± 1 degree diffraction peak are observed, it can be identified
as a known peak. For example, when a substance having a
peak with a misalignment of ± 1 degree from 22.491°, it can be
determined that the BiFeO3 described in the present invention
are included in the substance. The sputtering target which
comprises Bi, Fe, and O and has a diffraction peak at 22.380 ≤
29 ≤ 22.500 is particularly effective in recording properties.

Similarly, according to the standard data of Bi36Fe2O57,
Bi36Fe2O57 has a peak at 2θ = 27.681 degree. The above-noted
four targets were measured, and it was found that they had a
peak at 27.65°, 27.64°, 27.76°, and 27.67°.
According to the standard data of Bi25FeO40, it has a peak
at 2θ = 27.683 degree. These four targets were measured, and
it was found that they had a peak at 27.66°, 27.64°, 27.76°, and
27.68°, respectively.
Since misalignments of approx. ± 1 degree are caused in
the repeated measurements, it is considered that the
misalignments of ± 1 degree in 2θ of any of these substances fall
under the error range of measurement.
Preferably, the sputtering target does not further
comprise Bi2Fe4O9. With increasing content of Bi, the content
of Bi2Fe4O9 tends to lower. The presence of Bi2Fe4O9 can be
checked by means of X-ray diffraction. For the radiation source,
Cu is used to measure it with the angle of 29 of an X-ray
diffractometer from 5° to 60°. It is found that an optical
recording medium with a BiFeO layer formed for a recording
layer using a sputtering target in which the presence of
Bi2Fe4O9 is recognized by means of X-ray diffraction does not
satisfactorily exhibit recording properties and does not suit
high-density recording.
The sputtering target preferably has a content of Co, Ca

and Cr less than the detection limit. When a sputtering target
which comprises impurities is used, a formed layer also
comprises elements of impurities. In optical recording,
recording is performed by absorbing laser beam irradiation into
a recording layer to elevate the temperature at the recording
layer and to induce physical or chemical changes. The term
physical or chemical changes means changes such as
crystallization or the like. A layer used for a recording layer
containing impurities is not preferably used because
crystallization temperature varies, and the size of crystal differs
at the time of crystallization.
For detection of impurities, the composition is
quantitatively analyzed by means of the inductively coupled
plasma emission spectrometry. This analysis is suitable when
analyzing element in minute amount, and even when using this
analysis, the sputtering target preferably has a content of Co,
Ca and Cr less than the detection limit.
The sputtering target preferably has a packing density of
65% to 96%.
The higher the packing density of the sputtering target is,
the higher the strength of the target is, and the time for forming
a layer tends to be shortened because of its high-density
elements, at the same time, the difference in composition
between the target and the formed layer tends to increase.

There may be cases where the composition of the target can be
close to that of the layer by reducing the density of the target,
however, a density-reduced target causes problems with not only
delayed layer-forming speed but also causes a problem that the
target itself goes brittle to be broken when forming a layer.
Herein, the term packing density represents a value obtained as
density by comparing the weight of a target actually produced to
the weight of the target calculated when the weight of the target
is dominated by an intended substance at 100%.
Table 1 shows packing densities of a target, conditions of
the target during the time for forming a layer or after the
forming, and recording properties of a recording medium with a
layer formed therein when using a Bi10Fe5Ox target having a
diameter of 76.2mm and a thickness of 4mm. When the packing
density was 50% or less, the target was not able to be formed as
a sputtering target even after calcinations. With a packing
density of 61%, the target was able to be formed, however, it was
easily broken immediately after applying an electric power of
100W. With a packing density of 98%, the density was so high
and the target was so hard that it was easily broken. With a
packing density of 65% to 96%, the target was able to be formed
with no problem, and excellent recording properties were
exhibited. Even with a packing density of 61% and 98%,
excellent recording properties were obtained by forming a layer

under the conditions ensuring no occurrence of fracture of target
in forming a layer. As can be seen from the above results, the
packing density of the sputtering target is preferably 65% to
96%.

The sputtering target preferably has an atomic ratio of Bi
to Fe satisfying the relation of Bi/Fe ≥ 0.8. An optical recording
medium having a BiFeO-layer formed using the target exhibits
excellent properties and is particularly suitable for high-density
recording.
Theoretically, the upper limit of the ratio Bi/Fe is 25
based on the assumption that a sputtering target which
comprises Bi25FeO40 as the principal constituent is used,
however, virtually, it is 15 or so. A sputtering target
production method of the present invention relates to a method
for producing the BiFeO target of the present invention by

calcining powder of Bi2O3 and Fe2O3. Bi2O3 naturally exists as
an oxide of Bi, and Fe2O3 exists as an oxide of Fe. These
powers are crushed by a dry process or a wet process and then
classifying into a uniformly sized particle diameter. Next, the
powders are mixed, heated, and pressed to put into shape and
then calcined at a maintained temperature of 750°C in the
atmosphere. The strength of a sputtering target can be
improved by repeatedly performing the process of re-crushing a
calcined target and forming the shape by heating and pressing.
A sputtering target can be obtained by bonding the target
calcined as above to a backing plate made from oxygen-free
copper by means of metal bonding or resin bonding.
The present invention also proposes an optical recording
medium which comprises a BiFeO layer formed using the
sputtering target of the present invention. In the optical
recording medium, necessary layers are formed on a resin
substrate which comprises a polycarbonate or the like. Grooves
and pits may be formed on the resin substrate for controlling
tracking or the like. A BiFeO-layer is formed by applying
radiofrequency power while introducing an argon gas in vacuo.
Besides, a metallic layer and a protective layer for improving
properties may be disposed as a reflective layer.
The above paragraphs describe the sputtering target of
the present invention with a focus on optical recording media,

and the application of the sputtering target of the present
invention is not limited to optical recording media and may be
used for other purposes, as long as performance of the layer
meets requirements. For example, the sputtering target can be
used for forming a thin layer made from magnetic materials, for
forming a thin-layer for producing an isolator for optical
controlling, and for forming a thin layer for an optical switch.
For a rough production line of forming the sputtering
target, it is possible to take procedure steps of weighing of raw
materials, mixing by a dry-process ball mill, hot-pressing, shape
forming, and bonding can be used. It is also usable to take the
procedure steps of weighing of raw materials, mixing by a
dry-process ball mill, spray-drying, hot-pressing, shape forming,
and bonding.
According to the present invention, it is possible to
present a write-once-read-many recording medium capable of
recording small recording marks at higher degrees of modulated
amplitudes with stability. With the use of the additional
elements that have not been found so far, it is possible to
provide a write-once-read-many optical recording medium
having excellent recording and reproducing properties and
reliabilities.
Further, the present invention can provide a sputtering
target suitable for arbitrarily forming a layer having a stable

