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DESCRIPTION
MULTI-LAYERED PHASE-CHANGE OPTICAL
RECORDING MEDIUM
Technical Field
The present invention relates to a multi-layered
phase-change optical recording medium having a plurality of
phase-change recording layers, which enables to record,
reproduce and rewrite information by irradiating phase-change
recording layers with a light beam to thereby induce an optical
change in a recording layer material in the phase-change
recording layers.
Background Art
Typically, a phase-change optical recording medium
(phase-change optical disc) such as DVD+RW has a basic
structure in which a recording layer composed of a phase-change
material is formed on a plastic substrate, and a reflective layer
that allows for improving optical absorptance of the recording
layer and has a thermal diffusion effect is formed on the
recording layer, and such a typical phase-change optical
recording medium is irradiated with a laser beam from the
substrate surface side, thereby information can be record and
reproduced. A phase-change recording material is
phase-changed between an amorphous phase and a crystalline
phase by application of heat of laser beam irradiation and
subsequent cooling. When, after a rapid heating treatment of a
phase-change recording material, the phase-change recording
material is immediately quenched, the phase-change recording
material is changed into an amorphous phase, and when the
phase-change recording material is slowly cooled after the rapid
heating treatment, it is crystallized. These characteristics are
applied for recording and reproducing information in
phase-change optical recording media.
Further, for the purpose of preventing oxidation,
transpiration or deformation of a recording layer which are
caused by application of heat for light beam irradiation, a
phase-change optical recording medium is typically provided
with an under protective layer (also called as an under dielectric
layer) in between a substrate and a recording layer, and an
upper protective layer (also called as an upper dielectric layer)
in between the recording layer and a reflective layer. These
protective layers can respectively function to control optical
properties of the optical recording medium by adjusting the
thickness thereof, and the under protective layer also has a
function to prevent the substrate from softening due to heat
applied to the recording layer during recording.
In recent years, with increased information volume
handled with computers, recording capacities of rewritable
optical discs such as DVD-RAM, DVDRW and DVD+RW are
largely increased, and high-density recording of information is
advanced. DVD has a recording capacity of around 4.7GB,
however, in the days to come, requests for high-density
recording are further expected to increase. Along with
increased information volume, it should be considered that
enhancement of recording speed is also required. Presently, as
a rewritable DVD disc, an optical recording medium with a
single-recording layer that allows for 8X recording speed is
commercially available. As a method for allowing for
high-density recording using such a phase-change optical
recording medium, for example, there have been proposals to
shift a wavelength of a laser beam to be used toward shorter
wavelengths to blue region or to enlarge the numerical aperture
(NA) of an objective lens used for optical pick-up to record or
reproduce information to thereby change the spot size of the
laser beam applied to an optical recording medium to smaller,
and these methods have been studied, developed and put into
practical use.
For a technique to increase recording capacity by
improving an optical recording medium itself, various types of
two-layered phase-change optical recording media are proposed
that are produced such that two information layers each
composed of at least a recording layer and a reflective layer are
laid one on top of another on one surface of a substrate, and the
two information layers are bonded together with an ultraviolet
curable resin or the like. A separation layer (called an
intermediate layer in the present invention) which is the bonded
part between these information layers has a function to optically
separate the two information layers and is composed of a
material that less absorbs laser beam or avoid absorbing laser
beam unnecessarily because the laser beam used for recording
and reproducing needs to reach the innermost information layer
as viewed from the laser irradiation side.
There are still many problems existing in such a
two-layered phase-change optical recording medium.
For example, when an information layer (a first
information layer) disposed at the front side as viewed from the
laser beam irradiation side is not sufficiently transmissive to a
laser beam, it is impossible to record and reproduce information
on a recording layer of another information layer (a second
information layer) disposed at the innermost side as viewed
from the laser beam irradiation side, and thus a reflective layer
constituting the first information layer must be a ultrathin
semi-transparent reflective layer. As the result, it is difficult
to obtain light transmittance and heat dissipation effect, and it
is necessary to form a heat diffusion layer (a light transmissive
layer) so as to make contact with the semi-transparent reflective
layer to increase the light transmittance and supplement the
heat dissipation effect. Further, as compared to conventional
single-layer phase-change optical recording media or
conventional two-layered recordable optical recording media, the
reflectance of a two-layered phase-change optical recording
medium is extremely low of about one third of that of each of
these conventional media. Thus, it is considered that it may be
difficult to record and reproduce information on the first
information layer as well as on the second information layer
even with stable tracking ability provided. Further, since the
first information layer has a layer structure in which heat is
hardly dissipated, from the perspective of storage stability, it is
not conceivable that any materials may be used for the recording
layer just because a phase-change material is used. Actually, it
is necessary to limit materials and the composition of the
recording layer for the first information layer.
As a known technique, Patent Literature 1 discloses a
two-layered phase-change optical recording medium provided
with a substrate having a groove depth of 10 nm to 30 nm and
using a laser beam having a wavelength of 360 nm to 420 nm,
however, an upper protective layer in a first recording layer
structure of the two-layered phase-change optical recording
medium is composed of ZnS-SiO2, and the layer composition
differs from that of the multi-layered phase-change optical
recording medium of the present invention.
Patent Literature 2 discloses an optical recording medium
provided with a thermal diffusion layer containing an In oxide
and an Sn oxide as the main components, in which the content of
the In oxide is 90 mol% or more. Patent Literature 3 discloses
an optical recording medium provided with a thermal diffusion
layer containing an Sn oxide and an Sb oxide, and Patent
Literature 4 discloses an optical recording medium provided
with a thermal diffusion layer containing an In oxide and a Zn
oxide as the main components, in which the content of the In
oxide is 50% or more. However, these disclosed optical
recording layer respectively have a different layer composition
from that of the optical recording medium of the present
invention, and there is no description on groove depth of the
substrate, unlike the present invention.
Patent Literature 5 discloses a two-layered recordable
optical recording medium with recording layers composed of an
organic dye or organic dyes, in which the range of the groove
depth is 4XV16n to 7A./16n, however, the two-layered recordable
optical recording medium can ensure a reflectance of about 20%,
and the optical recording medium configuration basically differs
from that of a two-layered phase-change optical recording
medium, and the groove depth range also differs from that of a
two-layered phase-change optical recording medium.
Further, Patent Literature 6 describes a two-layered
optical recording medium in its claim 8, however, the optical
recording medium configuration is basically different from that
of the two-layered phase-change optical recording medium of the
present invention.
Patent Literature 1 Japanese Patent Application
LaidOpen (JPA) No. 2004005920
Patent Literature 2 Japanese Patent Application
Laid-Open (JPA) No. 2004-047034
Patent Literature 3 Japanese Patent Application
Laid-Open (JP-A) No. 2004047038
Patent Literature 4 Japanese Patent Application
Laid-Open (JPA) No. 2005004943
Patent Literature 5 Japanese Patent Application
Laid-Open (JP-A) No. 2005-004944
Patent Literature 6 International Publication No.
WO02/029787
Disclosure of Invention
The present invention is proposed in view of the present
situation and aims to solve the various conventional problems
and to achieve the following object.
The present invention aims to provide a multi-layered
phase-change optical recording medium that has a plurality of
information layer, each of the information layers other than an
information layer disposed at the innermost side as viewed from
the laser beam irradiation side has a high light transmittance to
allow stable recording and reproducing of information in the
respective information layers, and is excellent in repetitive use
durability and in storage stability.
The above-noted object can be achieved by the means
according to the following items to .
