DESCRIPTION OPTICAL RECORDING MEDIUM

Technical Field The present invention relates to an optical recording medium, and specifically to a single-side dual-layer optical recording medium.

Background Art Digital broadcasting has been already started and digital terrestrial broadcasting will entirely replace analogue broadcasting in 5 years, with an infrastructure for handling high volume digital image contents being prepared on a full-scale basis. Examples of high-speed, mass-storage devices to record these contents include hard discs, and hard discs having a capacity of one terabyte have been already realized.

For optical recording media, on the other hand, Bluτay discs having a capacity of 25GB or 50GB and recording apparatus therefor using a laser beam of 405nm wavelength have been introduced in the market. Bluτay ROM discs storing a movie content and reproducing apparatus therefor are now ready to enter the market.

Systems using a laser beam of 405nm wavelength include a Blu ray standard system and an HD DVD standard system,

differing in the storage capacity due to the difference in the numerical aperture NA of an objective lens adopted. When an objective lens with a numerical aperture NA of 0.65 is used for the HD DVD standard system, a single-side optical recording medium has a capacity of 15GB, and 30GB in a dual-layer, thus having less storage capacity than a Blu ray disc. However, by taking advantage of low disc manufacturing cost, a ROM disc and a reproducing apparatus therefor complying with the HD DVD standard have been recently released. HD DVD-based recording discs such as write-once type

"recordable" discs and "rewritable" discs are ready to be introduced in the market. Because the rewritable discs are used not only as optical recording media for recorders but also as optical recording media for personal computers for handling high- volume image files, a sequential recording system for sequentially recording from the inner tracks to the outer tracks of an medium, and a random recording system suitable for recording plural files of small capacities are available. The sequential recording system is applied to the write-once type recordable media. The random recording system can cause a recording medium to be in a condition that recorded areas and unrecorded areas randomly exit. A single-side single-layer medium can be recorded by any of the systems of sequential recording and random recording without any problems. A single-side

dual-layer medium such as a DVD+RW, DVD KW, BD KE U31uτay Disc Rewritable) can be recorded by random recording.

Patent Documents 1 to 3 disclose optical recording media which are recorded using a blue laser, but fail to disclose the relations between these techniques and the random recording.

Patent Literature Y- International Publication No. WO03/025922

Patent Literature 2- Japanese Patent Application Laid Open (JP-A) No. 2001-243655 Patent Literature 3: Japanese Patent (JP-B) No. 3561711

Disclosure of Invention

The present invention has been accomplished in view of the foregoing circumstances, and an object of the present invention is to solve the above-problems in the prior art and to achieve the following object.

In the case of a BD system using a wavelength of 405nm and an objective lens with a numerical aperture NA of 0.85, a single-side dual-layer optical recording medium has an intermediate layer having a thickness of 25μm, which separates two information layers. In this medium, when recorded areas (a crystal phase and amorphous phase) and unrecorded areas (a crystal phase alone) randomly exit in the first information layer disposed on the side of light incidence, recording and

reproduction can be performed without problems on portions ot the inner side second information layer that correspond to boundaries between the recorded areas and unrecorded areas of the first information layer. This means that the condition of the first information layer less influences the second information layer and crosstalk is small between the two layers.

On the other hand, in the case of an HD DVD system using a wavelength of 405nm, but an objective lens with a numerical aperture NA of 0.65, and a single-side dual-layer optical recording medium that has an intermediate layer having a thickness of 25μm, off-track occurs when recording is performed, while tracking the groove, on portions of the second information layer that correspond to the boundaries between the recorded areas and unrecorded areas randomly existing in the first information layer. This means that the condition of the first information layer largely influences the second information layer, and crosstalk is large between the layers under this condition of the system and the medium.