composition and structure, the production method thereof, and
can also provide a high-density optical recording medium using
the target.
Hereinafter, the present invention will be described in
detail on an optical recording medium which comprises a
recording layer in which a material of the present invention
represented by BiOx (0 write-once-read-many optical recording medium which comprises
Bi, M and oxygen as constituent elements of a recording layer,
referring to specific examples, however, the present invention is
not limited to the disclosed examples.
Example A-1
On a polycarbonate-substrate with a guide groove having
a groove depth of 21nm formed thereon, a layer having the
composition represented by BiOx (0 10nm was formed by sputtering to yield a write-once-read-many
optical recording medium of the present invention. This layer
was formed at a radiofrequency power of 100W and an Ar gas
flow rate of 40sccm using a sputtering target having the
composition of Bi2O3 and a diameter of 76.2mm.
Recording was performed to the optical recording medium
under the following conditions using an optical disk evaluation
system DDU-1000 manufactured by PULSTEC INDUSTRIAL
CO., LTD. having a lens numerical aperture of 0.65 at a

wavelength of 405nm.
• Modulation Mode: 1-7 modulation
• Recording linear density: The shortest mark length
(2T) = 0.231µm
• Recording linear velocity: 6.0m/s
• Waveform equalization: Normal equalizer
As a result, an excellent jitter value of 9.9% was obtained
in a consecutively recorded portion at a recording power of
5.2mW, and excellent binary recording properties having a
modulated amplitude of 55% were realized.
Example A-2
A write-once-read-many recording medium of the present
invention was produced by sputtering a layer which comprises
Fe and O, having a thickness of 10nm on a
polycarbonate-substrate with a guide groove having a groove
depth of 21nm formed thereon. This layer was formed at a
radiofrequency power of 100W and an Ar gas flow rate of 40sccm
using a sputtering target having the composition of Bi10Fe5Ox
and a diameter of 76.2mm. The target was prepared by
calcining a mixture of Bi2O3 and Fe2O3 at a ratio of 2:1.
Theoretically it should have been Bi10Fe5O22.5, however, the
amount of oxygen was unable to be measured with accuracy
because of leaked oxygen in a calcination process, and therefore
the oxygen is represented by Ox.

The optical recording medium was evaluated under the
following conditions using an optical disk evaluation system
DDU-1000 manufactured by PULSTEC INDUSTRIAL CO., LTD.
having a lens numerical aperture of 0.65 at a wavelength of
405nm.
• Modulation Mode- 1-7 modulation
• Recording linear density: The shortest mark length
(2T) = 0.231pm
• Recording linear velocity: 6.0m/s
• Waveform equalization: Normal equalizer
As a result, an excellent jitter value of 8.9% was obtained
in a consecutively recorded portion at a recording power of
5.8mW, and excellent binary recording properties having a
modulated amplitude of 52% were realized.
Example A-3
Reflection EELS measurements were performed using a
write-once-read-many optical recording medium produced and
used for recording in Example A-1. A scanning Auger electron
spectrometer PHI4300 manufactured by Perkin-Elmer was
modified for the measurements. EELS stands for Electron
Energy Loss Spectroscopy and is a spectroscopy system in which
electrons are irradiated to a sample to measure an energy
distribution of electrons scattered by interaction with the outer
surface of the sample. When a primary electron of certain

energy excites the inner shell of an atom to be measured, an
electron of certain energy is discharged, resulting in the
scattering of the primary electron. During this process, some
energy is lost by the interaction with the neighboring atoms.
Therefore, by examining the way the electrons are scattered,
information such as radial distribution function of neighboring
atoms can be obtained.
The radial distribution function in the vicinity of an
oxygen atom was measured based on the EELS spectrum
obtained by EELS measurements. A radial distribution
function represents a probability of existence of electron in the
vicinity of an atom and enables calculations and presumptions of
the valence and structure of the atom. Among analysis
software packages based on the photoelectron multiple
scattering theory, the FEFF software published by Washington
University are widely used. The number of valences and
structure thereof can be presumed by using this analysis
software and referring the calculated values against actually
measured values.
FIG. 1 illustrates values of radial distribution function
measured by the method as stated above.
FIG. 2 shows radial distribution functions calculated
using an FEFF. The diagram shows radial distribution
functions respectively in the case of Bitrivalent which could be

taken by Bi; the case of Bi-trivalent however taking a structure
of β-Bi2O3; and the case of BiO2 of Bi-tetravalence.
Comparing these radial distribution functions, peaks 1011
and 1012 in the vicinity of 6 angstrom in the recorded portion
are distinctively shown. When comparing the two diagrams,
the peaks 1011 and 1012 match each other. It proves that BiO2,
i.e. BiO2 of tetravalent Bi exists in the recorded portion.
A write-once-read-many optical recording medium having
such a recording mark enables excellent recording with higher
modulated amplitudes and realizing high-density recording.
Hereinafter, the present invention will be further
described in detail on a write-once-read-many optical recording
medium using the element L used in the present invention as an
additional element, referring to Examples and Comparative
Examples, however, the present invention is not limited to the
disclosed examples.
(Examples B-1 to B18)
A write-once-read-many optical recording medium was
produced by taking a structure in which a polyolefin-substrate
(ZEONOR manufactured by NIPPON ZEON CO., LTD.); a
recording layer which comprises bismuth as the principal
constituent of the elements constituting the recording layer and
a Bi oxide; a heat-insulating layer; and a reflective layer were
formed in a laminar structure, and using the following materials

for these respective layers.
A sputtering target was produced using raw materials in
which Bi2O3 and oxides of the element shown in Table 2 were
mixed at a ratio from 2:1 to 5:1, and then a recording layer was
formed using this sputtering target so as to have a thickness
approx. 7nm.
Table 2 shows resulting respective Pauling's
electronegativity values of each element L added to the
respective recording layer, and the standard enthalpy values of
formation ∆Hf0 of oxides of the element L, however, these values
are based on the definitions of the present invention. When the
Pauling's electronegativity value is 1.80 or more, the standard
enthalpy of formation ∆Hf° of oxides of the element L is not
important, and thus some elements in Table 2 have not their
values of ∆Hf0.
As described above, in the present invention, Pauling's
electronegativity values and standard enthalpy values of
oxide-formation of an element L were obtained with each valence
fixed to each element group. The individual valences of the
individual elements based on the definitions of the present
invention are also written in Table 2.
In Table 2, the term type A represents an element L
which falls under the definition (I) of the present invention, and
the term type B represents an element L which falls under the