A multi-layered phase-change optical recording
medium, including a first substrate disposed at the front side as
viewed from the laser beam irradiation side, a second substrate
disposed at the innermost side as viewed from the laser beam
irradiation side, each of the first substrate and the second
substrate having a serpentine spiral guide groove on a recording
surface side thereof, intermediate layers, and a plurality of
information layers each having a phase-change recording layer,
being disposed via each of the intermediate layers in between
the first substrate and the second substrate, each of the
information layers other than the information layer disposed at
the innermost side as viewed from the first substrate side
includes the following five layers: an under protective layer, the
phase-change recording layer, an upper protective layer, a
semi-transparent reflective layer, and a thermal diffusion layer
or a light transmissive layer, and the information layer disposed
at the innermost side as viewed from the first substrate side
includes an under protective layer, the phase-change recording
layer, an upper protective layer and a reflective layer, wherein
each of the thermal diffusion layers or light transmissive layers
of the respective information layers other than the information
layer disposed at the innermost side as viewed from the first
substrate side contains an In oxide, a Zn oxide, an Sn oxide and
an Si oxide, and when the contents of the In oxide, the Zn oxide,
the Sn oxide and the Si oxide are represented by "a", "b", "c" and
"d" [mol%] respectively, the following requirements are satisfied,
and when the refractive index of the first and second substrates
was represented by "n", the laser light wavelength is
represented by "A." and the depth of the groove guide of the first
and second substrates is represented by H, the depth of the
groove guide H satisfies the following requirement,
The multi-layered phase-change optical recording
medium according to the item , wherein each of the
phase-change recording layers contains at least three elements
of Ge, Sb and Te, and when the composition ratio of the Ge, Sb
and Te is represented by "a", "P" and "y" [atomic%], the following
requirements are satisfied,
The multi-layered phase-change optical recording
medium according to any one of the items to , wherein
each of the upper protective layer in the respective information
layers other than the information layer disposed at the
innermost side as viewed from the first substrate side contains
an In oxide, a Zn oxide, an Sn oxide and an Si oxide or a Ta
oxide, and when the contents of the In oxide, the Zn oxide, the
Sn oxide and Si oxide or the Ta oxide are represented by "e", "f,
"g" and "h" [mol%] respectively, the following requirements are
satisfied,
The multi-layered phase-change optical recording
medium according to any one of the items to , wherein
each of the semi-transparent reflective layers in the respective
information layers other than the information layer disposed at
the innermost side as viewed from the first substrate side
contains Cu as the main component.
The multi-layered phase-change optical recording
medium according to any one of the items to , wherein
each of the under protective layers in the respective information
layers contains ZnS and SiC>2.
Brief Description of Drawings
FIG. 1 is a view showing one example of a layer
configuration of a two-layered phase-change optical recording
medium.
FIG. 2 is a view showing intensity distributions of
reflected lights.
FIG. 3 is a graph showing a relation between a groove
and tracking error.
FIG. 4 is a graph showing a relation between groove
depth and tracking error signal amplitude.
FIG. 5 is an illustration of a photo-detector.
FIG. 6 is a comparative graph comparing a light
transmittance of a conventional thermal diffusion layer material
and a light transmittance of the thermal diffusion layer (light
transmissive layer) material of the present invention.
FIG. 7 is an illustration explaining a linear velocity
transition.
FIG. 8 is a graph, showing a relation between a content of
ZnO or SnO2 contained in the first upper protective layer of the
present invention, a linear velocity transition and a jitter value.
FIG. 9 is a graph showing a relation between a content of
SnO2 contained in the first upper protective layer of the present
invention and storage stability.
FIG. 10 is a graph showing a relation between storage
stability, an upper protective layer material and a thickness of
the upper protective layer.
FIG. 11 is a graph showing light absorbance, reflectance
and light transmittance of reflective layer materials.
FIG. 12 is a graph showing layer-thickness dependency of
light absorbance, reflectance and light transmittance of Cu
measured at a wavelength of 660 nm.
FIG. 13 is a graph showing layer-thickness dependency of
light absorbance, reflectance and light transmittance of Ag
measured at a wavelength of 660 nm.
FIG. 14 is a graph showing wavelength dependency of
light transmittance of Cu and Ag.
FIG. 15 is a graph showing recording properties of a first
recording layer in the case where a first reflective layer is
composed of Cu, Ag or Au.
FIG. 16 is an illustration showing a IT cycle recording
strategy for recording information on L0 layer.
FIG. 17 is an illustration showing a 2T cycle recording
strategy for recording information on LI layer.
FIG. 18 is an illustration showing measurement results of
light transmittance of the first information layer (LO layer)
within a wavelength range of 640 nm to 680 nm for the
two-layered phase-change optical recording media prepared in
Examples 1 to 3, 6, and Comparative Example 1.
FIG. 19 is a graph showing measurement results of a
relation between recording power and DOW10 jitter for the
two-layered phase-change optical recording media prepared in
Examples 1 to 3, 6 and Comparative Example 1.
FIG. 20 is a graph showing a relation between thermal
conductivity of the thermal diffusion layer (light transmissive
layer) of the respective two-layered phase-change optical
recording media prepared in Examples 1 to 3, 6 and
Comparative Example 1, the optimum recording power obtained
in FIG. 19, and light transmittance of each of the LO layers
measured at a wavelength of 660 nm obtained in FIG. 18.
Best Mode for Carrying Out the Invention
Hereinafter, the present invention will be described in
detail.
The present invention is a multi-layered phase-change
optical recording medium having a plurality of information
layers between a first substrate and a second substrate as
described above, in which thermal diffusion layers (light
transmissive layers) of respective information layers other than
an information layer disposed at the innermost side as viewed
from the laser beam irradiation side are composed on materials
of an In oxide, a Zn oxide, an Sn oxide and an Si oxide and the
content of these materials is set within a range specified in a
first embodiment of the present invention, thereby the light
transmittance of the respective information layers can be
increased and the recording sensitivity of the respective
information layers can be enhanced. As the result, the light
reflectance at the information layer disposed at the innermost
side as viewed from the laser beam irradiation side can be
increased, and thus information can be reduced and reproduced
with stable tracking ability. Here, a refractive index of the
first and second substrates is represented as "n", the wavelength
of a laser beam is represented as "X" and the depth of the groove
depth of the first and second substrates is represented as "H".
On a multi-layered phase-change optical recording medium
having a low light reflectance, information can be recorded and
reproduced with stable tracking ability by setting the groove
depth of the guide groove H within a range specified in the first
embodiment of the present invention to induce a phase change to
reflected light beams from concave and convex portions of the
guide groove (generally, concave portions are called "land
portion" and convex portions are called "groove portion") and
detecting the difference in quantity of reflected light beams.
Respective refractive indexes "n" and respective groove depths
"H" of the first and second substrates may be the same to each
other or may be different to each other, however, typically they
are respectively set to have the same value between the first
substrate and the second substrate. It should be noted that a
laser beam that can be used in the present invention has a
wavelength X within a red wavelength region of 645 nm to 665
nm. Details on material of the substrates will be described
below, and typically, a polycarbonate having a refractive index
"n" of 1.55 to 1.60 is used.
FIG. 1 shows one example of a layer configuration of an
optical recording medium having two layers of information layer
each of which includes a phase-change recording layer.
During rotation of an optical disc, an eccentric error of
several ten micrometers occurs due to eccentricity and
axis-variation etc. between the central hole and the track core of
the optical disc. Particularly, as can be seen in a multi-layered
optical recording medium, the eccentricity amount of an
information layer that is distantly located from an optical
pickup is usually greater than that of an information layer
located at the front side as viewed from the optical pickup. For
this reason, to constantly and accurately scan the information
tracks with a laser beam spot, it is necessary to detect a
tracking error with an optical pickup, drive a tracking actuator
connected to a servo circuit and control a fine position of the
object lens. Thus, designing of a groove depth formed on a
substrate is particularly important to an optical recording
medium having a low reflectance like a multi-layered
phase-change optical recording medium.
As typical examples of tracking error signals, push-pull
signals are exemplified. A reflected light beam is picked up
with an objective lens from a disc having a guide groove with a
track pitch "p", and a push-pull signal can be detected with a
photodiode split behind the objective lens. A periodically
arrayed guide groove is a sort of diffraction grating, and in
reflected lights, a zero-order light that goes straight on and a +
first-order light that is diffracted at an angle 9 are generated.
