JPH0724116B2 - Light disk - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an optical disc, in particular a rewritable optical disc.

2. Description of the Related Art There have been attempts to increase the number of recording layers to increase the S / N ratio of recorded signals. The aim is to make maximum use of the interference effect of light. As shown in FIG. 1 (b), a transparent layer, a light absorbing layer, a transparent layer, and a light reflecting layer are sequentially formed on the substrate. A laminated structure or, conversely, a laminated structure of a light reflecting layer, a transparent layer, a light absorbing layer, and a transparent layer is used. The film thickness of each layer is designed to maximize the change in reflectance before and after recording based on the optical constants. In the case of the structure of FIG. 1 (b), light is incident from the substrate side. In the case of the reverse structure, light is incident from the protective material side.

In this structure, the reflection layer is used mainly for the purpose of increasing the light absorption effect, and generally a metal thin film having a high light reflection coefficient such as Al and Au is uniformly formed.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention By the way, when an attempt is made to record a signal by irradiating a laser beam on a disc rotating at a constant number of rotations, the outer peripheral portion of the disc is naturally rotated at a higher rotational speed (peripheral speed) than the inner peripheral portion. Speed)
Therefore, even if light with the same pulse width is irradiated, the amount of light received per unit area in the outer peripheral portion of the disc is smaller than that in the inner peripheral portion, and it is difficult to raise the temperature.

This means that the outer peripheral portion of the disc has a lower recording sensitivity than the inner peripheral portion. As a means to overcome this, it is usual to change the laser power applied to the inner circumference and the outer circumference to increase the irradiation power toward the outer circumference, but for example, a chalcogenide glass thin film such as GeTe is used as the light absorption layer. In the case of constructing a rewritable optical disk that utilizes a reversible phase change between an amorphous phase and a crystalline phase, this method alone is not sufficient.

In other words, in order to change the amorphous phase to the crystalline phase, it is necessary to gradually hold the temperature above the glass transition temperature until the atomic arrangement is adjusted and the crystallinity is restored. Is shorter, the time required for this crystallization tends to be short at the same time, and crystallization is much less likely to occur than in the inner circumference. In order to lengthen the irradiation time, when crystallizing, a long and narrow light spot is used in the rotation direction of the disk and the relative irradiation time is lengthened, but tracking servo, optical transmission efficiency, etc. From the viewpoint of the above, since the length of the light spot is limited and cannot be unnecessarily lengthened, the relative irradiation time difference between the inner and outer peripheries cannot be eliminated even by this.

Means for Solving the Problems The present invention focuses on the thermal diffusivity of the light reflecting layer in order to solve the above problems, and the thermal diffusivity is reflected so that the thermal diffusivity continuously decreases from the inner circumference to the outer circumference of the disc. The layer thickness and material composition are formed.

Operation As a result, heat is less likely to escape in the outer peripheral portion than in the inner peripheral portion, and even if the same light is absorbed, it is possible to keep the temperature relatively high for a longer period of time. That is, the difference in sensitivity between the inner and outer circumferences can be reduced.

Practical Example FIG. 1 shows the configuration of an embodiment of the optical disc of the present invention. An optical disc is constructed by laminating a transparent body layer 2, a light absorbing layer 4, a transparent body layer 2 and a reflective layer in this order on a disc-shaped substrate 1, and a protective material 5 similar to that of the substrate is used on it with an adhesive. And fit them together. The order of stacking may be the reverse of the above. However, in that case, the incident direction of the laser beam is on the protective substrate side.

In the case of the structure shown in FIG. 1, the substrate needs to be transparent, and may be a glass, PMMA, polycarbonate or the like used for a normal optical disk. The function of the transparent layer is to sandwich the light absorbing layer, suppress destruction such as evaporation, and increase the light absorption efficiency by selecting an appropriate film thickness, and at the same time increase the S / N. Inorganic compound films such as oxides and sulfides, such as SiO ₂ and A
l ₂ O ₃ , ZnS or the like is used.

The reflective layer has a function of increasing the light absorption efficiency and a function of reducing the sensitivity difference between the inner and outer circumferences as described above.

There are the following two methods for reducing the sensitivity difference between the inner and outer circumferences of the reflective layer.

One is a method of continuously changing the thermal diffusivity K K = λ / ρC (λ: thermal conductivity, ρ: density, C: specific heat) of the reflective layer on the inner and outer circumferences. This is easily achieved by continuously changing from a composition in which heat easily escapes to a composition in which heat does not easily escape from the inner circumference to the outer circumference. As the material, one that can change its thermal diffusivity continuously with the change in composition, for example, an alloy thin film of Au and Cr is used, and the above effect can be obtained by increasing the Cr concentration in the outer circumference compared to the inner circumference. be able to.

"For example, the change in the thermal diffusivity due to the difference in the composition of the reflective film is
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The other is a method of continuously changing the heat capacity of the reflective layer on the inner and outer circumferences. The rate at which heat is diffused to the surroundings when irradiated with light is determined by the above-mentioned thermal diffusivity, but if the heat capacity of the reflective layer is small, it approaches a steady state thermally, and the heat gradually increases. The spread of is reduced. In this case, it is not necessary to use an alloy as the material, and Au, Al, Ni, Cr, Fe, Co, Pt or the like can be usually used as the metal material used for the reflective layer. Of course, these alloys can be used.