Therefore, an object of the present invention is to provide a single-side dual-layer optical recording medium allowed to be used for random recording, in which the second information layer can be recorded and reproduced without problems, not by changing the system, but by adjusting the condition of the optical recording medium, even though the recorded areas and the

unrecorded areas randomly exist in the first information layer. These problems are solved by the following <1> to <2>. <1> An optical recording medium including: a first information layer, a second information layer, and an intermediate layer disposed between the first information layer and the second information layer, wherein the first information layer and the second information layer are disposed in this order from the side of light incidence and the second information layer includes a second substrate, and wherein the form of a groove provided in the second substrate and the thickness of the intermediate layer are adjusted so that a push pull signal in the second information layer in an unrecorded condition is 0.22 or more and the relation between an amplitude "a" of a jump-back signal as measured upon tracking on a portion of the second information layer that corresponds to a boundary between a recorded area and an unrecorded area in the first information layer and an amplitude "b" of a defect signal which is locally generated by the influence of the first information layer satisfies the following requirement:

0 < b/a < 0.35. <2> The optical recording medium according to <1>, wherein elements constituting a recording layer material used for the first information layer and the second information layer comprise Ag, In, Sb, Te and Ge.

Brief Description of Drawings

Fig. 1 shows an example of a structure of a single-side dual-layer optical recording medium.

Fig. 2 shows an example of a structure of a first information layer.

Fig. 3 shows an example of a structure of a second information layer.

Fig. 4 shows an example of portions X, Y of the second information layer that correspond to boundaries between recorded areas and unrecorded areas randomly existing in the first information layer.

Fig. 5A shows a result of monitoring PD difference signal output in the boundary in the first information layer by a Push Pull Method when on track. Fig. 5B shows a result of monitoring PD difference signal output in the boundary in the first information layer by the Push Pull Method when off-track.

Fig. 6 shows a result of monitoring PD difference signal output by the Push Pull Method, when tracking is performed in the portion of the second information layer that corresponds to the boundary in the first information layer.

Best Mode For Carrying Out the Invention

The present invention will be explained hereinafter.

Optical recording media for use in an HD DVU standard system using a laser beam of 405 nm wavelength and an objective lens with a numerical aperture NA of 0.65 have the same substrate thickness and medium structure as DVD, and can be manufactured by using conventional equipment. Thus, they can be manufactured at lower cost than Blu-ray discs, so that their development has been promoted in terms of merit in the cost. The optical recording medium for the HD DVD standard has a capacity of 15GB, and 30GB in dual-layer, that is, larger than 25GB in a single-side single-layer Blu-ray disc. Therefore, dual-layer optical recording media are expected in view of capacity.

However, in the dual-layer optical recording medium, an intermediate layer, which separates a first information layer and a second information layer, is thin, and an influence of crosstalk between the two layers cannot be disregarded. Moreover, the recording sensitivity of the second information layer may differ depending on whether or not the first information layer is recorded. The variation in the recording sensitivity of the second information layer can be reduced by making light transmittance higher when the first information layer has been recorded, and making the difference in light transmittance between the recorded area and unrecorded area smaller. However, when the

variation in the recording sensitivity of the second information layer is large, the second information layer should be recorded while checking the condition of the first information layer. Otherwise, the characteristics of the second information layer largely varies. The light transmittance is made higher in order to make the recording sensitivity variation smaller. However, when recording is performed on portions of the second information layer that correspond to boundaries between the recorded areas and unrecorded areas which randomly exist in the first information layer, occurrence of off-track is a more serious problem than variation of recording characteristics.

The inventors of the present invention have found an optimal condition of medium characteristics of the single-side dual-layer optical recording medium, which the condition enables random recording even though the recorded areas and the unrecorded areas randomly exist in the first information layer.