Here, 9 (theta) is represented by sin_1(A,/p). Among reflected
lights to the objective lens, besides the zero-order light, part of
the + first-order light is picked up with the objected lens. At a
region where diffracted lights of the zero-order light and the
first-order light are overlapped, interference of light is
generated. The intensity of the lights varies due to track
misalignment of a laser beam applied to the optical recording
medium. A two-split photodiode (which may be a
quadruple-detector photodiode though) separately detects a
region where the zero-order light and the ± first-order light are
overlapped to read a difference signal to thereby generate a
tracking error signal. FIG. 2 shows intensity distributions of
reflected lights. When the center of a laser beam is in accord
with the center of a guide groove, the intensity distribution of
reflected light is a symmetrical distribution and the respective
output from the photodiode results in the equation, X = Y.
When a tracking misalignment occurs, the intensity distribution
of reflected light is an asymmetrical distribution and a value X
is greater than a value Y. When a tracking error signal Z is
defined as being equal to X - Y, i.e., Z = X - Y, the value Z is
represented by the following equation.
Z = 4(S1) (El)sin(l)sin(27tAp/p)
In the equation, SI represents an area in which the zero-order
light and the first-order light are overlapped on a detector; EO
and El respectively represent an amplitude of the zero-order
light and an amplitude of the first-order light; represents a
phase difference between the zero-order light and the first-order
light; "p" represents a track pitch; and Ap represents a track
misalignment amount. Since the tracking error signal Z has a
value that depends on the tracking misalignment amount Ap and
is an odd function, the value Z shows that the beam spot is
misaligned in either the plus direction or minus direction.
FIG. 3 shows a relation between a groove in an optical
recording medium and tracking error. When the laser beam
center is in accord with the groove center or the center of a land
portion, the tracking error signal is zero. When the laser beam
center is not in accord with the land center and the misaligned
position between the laser beam center and the land center is at
the inner position of the land portion or at the outer position of
the land portion as viewed from the radius direction of the
optical disc, the plus/minus sign of reflected light is reversed
and the tracking misalignment amount and the plus or minus
direction in the radius direction can be distinguished. And the
tracking misalignment amount is used for a servo signal. When
sin is a maximum value, the maximum amplitude of the
tracking error signal Z can be obtained. When the groove shape
is rectangular and the groove depth is ("n" is a refractive
index of a substrate), the value is equal to and
the maximum amplitude can be obtained. For this reason, the
dqpth of a guide groove of an optical disc is often set at near
λ/(8n) (see FIG. 4). However, optical properties and thermal
transfer characteristic depending on the layer configuration of
an optical recording medium affect recording properties of the
optical recording medium, and the recording properties are also
changed depending on the groove depth employed. Therefore, it
is necessary in the present invention that each groove depth H
of the first substrate and the second substrate be set to λ/(l7 x
n)
groove depth H of a first substrate and a second substrate to 0
to λ/(4n), however, when the each groove depth is set within the
range, it is difficult to satisfy both excellent recording
properties and highly stable tracking accuracy with the use of a
groove depth within the range. A higher push-pull signal
amplitude means more excellent property. However, in view of
a balance with recording properties, an excessively high
push-pull signal amplitude is not always preferable. Thus,
there is a need to design a groove depth taking into account both
of the properties. The results of push-pull signal in the present
invention was measured using a photodetector shown in FIG. 5
based on the expression, [(la + lb] - (lc + Id)) / [la + lb + lc + Id].
The first substrate needs to be transmissive to a light
beam irradiated for recording and reproducing, and material
conventionally known in the art can be used. Typically, glass,
ceramics or resins are used for the first substrate, and resins
are particularly preferable in terms of formability and cost.
Examples of the resin include polycarbonate resins, acryl resins,
epoxy resins, polystyrene resins, acrylonitrile-styrene copolymer
resins, polyethylene resins, polypropylene resins, silicon resins,
fluorine resins, ABS resins and urethane resins. Acryl resins
such as polycarbonate resins and polymethyl methacrylate
(PMMA) are preferable because they are excellent in formability,
optical properties and cost.
A surface of the first substrate with information layers
formed thereon has a helical or concentric wobbled groove, and a
convexo-concave pattern called land portion and groove portion
is formed thereon. Typically, a groove is transcribed on a
substrate surface using a stamper attached in a die by injection
molding method or a photopolymer method, thereby the
substrate surface is formed. The thickness of the first
substrate is preferably about 10 μm to 590 μm.
For material of the second substrate, the same material
as the one used for the first substrate may be used, or a
material that is opaque to recording and reproducing light beam
may be used, or the material and groove shape of the second
substrate may differ from those for the first substrate. The
thickness of the second substrate is not particularly limited,
however, it is preferable to adjust the thickness so that the total
thickness of the first and second substrates is 1.2 mm.
For a thermal diffusion layer (light transmissive layer),
there are known techniques using ITO[In2O3 (main components)
- SnO2] prepared by mixing an In oxide and an Sn oxide or using
IZO[In2O3 (main components) - ZnO] as a thermal diffusion
layer material of conventional optical discs, however, an In
oxide-rich material is highly expensive, and the use of an In
oxide_rich material is problematic in terms of production cost.
In addition, in the course of development of an optical recording
medium with the use of an In oxide_rich material, it was found
that the first information layer cannot have a sufficient light
transmittance and the recording sensitivity of the first
information layer is degraded because of its high thermal
conductivity. Therefore, to increase the light transmittance of
the first information layer and increase the light reflectance of
the second information layer to thereby enhance tracking
accuracy while ensuring a proper thermal conductivity value,
there is a need to find out a material having a high sputtering
rate and allowing for ensuring recording properties and
recording sensitivity, separately from ITO and IZO which have
been conventionally used for thermal diffusion layers.
Then, the present inventors studied a mixture of oxides
having a higher light transmittance on a thermal diffusion layer
(light transmissive layer) and enabling to more efficiently
enhance recording sensitivity than IZO and ITO. Consequently,
the study result showed that there is a need to mix an In oxide,
a Zn oxide, an Sn oxide and an Si oxide at the following
composition ratio when the respective contents of the In oxide,
An oxide, Sn oxide and Si oxide are represented by a, b, c and d
[moI%].
3
0
a+b+c+d= 100
Use of an In oxide of less than 3 mol% is unfavorable
because a sufficient thermal conductivity cannot be obtained and
the obtained thermal diffusion layer is hardly sputtered because
of decreased electrical conductivity. When the content of In
oxide is more than 50 mol%, a high light transmittance cannot
be ensured and the cost is very expensive. Furthermore, the
recording sensitivity of the first information layer is degraded
due to the high thermal conductivity. Use of an Si oxide within
the above-noted range is preferable because it can improve
repetitive recording durability of the first information layer.
The preferred content ratio for a Zn oxide and an Sn oxide is not
particularly limited, however, when either material is highly
contained, the sputtering rate tends to be high. These oxides
respectively have a high electrical conductivity and allows for
DC (direct current) sputtering, and thus when a thermal
diffusion layer (light transmissive layer) having a layer
thickness of around 60 nm is formed, the layer can be formed in
a short time. Further, by reducing the content of an In oxide, it
is possible to increase the light transmittance of the first
information layer and further to enhance recording sensitivity
thereof (see FIG. 20). All of the four types of oxides are
materials that do not accelerate deterioration of reflective
layers.
FIG. 6 shows measurement results of light transmittance
of a first information layer of an optical recording medium which
was prepared in the same manner as an optical recording
medium produced in Example 1, which will be described
hereinafter, except that material of the thermal diffusion layer
(light transmissive layer) used in Example 1 was changed to IZO.
It clearly shows that the optical recording medium of Example 1
has a higher light transmittance than the optical recording
medium using IZO for the thermal diffusion layer.
The layer thickness of the thermal diffusion layer (light
transmissive layer) is preferably within a range of 40 nm to 80
nm. When the thermal diffusion layer thickness is thinner
than 40 nm, heat dissipation ability is degraded and repetitive
use durability of the optical recording medium degrades. When
thicker than 80 nm, it is unfavorable because the light
transmittance is decreased.
