JP2967949B2 - Optical information recording medium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-speed, high-speed,
The present invention relates to a high-density and overwritable optical information recording medium.

[0002]

2. Description of the Related Art In recent years, as the amount of information has increased, there has been a demand for a recording medium capable of recording and reproducing a large amount of data at a high density and at a high speed. However, an optical disk is expected to exactly meet such a use. I have. Optical discs include a write-once type, which allows recording only once, and a rewritable type, which allows recording / erasing any number of times.

[0003] Examples of the rewritable optical disk include a magneto-optical recording medium utilizing a magneto-optical effect and a phase change medium utilizing a reflectance change accompanying a reversible change in crystal state. The phase change medium does not require an external magnetic field, and has the advantage that recording and erasing can be performed only by modulating the power of laser light, and the recording and reproducing apparatus can be downsized. Furthermore, so-called one-beam overwriting, in which erasing and re-recording are performed simultaneously with a single laser light beam, is possible.

As a recording layer material of such a phase change medium capable of one-beam overwriting, a chalcogen alloy thin film is often used. For example, GeTeSb
System, InSbTe system, GeSnTe system and the like.
In an actual medium, the recording layer is usually sandwiched between dielectric layers to prevent repetitive deterioration due to overwriting, and to improve the reflectance difference (contrast) by using an interference effect.

Generally, in a rewritable phase change recording medium, an unrecorded / erased state is changed to a crystalline state, and an amorphous bit is formed. The amorphous bit is formed by heating the recording layer to a temperature higher than the melting point and rapidly cooling the recording layer. In this case, the dielectric layer functions as a heat radiation layer for obtaining a sufficient supercooled state. On the other hand, erasing (crystallization) is performed by heating the recording layer to a temperature higher than the crystallization temperature of the recording layer and lower than the melting point.
In this case, the dielectric layer functions as a heat storage layer for keeping the temperature of the recording layer high until crystallization is completed.

In order to prevent deformation of the recording layer due to melting and volume expansion during heating and cooling as described above, to prevent thermal damage to the plastic substrate, and to prevent deterioration of the recording layer due to moisture, the above dielectric material is used. The protective layer consisting of layers is important. The protective layer material is transparent at the laser beam wavelength, has a high melting point and excellent heat resistance, is chemically stable against oxidation and the like, has an appropriate thermal conductivity, and is easy to form. Must meet a wide variety of requirements. For the practical use of a phase change medium, it is extremely important to select and improve this protective layer as well as the recording layer.

[0007]

The material for the protective layer is as follows:
A dielectric material having a high melting point and being mechanically and chemically stable is often used. However, due to reasons such as the protective layer not having sufficient heat resistance and mechanical strength, the recording layer, the protective layer, and the substrate may be deformed, cracked, or peeled during repeated recording / erasing. And the number of defects and noise increases with the number of repetitions of recording / erasing.

Whether or not a film having excellent physical properties as a protective layer largely depends on film forming conditions other than the material. For example, the present inventors have already shown that a medium having excellent repetition characteristics can be obtained by using tantalum oxide having a density of 7.25 g / cm ³ or more as the dielectric protective film. In addition, oxide / nitrides such as Si and Al, ZnS, ZnO, and mixtures thereof have been proposed as dielectric materials for the protective layer. (Japanese Patent Application Laid-Open Nos. 62-167090 and 63
These dielectric materials are generally formed by a sputtering method. However, in the case of a single oxide, nitride, sulfide or the like, the so-called atomicicening effect (J. Vac. Sc.
i. Tech. A7 (1989), 1105) tends to generate compressive stress, and this tendency becomes more pronounced as the pressure of the inert gas during sputtering is reduced.

According to the study of the present inventors, it has been found that the use of a dielectric film formed at a low pressure is less likely to cause deterioration due to repeated overwriting, but these films have extremely high compressive stress. Easy to blister and peel off,
Often there is a problem with stability over time. In particular, separation easily occurs between the upper dielectric protection layer and the recording layer, and it is necessary to reduce the compressive stress of the upper dielectric protection layer.

If the pressure during sputtering is increased to reduce the compressive stress, or if ZnS and a mixed film based on ZnS are used, swelling and peeling are less likely to occur.
The mechanical strength is not enough and the deterioration due to repeated overwriting is fast. Therefore, repeated overwrite characteristics,
Obtaining a dielectric protective layer that is excellent both in stability and aging stability is in opposition to each other, and the present inventors have conducted various studies to resolve this contradiction.

[0011]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a phase change type recording medium having a recording layer sandwiched between dielectric protection layers and a reflective layer provided on the recording layer. This layer is formed by laminating dielectric layers with different compressive stresses. The thickness of the dielectric layer with high compressive stress is made thinner than that of the dielectric layer with low compressive stress, and the heat resistance and mechanical properties of the dielectric layer are reduced. The compression stress of the upper dielectric protection layer is reduced as a whole without deteriorating the mechanical strength.