Fig. 1 shows an example of a structure of a single-side dual-layer optical recording medium containing a first information layer 1 disposed on the side of light incidence (an arrow), an intermediate layer 2, and a second information layer 3 disposed inside. Further, Fig. 2 shows an example of a structure of the first information layer 1, and Fig. 3 shows an example of a structure of the second information layer 3. In the first information layer 1, a first lower dielectric protective layer 11, a

first recording layer 12, a first upper dielectric protective layer 13, a first sulfuration preventing layer 14, a semitransmissive reflective layer 15, an optical adjustment layer 16 are disposed on a first substrate having a groove 4 in this order. The second information layer 3 contains a second lower dielectric protective layer 17, a second recording layer 18, a second upper dielectric protective layer 19, a second sulfuration preventing layer 20, a second reflective layer 21, and a second substrate having a groove 5. Materials made up of Sb Te eutectic composition containing approximately 70 atomic% Sb are preferably used for the first and second recording layers. Examples thereof include Ag-In-Ge-Sb Te. For high speed recording, examples thereof include those containing as a main component Ge-In-Sb and doped with at least one of Zn, Te and Ga, and those containing as a main component Ge-Sn-Sb and doped with at least one of Mn and Zn.

The first recording layer preferably has a thickness of 5 nm to 9 nm. When the thickness is less than 5 nm, the light transmittance may be too high, the recording sensitivity may be decreased, and the temperature may not rise to a level sufficient to effect melting of the recording layer. Thus, an amorphous phase is not easily formed and rewriting characteristics may become poor. When the thickness is more than 9 nm, the light

transmittance of the first information layer may be too low, and the recording sensitivity of the second information layer may be largely decreased. However, this shall not apply to a sufficiently high laser power of a recording apparatus. The second recording layer preferably has a thickness of

10 nm to 20 nm, and more preferably 10 nm to 15 nm. When the thickness is less than 10 nm, the light absorbance may be reduced, and the characteristics may become poor due to the reduction of the recording sensitivity and reflectivity. When the thickness is more than 20 nm, the overwrite characteristics may become poor.

Materials of the semitransmissive reflective layer are preferably Ag, and Ag alloys containing 0.2 % by mass to 5.0 % by mass of at least one metallic element selected from Bi, Cu, In, and Pd. The semitransmissive reflective layer preferably has a thickness of 7 nm to 12 nm. When the thickness is less than 7nm, the reflectivity and cooling rate may be decreased, and then an amorphous phase is not easily formed, the degree of modulation becomes smaller. When the thickness is 12 nm or more, the light transmittance may be too lower, the recording sensitivity of the second information layer may be significantly decreased.

The materials of the semitransmissive reflective layer can be used for the second reflective layer.

The second reflective layer preferably has a thickness of

100 nm to 160 nm. A thickness of less than 100 nm leads to a poor heat releasing capability, shorter mark length, smaller area, and poor characteristics. A thickness of more than 160nm leads to a lower recording sensitivity. The dielectric protective layer used for each information layer is preferably made of material that is transparent, has a higher melting point than the recording layer, and improves environmental resistance of the recording layer. In a single-side single-layer phase-change optical recording medium, a mixture of ZnS"Siθ2 is often used as the material for the dielectric protective layer, the ratio of ZnS to Siθ2 is preferably 80:20 (mole%).

However, since the single-side dual-layer optical recording medium has a thinner semitransmissive reflective layer than the single-side single-layer optical recording medium, its heat releasing capability may be decreased, and the amorphous phase may not be easily formed. It is better to use a material having as high a thermal conductivity as possible for the first upper dielectric protective layer. ZnS Siθ2 or an oxide having a higher heat releasing capability is selected to use depending on the recording linear velocity and recording sensitivity.

Examples of oxides having a higher heat releasing capability include metallic oxides such as ZnO, Snθ2, AI2O3, Tiθ2,

I112O3, MgO, Zrθ2, TaO, Ta2θs, and Nb2θs. A complex oxide such as a mixture of ZnO and AI2O3 which are selected from the metallic oxides may be used.

The first upper dielectric protective layer preferably has a thickness of 10 nm to 30 nm. When the thickness is less than 10 nm, the recording sensitivity may be decreased. When the thickness is more than 30 nm, the overwrite characteristics may be adversely affected and reflectivity may be decreased.

The second upper dielectric protective layer preferably has a thickness of 15 nm to 30 nm. When the thickness is less than 15 nm, a recording sensitivity may be decreased. When the thickness is more than 30 nm, an overwrite characteristics may be adversely affected and reflectivity may be decreased.