The thermal diffusion layer (light transmissive layer)
mentioned above can be formed by various vapor growth
methods such as vacuum evaporation method, sputtering method,
plasma CVD method, photo-CVD method, ion plating method and
electron beam deposition method. Of these, sputtering method
is excellent in mass-productivity, film quality and the like.
The intermediate layer preferably has small light
absorption coefficient to light of wavelengths irradiated for
recording and reproducing information. For materials of the
intermediate layer, resins are preferably used in terms of
formability and cost, and ultraviolet (UV) curable resins,
delayed resins, thermoplastic resins etc. can be used.
The intermediate layer is the one that enables an optical
pickup to identify the first information layer and the second
information layer to optically separate them, and the thickness
of the intermediate layer is preferably 10 μm to 70 μm. When
the thickness of the intermediate layer is thinner than 10 μm,
cross-talk phenomenon occurs between information layers.
When thicker than 70 μm, spherical aberration occurs when
information is recorded or reproduced on the second information
layer, and it tends to be difficult to record and reproduce
information.
Development on conventional materials for recording
layers is broadly divided into two streams. One stream
includes GeTe which is a material used for recordable recording
layers," Sb2Te3, i.e., an alloy between Sb and Te, which is
reversibly phase-changeable; and a recording layer material
consisting of a ternary alloy of GeSbTe prepared from a solid
solution or an eutectic composition of the above-noted two
materials. Another stream include recording layer materials
composed of an alloy between Sb and Te similarly to the above,
however, the alloy is an eutectic composition of Sb and Sb2Te3 in
which a trace amount of elements is added to SbTe having a Sb
content of around 70%.
In an optical recording medium having multiple recording
layers, particularly an information layer disposed at the front
side as viewed from the laser beam irradiation side is required
to have a high light transmittance in consideration of recording
and reproducing an information layer disposed at the innermost
side as viewed from the laser irradiation side. To meet the
need, it is necessary to make recording layers thin in parallel
with efforts to reducing light absorbance of metal layers. A
thinner layer thickness of a recording layer reduces a
crystallization rate thereof, therefore, it is advantageous to
select a recording layer material itself having a high
crystallization rate. Then, among the two streams of the
materials for recording layers, the latter material, i.e., a SbTe
eutectic composition containing Sb at around 70% is preferable
to the former material.
However, according to studies by present inventors, it
was found that when the content of Sb is increased to achieve a
higher crystallization rate, i.e., to achieve a higher linear
velocity, the crystallization temperature was decreased and the
storage stability of the optical recording medium was degraded.
In a multi-layered phase-change optical recording
medium, when an information layer disposed at the innermost
side as viewed from the laser beam irradiation side is
reproduced, there is a problem that the amplitude of
reproducing signals is low because of its low reflectance which is
attributable to light absorption or the like of an information
layer disposed at the front side as viewed from the laser beam
irradiation side. Taking the problem into account, a higher
reproducing light power is required than in reproducing an
optical recording medium having a single recording layer.
In use of the SbTe material, to achieve a high
crystallization rate, it requires only to increase the Sb content,
however, it causes a decreasing trend of the crystallization rate.
Therefore, when information is reproduced on an information
layer using SbTe disposed at the front side as viewed from the
laser beam irradiation side with a high reproducing light power,
it may cause a problem that amorphous marks are
re-crystallized and cannot be reproduced. It is also unfavorable
because a decreased crystallization temperature triggers an
unstable storage condition. Then, a crystallization rate can be
kept high at information layers by adding a third element Ge to
SbTe material, thereby it is possible to maintain an optical
recording medium stable during storage without causing
re-crystallization of amorphous marks even when reproducing
information with a high reproducing light power.
For a phase-change recording material allowing for
obtaining excellent recording properties, reproducing
information with a high reproducing light power and stabilizing
the storage condition, used for phase-change recording layers of
a multi-layered phase-change optical recording medium,
materials containing at least three elements of GeSbTe are
preferable.
Further, other elements may be added to the GeSbTe
ternary material. For the additive elements, Ag and In are
preferable, and they are typically used for storage stability.
The composition ratio of the total additive elements is
preferably set to 8 atomic% or less. When the composition ratio
is more than 8 atomic%, the crystallization rate of the recording
layer is slow, although the storage stability is enhanced,
resulting in a difficulty in recording at high speed. Further, it
is unfavorable because stability of recorded condition to a
reproducing light is degraded.
As defined in the item , i.e, a second embodiment of
the present invention, when the Sb content (p) is within the
range of 60
material, Sb material allows for recording and reproducing
information in a stable manner. When the Sb content (p) is less
than 60 atomic%, information cannot be record and reproduced
in a stable manner, and further, as a multi-layered phase-change
optical recording medium, it may result in a recording layer that
is not suited for high-speed recording. With the use of a Sb
content more than 75 atomic%, the crystallization temperature
is decreased and it is difficult to reproduce information with a
high reproducing light power, resulting in an unstable storage
condition, although the crystallization rate is increased.
Te has a function to accelerate generation of an
amorphous condition and to increase the crystallization
temperature. However, when combining only Te singularly with
Sb, these is a possibility that recorded amorphous marks are
lost due to long-term storage or storage at high temperature
because of the insufficient temperature increasing rate of the
crystallization temperature and poor stability of the amorphous
phase, although it is possible to adjust the crystallization rate
utilizing the amorphousis-accelerating effect. In contrast, use
of a combination of Te with Ge have advantages in that stability
of amorphous phase can be ensured by using Ge and stability of
a crystalline phase can also be enhanced. Generally, a
crystallized condition is a highly stable condition, however, in
the case of a material used for high-speed recording taken up
here, the crystallization progresses at high speed at the time of
initialization or recording, and thus it cannot be necessarily
said that a formed crystallized condition is stable. For this
reason, when information is recorded again after long-term
storage or storage at high temperature, it causes a problem that
recording properties and recording conditions are changed from
the condition before storage. The reason is considered that the
crystallized condition is changed by storage from the
crystallized condition before storage. However, an addition of
Te to materials for recording layer can reduce such variation of
recording properties and recording conditions caused by storage.
To obtain an effect of reducing variation of recording
properties and recording conditions before and after storage by
improving stability of crystal as described above, it is desirable
to add Te at 6 atomic% or more (6
large amount of Te addition causes a too slow crystallization
rate, causing a loss of ability to repeatedly record information at
high speed. When Te is used for the first recording layer of the
multi-layered phase-change optical recording medium of the
present invention, it is preferable to set the Te amount to 30
atomic% or less (γ
Further, use of a Ge (a) amount within a range of 2
20 (atomic%) enables to reproduce information with
high-reproducing light power and ensures an excellent storage
condition. Addition of a Ge amount of less than 2 atomic%, the
effect of Ge addition cannot be obtained and an excellent storage
condition cannot be obtained. In contrast, addition of a Ge
amount of more than 20 atomic%, the recording sensitivity is
degraded because of its high-melting point of the Ge itself,
although stability with a reproducing light and storage stability
are highly maintained because the crystallization temperature
can be set high.
The layer thickness of the first recording layer is
preferably within a range of 4 nm to 10 nm. When the first
recording layer has a layer thickness less than 4 nm, signal
quality is degraded due to an excessively low reflectance, and
the repetitive recording properties are degraded. When thicker
than 10 nm, it is unfavorable because the light transmittance is
decreased. The layer thickness of the second recording layer is
preferably within a range of 10 nm to 20 nm. When the second
recording layer has a layer thickness less than 10 nm, the
repetitive recording properties are degraded, and when thicker
than 20 nm, the recording sensitivity is degraded.