definition (II) of the present invention.
A material in which ZnS-SiO2 was used for a
heat-insulating layer at a ratio of 85:15 (mol%) and formed so as
to have a thickness of 15nm.
For a reflective layer, an Ag alloy was used and formed so
as to have a thickness of 100nm.
The track pitch of a polyolefin substrate was 0.437µm and
the thickness was 0.6mm.
Recording was performed to these optical recording media
under the following conditions using an optical disk evaluation
system DDU1000 manufactured by PULSTEC INDUSTRIAL
CO., LTD. having a lens numerical aperture of 0.65 at a
wavelength of 405nm.
As a result, extremely excellent recording and
reproducing properties, i.e. jitter values shown in Table 2 were
realized.

• Modulation Mode: 1-7 modulation
• Recording linear density: The shortest mark length
(2T) = 0.204pm
• Recording linear velocity: 6.6m/s
• Waveform equalization: Limit equalizer
• Reproducing power: 0.5mW
Next, these write-once-read-many optical recording media

were left under the conditions at a temperature of 80°C and a
relative humidity of 85% for 100 hours to measure the amount of
change in jitter value. The amount of change in jitter value is
calculated as follows:
(Jitter value after a storage test) - (Initial jitter value)
Table 2 shows the results.
As is clear from Table 2, the elements satisfying the
definition (I) for the element L of the present invention
respectively demonstrated an excellent initial jitter value and a
small degree of degradation in jitter value, suffering from the
storage test.
It was also demonstrated that the standard enthalpy of
formation of oxides "∆Hf0" may take any values, as long as the
definition (I) is satisfied.
Also, the elements satisfying the definition (II) for the
element L of the present invention respectively demonstrated an
excellent initial jitter value and a small degree of degradation in
jitter value suffering from the storage test.

In the Examples above, the wavelength for recording and
reproducing was set at 405nm, however, recording was also
excellently performed with a jitter value of 9% or less for a laser
beam at a wavelength of 660nm by adjusting the thickness of a
heat-insulation layer from 15nm to 120nm. In the test, the
track pitch of the substrate was 0.74µm, and the recording and
reproducing conditions were based on those of DVD+R. The
amount of increased jitter value obtained in the same storage
test as the Examples above gave results substantially similar to
those shown in Table 2.
(Comparative Example B-1)
A write-once-read-many optical recording medium was

produced in the same manner as Example B-1, provided that the
principal constituent is bismuth, and a recording layer formed
by sputtering through the use of a metallic bismuth target was
used instead of the recording layer containing a Bi oxide. The
optical recording medium was then evaluated.
The analysis by X-ray photoelectron spectroscopy showed
that no bismuth oxide was detected in the recording layer except
in the interface between the substrate and the recording layer
and the interface between the recording layer and ZnS-SiO2.
Thus, it proved that the recording layer produced in
Comparative Example B-1 did not include a Bi oxide.
As a result of the measurements of recording and
reproducing properties, the initial jitter value exceeded 15%,
and it was impossible to measure jitter after the storage test.
From the results, the importance of a recording layer
which comprises not only bismuth as the principal constituent
but also a Bi oxide is confirmed.
(Example B-19)
On a polycarbonate substrate with a guide groove having
a groove depth of 50nm and a track pitch of 0.40µm formed
thereon, a ZnS-SiO2 layer, i.e. an under coating layer having a
thickness of 65nm and a BiPdO layer, i.e. a recording layer
having a thickness of 15nm were disposed in this order in a
laminar structure by sputtering. The atomic number ratio of Bi

to Pd, or Bi:Pd was approx. 3:1.
Next, on the recording layer, an organic-material layer,
i.e. an upper coating layer which comprises a pigment or a dye
represented by the following Chemical Formula 1 was formed by
spin-coating so as to have the average thickness of approx. 30nm.
On the organic-material layer, a reflective layer which comprises
Ag, having a thickness of 150nm was disposed by sputtering,
and then a protective layer made from an ultraviolet curable
resin having a thickness of 5pm was further disposed on the
reflective layer by spin-coating so as to thereby yield a
write-once-read-many recording medium of the present
invention.
A pigment or a dye represented by Chemical Formula 1 is
typically used for materials of conventional DVD±R, and such
materials respectively have little absorption in blue-laser
wavelengths.

Recording and reproducing were performed to the optical
recording medium under the recording and reproducing
conditions in accordance with HD and DVD-R to evaluate it
using an optical disk evaluation system DDU-10 having a
numerical aperture of 0.65 manufactured by PULSTEC
INDUSTRIAL CO., LTD. at a wavelength of 405nm..
The measurement of the optical recording medium
resulted in an excellent value of PRSNR 22 at a recording power
of 5.8mW, and excellent recording and reproducing properties
were achieved.
(Example B-20)
On a polycarbonate substrate with a guide groove having
a groove depth of 20nm and a track pitch of 0.32µm formed
thereon, a reflective layer made from Ag having a thickness of
100nm, a ZnS-SiO2 layer, i.e. an upper coating layer having a
thickness of 16nm and a BiPdO layer or a recording layer having
a thickness of 7nm were disposed in this order in a laminar
structure by sputtering. The atomic number ratio of Bi to Pd,
or Bi:Pd was approx. 3:1.
Next, a cover layer made from a resin having a thickness
of 0.1mm was laminated on the recording layer to yield a
write-once-read-many recording medium of the present
invention.
Recording and reproducing were performed to the optical

recording medium under the recording and reproducing
conditions in accordance with BD-R to evaluate it using an
optical disk evaluation system having a numerical aperture of
0.85 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a
wavelength of 405nm..
The measurement of the optical recording medium
resulted in an excellent jitter value of 6.0% at a recording power
of 7.0mW, and excellent recording and reproducing properties
were achieved.
(Example B-21)
A write-once-read-many optical recording medium of the
present invention was produced in the same manner as Example
B-19, provided that a BiBO layer was used for the recording
layer. The write-once-read-many optical recording medium was
then evaluated. The atomic number ratio of Bi to B, or Bi:B
was approx. 2:1.
The measurement of the optical recording medium
resulted in an excellent value of PRSNR 23 at a recording power
of 5.6mW, and it exemplified that excellent recording and
reproducing properties are realizable with the optical recording
medium.
(Example B-22)
A write-once-read-many optical recording medium of the
present invention was produced in the same manner as Example