The first lower dielectric protective layer preferably has a thickness of 40 nm to 60 nm. When the thickness is less than 40 nm, the overwrite characteristics may be adversely affected and the quality of recording marks may be significantly decreased under a high-temperature environment. When the thickness is more than 60 nm, the reflectivity may be increased, and the recording sensitivity may be decreased.

The second lower dielectric protective layer preferably has a thickness of 40 nm to 75 nm. When the thickness is less than 40 nm, the overwrite characteristics may be adversely affected and the quality of recording marks may significantly decrease

under a high-temperature environment. When the thickness is more than 75 nm, the reflectivity may be increased, and the recording sensitivity may be decreased.

When the semitransmissive reflective layer or the second reflective layer contains Ag, and a material containing S is used for the upper dielectric protective layer, the sulfuration preventing layer must be disposed in order to prevent the reaction between Ag and S. The material of the sulfuration preventing layer is preferably an oxide having a small light absorbance. Examples thereof include oxides containing Nb2θs as a main component.

The sulfuration preventing layer preferably has a thickness of 2 nm to 7 nm. When the thickness is less than 2nm, a prevention effect may be lost due to non-uniformity of the layer. When the thickness is more than 7 nm, the reflectivity and recording sensitivity may be decreased.

Since the optical adjustment layer quenches the first recording layer irradiated with light, materials having a higher thermal conductivity and light transmittance, smaller light absorption and higher refractive index are preferably used for the optical adjustment layer. Examples thereof include InSnOx, InZnOx, Tiθ2, Bi2θ3, Li2θ3, WO3 and mixtures thereof.

The optical adjustment layer preferably has a thickness of 10 nm to 40 nm, and more preferably 15 nm to 30 nm. When the

thickness is less than 10 nm, the transmittance ot the tirst information layer may be decreased, and the recording power required for the second information layer becomes stronger than necessary. A thickness of more than 40 nm results in the same situation.

The first and second substrates are necessary to be made of material that sufficiently pass through light for recording and reproduction, and those known in the art may be applied.

That is, glass, ceramics or resins may be used, and resins are particularly preferred in terms of their formability and cost.

Examples of the resins include polycarbonate resins, acrylic resins, epoxy resins, polystyrene resins, acrylonitrile-styrene copolymer resins, polyethylene resins, polypropylene resins, silicone resins, fluorine resins, ABS resins, and urethane resins. Among these, polycarbonate resins and acrylic resins such as polymethyl methacrylate (PMMA) are particularly preferable in terms of their formability, optical properties and cost.

On a surface of the first substrate on which a first information layer is to be disposed, a concavo-convex pattern such as a spiral or concentric groove, or a guide groove is formed. The concavo-convex pattern is generally formed by injection molding or photopolymer method.

The first substrate preferably has a thickness of 590 mm

to 610 mm. The first substrate preferably has a groove depth ot 23 nm to 29 nm.

The second substrate preferably has a thickness of 0.595 mm to 0.605 mm. The second substrate preferably has a groove depth of 23 nm to 27 nm.

The groove depth in the above range allows both the first and second information layers to surely have a reflectivity of 3% or more. The above range is also preferred to ensure a push pull signal as a groove signal. A groove depth of less than 23 nm leads to a larger reflectivity, but a push pull signal of smaller than 0.22. When the push pull signal is smaller than 0.22, tracking cannot be stably performed in a certain track (groove), and the smaller the push pull signal is, the higher the possibility that off-tracking occurs. When the groove depth is more than 27 nm, the push pull signal becomes larger, but the reflectivity becomes less than 3%. The push pull signal is necessary to exceed at least 0.22 to achieve the object of the present invention, that is, to ensure that a light beam that has passed through the boundary between a recorded area and unrecorded area in the first information layer is focused on the second information layer, and tracking is stable even influenced by the first information layer.