In a third embodiment of the present invention, when the
upper protective layers of respective information layers other
than the information layer disposed at the innermost side as
viewed from the first substrate side contains an In oxide, a Zn
oxide, an Sn oxide and an Si oxide or a Ta oxide and each of
contents of the In oxide, Zn oxide, Sn oxide and Si oxide or Ta
oxide is represented as e, f, g and h [mol%], the following
requirements are satisfied:
3
50
0
e + f + g+ h = 100
Use of a composition satisfying all the expressions stated
above is preferable because it enables to increase recording
speed by containing a Zn oxide or an Sn oxide at 50 mol% or
more in the upper protective layers to thereby accelerate
crystallization of the recording layers. An alternative property
to crystallization rate is transition linear velocity. Transition
liner velocity is a linear velocity, specifically, when a reflectance
of a recording layer after being irradiated with a light beam
continuously is monitored at a constant linear velocity,
transition linear velocity means a linear velocity at which the
reflectance starts changing. In FIG. 7, the transition linear
velocity is 21m/s. Generally, a faster transition linear velocity
increases the recording speed. In FIG. the continuous light
power is set at 15mW.
As shown in FIG. 8, by containing a Zn oxide or an Sn
oxide at 50 mol% or more in an upper protective layer, a DOW10
jitter value of lower than 9% can be realized, and excellent jitter
property can be obtained at high-speed recording.
Further, as shown in FIG. 9, when the content of a Zn
oxide or an Sn oxide is set at 90 mol% or less in an upper
protective layer, it is preferable because variation of DOW10
jitter value can be suppressed to 1% or less and the storage
stability is improved. When the content of a Zn oxide or an Sn
oxide is 90 mol% or more, excellent storage reliability cannot be
obtained.
For a material used for an upper protective layer in a
conventional single-layer phase-change optical recording
medium, the upper protective layer is preferably formed of a
material that is transparent, transmissive to light, has a higher
melting point than the recording layer and has functions to
prevent deterioration of the recording layer, to enhance the
bonding strength with the recording layer and enhance the
recording properties. It has been known that particularly
ZnS-SiO2 is preferably used for upper protective layer, and for
the mixture ratio, ZnS"SiC>2 = 80^20 (molar ratio) is most
preferable.
However, in the case of a multi-layered phase-change
recording medium, when recording information on a first
recording layer, the heat dissipation property of the first
recording layer is degraded because of the thin thickness of the
first reflective layer to cause a problem that there is a difficulty
in recording. To avoid the problem, it is preferable to use a
material having excellent thermal conductivity. When
ZnS-SiO2is used for the first upper protective layer, the
recording properties are degraded because of the low thermal
conductivity and the storage stability after recording is poor,
and thus ZnS"SiO2 is not suitable for the first upper protective
layer in a multi-layered phase-change optical recording medium.
FIG. 10 shows a result of measuring a DOW10 jitter variation
obtained when a phase-change optical recording medium of
Example 1 to be hereinafter described and another phase-change
optical recording medium prepared in the same manner as in
Example 1 except that material of the first upper protective
layer was changed to ZnS-SiO2 (80:20 mol%) were stored at 80oC
and a relative humidity (RH) of 85% for 100 hours. The result
shown in FIG. 10 demonstrated that in the optical recording
medium using ZnS"SiO2, the jitter variation was not reduced
irrespective to the layer thickness.
Summarizing above-mentioned results, it is preferable to
use a Zn oxide and an Sn oxide having higher heat dissipation
property than ZnSSiO2. When either Zn oxide or Sn oxide is
used as simple material, excellent storage stability cannot be
obtained. Therefore, an In oxide and an Si oxide or a Ta oxide
are preferably contained therein. When these metal oxides are
used for a first information layer of a multi-layered
phase-change optical recording medium, high light
transmittance, excellent recording properties and storage
stability can be obtained because these metal oxides are
transparent to light and have high thermal conductivity.
Since a first upper protective layer formed using these
metal oxides can ensure a sufficient modulation degree and a
sufficient reflectance with a thin layer of around 5 nm, (in, Zn,
Sn) oxides, which can be sputtered by RF sputtering or DC
sputtering, plus Si oxide may be used. When the use amount of
Sn oxide or Zn oxide is large in a first upper protective layer,
the crystallization rate of the recording layer is accelerated, and
thus the recording speed can be increased.
For the second upper protective layer, ZnS"SiO2 may be
used as usual. The reason is that when information is recorded
on the second recording layer, sufficient heat dissipation
property can be obtained because the second reflective layer can
be thickly formed. However, when ZnS-SiC*2 is used for the
second upper protective layer and Ag is use for the second
reflective layer, it is preferable that an interface layer formed of
TiOTi02 is inserted in between the second upper protective
layer and the second reflective layer.
The layer thickness of the first upper protective layer is
preferably in a range of 2 nm to 15 nm. When the first upper
protective layer has a thickness thinner than 2 nm, the
modulation degree is reduced due to excessively high light
transmittance, and when thicker than 15 nm, it is unfavorable
because the light transmittance is lowered, heat hardly escapes,
and thus the recording properties are degraded.
The layer thickness of the second upper protective layer
is preferably within a range of 3 nm to 30 nm. When the second
upper protective layer has a thickness thinner than 3 nm, the
recording properties are degraded, and when thicker than 30 nm,
heat easily stays at the second upper protective layer, resulting
in degraded recording properties.
In a fourth embodiment of the present invention, Cu is
used as the main component for semi-transparent reflective
layers of information layers other than that of an information
layer disposed at the innermost side as viewed from the laser
irradiation side. This configuration enables enhancing
recording properties and storage stability of the first recording
layer. Here, the term "main component" means that Cu is
contained at 95% by weight or more. The reason why the first
reflective layer containing Cu as the main component is
preferable is described below.
In a phase-change optical recording medium having two
layers of recording layer as shown in FIG. 1, a second
information layer needs to be irradiated with as much light as
possible using a recording and reproducing laser light.
Therefore, a material that hardly absorbs laser light and is
easily transmissive to laser light is preferably used for the first
reflective layer.
Then, the present inventors measured optical properties
for various types of reflective layers at a wavelength of 660 nm.
Here, data of A (absorbance), R (reflectance) and T (light
transmittance) was obtained. For measurement samples,
polycarbonate substrates each having a thickness of 0.6 mm and
covered with various metal films having a layer thickness of 10
nm formed were used. The results are shown in FIG. 11. From
the results, it can be assumed that Pt, Pd and Ti are unsuitable
for a first reflective layer because these materials have a low
light transmittance and a high light absorbance.
Next, Cu and Ag, which has a relatively high light
transmittance and a relatively low absorbance, were respectively
formed on a polycarbonate substrate, A (absorbance), R
(reflectance) and T (light transmittance) were measured at a
wavelength of 660 nm while varying the layer thickness, the
results as shown in FIG. 12 (Cu) and FIG. 13 (Ag) were obtained.
The measurement results showed that Ag has greater variations
by layer thickness than Cu. This shows that Cu has more
excellent stability of optical constant to layer thickness of a
layer formed.
Further, FIG. 14 shows the measurement results of
spectral transmittance when Ag and Cu were formed to be 8 nm
in thickness. The results showed that light transmittance lines
of Ag and Cu cross at a wavelength range of around 450 nm.
These facts showed that Cu has a higher light
transmittance than Ag in a wavelength range longer than 450
nm or so, and Cu is preferably used to a laser light having a
wavelength of around 660 nm for the first reflective layer.
Further, in respective recording media using Cu, Ag or
Au for each of the first reflective layers, a 3T single pattern was
recorded on the first recording layer at a wavelength of 660 nm,
and the C/N ratio was measured. The results are shown in FIG.
15. The highest C/N ratio was obtained when using Cu. From
the perspective of recording properties, Cu is also shown to be
suitable for the first reflective layer. Note that respective plots
shown in FIG.14 are the ones that a plurality of experimental
data units is arranged along the horizontal axis.
The second reflective layer is not necessarily
semi-transparent, like the first reflective layer.
The layer thickness of the first reflective layer is
preferably within a range of 6 nm to 12 nm. When the layer
thickness is thinner than 6 nm, signal quality is degraded due to
the excessively low reflectance, and repetitive recording
property is degraded because the heat dissipation property is
degraded. When thicker than 12 nm, it is unfavorable because
the light transmittance is lowered.
The layer thickness of the second reflective layer is
preferably within a range of 100 nm to 200 nm. When the layer
thickness is thinner than 100 nm, sufficient heat dissipation
property cannot be obtained and then repetitive recording
property is degraded, and when the layer thickness thicker than
200 nm, a layer having a wasteful thickness is formed, although
the heat dissipation property is unchanged, and mechanical
property of the recording medium itself is degraded.
