JP2005119116A - Optical recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical recording medium employing a phase change material mainly composed of GaSb in a recording layer, enabling initialization excellently and capable of corresponding to the linear recording speed of four times or higher at a recording density equal to DVD. <P>SOLUTION: The optical recording medium has a constitution wherein at least a lower protective layer, a recording layer composed of a phase change material, an upper protective layer and a reflecting layer are formed on a transparent base, and rewriting and recording are conducted by causing a phase change between a crystal phase and an amorphous phase of the recording layer by irradiation with a laser light. The phase change material is represented by the following composition formula: GaαSbβBiγ, 0.10<α<0.18, 0.01<γ<0.10, β=1-(α+γ), (wherein α, β, γ denote an atomic ratio). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical recording medium using a phase change material capable of high-speed recording and compatible with a wide range of recording linear speeds.

In recent years, optical discs (hereinafter referred to as phase change discs) using a phase change material as a recording layer have been actively developed.
In general, in a phase change optical disk, a specific groove is formed on a transparent plastic substrate, and a thin film is formed thereon. The plastic material used for the substrate is mainly polycarbonate, and injection molding is often used to form the grooves. The thin film formed on the substrate is a multilayer film, and basically includes a lower protective layer, a recording layer, an upper protective layer, and a reflective layer in order from the substrate side. Oxide in the lower and upper protective layer, a nitride, although such sulfides are used, often used ZnS-SiO ₂ mixed inter alia ZnS and SiO _2. A phase change material containing SbTe as a main component is often used for the recording layer. Specific examples include Ge—Sb—Te, In—Sb—Te, Ag—In—Sb—Te, Ge—In—Sb—Te, Ge—Sn—Sb—Te, and the like. Ge—Te, In—Sb, Ga—Sb, Ge—Sb, or the like is used. Although a metal material is used for the reflective layer, metal materials such as Al, Ag, Au, Cu, and their alloy materials are often used from the viewpoint of optical characteristics and thermal conductivity.
As a method for forming these multilayer films, various film forming methods such as resistance wire heating, electron beam vapor deposition, sputtering, and CVD can be used. Is often used. After forming these multilayer films, the resin layer is coated by spin coating in order to protect the thin film.

In the phase change optical disk thus manufactured, the phase change material used for the recording layer is in an amorphous state, and it is general to perform a so-called initialization process in which the phase change material is crystallized. This initialization is performed by irradiating laser light from a semiconductor laser having a width of several μm and a length of several tens to several hundreds of μm while rotating the disk, and moving the laser light in the radial direction. In many cases, laser beam irradiation is performed more efficiently by providing a focusing function.
The phase change optical disk thus manufactured can form an arbitrary amorphous mark by irradiating an arbitrarily determined laser emission pattern (hereinafter referred to as “strategy”). Furthermore, the phase change disk can perform so-called direct overwrite (hereinafter referred to as DOW) recording in which erasing and recording are performed simultaneously. Incidentally, erasing is to crystallize an amorphous mark, and recording is to form an amorphous mark from a crystalline state.
As a frequently used strategy, there is ternary control (Pw>Pe> Pb) of recording power (Pw), erasing power (Pe), and bias power (Pb). A specific mark length is recorded by combining these and various pulse widths. As the data recording / reproducing modulation system, the EFM modulation used in the CD and the EFM + modulation used in the DVD are mark edge recording systems, and therefore, the control of the mark length is very important. Jitter characteristics are generally used for evaluating the mark length control.

