Title of Invention

DUAL-STACK OPTICAL DATA STORAGE MEDIUM FOR WRITE ONCE RECORDING

Abstract

A dual-stack optical data storage medium (10) for write-once recording using a focused radiation beam (9) having a wavelength &lgr; of approximately 655 nm is described. The radiation beam enters through an entrance face (8) of the medium (10) during recording. The medium comprises at least one substrate (1, 7) with present on a side thereof: a first recording stack (6), named L0, comprising a write-once type L0 recording layer, said first recording stack L0 having an optical reflection value RL0 and an optical transmission value TL0, a second recording stack (3), named L1, comprising a write-once type L1 recording layer, said second recording stack L1 having an effective optical reflection value RLleff. The first recording stack (6) is present at a position closer to the entrance face (8) than the second recording stack. (3). A transparent spacer layer (4) is sandwiched between the recording stacks (3, 6). The reflection values RL0 and RL1 eff are within the following ranges: 0.12 8804; RL0 8804; 0.18 and 0.128804; RL1eff 8804; 0.18 by which an improved sensitivity of the dual stack medium is achieved.

Full Text

Dual-stack optical data storage medium for write once recording The invention relates to a dual-stack optical data storage medium for write-once recording using a focused radiation beam having a wavelength X of approximately 655 nm and entering through an entrance face of the medium during recording, comprising: at least one substrate with present on a side thereof: a first recording stack, named LO, comprising a write-once type ID recording layer, said first recording stack LO having an optical reflection value RLO and an optical transmission value TLO> a second recording stack, named LI, comprising a write-once type LI recording layer, said second recording stack LI having an effective optical reflection value Ruefr, said first recording stack being present at a position closer to the entrance face than the second recording stack, a transparent spacer layer sandwiched between the recording stacks. An embodiment of an optical recording medium as described in the opening paragraph is known from Japanese Patent Application JP-11066622. Recently the Digital Versatile Disk (DVD) has gained market share as a medium with a much higher data storage capacity than the CD. This format is available in a read only (ROM), recordable (R) and a rewritable (RW) version. For recordable and rewritable DVD, there are at present several competing formats: DVD+R, DVD-R for recordable and DVD+R W, DVD-RW, DVD-RAM for rewritable. An issue for both the recordable and rewritable DVD formats is the limited capacity and therefore recording time because only single-stacked media are present with a maximum capacity of 4.7 GB. Note that for DVD-Video, which is a ROM disk, dual layer media with 8.5 GB capacity, often referred to as DVD-9, already have a considerable market share. Consequently, recordable and rewritable DVD's with 8.5GB capacity are highly desired. One of the most important concerns for DVD+RW and DVD+R is to obtain backwards compatibility with existing DVD-ROM/DVD-video players. It is expected that dual-layer DVD+R, which is currently being developed, can achieve high compatibility with existing dual-layer DVD-ROM media; an effective reflection from both layers above 18% and signal modulation of 60 % as demanded by DVD-ROM-DL, has been demonstrated in experiments. Note that the wordings "dual-layer" and "dual-stack" are often used interchangeably. In fact when dual-layer is written actually dual-stack is meant. The same holds for the expressions "single-stack" and "single-layer". In order to obtain a dual-stack recordable DVD medium which is compatible with the dual-layer (=dual-stack) DVD-ROM standard, the effective reflectivity of both the upper ID layer and the lower LI layer should be at least 18%, i.e. the minimum effective optical reflection level in order to meet the specification is Rmin = 0.18. Effective optical reflection means that the reflection is measured as the portion of effective light coming back from the medium when e.g. both stacks ID and LI are present and focusing on IO and LI respectively. The minimum reflection Rmm = 0.18 is a requirement of the DVD-ROM dual layer (DL) standard. It can be expected that, similar to single-stack media, recording speed will become a very important issue for DL-media as well. Especially, since the doubled capacity implies doubled waiting time for consumers before a complete disc is recorded. Thus, a future speed-race for DL-media may be even more important than it is now for single layer (SL) media. A recurrent issue in the speed-race is the required write-power. For dye based write-once discs, there is a nearly linear relationship between recording speed and required laser power. Therefore, the maximum speed is limited by the capabilities of existing (or future) laser diodes. Obviously, the