The first semi-transparent reflective layer and the
second reflective layer as described above can be formed by
various vapor deposition methods, for example, vacuum
evaporation method, sputtering method, plasma CVD method,
optical CVD method, ion-plating method and electron beam
deposition method. Of these, sputtering method is excellent in
mass productivity and layer quality and the like.
The first lower protective layer and the second lower
protective layer are preferably formed of a material that is
transparent, is transmissive to light and having a higher
melting point than the recording layer and has functions to
prevent deterioration of the recording layer, to enhance the
bonding strength with the recording layer and enhance the
recording properties. Metal oxides, nitrides, sulfides and
carbides are mainly used. Specific examples thereof include
metal oxides such as SiO, SiO2, ZnO, SnO2, A12O3, TiO2, ln2O3,
MgO and ZrO2; nitrides such as Si3N4, AIN, TiN, BN and ZrN;
sulfides such as ZnS, In2S3 and TaS4.' carbides such as SiC, TaC,
B4C, WC, TiC and ZrC; diamond carbons or mixtures thereof.
Each of these materials may be used alone or in combination
with two or more. Further, each of these materials may contain
impurities in accordance with necessity. For example, a
mixture of ZnS with SiO2 and a mixture of Ta2O5 with SiO2 are
exemplified. Particularly, ZnS-SiC>2 is often used, and the
mixture ratio of ZnS:SiO2 = 80:20 is most preferable. This
material is capable of increasing light absorption efficiency of
recording layers because it has a high refractive index n and an
extinction coefficient of nearly zero, and this material can
appropriately suppress diffusion of heat generated by light
absorption because of its small thermal conductivity, and thus
the use of ZnS:SiC>2 enables to raise the temperature of
recording layers to a temperature at which the recording layers
can be melted.
The layer thickness of the first lower protective layer is
preferably within a range of 40 nm to 80 nm. When the first
lower protective layer has a thickness thinner than 40 nm, it is
unfavorable because the repetitive recording durability and
recording properties are degraded and the light transmittance is
lowered. When thicker than 80 nm, it is unfavorable because
the light transmittance is lowered. The layer thickness is more
preferably within a range of 60 nm to 80 nm. By adjusting the
layer thickness of the first lower protective layer within the
above-mentioned range, the repetitive recording durability is
greatly enhanced.
The layer thickness of the second lower protective layer
is preferably within a range of 110 nm to 160 nm. When the
second lower protective layer has a thickness thinner than 110
nm, the reflectance of light from the second information layer is
lowered, resulting in degraded reproducing signal quality and
degraded repetitive recording durability. When thicker than
160 nm, the reflectance of light from the second information
layer is lowered, resulting in degraded reproducing signal
quality and degraded mechanical property of the recording
medium itself.
The first and the second upper protective layers and the
first and second lower protective layers described above can be
formed by various vapor deposition methods, for example,
vacuum evaporation method, sputtering method, plasma CVD
method, optical CVD method, ion-plating method and electron
beam deposition method. Of these, sputtering method is
excellent in mass productivity and layer quality and the like.
A two-layered phase-change optical recording medium
according to the present invention can be typically produced
through a layer forming step, an initialization step and a
bonding step.
In. the layer forming step, a first information layer is
formed on a surface of a first substrate (see FIG. l) with a
groove formed thereon, and a second information layer is formed
on a surface of a second substrate with a groove formed thereon.
The first information layer and the second information layer can
be formed by various vapor deposition methods, for example,
vacuum evaporation method, sputtering method, plasma CVD
method, optical CVD method, ion-plating method and electron
beam deposition method. Of these, sputtering method is
excellent in mass productivity and layer quality and the like.
In a sputtering method, a layer is formed with streaming an
inactive gas such as argon, and at this point in time, the layer
may be reactively sputtered while incorporating oxygen and
nitrogen etc.
In the initialization step, the entire surface of the optical
recording medium is initialized by irradiating the first
information layer and the second information layer with energy
light such as laser light, i.e, the recording layers are
crystallized. When there is a possibility that a film floats by
laser light energy during initialization step, the first
information layer and the second information layer may be
spin-coated with a UV curable resin etc. before the initialization
step and then cured by irradiating with ultraviolet ray so as to
be overcoated. Further, after the bonding step to be described
below is preliminarily carried out, the first information layer
and the second information layer may be initialized from the
first substrate side.
In the bonding step, the first substrate and the second
substrate are bonded together via an intermediate layer in a
condition where the first information layer faces the second
information layer. For example, a UV curable resin is applied
to any one of the layer surfaces of the first information layer
and the second information layer, the both substrates are
pressurized and bonded in the condition where the first
information layer surface and the second information layer
surface face to each other, and the ultraviolet curable resin is
irradiated with an ultraviolet ray to thereby cure the UV
curable resin.
A three-layered phase-change optical recording medium
having three information layers is produced basically in the
same manner as in the two-layered phase-change optical
recording medium stated above, however, since the number of
information layers is increased by one, it is produced, for
example, in the order of the following steps.
First, in a first layer forming step, a first information
layer is formed on a first substrate, and a third information
layer is formed on a second substrate.
Next, in an intermediate layer forming step, a second
intermediate layer is formed on the third information layer.
Next, in a second layer forming step, a second
information layer is formed on the second intermediate layer.
Next, in a bonding step, the first substrate and the
second substrate are bonded together via a first intermediate
layer in a condition where the first information layer and the
second information layer face to each other.
Subsequently, in an initialization step, the first
information layer to the third information layer are initialized
in the same manner as in the two-layered phase-change optical
recording medium.
The present invention can provide a multi-layered
phase-change optical recording medium having excellent
repetitive recording durability and excellent storage stability, in
which respective information layers other than an information
layer disposed at the innermost side as viewed from the laser
beam irradiation side are made to have a high light
transmittance to allow for recording and reproducing
information on the respecting information layers with stable
tracking accuracy.
Examples
Hereinafter, the present invention will be further
described in detail referring to specific Examples, however, the
present invention is not limited to the disclosed Examples.
For an evaluation apparatus, ODU1000 manufactured by
PULSTEC INDUSTRIAL CO., LTD was used. A laser
wavelength used to irradiate recording layers at the time of
recording was 660 nm, and a numerical aperture (NA) of an
objective lens was 0.65. A recording linear velocity used at the
time of recording was 9.2 m/s, and recorded information was
reproduced with a reproducing light power of 1.2 mW. For a
light waveform generation unit, MSG3A manufactured by
PULSTEC INDUSTRIAL CO., LTD was used. As to recording
method, for a first information layer (L0 layer) disposed at the
front side as viewed from the laser beam irradiation side, IT
cycle recording strategy was used , and for a second information
layer (LI layer) disposed at the innermost side as viewed from
the laser beam irradiation side, 2T cycle recording strategy was
used. The recording strategies used in the test are shown in
FIG. 16 and FIG. 17, and parameters used in the test are shown
in Tables 7-A and 7B. The wording "el" shown in Tables 7-A
and 7-B represents a ratio (Pe/Pw) of an erasing power Pe to a
recording power Pw. In FIG. 17, "dTx" is expressed as a
positive value when leading to the related clock edge, and is
expressed as a negative value when lagging to the related clock
edge.
(Example 1)
On a first substrate composed of a polycarbonate resin
having a diameter of 12 cm, a thickness of 0.575 mm, and having
convexoxoncaves (groove depth-' A./11.5n)(n= 1.55) of a tracking
guide formed with a serpentine continuous groove with a track
pitch of 0.74 μm and having a groove width of convex portions
(groove) of 0.25 urn on one surface thereof, a first lower
protective layer composed of ZnS-SiO2 (80 : 20 (mol%)) having a
layer thickness of 70 nm was formed by RF magnetron
sputtering method under the conditions of a sputtering power of
4kW and an Ar flow rate of 15 seem.