B-20, provided that a BiBO layer was used for the recording
layer. The write-once-read-many optical recording medium was
then evaluated. The atomic number ratio of Bi to B, or Bi:B
was approx.. 2:1.
The measurement of the optical recording medium
resulted in an excellent jitter value of 5.9% at a recording power
of 6.7mW, and excellent recording and reproducing properties
were achieved.
(Example B-23)
On a polycarbonate substrate with a guide groove having
a groove depth of 50nm and a track pitch of 0.32pm formed
thereon, a reflective layer made from Ag having a thickness of
100nm was formed by sputtering, an organic-material layer, i.e.
an upper coating layer which comprises a pigment or a dye
represented by Chemical Formula 1 was formed by spin-coating
so as to have the average thickness of approx. 30nm, then a
BiBO layer, i.e. a recording layer having a thickness of 15nm,
and a ZnS-SiO2 layer, i.e. an under coating layer were disposed
in this order in a laminar structure by sputtering. The atomic
number ratio of Bi to B, or Bi:B was approx. 2:1.
Next, a cover layer made from a transparent resin having
a thickness of 100nm was laminated on the recording layer to
thereby yield a write-once-read-many recording medium of the
present invention.

Recording and reproducing were performed to the optical
recording medium under the recording and reproducing
conditions in accordance with BD-R to evaluate it using an
optical disk evaluation system having a numerical aperture of
0.85 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a
wavelength of 405nm..
The measurement of the optical recording medium
resulted in an excellent jitter value of 6.5% at a recording power
of 4.8mW, and excellent recording and reproducing properties
were achieved.
(Example B24)
On a polycarbonate substrate with a guide groove having
a groove depth of 40nm and a track pitch of 0.74pm formed
thereon, a BiBO layer, i.e. a recording layer having a thickness
of 15nm and a ZnS-SiO2 layer, i.e. an upper coating layer having
a thickness of 40nm were disposed in this order in a laminar
structure by sputtering. The atomic number ratio of Bi to B, or
Bi:B was approx.. 2:1.
Next, on the recording layer, a reflective layer made from
Ag having a thickness of 100nm and a protective layer made
from an ultraviolet curable resin having a thickness of approx.
5pm were disposed to thereby yield a write-once-read-many
recording medium of the present invention.
Recording and reproducing were performed to the optical

recording medium under the recording and reproducing
conditions in accordance with DVD+R to evaluate it using an
optical disk evaluation system DDU-1000 having a numerical
aperture of 0.65 manufactured by PULSTEC INDUSTRIAL CO.,
LTD. at a wavelength of 660nm.
The measurement of the optical recording medium
resulted in a jitter value of 7.2% at a recording power of 12.0mW,
and excellent recording and reproducing properties were
achieved.
(Example B-25)
A write-once-read-many optical recording medium of the
present invention was produced in the same manner as Example
B-24, provided that Sb was added to the materials of the
recording layer. The write-once-read-many optical recording
medium was then evaluated. The atomic number ratio of Bi to
Sb, or Bi:Sb was approx.. 4:1.
The measurement of the optical recording medium
resulted in a jitter value of 7.6% at a recording power of 10.0mW,
and excellent recording and reproducing properties were
achieved.
(Example B-26)
On a polycarbonate substrate with a guide groove having
a groove depth of 20nm and a track pitch of 0.437µm formed
thereon, a BiPdO layer, i.e. a recording layer having a thickness

of 5nm and a ZnS-SiO2 layer, i.e. an upper coating layer having
a thickness of 15nm were disposed in this order in a laminar
structure by sputtering. On the ZnS_SiO2 layer, a reflective
layer which comprises Ag, having a thickness of 100nm was
disposed by sputtering, and a protective layer which comprises
an ultraviolet curable resin, having a thickness of approx. 5pm
was further disposed by spin-coating to thereby yield a
write-once-read-many optical recording medium of the present
invention.
In Example B-26, the atomic number ratio of the total
amount of Pd to bismuth was changed in the recording layer to
evaluate jitter value. The recording and reproducing conditions
were same as those in Examples B-l to B-18.
The measurement of the optical recording medium, as
shown in FIG. 3, exemplified that an excellent jitter value can
be obtained in the range where the atomic number ratio of the
total amount of Pd relative to bismuth is 1.25 or less. The
value 1.25 is represented by the dotted-line in FIG. 3. In
addition, it was exemplified that the elements defined in the
present invention except for Pd respectively have a tendency
similar to the above.
Next, the present invention relating to the sputtering
target will be specifically described referring to examples and
comparative examples, however, the present invention is not

limited to the disclosed examples.
Example C-1
Powders of Bi2O3 and Fe2O3 were mixed so that the
atomic ratio of Bi to Fe was 6:5 and then mixed in a ball mill by
dry-process for 1 hour. The mixed powder was formed by
pressing at 100MPa to 200MPa and then calcined at 750°C for 5
hours in the atmosphere to yield a sputtering target. The
target had a diameter of 152.4φ and a thickness of 4mm. The
target was bounded to a backing plate made from oxygen-free
copper by metal bonding to yield a sputtering target 1. The
sputtering target had a packing density of 75%.
X-ray diffraction pattern of the sputtering target was
measured. The measurement conditions are as shown in Table
3. FIG. 4 shows the measurement results.
To identify diffraction peak positions obtained in the
measurement, the diffraction peak positions were searched and
checked against those of known substances. The diagram
marked with (a) at the top of FIG. 4 represents the diffraction
pattern of target 1. The diagram marked with (b) represents
diffraction peak positions of BiFeO3 which are based on known
data. In X-ray diffraction analysis, data on the position of
diffraction lines and on the intensities of substances have been
compiled in a database from the source of data on X-ray
diffraction measured in the past. Therefore, the measured

substance can be identified by comparing the measured
diffraction result with the previous data. As a result of a
search after comparing BiFeO3 data marked with (b) with the
measured data marked with (a), it was found that diffraction
peaks with a mark "°" were those of BiFeO3. Similarly, the
diagram marked with (c) is the known data of Fe2O3, and the
diagram marked with (d) is the known data of Bi2O3. Similarly,
diffraction peaks of Bi2O3 and Fe2O3 were identified. The
greatest peak was the one corresponding to BiFe2O3, and this
demonstrated that this compound is the principal component.
Further, the sputtering target was subjected to the ICP analysis,
i.e. the inductively coupled plasma emission spectrometry. Part
of the sputtering target was dissolved in an aqua regia as a
sample and then diluted with superpure water for the analysis.
For the solution, elements of Co, Ca, and Cr were respectively
analyzed. The result of analysis was that the content of the
respective elements was less than the detection limit.