The phase change optical disk produced in this way is now widely used as a rewritable medium for DVD. There are three types of DVD rewritable media: DVD-RAM, DVD-RW, and DVD + RW. These recording capacities are all the same and are 4.7 GB, but the recording linear velocities are different. In particular, DVD + RW achieves a maximum recording linear velocity of 14 m / s, which is four times the standard linear velocity of DVD of 3.5 m / s, and is the fastest recording among rewritable DVDs. However, for the purpose of further reducing the data recording time, a medium capable of higher linear velocity recording has been actively developed in each method.
As a method for realizing high linear velocity recording, it is necessary that the phase change material used for the recording layer has a sufficiently high crystallization speed and that a crystalline state can be obtained even at a high linear velocity. The most effective way to increase the crystallization rate of the phase change material is to adjust the phase change material itself. For example, as a recording layer material used in a rewritable phase change optical disk that has been commercialized so far, Sb-Te system represented by Ag-In-Sb-Te and Ge-Sb-Te is mainly used. However, in this system, a method of increasing the amount of Sb or a method of adding In, Ga, Sn or the like for improving the crystallization speed can be used. However, these methods cause problems such as difficulty in crystallization by initialization and deterioration in storage reliability, and trade-off between these problems and high-speed recording becomes a problem. In particular, this tendency becomes more prominent when the recording density becomes high, and it is very difficult to realize a recording linear velocity of 4 times or more at a recording density equivalent to DVD.

Under these circumstances, the present inventors recently reported on a medium that uses GaSb as a recording layer material and achieves a recording linear velocity of 4 times or higher at a recording density equivalent to DVD (non-non-standard). Patent Document 1). GaSb itself has been reported as a phase change material having the possibility of high-speed recording so far (for example, Non-Patent Document 2), but the crystallization temperature is very high and crystallization by initialization is difficult. The overwrite characteristics, modulation degree, and storage reliability in high-speed recording at a recording density equivalent to DVD were not satisfied at the same time. The inventors pay attention to the composition ratio of Ga and Sb, and the crystallization temperature can be lowered to less than 200 ° C. by using the composition of Ga ₁₂ Sb ₈₈ (number is atomic%) which is a eutectic point from the phase diagram. I found. Furthermore, it was reported that both high-speed recording and storage reliability can be achieved, and the possibility of high-speed recording at a recording density equivalent to DVD was presented. However, as a result of repeated experiments, it was found that the Ga ₁₂ Sb ₈₈ composition has a large distribution in the periphery of the reflected signal after initialization, which becomes noise and degrades the jitter characteristics. The cause of this is considered to be that the crystal phase of Sb alone increases due to the large amount of Sb, and this phase itself appears in the reflected signal. For this reason, this problem cannot be solved by initialization conditions, methods, apparatuses, etc., and it is necessary to reexamine the composition ratio of Ga and Sb. As a result, it has been found that the above-mentioned problems can be solved within a range of Sb86% or less. However, there is a problem that the crystallization speed is also reduced due to the decrease in the amount of Sb, and high-speed recording itself becomes difficult.

Proceeding of The 14th Symposium on Phase Change Optical Information Storage PCOS2002 p. 11: “Characterization of GaSb Phase-Change Material for High-Speed Re-Writable Media”) Applied Optics / vol. 26, No22115 November 1987: “Phase-change Optical data storage in GaSb”.

  The present invention uses a phase change material containing GaSb as a main component for the recording layer, can be satisfactorily initialized, and is capable of supporting a recording linear velocity of 4 × or higher at a recording density equivalent to DVD. The purpose is to provide a recording medium.

The above problems are solved by the following inventions 1) to 4) (hereinafter referred to as the present invention 1 to 4).
1) At least a lower protective layer, a recording layer made of a phase change material, an upper protective layer, and a reflective layer are provided on a light-transmitting substrate, and the recording layer is separated into a crystalline phase and an amorphous phase by laser light irradiation. An optical recording medium on which rewrite recording is performed by causing a phase change, wherein the phase change material is represented by the following composition formula (where α, β, and γ are atomic ratios): recoding media.
GaαSbβBiγ
0.10 <α <0.18
0.01 <γ <0.10
β = 1− (α + γ)
2) The thickness of the lower protective layer is 50 to 100 nm, the thickness of the recording layer is 10 to 20 nm, the thickness of the upper protective layer is 3 to 15 nm, and the thickness of the reflective layer is 100 to 300 nm. 1) The optical recording medium as described.
3) Description of 1) or 2), wherein the material of the lower and upper protective layers in contact with the recording layer is composed of a mixture of ZnS and SiO ₂ and the mixing ratio Z (mol%) of SiO ₂ is in the following range. Optical recording media.
10 <Z <40
4) The substrate has meandering grooves having a groove pitch of 0.74 ± 0.03 μm, a groove depth of 22 to 40 nm, and a groove width of 0.2 to 0.4 μm. The optical recording medium described.