Next, on the first lower protective layer, a first recording
layer composed of Ago.2ln3.5Ge7Sb68.7Te20.6 having a layer
thickness of 7.5 nm was formed by DC magnetron sputtering
method under the conditions of a sputtering power of 0.4 kW and
an Ar flow rate of 35 seem.
Next, on the first recording layer, a first upper protective
layer composed of In203-ZnO-SnO2-Ta2O5 (7.5 : 22.5 : 60 : 10
(mol%)) having a layer thickness of 5 nm was formed by RF
magnetron sputtering method under the conditions of a
sputtering power of lkW and an Ar flow rate of 15 seem.
Next, on the first upper protective layer, a first
semi-transparent reflective layer composed of Cu-Mo (l.l mol%)
having a layer thickness of 8 nm was formed by DC magnetron
sputtering method under the conditions of a sputtering power of
0.5 kW and an Ar flow rate of 20 seem.
Next, on the first semi-transparent reflective layer, a
thermal diffusion layer composed of In203"ZnO-Sn02'SiO2 (8.8 -
41.7 : 35.2 : 14.3 (mol%)) having a layer thickness of 65 nm was
formed by DC magnetron sputtering method under the
conditions of a sputtering power of 2kW and an Ar flow rate of
15 seem to thereby form a first information layer (L0 layer).
In the meanwhile, on a second substrate composed of a
polycarbonate resin having a diameter of 12 cm, a thickness of
0.6 mm, and having convexo-concaves (groove depth- AV11.5n)
(n= 1.55) of a tracking guide formed with a serpentine
continuous groove with a track pitch of 0.74 μm and having a
groove width of convex portions (groove) of 0.24 μm on one
surface thereof, a second reflective layer composed of Ag having
a layer thickness of 40 nm was formed by DC magnetron
sputtering method under the conditions of a sputtering power of
3kW and an Ar flow rate of 15 seem.
Next, on the second reflective layer, an interface layer
composed of TiC"TiO2 (70 - 30 (mol%)) having a layer thickness
of 4 nm was formed by DC magnetron sputtering method under
the conditions of a sputtering power of 2kW and an Ar flow rate
of 15 seem.
Next, on the interface layer, a second upper protective
layer composed of ZnS"SiO2 (80 : 20 mol%) having a layer
thickness of 20 nm was formed by RF magnetron sputtering
method under the conditions of a sputtering power of 1.5 kW and
an Ar flow rate of 15 seem.
Next, on the second upper protective layer, a second
recording layer composed of Ago.2ln3.5Sb71.4Te21.4Ge3.5 having a
layer thickness of 15 nm was formed by DC magnetron
sputtering method under the conditions of a sputtering power of
0.4 kW and an Ar flow rate of 35 seem.
Next, on the second recording layer, a second lower
protective layer composed of ZnS"SiO2 (80 : 20 mol%) having a
layer thickness of 140 nm was fromed by RF magnetron
sputtering method under the conditions of a sputtering power of
4kW and an Ar flow rate of 15 seem to thereby form a second
information layer (LI layer).
For a sputtering apparatus, DVD SPRINTER
manufactured by Unaxis Japan Co., Ltd. was used.r
Next, an ultraviolet curable resin (KARAYAD DVD802,
manufactured by Nippon Kayaku Co., Ltd.) was applied over the
layer surface of the first information layer, the layer surface of
the second information layer was bonded to the layer surface of
the first information layer, and the first information layer and
the second information layer were bonded by spin-coating.
Then, the ultraviolet curable resin was irradiated with an
ultraviolet ray for curing to form an intermediate layer having a
layer thickness of 50 μm, thereby a two-layered phase-change
optical recording medium having two information layers as
shown in FIG. 1 was prepared.
Next, the two-layered phase-change optical recording
medium was irradiated with a laser beam in the order of the
second information layer and the first information layer to
initialize the information layers. The first recording layer and
the second recording layer were initialized by converging a laser
beam emitted from a semiconductor laser (emission wavelength
810 ± 10 nm) onto the recording layers by an optical pickup (NA
= 0.55). For conditions for initializing the second recording
layer, the disc was rotated in CLV (constant leaner velocity)
mode at a linear velocity of 7m/s, a feed rate of 40 um/revolution,
a radius position of 22 mm to 59 mm, with an initialization
power of 2,000 mW. For conditions for initializing the first
recording layer, the disc was rotated in CLV (constant leaner
velocity) mode at a linear velocity of 6m/s, a feed rate of 60
um/revolution, a radius position of 22 mm to 59 mm with an
initialization power of 1,100 mW.
For the reflectance of the crystalline phase after the
initialization, the LO layer had a reflectance of 6.2%, the LI
layer had a reflectance of 5.8%, and the two-layered
phase-change optical recording medium had well-balanced
reflectance.
(Examples 2 to 13 and Comparative Examples 1 to 4)
Two-layered phase-change optical recording media each
having the same substrates and the same layer configuration as
those of Example 1 were prepared except that the material
composition of the thermal diffusion layer (light transmissive
layer) was changed to the material composition shown in Table 1.
The two-layered phase-change optical recording media were
tested.
The acceptance level of light transmittance of the first
information layer was determined to be 42%. Further, to
determine the reliability of repetitive recording durability, a
jitter value (DOW 500 jitter) after repeatedly recording at 500
times was evaluated. The acceptance level of jitter value was
determined to be 10%.
Table 1 shows the evaluation results, and when Sn oxide
and Si oxide were not contained in the material composition for
thermal diffusion layer, the light transmittance and the DOW 10
jitter were not within acceptable level. When In oxide was
contained at 3 mol% or less, the DOW 500 jitter value was
increased, although the light transmittance was excellent.
When In oxide was contained at 50 mol% or more, the light
transmittance tended to be lowered, although the DOW 500
jitter value was decreased. Further, when Si oxide was
contained at 30 mol% or more, the sputtering rate tended to be
lowered.
(Examples 14 to 40 and Comparative Examples 5 to 13)
Two-layered phase-change optical recording media of
Examples 14 to 40 and Comparative Examples 5 to 13 were
prepared in the same manner as in Example 1, except that the
groove depths of the first and the second substrates were
changed to values shown in Tables 2-A, 2-B and 2-C. There is a
tendency that the shallower the groove depth is, the higher the
reflectance is, however, actually, there was little difference in
reflectance with the groove depths employed in Examples, the
reflectances of the L0 layer and LI layer were well balanced in
any of the optical recording media of Examples 14, 23 and 32
respectively formed with a deep groove and the optical recording
media of Example 22, 31 and 40 respectively formed with a
shallow groove.
The two-layered phase-change optical recording media of
Examples 14 to 40 and Comparative Examples 5 to 13 were
evaluated with a laser wavelength of 660 nm, 645 nm and 665
nm, respectively. Tables 2-A, 2-B and 2-C show the evaluation
results. When the groove depth was shallower than the lower
limit value (λ/17n) as in the optical recording media of
Comparative Examples 6 to 7, 9 to 10 and 12 to 13, the
amplitude of a push-pull signal was decreased, and stable
tracking accuracy could not be obtained. When the groove
depth was deeper than the upper limit value (λ/11.5n) as in the
optical recording media of Comparative Examples 5, 8 and 11,
the jitter value was increased.
Marks were repeatedly recorded 10 times at three
adjacent tracks in the recording layer and, of these three tracks,
the middle track was reproduced to evaluate properties of
optical recording media of Examples 14 to 40 and Comparative
Examples 5 to 13. The evaluation of properties was carried out
based on a jitter obtained when 3T to 11T and 14T marks and
spaces were randomly recorded. The "jitter" represents a time
deviation between boundary and time clock when reflectance
levels of marks and spaces were binarized on a specific slice
level. The lower the jitter value is, the more excellent the
recording property is. The acceptance level of jitter value was
9% or less. The modulation degree after recording was
measured, and both the L0 layer and LI layer had a modulation
degree of 63%. As to the recording power Pw at that time, the
recording power (Pw) of the L0 layer was 36 mW, and the
recording power (Pw) of the LI layer was 38 mW. A modulation
degree is the one represented by (Rtop - Rbot)/Rtop when the
reflectance of a crystalline phase is represented by Rtop and the
reflectance of an amorphous phase is represented by Rbot.