Example 02
An optical recording medium was produced using the
sputtering target prepared in Example C-1.
On a polycarbonate substrate with a guide groove having
a groove depth of 50nm and a track pitch of 0.44pm formed
thereon, a BiFeO layer having a thickness of 15nm was formed
by sputtering, an organic-material layer containing a pigment or
a dye represented by the following Chemical Formula 1 was
formed on the BiFeO layer by spin-coating so as to have the
average thickness of approx. 30nm, a reflective layer made from
Ag having a thickness of 150nm was disposed on the
organic-material layer by sputtering, and then a protective layer
made from an ultraviolet curable resin having a thickness of
approx. 5pm was further disposed on the reflective layer by
spin-coating to thereby yield a write-once-read-many recording
medium of the present invention. A pigment or a dye
represented by Chemical Formula 1 is typically used for
materials of conventional DVD±R, and such materials
respectively have little absorption in blue-laser wavelengths.

Binary recording was performed to the optical recording
medium under the following conditions using an optical disk
evaluation system DDU-1000 having a numerical aperture of
0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a
wavelength of 405nm.

• Modulation Mode: 8-16 modulation
• Recording linear density: 1T = 0.0917pm
The shortest mark length (3T) = 0.275pm
• Recording linear velocity: 6.0m/s
• Waveform equalization: Normal equalizer
As a result, as shown in FIG. 5, an excellent jitter value
of 10.2% was obtained at a recording power of 6.1mW, and
excellent recording properties were realized.

Example C-3
Sputtering target 2 was prepared in the same manner as
Example C-1, provided that powders of Bi2O3 and Fe2O3 were
mixed so that the atomic ratio of Bi to Fe was 35:5. The
sputtering target had a packing density of 67%.
X-ray diffraction pattern of the sputtering target was
measured. The measurement conditions were as shown in Table
3. FIG. 6 shows the measurement result.
To identify diffraction peak positions obtained in the
measurement, the diffraction peak positions were referred to
those of known substances. As shown in FIG. 6, most if not all
peaks of the diffraction pattern matched to those of Bi25FeO40-
As a matter of course, the highest peak was that of Bi25FeO40,
and it proves that this compound was the principal constituent.
Example C-4
An optical recording medium was produced using the
sputtering target prepared in Example C-3.
On a polycarbonate substrate with a guide groove having
a groove depth of 50nm and a track pitch of 0.44pm formed
thereon, a ZnSSiO2 layer having a thickness of 50nm and a
BiFeO layer having a thickness of 15nm were formed in this
order in a laminar structure by sputtering, an organic-material
layer containing a pigment or a dye represented by the following
Chemical Formula 1 was formed on the BiFeO layer by

spin-coating so as to have the average thickness of approx. 30nm,
a reflective layer which comprises Ag, having a thickness of
150nm was disposed on the organic-material layer by sputtering,
and then a protective layer made from an ultraviolet curable
resin having a thickness of approx. 5pm was further disposed on
the reflective layer by spin-coating to thereby yield a
write-once-read-many recording medium. A pigment or a dye
represented by Chemical Formula 1 is typically used for
materials of conventional DVD±R, and such materials
respectively have little absorption in blue-laser wavelengths.
Binary recording was performed to the optical recording
medium under the following conditions, using an optical disk
evaluation system DDU-1000 having a numerical aperture of
0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a
wavelength of 405nm.

• Modulation Mode: 8-16 modulation
• Recording linear density: IT = 0.0917pm
The shortest mark length (3T) = 0.275pm
• Recording linear velocity: 6.0m/s
• Waveform equalization: Normal equalizer
As a result, as shown in FIG. 7, an excellent jitter value
of 8.6% was obtained at a recording power of 7.0mW, and
excellent recording properties were realized.

Even when the recording power exceeded the optimum
recording power, a write-once-read-many optical recording
medium having a high modulated amplitude and a wide range of
recording power margin was enabled without a great change in
reproducing signal level or RF level of the recorded portions.
Example 05
Sputtering target 3 was obtained in the same manner as
Example C-1, provided that powders of Bi2O3 and Fe2O3 were
mixed so that the atomic ratio of Bi to Fe was 1-5. The target
had a packing density of 67%.
X-ray diffraction pattern of the sputtering target was
measured. The measurement conditions are as shown in Table
3. FIG. 8 shows the measurement result.
To identify diffraction peak positions obtained in the
measurement, the diffraction peak positions were searched and
checked against those of known substances. As a result, it was
found that the sputtering target had diffraction peaks
corresponding to those of Bi2Fe4O9, Bi2O3, and Fe2O3, however,
the other diffraction peaks of the sputtering target did not
match to those of known substances. The greatest peak was the
one corresponding to Fe2O3, and this demonstrated that this
compound was the principal constituent.
Example 06
An optical recording medium was produced using the

sputtering target prepared in Example C-5.
On a polycarbonate substrate with a guide groove having
a groove depth of 50nm and a track pitch of 0.44µm formed
thereon, a ZnS-SiO2 layer having a thickness of 50nm and a
BiFeO layer having a thickness of 10nm were disposed in this
order in a laminar structure by sputtering, an organic-material
layer containing a pigment or a dye represented by Chemical
Formula 1 was formed on the BiFeO layer by spin-coating so as
to have the average thickness of approx. 30nm, a reflective layer
made from Ag having a thickness of 150nm was disposed on the
organic-material layer by sputtering, and then a protective layer
which comprises an ultraviolet curable resin, having a thickness
of approx. 5pm was further disposed on the reflective layer by
spin-coating to thereby yield a write-once-read-many recording
medium. A pigment or a dye represented by Chemical Formula
1 is typically used for materials of conventional DVD±R, and
such materials respectively have little absorption in blue-laser
wavelengths.
Binary recording was performed to the optical recording
medium under the following conditions using an optical disk
evaluation system DDU-1000 having a numerical aperture of
0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a
wavelength of 405nm.