Hereinafter, the contents of the present invention will be described in more detail. Each layer of the present invention can be formed by a sputtering method or a vapor deposition method. However, it is preferable to use a sputtering method which is excellent in mass productivity and to form a film in an in-line apparatus which forms a film consistently in a vacuum. The thickness of each layer is selected so as to increase the reflectance difference (contrast) between the crystalline state and the amorphous state in consideration of the optical interference effect, in addition to the reasons described below.

Examples of the substrate include transparent resins such as polycarbonate, acrylic and polyolefin, and glass. The first dielectric protection layer provided on the substrate is preferably a dielectric excellent in adhesion to the substrate, heat resistance and mechanical strength. For example, a metal oxide film such as tantalum oxide, or oxidation of ZnS and other metals is preferable. And a mixture with a product. According to the study of the present inventors, since the first dielectric protective layer cannot expect the heat dissipation effect of the metal reflection layer, a dielectric having better heat resistance and mechanical strength than the upper protective layer is desirable.
For example, tantalum oxide is preferable because it has excellent adhesion to a substrate and is unlikely to swell or peel off. In particular, it is desirable to use a high-density and high-compression-stress film obtained by lowering the pressure during sputtering, and a high-compression-stress film of 2 × 10 ⁹ dyn / cm ² or more. Further, the thickness is desirably in the range of 100 ° to 5000 °. If the thickness is less than 100 °, the effect of preventing deformation of the substrate and the recording film is insufficient, and if it is more than 5000 °, cracks are likely to occur.

In the present invention, the material of the recording layer is not particularly limited, but Ge, Sb, Te
And the like. For example, ternary alloys such as InSbTe, GeSbTe, and GeSnTe, and alloys to which Ta, Co, Ag, and the like are further added. In particular, the GeSbTe ternary alloy system has a high crystallization rate, excellent stability over time of amorphous bits, and has practically sufficient characteristics. The thickness of the recording layer is usually 1
It is selected in the range of 00 ° to 1000 °. If the thickness of the recording layer is less than 100 °, a sufficient difference in reflectance between the crystalline state and the amorphous state cannot be obtained, while if it exceeds 1000 °, cracks are likely to occur.

Now, with respect to the second dielectric protection layer which is the point of the present invention, as the high compression stress dielectric layer, Si,
An oxide dielectric of a metal such as Al or Ta is desirable, and as the low compressive stress dielectric layer, ZnS or a mixture of ZnS and a dielectric of the above-mentioned metal oxide is desirable. When a metal oxide dielectric is mixed with ZnS, 5 to 5 metal oxides are used.
0 mol%, preferably about 10 to 30 mol% is good. In particular, as a metal oxide having a high compressive stress, tantalum oxide has excellent heat resistance, mechanical strength, and chemical stability, and is suitable as a dielectric layer used in the present invention. When a high compressive stress film of 2 × 10 ⁹ dyn / cm ² or more is used as tantalum oxide, the overwrite characteristics are further improved. Conversely, the compressive stress of ZnS or a mixture of ZnS and a metal oxide dielectric is desirably less than 2 × 10 ⁹ dyn / cm ² .

In order to improve the repetitive overwrite characteristics, the dielectric layer facing the recording layer exposed to a high temperature is made of a dielectric material having a high compressive stress, and the total thickness of the upper dielectric layer is set to 1000 ° or more. Is desirable. If the total film thickness is less than 1000 °, it is said that it is practically necessary 10 ⁵
The above repetitive overwriting cannot be tolerated. Further, when the thickness of the dielectric layer 4a is less than 500 °, the entire compressive stress can be reduced more effectively, and swelling and peeling can be prevented.

[0017]

The present invention will be further described below with reference to examples.
You. 1.2mm thick substrate with grooves on the surface
Was used. As a recording layer,
Ge ₁₄Sb₃₄Te₅₂(Atomic%) with a ternary alloy of D
A film was formed to a thickness of 700 ° by the C sputtering method. Also,
4mm thick UV curable resin for hard coat layer
Using.

Example 1 High compressive stress dielectric of the second dielectric protective layer facing the recording layer
150mm thick tantalum oxide as body layer, low pressure on it
1350 ° thick ZnS and SiO as compressive stress dielectric layer
_Two(SiO_Two20 mol%), and
(1) 1100 ° thick tan oxide as a dielectric protective layer
Tal was used. These dielectrics are radio frequency sputtering
(Frequency 13.56 MHz). During film formation
Was set to 0.28 Pa. Oxidation at this time
The compressive stress of tantalum is 5 × 10⁹dyn / cm^Two, ZnS
・ SiO_TwoThe compressive stress of the mixed film is 1.4 × 10⁹dyn /
cm ^TwoMet. The compressive stress is separately measured on the Si wafer.
Was formed with only a dielectric layer, and the thickness was determined from the warpage of the wafer.