starting point for a future DL speed-race is quite unfavorable when the currently developed 2.4X media already require very high write power. As a benchmark for available power budget we can take a current 4X drive that has 30 mW maximum output power and the future 8X drive, which is expected to have over 40 mW maximum power. To allow some margins, e.g. heating in drive, variations in media, wavelength-dependent sensitivity variation, etc., the nominal write power for 4X and 8X single-layer media should be considerably below this value, i.e. DVD+R is twice as high as for single layer DVD+R: PDL = 2*P$L. It means that the starting point (from power point-of-view) of dual-layer DVD+R speed race is very unfavorable, see Figure 2. The problem with DVD+R-DL is that there is nearly twice as much storage capacity but a limitation in available recording speed. For instance DVD+R single-layer is now recordable at 8X, while DVD+R-DL is limited to 2 AX. It would be very favorable for the acceptance of DVD+R-DL, if the DVD+R-DL can keep pace with the DVD+R single-layer speed-race. The current DVD+R-DL media are too unsensitive to keep up with this speed race due to laser power limitations. It is an object of the invention to provide a dual stack optical data storage medium of the type mentioned in the opening paragraph which has an improved recording sensitivity. This object is achieved with the optical data storage medium according to the invention which is characterized in that 0.12 Said higher sensitivity enables a higher writing speed without the need for higher laser powers. It is especially advantageous when 0.15 improvements of e.g. optical pick-up units (OPU's) for DVD players, lower-reflection discs will be played back more easily in the near future. The reflection and transmission of ID stacks is tuned mainly by variation of the thickness du>M of the semitransparent mirror, e.g. Ag or an Ag-alloy, and to a lesser extend by the absorptivity of the dye. E.g. for the case of Ag it turns out that, over the Ag thickness range of interest, the reflection and transmission depend approximately linearly on the Ag thickness; for the stack-design currently in use the following relations are found: TLo(dLOAg) = -3.7*dL0Ag + 105 (in %) and Ru)(dL0Ag) = 2* du)Ag - 8.8 (in %)> note that du)Ag is measured in nanometers, see figure 4. The contribution of the first recording layer thickness (dye) to the total absorption of the ID stack is rather small. Thus, reflection and transmission of ID are to a large extent determined by the choice of Ag-alloy thickness. Unfortunately, the large fraction of incident laser power that is directly dissipated in the semitransparent mirror does not contribute to the recording of the dye layer: heat generated in the mirror does not flow in to the dye due to the very low heat conductivity of the latter and the very high heat conductivity of the former. It implies that over a large range of RLO (and TLO) values, the required write power for ID stays remarkably constant, see Fig. 3. A high reflection of LI can only be achieved in combination with a high transmission of LO, because the effective LI reflection depends quadratically on TLO* Rueff= RLI*TLO . It is advantageous when RLO is substantially equal to RLicff. In this way a balanced reflection is seen from both stacks of the medium by a read out radiation beam of an optical drive. Preferably the effective reflections of ID and LI are equal, i.e. RLicff - RLO, and hence the maximum allowed absorption in L1 is limited to ALimax = 1 - RLO/TLO2- In reality ALimax will be lower because the reflection of LI is also influenced by diffraction effects. The write power for LI in a dual-layer disc will be proportional to (ALI *TLO)"1. With this in mind it is possible to estimate the dependence of LI write power on the effective reflection level R*ff of the dual-layer disc, given that for RLieff = 18% the write power PLI >eff = 30 mW. When using the experimental relation for TLO and RLO given above, a balanced effective reflection of ID and L19 and assuming that ALI - 1 - Ru, it is found that at an effective reflection of 12%, the required write power for LI could be halved, i.e. of the same magnitude as for single-layer media! It is noted that the sensitivity of ID can be improved by using a dye with larger absorption value k. Calculations show that the increasing sensitivity of ID implies that a transmission of about 60% can be achieved in practice. A TLO of 60 % or more can be achieved when the first recording stack comprises a first reflective layer with a thickness dLOM and an absorption coefficient kLOM and the ID recording layer has an absorption coefficient ku)R and a thickness dLOR and where (ku>R* ^LOR + ktoM* In order to balance the effective reflection and sensitivity of the two layers, it is favorable when the second recording stack comprises a second reflective layer and the LI recording layer has an absorption coefficient kuR and where the intrinsic reflection Ru ofthe