• Modulation Mode: 8-16 modulation
• Recording linear density: 1T = 0.0917µm
The shortest mark length (3T) = 0.275µm
• Recording linear velocity: 6.0m/s
• Waveform equalization: Normal equalizer
The recording resulted in a degraded jitter value of 22.6%
at a recording power of 8.1mW.
Further, the thickness of the BiFeO layer was changed to
15nm and 20nm respectively, however, the resulting jitter values
were further worsened, and the measurement was impossible.
Example C-7
Sputtering target 4 was obtained in the same manner as
Example C-1, provided that the powders of Bi2O3 and Fe2O3 were
mixed so that the atomic ratio of Bi to Fe was 4:5. Sputtering
target 4 has a packing density of 77%.
X-ray diffraction pattern of the sputtering target was
measured. The measurement conditions are as shown in Table
3. FIG. 9 shows the measurement results.
The diffraction pattern obtained in the measurement is
shown at (a) in FIG. 9. Diffraction peak positions of the
sputtering target were referred to those of known substances of
(b) BiFeO3 and (c) Bi2Fe4O9. The search result showed that the
sputtering target had only diffraction peaks corresponding to
those of BiFeO3 and Bi2Fe4O9. The greatest peak was the one

corresponding to BiFe2O3, and this demonstrated that this
compound was the principal constituent.
Example C-8
An optical recording medium was produced using the
sputtering target prepared in Example C-7.
On a polycarbonate substrate with a guide groove having
a groove depth of 50nm and a track pitch of 0.44pm formed
thereon, a ZnSSiO2 layer having a thickness of 50nm and a
BiFeO layer having a thickness of 10nm were disposed in this
order in a laminar structure by sputtering, an organic-material
layer containing a pigment or a dye represented by Chemical
Formula 1 was formed on the BiFeO layer by spin-coating so as
to have the average thickness of approx. 30nm, a reflective layer
made from Ag having a thickness of 150nm was disposed on the
organic-material layer by sputtering, and then a protective layer
made from an ultraviolet curable resin having a thickness of
approx. 5pm was further disposed on the reflective layer by
spin-coating to thereby yield a write-once-read-many recording
medium. A pigment or a dye represented by Chemical Formula
1 is typically used for materials of conventional DVD±R, and
such materials respectively have little absorption in blue-laser
wavelengths.
Binary recording was performed to the optical recording
medium under the following conditions using an optical disk

evaluation system DDU-1000 having a numerical aperture of
0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a
wavelength of 405nm. The shortest mark length was set at
0.205µm to examine the capability of high density recording.

• Modulation Mode: 1-7 modulation
• Recording linear density: The shortest mark length
(2T) = 0.205µm
• Recording linear velocity: 6.0m/s
• Waveform equalization: Normal equalizer
FIG. 10 shows the result.
Example C-9
An optical recording medium was produced in the same
manner as Example C-8 using a sputtering target prepared in
the same manner as Example C-7, provided the atomic ratio of
Bi to Fe was changed to prepare the sputtering target. The
optical recording was measured under the same recording
conditions as Examples. FIG. 10 shows the measurement result.
As shown in FIG. 10, an optical recording medium using a
sputtering target having an atomic ratio represented by Bi/Fe >
0.8, i.e. the sputtering target having an atomic ratio of Bi/(Bi +
Fe) ≥ 4/9 in FIG. 10, demonstrated a jitter value of approx. 14%,
and it was demonstrated that an excellent jitter value can be
obtained even at high density recording. The jitter value was

substantially improved even with an optical recording medium
using a sputtering target having an atomic ratio represented by
Bi/(Bi + Fe) ≥ 3/8, and it turned out to be effective.
Example C-10
Sputtering target 5 was prepared in the same manner as
Example 01, provided that powders of Bi2O3 and Fe2O3 were
mixed so that the atomic ratio of Bi to Fe was 10:5. The target
had a packing density of 85%.
X-ray diffraction pattern of the sputtering target was
measured. The measurement conditions are as shown in Table
3. FIG. 11 shows the measurement result.
To identify diffraction peak positions of the diffraction
pattern (a) obtained in the measurement, the diffraction peak
positions were referred to those of known substances (b) to (e).
As a result, there were diffraction peaks corresponding to those
of Bi25FeO40, BiFeO3, Bi2O3, and Fe2O3, however, the other
diffraction peaks of the sputtering target did not match to those
of other substance. The greatest peak was the one
corresponding to Bi25FeO40, and this demonstrated that this was
the principal constituent.
The sputtering target was subjected to the ICP analysis,
i.e. inductively coupled plasma-atomic emission spectroscopy.
Part of the sputtering target was dissolved in an aqua regia as a
sample and then diluted with superpure water for the analysis.

Elements of Co, Ca, and Cr were respectively analyzed for the
solution. The result of analysis was that the content of the
respective elements was less than the detection limit.
In addition, sputtering target 6 in which 0.003% by mass
of Al and 0.001% by mass of Co had been detected as impurity
was prepared in the same manner as above.
Example C-11
Optical recording media were prepared using the
sputtering targets 5 and 6 prepared in Example C-10,
respectively.
On a polycarbonate substrate with a guide groove having
a groove depth of 50nm and a track pitch of 0.44pm formed
thereon, a ZnS-Si02 layer having a thickness of 50nm and a
BiFeO layer having a thickness of 15nm were disposed in this
order in a laminar structure by sputtering, an organic-material
layer containing a pigment or a dye represented by Chemical
Formula 1 was formed on the BiFeO layer by spin-coating so as
to have the average thickness of approx. 30nm, a reflective layer
made from Ag having a thickness of 150nm was disposed on the
organic-material layer by sputtering, and then a protective layer
made from an ultraviolet curable resin having a thickness of
approx. 5µm was further disposed on the reflective layer by
spin-coating to thereby yield a write-once-read-many recording
medium. A pigment or a dye represented by Chemical Formula

1 is typically used for materials of conventional DVD±R, and
such materials respectively have little absorption in blue-laser
wavelengths.
Binary recording was performed to the optical recording
media under the following conditions using an optical disk
evaluation system DDU-1000 having a numerical aperture of
0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a
wavelength of 405nm.