However, a large groove depth leads to stable tracking, but the influence of tracking error signals locally generated by

the influence of the first information layer becomes significant. Thus, the push pull signal is required to be larger than 0.22 which is a lower limit for stable tracking. Under this condition, it is confirmed in an experiment that off-track does not occur during sequential tracking feed, recording and reproduction in the second information layer, as long as no error occurs, or even when an error occurs, as long as b/a is 0.35 or less (b/a < 0.35), where b/a is the ratio of an amplitude "b" of a defect signal (an amplitude "b" of a tracking error signal) which is locally generated by the influence of the first information layer to an amplitude "a" of a jump back signal as measured upon tracking on a portion of the second information layer.

The light absorbance of the intermediate layer is preferably small at a selected light wavelength for recording and reproduction. Materials of the intermediate layer are preferably resins in terms of their formability and cost and examples include ultraviolet curable resins, slow-setting resins and thermoplastic resins. A concavo-convex pattern such as a guide groove formed by injection molding or photopolymer method may be formed on the intermediate layer.

The intermediate layer preferably has a thickness "e" of 25 μm to 33 μm. When the intermediate layer is thinner than 25 μm, the error signal amplitude "b" becomes larger and the value of b/a exceed 0.35 at an optimal groove depth by the influence on

the second information layer, caused by a light beam that has passed through the first information layer. On the other hand, as the intermediate layer becomes thicker than 33 μm, the push pull signal itself in the second information layer becomes smaller than 0.22 due to aberration. The error signal amplitude "b" also becomes smaller.

The first and second information layers each preferably has a reflectivity of 3.0% to 6.0%. When the reflectivity is smaller than 3.0%, focusing and tracking may not be possibly performed by an information recording reproducing apparatus. On the other hand, it is difficult to have a reflectivity of 6.0% or more in a phase-change optical recording medium. Particularly, the first and second information layers layer cannot together have a reflectivity of 6.0% or more. Even if it succeeded in allowing either the first information layer or the second information layer to have a reflectivity of 6.0% or more, a reflectivity difference may be larger, and focusing is not well performed when it is switched from the first information layer to the second information layer. The first and second information layers preferably together have a reflectivity of 3% to 4%.

The single-side dual-layer optical recording medium of the present invention is needed to be capable of recording and reproduction in a system using a laser beam of 405 nm wavelength and an objective lens with a numerical aperture NA of

0.65, and of allowing a user to achieve random recording of image and character information on blank first and second information layers. For this purpose, the intermediate layer preferably has a thickness of 25±5 μm. Examples of troubles occurring under this condition include, as shown in Fig. 4, occurrence of off-track at the time of recording/reproduction on/or from portions X, Y of the second information layer that correspond to the boundaries between the recorded areas and unrecorded areas randomly existing in the first information layer. In Fig. 4, the recorded area is a center part of the first information layer, and the unrecorded areas are both sides of the first information layer. This is because a PD (photodiode) in an optical pick-up receives the crosstalk of the first information layer when information is reproduced from the second information layer by irradiation with light passing through the boundaries between the recorded areas and unrecorded areas in the first information layer.

Namely, in Differential Push Pull Method that is a tracking method in which an incident beam is divided into three beams, when sub beams positioned on the land parts adjacent to a main beam positioned on a groove pass through the boundary, the difference signal between signals detected by a two-split PD is generally 0, as long as groove tracking is successfully performed. Specifically, PD difference signal output is monitored though a

30KHz lowpass filter- when tracking can be performed lon-trackj, a signal is obtained as shown in Fig. 5A; and when off-track occurs, a tracking error signal is obtained as shown in Fig. 5B (tracking is performed and a signal for one cycle of a track). However, when tracking is performed in a portion of the second information layer that corresponds to the boundary between a recorded area and unrecorded area in the first information layer, a signal is obtained as shown in Fig. 6. That is, an error signal E is also generated, as if remaining error is generated. The error signal is generated because information in the boundary between the recorded area and unrecorded area is transmitted as a result of crosstalk and then an offset isignal is generated when the light passes through the first information layer. This is simply referred to as "an error signal", hereinafter. In Examples, an error signal amplitude and an amplitude at off-track in Fig. 5B are referred to as "Rpp".