Hereinafter, the present invention will be described in detail.
As a result of investigations by the present inventors, the addition of Bi in the composition range specified in the present invention 1 or 2 to a GaSb alloy can eliminate a decrease in the crystallization rate due to an increase in Ga, and increase Ga. It has been found that there is no adverse effect on the distribution of the reflected signal in the circumference after the improvement. Although the details of the reason why the crystallization speed is improved by adding Bi are unknown, it is considered as follows.
When the phase change material changes from amorphous to crystalline, if the material contains a component that becomes a crystal nucleus, crystallization is considered to occur easily around the crystal nucleus. According to the phase diagram, Bi does not form an alloy with Ga and Sb, and Bi is considered to exist alone. It is known that Bi itself tends to be in a crystalline state, and Bi itself becomes a crystal nucleus, and as a result, it is considered that the crystallization speed is improved.
From the above, the crystallization speed is improved with the addition of Bi, and a phase change optical recording medium suitable for high-speed recording can be provided. However, when the amount of Bi added is excessive, crystallization is facilitated, and the storage characteristics are improved. An adverse effect comes out. Therefore, the composition range specified in the present invention 1 is desirable. More preferably, γ is in the range of 0.02 to 0.08.
The range specified in the present invention 1 is also desirable for the Ga content. When α is 0.10 or less, the crystallization speed is too fast, and when α is 0.18 or more, the crystallization speed is slow. More desirably, α is in the range of 0.12 to 0.16.

Next, the film thickness of each layer is preferably within the range specified in the present invention 2.
The lower protective layer has a function of adjusting the reflectance of the optical recording medium, and is preferably 50 to 100 nm, but more preferably 50 to 80 nm. If the thickness is less than 50 nm, it is difficult to produce stably because the change in reflectance with respect to the film thickness is large.
If the thickness of the recording layer is less than 10 nm, problems such as repeated deterioration of recording characteristics occur, and if it is more than 20 nm, the jitter characteristics of the initial recording are deteriorated. A more desirable film thickness is 12 to 18 nm. In the present invention, it has also been found that when the thickness of the lower protective layer is 70 nm or more, good recording characteristics can be obtained even if the recording layer is made as thin as 10 nm. This is presumably because the deterioration of the repeated recording characteristics caused by the thin recording layer can be prevented by increasing the thickness of the lower protective layer.
If the film thickness of the upper protective layer is less than 3 nm, problems such as poor recording sensitivity and a low degree of modulation occur. On the other hand, if it is thicker than 15 nm, the heat dissipation effect is lost, and the jitter characteristics and the repeated recording characteristics deteriorate. More desirably, it is 5 to 10 nm.

The thickness of the reflective layer is preferably in the range of 100 to 300 nm, more preferably in the range of 100 to 200 nm, and still more preferably in the range of 120 to 150 nm. If the thickness is less than 100 nm, the heat dissipation effect may not be obtained. Further, even if it is thicker than 300 nm, the heat dissipation effect does not change, and a film having a thickness that is not necessary is simply formed.
As the reflective layer material, metal materials such as Al, Ag, Au, and Cu and their alloy materials can be used from the viewpoint of optical characteristics and thermal conductivity. In particular, since a rapid cooling structure is desirable in the present invention, Ag having the highest thermal conductivity or alloys thereof are suitable. When Ag is used and a material containing sulfide is used for the upper protective layer, the sulfur sulfidation of Ag due to the sulfur component becomes a problem. Therefore, it is necessary to provide a sulfidation preventing layer between the upper protective layer and the reflective layer. As a material for the sulfidation prevention layer, it is necessary to use a material resistant to sulfidation, and specifically, a metal film such as Si or Al, a nitride such as SiN or AlN, a carbide such as SiC or TiC, or the like is used. . The film thickness is preferably about 2 to 5 nm. More desirably, the thickness is 3 to 5 nm. If the thickness is less than 2 nm, the effect of preventing sulfidation is likely to be lost, and if the thickness is more than 5 nm, the heat dissipation effect and optical influence may be increased.