• Modulation Mode: 8-16 modulation
• Recording linear density: IT = 0.0917pm
The shortest mark length 3T = 0.275pm
• Recording linear velocity: 6.0m/s
• Waveform equalization: Normal equalizer
As a result, excellent jitter values were obtained.
Specifically, the optical recording medium using target 5 had a
jitter value of 8.4% at a recording power of 5.0mW, and the
optical recording medium using target 6 had a jitter value of
8.2% at a recording power of 5.0mW, and both optical recording
media achieved excellent recording properties.
Even when the recording power exceeded the optimum
recording power, a write-once-read-many optical recording
medium having a high modulated amplitude and a wide range of
recording power margin was enabled without a great change in

reproducing signal level or RF level of the recorded portions.
Example C-12
Sputtering target 7 was obtained in the same manner as
Example C-1, provided that powders of Bi2O3 and Fe2O3 were
mixed so that the atomic ratio of Bi to Fe was 10:5. The
sputtering target had a packing density of 71%.
X-ray diffraction pattern of the sputtering target was
measured. The measurement conditions are as shown in table 3.
FIG. 12 shows the measurement result.
To identify diffraction peak positions obtained in the
measurements the diffraction peak positions were referred to
those of known substances. The diagram marked with (a)
represents the diffraction pattern of target 7, and the diagram
marked with (b) represents the diffraction pattern with known
data of Bi36Fe2O57. The diagram marked with (c) shows known
data of BiFeO3, and the diagram marked with (d) shows known
data of Fe2O3. The greatest peak was the one corresponding to
Bi36Fe2O57, and this demonstrated that this compound was the
principal constituent.
Example C-13
An optical recording medium was produced using the
sputtering target prepared in Example C-12.
On a polycarbonate substrate with a guide groove having
a groove depth of 21nm and a track pitch of 0.44µm formed

thereon, a BiFeO layer having a thickness of 5nm, a ZnS-SiO2
layer having a thickness of 14nm, and a reflective layer made
from Ag having a thickness of 100nm were disposed in this order
in a laminar structure by sputtering. On the reflective layer, a
protective layer made from an ultraviolet curable resin having a
thickness of approx. 5pm was disposed by spin-coating to yield a
write-once-read-many optical recording medium.
Binary recording was performed to the optical recording
medium under the following conditions using an optical disk
evaluation system DDU-1000 having a numerical aperture of
0.65 manufactured by PULSTEC INDUSTRIAL CO., LTD. at a
wavelength of 405nm.

• Modulation Mode: 8-16 modulation
• Recording linear density: IT = 0.0917µm
The shortest mark length 3T = 0.275µm
• Recording linear velocity: 6.0m/s
• Waveform equalization: Normal equalizer
As a result, an excellent jitter value of 6.2% was obtained
at a recording power of 10.1mW, and excellent recording
properties were enabled by the write-once-read-many optical
recording medium.
Example C-14
Sputtering target 8 was obtained in the same manner as

Example C-1, provided that powders of Bi2O9 and Fe were mixed
so that the atomic ratio of Bi to Fe was 35:5. The sputtering
target had a packing density of 69%.
X-ray diffraction pattern of the sputtering target was
measured, and very slight peaks of Bi and Fe were observed, and
it was clarified that a compound corresponding to Bi25FeO40 or a
compound corresponding to Bi36Fe2O57 was the principal
constituent.
Example C-15
An optical recording medium was produced using the
sputtering target prepared in Example C-14.
On a polycarbonate substrate with a guide groove having
a groove depth of 50nm and a track pitch of 0.44pm formed
thereon, a ZnS-SiO2 layer having a thickness of 20nm and a
BiFeO layer having a thickness of 13nm were disposed in this
order in a laminar structure by sputtering. The BiFeO layer
was formed with introducing oxygen thereto at a flow rate of 2%
relative to Ar at the same time. On the BiFeO layer, a
ZnS-SiO2 layer having a thickness of 20nm was formed, and an
Alalloy reflective layer having a thickness of 100nm was formed
on the ZnS-SiO2 layer by sputtering, and a protective layer made
from an ultraviolet curable resin having a thickness of approx.
5pm was further disposed on the Al-alloy reflective layer by
spin-coating to yield a write-once-read-many optical recording

medium.
Binary recording was performed to the
write-once-read-many optical recording medium under the
following conditions using an optical disk evaluation system
DDU-1000 having a numerical aperture of 0.65 manufactured by
PULSTEC INDUSTRIAL CO., LTD. at a wavelength of 405nm.

• Modulation Mode: 8-16 modulation
• Recording linear density- IT = 0.0917µm
The shortest mark length 3T = 0.275µm
• Recording linear velocity- 6.0m/s
• Waveform equalization: Normal equalizer
As a result, an excellent jitter value of 7.6% was obtained
at a recording power of 9.0mW, and excellent recording
properties were enabled by the write-once-read-many optical
recording medium.
It was possible to achieve a write-once-read-many optical
recording medium having a high modulated amplitude and a
wider recording power margin without great changes in
reproducing signal level or RF level even when the recording
power exceeded the optimum recording power.
Example C-16
For a sputtering target of Bi10Fe5Ox, four targets thereof
were produced in the same manner. X-ray diffraction patterns

of these sputtering targets were measured. Table 4 shows the
values of 2θ of the diffraction peaks of these Bi10Fe5Ox targets
detected as a result of the measurement. Targets having such a
peak at those angles of 2θ shown in Table 4 can be presented as
examples of the sputtering target of the present invention.

WE CLAIM:
1. A write-once-read-many optical recording medium comprising:
a substrate,
a recording layer, and
a reflective layer,
wherein the recording layer comprises a material represented by BiOx (0 1.5), and a recording mark with information recorded therein comprises crystal of Bi and/
or crystal of a Bi oxide.
2. The write-once-read-many optical recording medium as claimed in claim 1,
wherein the recording layer comprises a material represented by BiOx (0 recording mark with information recorded therein comprises tetravalent Bi in form of
crystal of Bi oxide.
3. The write-once-read-many optical recording medium as claimed in claim 1,
wherein the recording layer comprises Bi, O, and M as constituent elements,
wherein M represents at least one element selected from the group consisting of Mg, Al,
Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and Ta, and a recording
mark with information recorded therein comprises crystal of the one or more elements
contained in the recording layer, tetravalent Bi in form of a crystal of Bi oxide and crystal
of an oxide of M.
4. The write-once-read-many optical recording medium as claimed in claim 3,
wherein the atomic number ratio of the total amount of the element M to bismuth is 1.25
or less.
5. The write-once-read-many optical recording medium as claimed in claim 3,
wherein the write-once-read-many optical recording medium has any one of laminar
structures of a laminar structure in which at least the recording layer, an upper coating
layer, and the reflective layer are disposed on the substrate in this order, and a laminar