second recording stack is in the range 0.30 - 0.60 and where 0.075 25 nm and preferably the thickness ofthe dye layer duR is in the range of 0 In an embodiment the first reflective layer has a thickness du)M 5 16 nm, preferably du>M I 12 nm and mainly comprises one selected from Ag, Au or Cu. For this stack, a relatively thin first reflective layer is placed between the dye and the spacer. The first reflective layer serves as a semi-transparent layer to increase the reflectivity. A maximum thickness and suitable material must be specified to keep the transmission ofthe first metal reflective layer sufficiently high. For the metal layer e.g. Ag, Au, Cu, and also Al, or alloys of all thereof, or doped with other elements, can be used. In order to obtain a sufficiently transparent stack, the preferred thickness ofthe first reflective layer is as specified above. Preferably ku>R > 0.025, more preferably > 0.050. By increasing the k ofthe ID recording layer a higher sensitivity may be achieved. The contribution ofthe first recording layer thickness (dye) to the total absorption ofthe ID stack is rather small. Thus, reflection and transmission of ID are to a large extent determined by the choice of Ag(-alloy) thickness. Therefore, using a dye with a higher absorption will increase the sensitivity of the ID recording stack, with little adverse effects on the transmission and reflection. The present invention can be applied to all dual layer DVD recordable (R) formats. The dye material of the recording layers intrinsically has a high transmission at the recording wavelength h. Typical dyes that can be used are cyanine-type, azo-type, squarylium-type,or other organic dye material having the desired properties. In the dual stack optical data storage medium guide grooves for guiding the radiation beam may be present in both the ID and the LI stack. A guide groove for the ID stack is normally provided in the substrate closest to the entrance face. In an embodiment a guide groove (G) for LI is provided in the transparent spacer layer. This embodiment is called type 1. In another embodiment a guide groove (G) for LI is provided in the substrate. This embodiment is called type 2. The invention will be elucidated in greater detail with reference to the accompanying drawings, in which: Fig. 1 shows a schematic layout of an embodiment of the optical data storage medium according to the invention including the two stacks ID and LI; Fig. 2 shows the dependence of write power on recording speed for single layer DVD+R (circles) and dual-layer DVD+R (square) and estimated dependence for dual-layer DVD+R at 18 % reflection level (dashed line). Fig. 3 showsjitter versus write power for ID stacks having different reflection. Circles: 7 % reflection, Squares: 9 % reflection, Crosses: 18% reflection.. Fig. 4 shows the dependence of ID transmission (squares) and ID reflection (checkers) on Ag-alloy thickness for a specific stack design, i.e. groove depth = 140nm, groove width = 300 nm, dye thickness in groove = 80 nm, IX AZO-dye; Fig. 5 shows the theoretical write power dependence of LI on effective reflection of ID and LI; Fig. 6 shows the playability of dual-layer DVD media having reduced reflection level on existing DVD players; Fig. 7 shows the absorption coefficient k as a function of h for two dyes used in DVD+R (DL); Figs. 8-1 and 8-2 show the calculated reflection (RLO), modulation (MLO)> transmission (Tto)5 and modulation x reflection (RLO*MLO) product for a ID stack as function of Ag thickness; Fig. 9 shows the transmission TLO through LO as function of (ku)R* ^LOR + ku>M* du>M)A-; Fig. 10 shows a type 1 optical data storage medium; Fig. 11 shows a type 2 optical data storage medium. Fig. 12 shows the intrinsic LI reflection, RLI, as a function of dye thickness for k values of 0.05,0.15 and 0.25, respectively, assuming two different leveling parameters (L=0.375, L=0.3). In Fig 1 a dual-stack optical data storage medium 10 for recording using a focused radiation beam 9, e.g. a laser beam, having a wavelength 655 nm is shown. The laser beam 9 enters through an entrance face 8 of the medium 10 during recording. The medium 10 comprises a substrate 7 with present on a side thereof a first recording stack 6 named LO, comprising a write-once type ID recording layer 5 having a complex refractive index fiu)= nu> -i.ku) and having a thickness dLo. The first recording stack ID has an optical reflection value RLO and an optical transmission value TLO. A second recording stack 3 named LI comprising a write-once type LI recording layer having a complex refractive index fici = nLi - i.ku and having a thickness duR is present. The second recording stack LI has an optical reflection value RLI. The optical parameters are all measured at the laser beam wavelength. The first recording stack 6 is present at a position closer to the entrance face 8 than the second recording stack 3. A transparent spacer layer 4 is sandwiched between the recording stacks 3 and 6. The transparent