The repetitive overwrite characteristic has a linear velocity of 10
m / s, recording power 15 mw, erasing power 8 mw, and a single frequency (4 MHz, duty 50%). After performing the overwriting a predetermined number of times, the C / N ratio was measured, and an erasing laser beam was applied once, and the erasing ratio was determined from the decrease in the carrier level. In the recording medium of the above embodiment, an initial CN / N ratio of 55 dB and an erase ratio of 25 dB can be obtained. The C / N ratio after repeated overwriting 10 ⁶ times is 5
0 dB and the erasing ratio were 22 dB, and extremely good repetition characteristics were obtained. When an acceleration test was performed on the medium under the conditions of a temperature of 85 ° C. and a relative humidity of 85% RH,
No increase in defects was observed even after 1000 hours, and no deterioration was observed in the C / N ratio, the erase ratio, and the repetition characteristics.

EXAMPLE 2 A 1350 ° -thick mixed film of ZnS and tantalum oxide (containing 20 mol% of tantalum oxide) as a low-compression-stress dielectric layer
And a recording medium was produced in the same manner as in Example 1 except for the above. The compressive stress of the mixed film is 1.7 × 10 ⁹ dy
n / cm ² . C / N ratio 56d as initial characteristics
B, an erasing ratio of 27 dB was obtained, and the C / N ratio after 10 ⁶ times was 50 dB, and the erasing ratio was 25 dB, showing good characteristics.
Further, even after 1000 hours of the accelerated test, the number of defects increased and C /
No deterioration of the N ratio, the erase ratio, and the repetition characteristics was observed.

COMPARATIVE EXAMPLE 1 The second dielectric protective layer was formed to a thickness of 1500 ° and a compressive stress of 5 × 10
Example 1 only ⁹ dyn / cm ² tantalum oxide
The same recording medium as in Example 1 was produced. Repeatability is 10
^Although it was good up to ^six times, in the accelerated test, peeling and swelling occurred between the recording layer and the upper tantalum oxide layer after 100 hours.

COMPARATIVE EXAMPLE 2 The second dielectric protective layer was formed to a thickness of 1500 ° and a compressive stress of 1.4 ×.
A recording medium was prepared in the same manner as in Example 1 except that only a 10 ⁹ dyn / cm ² ZnS and SiO ₂ mixed film was used. In the accelerated test, no deterioration was observed even after 1000 hours. In repeated overwriting, the initial C / N
Ratio 55dB is after 10 ⁶ times was reduced to 45dB.

Comparative Example 3 The thickness of the high compressive stress tantalum oxide dielectric layer on the side facing the recording layer of the second dielectric protective layer was 1000 °, and the thickness of the low compressive stress ZnS / SiO ₂ mixed film was 1000 nm. As 500Å
Otherwise, the same recording medium as in Example 1 was produced. Although the repetitive overwrite characteristics were good, peeling and swelling occurred 100 hours after the accelerated test.

COMPARATIVE EXAMPLE 4 The side of the second dielectric layer facing the recording layer was a mixed film of ZnS and SiO ₂ having a thickness of 150 ° and a thickness of 1350 was formed thereon.
Å, a tantalum oxide film having a compressive stress of 4 × 10 ⁹ dyn / cm ² was provided. Although the initial C / N ratio was 55 dB,
0 by ⁵ times repeated overwriting of degraded down to 45dB.

[0025]

As described above, by using the layer structure of the present invention, a phase change type information recording medium having excellent repetition characteristics and stability over time can be obtained.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Kenichi Uchino 1000 Kamoshita-cho, Midori-ku, Yokohama-shi, Kanagawa Prefecture Inside the Mitsubishi Chemical Research Institute (72) Inventor Takashi Ohno 1000 Kamoshita-cho, Midori-ku, Yokohama-shi, Kanagawa Prefecture Mitsubishi Chemical Inside the Research Institute, Inc. (72) Inventor Natsuko Suzuki 1000, Kamoshida-cho, Midori-ku, Yokohama-shi, Kanagawa Prefecture Inside the Mitsubishi Chemical Research Institute (56) References JP-A-3-73436 (JP, A)

Claims (4)

(57) [Claims] 1. A first dielectric protective layer on a transparent substrate, a recording layer for recording information using an optical change accompanying a reversible phase change between a crystal and an amorphous phase by irradiation with a laser beam, Second
In the optical information recording medium in which a dielectric protective layer and a reflective layer are sequentially formed, the second dielectric protective layer is a multilayer protective layer in which dielectric layers made of materials having different compressive stresses are laminated, and Among the layers constituting the second dielectric layer, a dielectric layer having a high compressive stress is provided on the side facing the recording layer, and the thickness thereof is smaller than that of the dielectric layer having a low compressive stress. Characteristic optical information recording medium. 2. A high-compression-stress dielectric layer facing the recording layer having a thickness of 5 out of the dielectric layers constituting the second dielectric protection layer.
Claim 1 characterized by being less than 00 °
The optical information recording medium according to the item. 3. The dielectric layer constituting the second dielectric protection layer, wherein the dielectric layer having a high compressive stress is tantalum oxide, and the dielectric layer having a low compressive stress is a mixture of ZnS and another dielectric. 3. The optical information recording medium according to claim 2, wherein the medium is a film composed of: 4. The optical device according to claim 3, wherein the film made of a mixture of ZnS and another dielectric is a film made of a mixture of ZnS and silicon oxide or tantalum oxide. Information recording medium.