The push-pull signals shown in Tables 2-A, 2-B and 2-C
are the ones that [(la + lb) - (lc + Id)] / [la + lb + lc + Id] are
measured using a photodetector shown in FIG. 5. The
frequency was filtered with a low-pass filter having a cutoff
frequency of 30 kHz (-3dB) so that serpentine wobble frequency
components (about 820 kHz) and other noise components were
not mixed at the time of measurement. The acceptance level of
push-pull signal was determined to be 0.28 or more.
The same results could also be obtained with the optical
recording media of Examples 2 to 13 in which each of the
material of the thermal diffusion layer (light transmissive layer)
was changed.
(Examples 41 to 49 and Reference Examples 14 to 18)
Two-layered optical recording media each having the
same substrates and the same layer configuration as those of
Example 1 were prepared, except that the material composition
of the recording layer was changed to the material composition
shown in Table 3. The prepared optical recording media were
tested as to recording properties and storage stability.
A jitter (DOW10 jitter) after repeatedly recording 10
times at a recording linear velocity of 9.2 m/s was measured, a
jitter value less than 10% was evaluated as "A" and a jitter
value of 10% or more was evaluated as "B". Further, when a
jitter variation obtained after being left at 80°C for 300 hours
was less than 2%, it was evaluated as "A", and when a jitter
variation obtained after being left at 80°C for 300 hours was 2%
or more, it was evaluated as "B". The two-layered
phase-change optical recording medium was stored in a
thermostatic bath set at 80°C and a relative humidity of 85% for
300 hours to thereby carry out the storage test.
As can be seen from Table 3, when the content of Ge was
smaller than 2%, the storage stability of the optical recording
medium was degraded because of the low crystallization
temperature. When the content of Ge was greater than 20%,
the repetitive recording property of the optical recording
medium was degraded, although the crystallization temperature
was sufficiently high. It can be considered that when the
content of Sb is smaller than 60%, high-speed recording cannot
be carried out because of its low transition linear velocity at the
recording layer. When the content of Sb was greater than 75%,
recording could not be done because of its excessively high
transition linear velocity, and the storage stability of the optical
recording medium was poor because the crystallization
temperature was decreased. When the content of Te was
smaller than 6%, it was difficult to initialize the recording layer,
and the jitter value was increased. When the content of Te was
greater than 30%, the transition linear velocity was slow and
the recording properties tended to be degraded
(Examples 50 to 65}
Two-layered phase-change optical recording media each
having the same substrates and the same layer configuration as
those of Example 1, except that the material composition of the
first upper protective layer was changed to the composition as
shown in Tables 4 and 5. The prepared optical recording media
were tested.
A transition linear velocity of the first information layer
and a jitter value obtained after repeatedly recording 10 times
were evaluated. The evaluation results showed that the
sputtering rate was almost the same, however, when any one of
ZnO or SnO2 was contained at 50 mol% or more, the recording
properties of the optical recording medium were enhanced
because the transition linear velocity was improved.
(Example 1 and 66 to 77)
Two-layered phase-change optical recording media each
having the same substrates and the same layer configuration as
those of Example 1, except that the material composition of the
first semi-transparent reflective layer was changed to the
material shown in Table 6. The prepared optical recording
media were tested.
The measurement results of Examples 66 to 77 were
shown together with the results of Example 1 in Table 6. The
jitter variation of the optical recording media after being stored
at 80°C and a relative humidity of 85% for 300 hours was
evaluated, and any of the optical recording media had a jitter
variation less than 1%.
Example 78
For the respective two-layered phase-change optical
recording media prepared in Examples 1 to 3, 6 and
Comparative Example 1, a light transmittance of the first
information layer (LO layer) within a wavelength range of 640
nm to 680 am was measured. FIG. 18 shows the measurement
results of the light transmittance. FIG. 19 shows measurement
results of a relation between recording power and DOW10 jitter
for the two-layered phase-change optical recording media-
Further, FIG. 20 shows a relation between thermal
conductivity of the thermal diffusion layer (light transmissive
layer) of the respective two-layered phase-change optical
recording media, the optimum recording power obtained in FIG.
19 (a recording power with which the lowest jitter value was
obtained), and light transmittance of each of the LO layers
measured at a wavelength of 660 nm obtained in FIG. 18.
The results shown in FIG. 20 verified that the smaller
the amount of In oxide was contained in the thermal diffusion
layer (light transmissive layer), the higher the light
transmittance of each of the LO layers was and the lower the
optimum recording power was.
CLAIMS
1. A multi-layered phase-change optical recording medium,
comprising-
a first substrate disposed at the front side as viewed from
the laser beam irradiation side,
a second substrate disposed at the innermost side as
viewed from the laser beam irradiation side,
each of the first substrate and the second substrate
having a serpentine spiral guide groove on a recording surface
side thereof,
intermediate layers, and
a plurality of information layers each having a
phase-change recording layer, being disposed via each of the
intermediate layers in between the first substrate and the
second substrate,
each of the information layers other than the information
layer disposed at the innermost side as viewed from the first
substrate side comprising the following five layers',
an under protective layer,
the phase-change recording layer,
an upper protective layer,
a semi-transparent reflective layer, and
a thermal diffusion layer or a light transmissive layer,
and.
the information layer disposed at the innermost side as
viewed from the first substrate side comprising:
an under protective layer,
the phase-change recording layer,
an upper protective layer, and
a reflective layer,
wherein each of the thermal diffusion layers or light
transmissive layers of the respective information layers other
than the information layer disposed at the innermost side as
viewed from the first substrate side comprises an In oxide, a Zn
oxide, an Sn oxide and an Si oxide, and when the contents of the
In oxide, the Zn oxide, the Sn oxide and the Si oxide are
represented by "a", "b", V and "d" [mol%3 respectively, the
following requirements are satisfied, and when the refractive
index of the first and second substrates was represented by V,
the laser light wavelength is represented by "A," and the depth of
the groove guide of the first and second substrates is
represented by H, the depth of the groove guide H satisfies the
bllowing requirement,
2. The multi-layered phase-change optical recording
medium according to claim 1, wherein each of the phase-change
recording layers comprises at least three elements of Ge, Sb and
Te, and when the composition ratio of the Ge, Sb and Te is
represented by "", "" and "γ" [atomic%], the following
requirements are satisfied,
3. The multi-layered phase-change optical recording
medium according to any one of claims 1 to 2, wherein each of
the upper protective layer in the respective information layers
other than the information layer disposed at the innermost side
as viewed from the first substrate side comprises an In oxide, a
Zn oxide, an Sn oxide and an Si oxide or a Ta oxide, and when
the contents of the In oxide, the Zn oxide, the Sn oxide and Si
oxide or the Ta oxide are represented by "e", "f", "g" and "h"
[mol%3 respectively, the following requirements are satisfied,
3
50
0
e + f+g + h= 100.
4. The multi-layered phase change optical recording
medium according to any one of claims 1 to 3, wherein each of
the semi-transparent reflective layers in the respective
information layers other than the information layer disposed at
the innermost side as viewed from the first substrate side
comprises Cu as the main component.
5. The multi-layered phase-change optical recording
medium according to any one of claims 1 to 4, wherein each of
the under protective layers in the respective information layers
comprises ZnS and SiO2.
A multi-layered phase-change optical recording medium having a first substrate and a second substrate, and a plurality
of information layers, wherein each of thermal diffusion layers of information layers other than an information layer disposed
at the innermost side as viewed from the first substrate side has In oxide, Zn oxide, Sn oxide and Si oxide, and when the contents of
thereof are represented by "a", "b", "c" and "d" [mol%] respectively, the following requirements are satisfied, and when the refractive
index of the first and second substrates was represented by "n", the laser light wavelength is represented by " " and the depth of the
groove guide of the first and second substrates is represented by H, the II satisfied the following requirement, 3
a + b + c + d=100 λ/17n |