As the light absorbing layer, both a write-once type material (irreversible material) and a rewritable material (reversible material) are effective, but a rewritable type is more effective. The write-once materials, TeOx, TeOx-Pd, TeOx -Au, Se-Te-Pb, CS 2 -Te, Te-
In addition to Te-based compounds such as C- (H), dye-based materials, Bi, Au
A metal thin film such as is used. T as a rewritable material
A chalcogenide glass thin film based on e, Te, a magnetic thin film in which a rare earth element and a transition metal element are combined, and the like can be used.

Each layer other than the reflective layer constituting the optical disk of the present invention is uniformly formed on the disk surface by a method suitable for the composition such as vacuum deposition, sputtering, CVD, and plasma polymerization.

When a uniform film is formed by vapor deposition from a point source on a rotating disk substrate, the difference in rotational speed between the inner and outer circumferences is taken into consideration, and generally, the outer circumference has a shape as shown in FIG. A narrow mask is used, but when forming the reflective layer, a mask having a shape in which the film thickness is intentionally changed is used as described below.

Example 1 A PMMA resin substrate having a diameter of 200 mm and a thickness of 1.2 mm was used as a substrate,
While rotating it, ZnS was deposited on it about 850Å, Te ₄₅ Ge ₁₅ Sb
_{A 30} Se ₁₀ quaternary film was formed by depositing approximately 400 Å and ZnS at 1700 Å one after another. The degree of vacuum is ~ 10 ^-5 Torr.

Next, Au was evaporated as a reflective layer through the mask shown in FIG. The positional relationship between the source, substrate and mask is the third
This is as shown in FIG. At this time, the film thickness of the reflective layer continuously changed in inverse proportion to the peripheral speed, and when the position of φ100 was set to 1, it could be halved at the position of φ200.

Example 2 When the composition itself of the reflective layer is continuously changed, for example, when an AuCr alloy is used, co-evaporation is performed from two cases of Au and Cr.

As in Example 1, an Au—Cr alloy layer was formed in the positional relationship shown in FIG. 5 on the sequentially stacked discs of ZnS, Te ₄₅ Ge ₁₅ Sb ₃₀ Se ₁₀ , and ZnS. At this time, the Au side mask is selected such that the entire surface is vapor-deposited with a uniform film thickness as shown in FIG. 2, and the Cr side mask is the outer peripheral side as the inner peripheral side as shown in FIG. By using a mask that is relatively wider than the mask on the Au side compared to the above, an Au-Cr reflective layer in which the Cr concentration continuously increases from the inner peripheral portion to the outer peripheral portion can be obtained. .

The Cr concentration gradient can be freely selected by adjusting the opening degree of the mask. The vapor deposition rate is controlled so that the Cr concentration is 5 at% at the inner circumference, and the Cr concentration is 15 at% at the outer circumference.
Designed to be.

Example 3 The disks of Examples 1 and 2 were laminated with the same PMMA resin plate as the substrate using an epoxy adhesive to complete a disk having a close contact structure. Table 1 shows the results of sensitivity comparison.
No. 1 in the table is of the conventional structure, and disc Nos. 2 and 3 are of Example 1
It corresponds to 2.

The evaluation methods are the minimum recording power (recording spot is 0.9 μmφ) and the lowest bottom erasing power (erasing spot is 8 μm × 1 μm oval) when rotating at 900 rpm. Both the recording and erasing spots have a Gaussian power intensity distribution.

From Table 1, it was found that Examples 1 and 2 significantly reduced the difference in sensitivity between the inner and outer circumferences.

EFFECTS OF THE INVENTION As described above, the present invention enables the construction of an optical disc having a small difference in sensitivity between the inner and outer circumferences.

[Brief description of drawings]

1 (a) and 1 (b) are a plan view and a cross-sectional view of an embodiment of an optical disc of the present invention, and FIG. 2 is a plan view of a vapor deposition mask used for uniformly forming a reflection layer on both the inner and outer circumferences, 3 (a) and 3 (b) are plan views of a vapor deposition mask for continuously thinning the film thickness of the reflection layer from the inner circumference to the outer circumference, and FIG. 3 (c) is the position with respect to the source and the disk substrate. FIG. 4 is a plan view showing the shape of a vapor deposition mask for forming a reflective layer of an optical disk of the present invention continuously and thickly from the inner circumference to the outer circumference, FIG. 4B is a plan view of the mask of the optical disc in the embodiment of the present invention, and FIG. 8B is a diagram showing the positional relationship between the mask and the substrate.

Claims (4)

[Claims] 1. A light-absorbing layer which absorbs light, resulting in a physical or chemical change, a light-reflecting layer for increasing the light-absorbing efficiency of the light-absorbing layer, and an intermediate layer between these two layers. In an optical disc having a structure in which a transparent material layer for adjusting the optical path length of light and increasing the amount of reflected light before and after recording is provided on a disc substrate, the thermal diffusivity or heat capacity of the reflective layer is An optical disc, wherein the optical disc is formed so as to decrease continuously from the innermost peripheral portion to the outermost peripheral portion. 2. A reflective layer material of Au ^100- xCrx (0 <x <10
The optical disk according to claim 1, wherein an alloy thin film is used and the Cr concentration x is continuously increased from the innermost peripheral portion of the disk toward the outermost peripheral portion thereof. 3. The optical disk according to claim 1, wherein the reflective layer material is formed so that the film thickness becomes thinner from the innermost peripheral portion of the disk toward the outermost peripheral portion thereof. 4. The optical disk according to claim 1, wherein the light absorption layer is composed of a material thin film based on a reversible phase transformation between a crystalline phase and an amorphous phase.