In the single-side dual-layer recording medium, the first information layer preferably satisfies the following light transmittance requirement: the light transmittance of amorphous portion Ta (%) and light transmittance of crystalline portion Tc (%) satisfy the following requirement: 0< Tc-Ta < 6 (%); and the light transmittance of crystalline portion Tc is 40% to less than 50%. When Tc is less than 40%, a power that is higher than the standard recording power (here, 13mW as measured at

disc surface) is needed for a recording power tor the second information layer, and recording sensitivity thus decrease. On the other hand, Tc of more than 50% is impossible in view of recording conditions. Tc of 42% to 45% is still more preferable. When Tc is more than 45%, the first information layer is inadequately initialized (crystallized), and the high reflectivity cannot be obtained, bringing to uneven reflection and substantially lower transmittance.

When the difference between Tc and Ta exceeds 6%, a power difference of ImW or more may occur between the recording power used for the second information layer after the first information layer has been fully recorded from the inner track to outer track, and the recording power used for the second information layer when the first information layer is unrecorded. Thus, under the condition that recorded areas and unrecorded areas randomly exist in the first information layer, if the second information layer is recorded at a recording power used when the first information layer is unrecorded, it results in the formation of good quality areas and poor quality areas recorded under the condition of poor recording sensitivity.

Generally, Tc is 42% to 43%, and Ta is 37% to 38%. The recording power for the second information layer is 12mW when the first information layer is recorded, while the recording power for the second information layer is 12.5mW when the first

information layer is unrecorded. The difference of the recording powers is relatively small, approximately 0.5mW.

The present invention can provide a single-side dual-layer optical recording medium allowed to be used for random recording, in which the second information layer can be recorded and reproduced without problems, not by changing the system, but by adjusting the condition of the optical recording medium, even though the recorded areas and the unrecorded areas randomly exist in the first information layer.

Examples

Hereinafter, with referring to Examples, the present invention will be explained in detail and the following Examples should not be construed as limiting the scope of the invention. (Example l)

On a first substrate made of polycarbonate resin having a diameter of 12 cm and an average thickness of 0.595 mm, in which a continuous wobbled groove having a groove depth of 26 nm, a groove width of 0.2 μm, and a track pitch of 0.40 μm is formed on a single-side, a first lower dielectric protective layer having a thickness of 44 nm and containing ZnS and SiO2 where 80 ^: 20 (mole%), a first recording layer having a thickness of 8.5 nm and containing Ago.2ln3.sSb69.8Te22Ge4.5, a first upper dielectric protective layer having a thickness of 15 nm

and containing ZnS and Siθ2 where ZnS^SiO ₂ = iH) ^' 2l) KmoleVo), an interfacial layer having a thickness of 15nm and containing Tiθ2, a semitransmissive reflective layer having a thickness of 10 nm and containing Ag, an optical adjustment layer having a thickness of 15 nm and containing Tiθ2 were formed in this order by magnetron sputtering in Ar gas atmosphere to obtain a first information layer.

Meanwhile, on a second substrate made of polycarbonate resin having a diameter of 12 cm and an average thickness of 0.600 mm, in which a continuous wobbled groove having a groove depth of 26 nm, a groove width of 0.2 μm, and a track pitch of 0.40 μm is formed on a single-side, a second reflective layer having a film thickness of 140 nm and containing AgBi (Bi 0.5 mass%), a second interfacial layer having a thickness of 3 nm and containing Nb ₂ O ₅ and ZrO ₂ where Nb ₂ O ₅ ^rO ₂ = 70:30 (mole%), a second upper dielectric protective layer having a thickness of 21 nm and containing ZnS and SiO ₂ where ZnS^SiO ₂ = 80^20 (mole%), a second recording layer having a thickness of 15 nm and containing Ago. ₂ In3.sSb69.8Te ₂₂ Ge-1.5, a second lower dielectric protective layer having a thickness of 65 nm and containing ZnS and SiO ₂ where ZnS^SiO ₂ = 80^20 (mole%) were formed in this order by magnetron sputtering in Ar gas atmosphere to obtain a second information layer.