spacer layer 4 has a thickness substantially larger than the depth of focus of the focused radiation beam 9. The stacks are tuned such as to meet the following requirements 0.12 A more detailed description: Medium of type 1 (see Fig 10), with ID stack: 80 nm azo-dye in groove / 12 nm Ag-alloy and LI stack: lOOnm azo-dye/ 120nm Ag-alloy. The transparent spacer 4 has a thickness of 55 Jim. Optical reflection RLO of ID is 15%, transmission TLO of ID is 61 %, effective reflection RLieff (through LO) of LI is 15 %. By using dyes as recording layer, which dyes are relatively transparent at the laser recording wavelength, recording stacks with high transmission suitable for multi-stack media can be fabricated. This is typically the case in write-once optical media such as CD-R and DVD+R. The IO stack has a guide groove with a depth of 145 nm and a width of 325 nm (FWHM). The LI stack has a guide groove G with a depth of 170nm and a width of 370 ran (FWHM). The guide groove G is provided in the transparent spacer layer 4. The ID recording layer is a 80 nm thick azo-dye having a refractive index fiu) = 2.45 - i.0.08. The wavelength X of the focused laser beam 9 is approximately 655nm. (kujR* dL0R + ku>M* dL0M) / X = (0.08*80 + 3.75* 12)/ 655 = 0.078, which is indeed smaller than 0.08. Similar reflection values may be obtained using Au, Cu or alloys of these metals as reflective layer material. In Fig. 2 the dependence of maximum write power Pwmax on recording speed for single layer DVD+R (circles) is shown as well as Pwmax for dual-layer DVD+R (square) at 2.4 X and the estimated dependence for dual-layer DVD+R (dashed line) at 18 % reflection level. It is clear that a relatively high write power Pw is required for such a dual stack (18% reflection) write once recording medium at higher recording speeds, i.e. 4X (14 m/s) or higher. At 8X speed 60 mW is required which is more than what is available at this moment in consumer recorders and drives. Hence it is clear that there is a need for a more sensitive dual-layer DVD+R medium. In Fig. 3 the average jitter versus write power for ID stacks having different reflection is shown. The average jitter is a measure for the deviation of the position of written marks from their optimum position. The average jitter is minimal at optimum write power. Circles: 7 % reflection, Squares: 9 % reflection, Crosses: 18% reflection. It is noticeable that over a large range of RLO values the optimum write power stays remarkably constant. In Fig. 4 the dependence of ID transmission TLO (squares) and ID reflection RLO (checkers) on Ag thickness for a specific stack design, i.e. groove depth =140 nm, groove width = 300 nm, dye thickness in groove = 80 nm and a IX AZO-dye is shown. The reflection and transmission of ID stacks is tuned mainly by variation of the thickness d^oM of the semitransparent mirror, e.g. Ag or a Ag-alloy, and to a lesser extend by the absorptivity k of the dye. E.g. for the case of Ag it turns out that, over the Ag-alloy thickness range of interest, the reflection and transmission depend approximately linearly on the Ag-alloy thickness; for the stack-design currently in use the following relations are found: Tu)(dL0Ag) = -3.7*du)Ag + 105 (in %) and RLo(du>Ag) * 2* du>Ag - 8.8 (in %), note that dLoAg is measured in nanometers. In Fig. 5 the theoretical dependence of write power of LI: Punom on effective reflection of ID and LI is shown. A high reflection of LI can only be achieved in combination with a high transmission of LA because the effective LI reflection Rueff depends quadratically on TLO: Rueff- RLI*TLO . Because preferably the reflection of ID and LI is balanced, i.e. Rueff- RLO, the maximum allowed absorption in LI is limited to Aumax = 1 - RU/TLD • In reality ALimax will be lower because the reflection of LI is also influenced by diffraction effects. Punorm in a dual-layer disc will be proportional to (ALI*TLO)" • With this in mind it is possible to estimate the dependence of LI write power on the effective reflection level Rcfr of the dual-layer disc, given that for Ruefr ~ 18% the write power Pueff = 30 mW. When the experimental relations for TLO and RLO given above : TLo(du)Ag) ~ -3.7*du)Ag + 105 (in %) and RLo(dLOAg) = 2* dLOAg - 8.8 (in %), and the assumptions that Au = 1 - RLI and RLO - Rueff are used, it is found that at an effective reflection level Rueff of 12%, the required optimal write power for the LI recording layer is halved, i.e. of the same magnitude as for single-layer media. It is noted that the sensitivity can of ID can be improved by using a dye with larger absorption value k, with little adverse effects on the reflection and transmission ofLO. Calculations show that a transmission of about 60% can be achieved in practice. In Fig. 6 the payability of dual-layer DVD media having reduced reflection level on existing DVD players is shown. Payability is defined as the percentage of existing DVD players that will correctly read the data from the inserted medium. In Fig. 7 the