structure in which at least the reflective layer, an upper coating layer, the recording layer,
and a cover layer are disposed on the substrate in this order.
6. The write-once-read-many optical recording medium as claimed in claim 3,
wherein the write-once-read-many optical recording medium is produced using a
sputtering target which comprises one or more selected from BiFeO3, Bi25FeO40, and
Bi36Fe2O57.
7. The write-once-read-many optical recording medium as claimed in claim 1,
wherein the recording layer comprises Bi, O, and L as constituent elements, and the
recording layer comprises a Bi oxide, and
wherein L represents at least one element selected from the group consisting of B, P, Ga,
As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At, and Cd, and a recording
mark with information recorded therein comprises tetravalent Bi in form of crystal of Bi
oxide.
8. The write-once-read-many optical recording medium as claimed in claim 7,
wherein the element L represents at least one element selected from the group consisting
of B, P, Ga, Se, Pd, Ag,.Sb, Te, W, Pt, and Au.
9. The write-once-read-many optical recording medium as claimed in claim 7,
wherein the atomic number ratio of the total amount of the element L to bismuth is 1.25
or less.
10. The write-once-read-many optical recording medium as claimed in claim 7,
wherein the write-once-read-many optical recording medium comprises an upper coating
layer and has a laminar structure in which the recording layer, the upper coating layer,
and the reflective layer are disposed on the substrate in this order.
11. The write-once-read-many optical recording medium as claimed in claim 10,
wherein the write-once-read-many optical recording medium comprises an under coating

layer and has a laminar structure in which the under coating layer, the recording layer, the
upper coating layer, and the reflective layer are disposed on the substrate in this order.
12. The write-once-read-many optical recording medium as claimed in claim 7,
wherein the write-once-read-many optical recording medium comprises an upper coating
layer and a cover layer and has a laminar structure in which the reflective layer, the upper
coating layer, the recording layer, and the cover layer are disposed on the substrate in this
order.
13. The write-once-read-many optical recording medium as claimed in claim 12,
wherein the write-once-read-many optical recording medium comprises an under coating
layer and has a laminar structure in which the reflective layer, the upper coating layer, the
recording layer, the under coating layer, and the cover layer are disposed on the substrate
in this order.
14. The write-once-read-many optical recording medium as claimed in claim 7,
wherein the write-once-read-many optical recording medium comprises at least one of an
under coating layer and an upper coating layer, and at least any one of the under coating
layer and the upper coating layer comprises ZnS and/or SiO2.
15. The write-once-read-many optical recording medium as claimed in claim 7,
wherein the write-once-read-many optical recording medium comprises at least one of an
under coating layer and an upper coating layer, and at least any one of the under coating
layer and the upper coating layer comprises an organic material.
16. The write-once-read-many optical recording medium as claimed in claim 7,
wherein recording and reproducing are enabled with a laser beam having a wavelength of
680 nm or less.
17. A process for the production of write-once-read-many optical recording medium
as claimed in claim 3, wherein a sputtering target comprises:

Bi, Fe, and O,
wherein the said sputtering target comprises one or more selected from BiFeO3,
Bi25FeO40, and Bi36Fe2O57.
18. The process as claimed in claim 17, wherein the sputtering target consists of Bi,
Fe, and O.
19. The process as claimed in any one of claims 17 to 18, wherein the sputtering
target is used for forming a recording layer for an optical recording medium in which
recording and reproducing are performed with a laser beam at a wavelength of 550 nm or
less.
20. The process as claimed in any one of claims 17 to 19, wherein the sputtering
target comprises a Bi oxide and a Fe oxide, or comprises a complex oxide of Bi and Fe.
21. The process as claimed in claim 20, wherein the sputtering target comprises the
complex oxide of Bi and Fe and comprises one or more selected from the Bi oxide and
the Fe oxide.
22. The process as claimed in claim 17, wherein the sputtering target comprises one
or more selected from a Bi oxide, a Fe oxide, and a complex oxide of Bi and Fe, and the
oxide is an oxide having a smaller amount of oxygen compared to the stoichiometric
composition.
23. The process as claimed in any one of claims 17 to 22, wherein the sputtering
target comprises Bi2O3 and/or Fe2O3.
24. The process as claimed in any one of claims 17 to 23, wherein the sputtering
target does not comprise Bi2Fe4O9.
25. The process as claimed in any one of claims 17 to 24, wherein the content of Co,

Ca, and Cr is less than the detection limit of the inductively coupled plasma emission
spectrometry.
26. The process as claimed in any one of claims 17 to 25, wherein the sputtering
target has a packing density of 65% to 96%.
27. The process as claimed in any one of claims 17 to 26, wherein the atomic ratio of
Bi and Fe satisfies the requirement of Bi/Fe ≥ 0.8.
28. A method of producing a sputtering target comprising:
calcining powders of Bi2O3 and Fe2O3 in the process as claimed in any one of
claims 17 to 27.
29. An optical recording medium as claimed in claim 1 comprising:
a substrate, a recording layer, and a reflective layer,
wherein the recording layer is formed using a sputtering target which comprises one or
more selected from BiFeO3, Bi25FeO40, and Bi36Fe2O57.

ABSTRACT

WRITE-ONCE-READ-MANY OPTICAL RECORDING MEDIUM,
SPUTTERING TARGET AND THE PRODUCTION METHOD THEREOF
A write-once-read-many optical recording medium enabling excellent
recording and reproducing properties at a wavelength of blue-laser wavelengths or
shorter, i.e. 500nm or less, particularly at wavelengths of near 405nm and high
density recording. To this end, a write-once-read-many optical recording medium of
the invention comprises a recording layer using a material represented by BiOx (0 Another write-once-read-many optical recording medium comprises a recording
layer which comprises Bi, oxygen, and M (M represents at least one element selected
from Mg, Al, Cr, Mn, Co, Fe, Cu, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Mo, V, Nb, Y, and
Ta), in which a recorded mark comprises crystal of the elements contained in the
recording layer and crystal of an oxide of the elements.

WRITE-ONCE READ-MANY OPTICAL RECORDING MEDIUM, SPUTTERING TARGET AND THE PRODUCTION METHOD THEROF

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