Next, as a material for the lower protective layer and the upper protective layer, a mixture of ZnS and SiO ₂ is desirable. In the prior art, both protective layers use dielectric materials such as oxides, nitrides, sulfides and carbides, or mixtures thereof, and are composed of a single layer or a plurality of layers. In the present invention, it has been found that the recording characteristics are improved when a dielectric material composed of a mixture of ZnS and SiO ₂ contacts the recording layer. Although the reason for this is unknown, it is considered as follows.
Since GaSbBi has a high crystallization speed, it is considered that the crystallization state is instantaneously provided by providing an effect of promoting crystallization. Some protective layer materials have a crystallization promoting effect. When such a protective layer is in contact with GaSbBi, it is considered that the formation of an amorphous state is hindered and the recording characteristics are deteriorated. In particular, during repetitive recording, heat tends to be easily generated, and there is a tendency that amorphization becomes difficult. Therefore, it is considered that a mixture of ZnS and SiO ₂ having a relatively small crystallization promoting effect is suitable.
The mixing ratio of SiO ₂ was found that the range defined in the present invention 3 is desirable. More preferably, it is 20-30 mol%. If it is 10 mol% or less, ZnS crystallization occurs due to repeated recording or initialization, which inhibits the recording layer from becoming amorphous. On the other hand, if it is 40 mol% or more, the refractive index becomes small, and sufficient optical characteristics cannot be obtained.
By using the substrate having the groove defined in the present invention 4 as the substrate of the optical recording medium of the present invention 1 to 3 described above, DVD + RW capable of high-speed recording at 4 × speed or more in accordance with the current DVD + RW medium standard. Media can be provided. The purpose of meandering the groove is to access a specific unrecorded track and to rotate the substrate at a constant linear velocity.

According to the first and second aspects of the invention, it is possible to provide an optical recording medium that exhibits excellent recording characteristics with respect to high linear velocity recording.
According to the third aspect of the present invention, it is possible to provide an optical recording medium having further excellent repeated recording characteristics.
According to the fourth aspect of the present invention, it is possible to provide a DVD + RW medium capable of high-speed recording at a quadruple speed or higher.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further more concretely, this invention is not restrict | limited at all by these Examples. The effects of the present invention are not limited to the protective layer material and the reflective layer material used in the examples and comparative examples, the composition of the recording layer, the manufacturing apparatus, the manufacturing method and layer configuration, and the evaluation apparatus.

<Examples 1-4 and Comparative Examples 1-4>
FIG. 1 shows a schematic structure of an optical recording medium according to this embodiment.
A polycarbonate substrate having a groove shape of 120 mmφ, a thickness of 0.6 mm, a track pitch of 0.74 μm, a groove (concave) width of 0.3 μm, and a depth of about 30 nm was used.
The lower protective layer is made of ZnS (80 mol%) SiO ₂ (20 mol%) at a film formation rate of 9 nm / sec and a thickness of 60 nm, and the recording layer is formed of the phase change material shown in Table 1 at a film formation rate of 5 nm / sec. The upper protective layer is made of ZnS (80 mol%) SiO ₂ (20 mol%) at a film formation rate of 4 nm / sec and a thickness of 11 nm, and the sulfidation prevention layer is made of SiC at a film formation rate of 1 nm / sec and a thickness of 4 nm. In the reflective layer, Ag was deposited to a thickness of 140 nm at a deposition rate of 35 nm / sec. The reason why SiC is used as the sulfidation prevention layer is to prevent reaction between Ag as the reflection layer and sulfur contained in the upper protection layer.
An RF magnetron sputtering method was used for the film formation of ZnS (80 mol%) SiO ₂ (20 mol%), and a DC magnetron sputtering method was used for the film formation of the recording layer, SiC, and Ag.
Next, a UV curable resin was applied as an environmental protection layer, and finally a similar substrate (not shown) was bonded to obtain an optical recording medium having a thickness of about 1.2 mm.
Next, this recording medium is an initialization device (POP120-7AH manufactured by Hitachi CP) having a laser head in which a focusing function is added to laser light having an output wavelength of 830 nm, a width of about 1 μm, a length of about 75 μm, and a maximum output of about 2 W. It was initialized using. The initialization conditions were an initialization power of 1400 mW, a linear speed of 11 m / s, and a constant head feed speed of 37 μm.