absorption coefficient k as a function of \ for two dyes used in DVD+R (DL) is shown. Dye 2 has a larger absorption value k than dye 1. In Fig. 8 the calculated reflection, modulation, transmission, and modulation x reflection product for an ID stack as function of Ag thickness are shown for dye 1 and dye 2. In Fig. 9 the transmission TLO as a function of (kLOR* du»R +ku)M* dtoM)/^ is shown. A TLO of more than 60 % can be achieved when (kLoR* dLOR + kLOM* duM)^ In Fig. 10a so-called type 1 medium is shown. An optical recording stack (LO), optically semi-transparent at the laser wavelength, is applied to a transparent, pre-grooved substrate 7. A transparent spacer layer 4 is attached to the ID stack. The spacer layer 4 either contains pregrooves (G) for LI or pregrooves (G) for LI are mastered into the spacer layer 4 after application to LO. Second recording stack LI is deposited on the grooved spacer layer 4. Finally, a counter substrate 1 is applied. In Fig. lla so-called type 2 medium is shown. An optical recording stack (LO), optically semi-transparent at the laser wavelength, is applied to a transparent, pre-grooved substrate 7. A second optical recording stack LI,reflective at the laser wavelength, is applied to a second transparent pre-grooved (G) substrate l.This substrate 1 with LI is attached to the substrate 7 with ID with a transparent spacer layer 4 in between. Preferred spacer-layerthickness for both disc types is 40 jxm to 70 \im. In Fig. 12 the intrinsic LI reflection, Ru» as a function of dye thickness dR for k values of 0.05,0.15 and 0.25, respectively, assuming two different leveling parameters (L=0.375,L=0.3) is shown. The stacks proposed in this document are not restricted to use in DVD+R-DL and can be applied in any (multi-stack) organic-dye based optical recording medium. The thickness and optical constant ranges specified, however, are such as to meet the requirements for an LO- and LI -stack of a DVD+R-DL medium. It should be noted that the actual recording of marks does not necessarily take place in the groove G but may take place in the area between grooves, also referred to as on-land. In this case the guide groove G merely serves as a servo tracking means with the actual radiation beam recording spot being present on-land. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. CLAIMS: 1. A dual-stack optical data storage medium (10) for write-once recording using a focused radiation beam (9) having a wavelength X of approximately 655 nm and entering through an entrance face (8) of the medium (10) during recording, comprising: at least one substrate (1,7) with present on a side thereof: a first recording stack (6), named LO, comprising a write-once type ID recording layer, said first recording stack ID having an optical reflection value RLO and an optical transmission value TLO> a second recording stack (3), named LI, comprising a write-once type LI recording layer, said second recording stack L1 having an effective optical reflection value RLleff, said first recording stack being present at a position closer to the entrance face than the second recording stack, a transparent spacer layer (4) sandwiched between the recording stacks (3, 6), characterized in that 0.12 2. A dual-stack optical data storage medium as claimed in claim 1, wherein 0.15 3. A dual-stack optical data storage medium as claimed in any one of claims 1 or 2, wherein RLO is substantially equal to RLicff. 4. A dual-stack optical data storage medium as claimed in any one of claims 1,2 or 3, wherein the first recording stack comprises a first reflective layer (5) with a thickness du)M and an absorption coefficient kLOM and the ID recording layer has an absorption coefficient kLOR and a thickness dLOR and where (ktoR* dL0R + kL0M* dun*) 5. A dual-stack optical data storage medium as claimed in any one of claims 1, 2, 3 or 4, wherein the second recording stack comprises a second reflective layer (2) and the LI recording layer has an absorption coefficient kuR and where the intrinsic reflection Ru of the second recording stack is in the range 0.30 - 0.60 and where 0.075 6. A dual-stack optical data storage medium as claimed in any one of claims 4 or 5, wherein the first reflective layer (5) has a thickness du>M £ 16nm and mainly comprises one selected from Ag, Au or Cu. 7. A dual-stack optical data storage medium as claimed in claim 6, wherein the first reflective layer (5)has a thickness dtoM 8. A dual-stack optical data storage medium as claimed in any one of claims 1 - 7, wherein kLoR> 0.025. 9. A dual-stack optical data storage medium as claimed in claim 8, wherein ku>R > 0.050 10. A dual-stack optical data storage medium as claimed in any one of claims 1 to 9, wherein a guide groove (G) for LI is provided in the transparent spacer layer (4). 11. A dual stack optical data storage medium as claimed in any one of claims 1 to 9, wherein a guide groove (G) for LI is provided in the substrate (1).

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