Next, an ultraviolet curable resin (KARAYAD DVD 003M

manufactured by Nippon Kayaku Co., Ltd.) was coated on the surface of the optical adjustment layer, and bonded to the second lower dielectric protective layer, and then an ultraviolet light was irradiated from the first substrate side so as to cure the ultraviolet curable resin formed as an intermediate layer, thereby producing a single-side dual-layer phase-change optical disc having two information layers. The intermediate layer had a thickness of 25±3μm (as measured from the inner periphery to outer periphery). Next, a laser beam was irradiated from the first substrate side by an initialization apparatus, and the second recording layer and the first recording layer were initialized in this order. The initialization was performed in a manner that a laser beam (oscillation wavelength of 810±10nm) from a semiconductor laser device was focused on each recording layer by means of an objective lens with a numerical aperture NA of 0.55.

The second recording layer was initialized under the condition of an optical disc rotated by CLV (Constant Linear Velocity) Mode, a linear velocity of 3m/s, a feed per revolution of 36μm/rev., a radial position (a distance from the rotation center) of 22mm to 58mm, and an initialization power of 35OmW. The first recording layer was initialized under the condition of an optical disc rotated by CLV (Constant Linear Velocity) Mode, a linear velocity of 5m/s, a feed per revolution of

50μm/rev., a radial position (a distance from the rotation center^ of 23mm to 58mm, and an initialization power of 50OmW.

Before the intermediate layer was formed to obtain the dual-layer optical recording medium, the light transmittance of the first information layer alone was measured using a spectrophotometer, ETA Optik, manufactured by Steag- the light transmittance of crystal portion was 42.4%> ^' the light transmittance of amorphous portion before initialization was 37%. Therefore, the difference of the light transmittance was 5.4%. (Examples 2 to 7)

Single-side dual-layer phase-change optical discs were prepared and initialized in the same manner as in Example 1, except that the thickness of the intermediate layer and the groove depth of the second substrate were changed to the values as shown in Examples 2 to 7 in Table 1.

In the first information layer of each of these optical discs, a random pattern was recorded in a radial position in a range of 40 mm to 41 mm, with the shortest mark length of 0.2μm, at a recording linear velocity of 6.6m/s, and a recording power of 10 mW by an ETM (8- 12 modulation) modulation method.

Next, a random pattern was recorded in a radial position in a range of 39.5mm to 40.5mm in the second information layer.

Push pull signal characteristics (push pull signal) which is a groove signal characteristics of the first information layer

before recorded, that is, a value in which under focused beam condition, PD difference signal output was passed though a 30KHz lowpass filter to obtain a push pull signal amplitude, and it was normalized by PD sum signal, and values of defect signal amplitude "b" (tracking error signal amplitude "b"), jump-back signal amplitude "a", and ratio (b/a) are shown in Table 1. Additionally, values of groove depth and reflectivity are shown in Table 1.

When sequential recording was successful under the above condition in the above radius range, it was evaluated as OK, and when off-track occurred during the course of sequential recording, it was evaluated as NG. Subsequently, sequential tracking was performed in the above range, and no off-track was evaluated as OK, and occurrence of off-track was evaluated as NG.

The results are shown in Table 1. When the push pull signal was 0.22 or more and b/a was 0.35 or less, at least sequential recording was determined to be OK. (Comparative Examples 1 to 4) Single-side dual-layer phase-change optical discs were prepared and initialized in the same manner as in Example 1, except that the thickness of the intermediate layer and the groove depth of the second substrate were changed to the values as shown in Comparative Examples 1 to 4 in Table 1.

These optical discs were recorded and evaluated in the same manner as in Examples 2 to 7.

The results are shown in Table 1. The push pull signal of less than 0.22, and b/a of more than 0.35 resulted in NG.

Table 1

to