The medium produced as described above was evaluated for DOW characteristics at a recording linear velocity of 28 m / s (equivalent to 8 × speed of DVD). The evaluation apparatus used was an optical disk evaluation apparatus (DDU-1000 manufactured by Pulstec Corp.) having a pickup with a wavelength of 650 nm and NA of 0.65. Evaluation was performed by recording on five adjacent tracks and reproducing the middle track. The recording method was a pulse modulation method, and the EFM + [8/16 (2,10) RLL] modulation method was used. The recording linear density was 0.267 μm / bit and recorded in the groove. Optimal conditions were used for the recording power Pw and the erasing power Pe. The bottom power Pb was constant at 0.1 mW.
The data to clock (Data to Clock) jitter σ / Tw (Tw: window width) of the signal thus recorded was measured and evaluated. With such a method, the change in jitter in one-time recording, two-time recording, ten-time recording, and 100-time recording was evaluated for each medium. The evaluation results are shown in FIG. 2. As can be seen from the figure, the medium of <Comparative Example> has poor initial characteristics and repeatability compared to the medium of <Example>.
From the above facts, it can be seen that the recording characteristics at a high linear velocity can be improved by employing the configuration of the present invention.

<Examples 5 to 13>
Except for using the material shown in Table 2 for the upper protective layer, an optical recording medium was prepared in the same manner as in <Example 3>, and the evaluation results in the same manner as in <Example 3> are shown in FIG. In Table 2 and FIG. 3, the data of Example 3 are also shown.
As can be seen from the results of FIG. 3, the mixing ratio of SiO ₂ in ZnSSiO ₂ is preferably in the range of 10 to 40 mol%, and, ZnSSiO ₂ of this range is superior to other materials.

1 is a diagram illustrating a schematic structure of an optical recording medium according to an example. The figure which shows the evaluation result of Examples 1-4 and Comparative Examples 1-4. The figure which shows the evaluation result of Example 3 and 5-13.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Board | substrate 2 Lower protective layer 3 Recording layer 4 Upper protective layer 5 Antisulfuration layer 6 Reflective layer 7 Environmental protective layer

Claims (4)

A transparent protective substrate has at least a lower protective layer, a recording layer made of a phase change material, an upper protective layer, and a reflective layer. The recording layer undergoes a phase change between a crystalline phase and an amorphous phase by laser light irradiation. An optical recording medium for performing rewrite recording by causing a phenomenon in which the phase change material is represented by the following composition formula (where α, β, and γ are atomic ratios): .
GaαSbβBiγ
0.10 <α <0.18
0.01 <γ <0.10
β = 1− (α + γ)   The thickness of the lower protective layer is 50 to 100 nm, the thickness of the recording layer is 10 to 20 nm, the thickness of the upper protective layer is 3 to 15 nm, and the thickness of the reflective layer is 100 to 300 nm. The optical recording medium according to 1. 3. The light according to claim 1, wherein the material of the lower and upper protective layers in contact with the recording layer is made of a mixture of ZnS and SiO ₂ , and the mixing ratio Z (mol%) of SiO ₂ is in the following range. recoding media.
10 <Z <40 4. The substrate according to claim 1, wherein the substrate has meandering grooves having a groove pitch of 0.74 ± 0.03 μm, a groove depth of 22 to 40 nm, and a groove width of 0.2 to 0.4 μm. Optical recording medium.

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