JPH0757299A - Optical information recording medium - Google Patents

Abstract

PURPOSE:To obtain a high speed and high density optical information recording medium utilizing the reversible phase shift of the recording layer between crystal and amorphous phases by irradiation with laser light and capable of overwriting. CONSTITUTION:A 1st dielectric protective layer, a phase shift type recording layer, a 2nd dielectric protective layer and a reflecting layer are successively disposed on a transparent substrate to obtain the objective optical information recording medium. Both the dielectric protective layers are protective films each made of a film of a ZnS-metal oxide mixture having <50mol% metal oxide content in the side facing the recording layer and >=90mol% metal oxide content in the side farthest from the recording layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is for high speed, high density and overwritable optical information recording utilizing the reversible phase change of a recording layer between crystalline and amorphous by irradiation of laser light. It concerns media.

[0002]

2. Description of the Related Art In recent years, a recording medium capable of recording and reproducing a large amount of data at high density and at high speed has been demanded as the amount of information has increased. Optical discs are expected to meet such applications. There is. Optical disks are classified into a write-once type that allows recording only once and a rewritable type that allows recording / erasing as many times as desired. Examples of the rewritable optical disk include a magneto-optical recording medium that utilizes the magneto-optical effect, and a phase change medium that utilizes the reflectance change associated with the reversible change of the crystalline state.

The phase change medium does not require an external magnetic field, and recording / erasing can be performed only by modulating the power of laser light, and the recording / reproducing apparatus can be miniaturized. Furthermore, so-called 1-beam overwriting is possible, in which erasing and re-recording are performed simultaneously with a single laser light beam. A chalcogen-based alloy thin film is often used as the recording layer material of such a phase change medium capable of one-beam overwriting. For example, GeTeSb system, InS
Examples thereof include bTe type and GeSnTe type.

In an actual medium, a recording layer is sandwiched between dielectric layers to prevent deterioration due to repeated overwriting, and an interference effect is utilized to improve a reflectance difference (contrast). . Generally, in a rewritable phase change recording medium, an unrecorded / erased state is set to a crystalline state to form an amorphous bit. Amorphous bits are formed by heating the recording layer to a temperature higher than the melting point and quenching.

In this case, the dielectric layer functions as a heat dissipation layer for obtaining a sufficient supercooled state. On the other hand, erase (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 that keeps the temperature of the recording layer at a high temperature until crystallization is completed. In order to prevent deformation due to melting and volume expansion of the recording layer during heating / cooling as described above, thermal damage to the plastic substrate, and deterioration of the recording layer due to moisture, the dielectric layer is used. The protective layer is important.

The protective layer material is transparent at the laser light wavelength, has a high melting point and excellent heat resistance, is chemically stable against oxidation and the like, has an appropriate thermal conductivity, and further has a film property. It must meet a wide range of requirements, including ease of formation. In order to put the phase-change medium into practical use, it is extremely important to select and improve the protective layer as well as the recording layer.

[0007]

As the protective layer material,
A dielectric material having a high melting point and mechanically and chemically stable is often used. However, due to the reason that the protective layer does not have sufficient heat resistance and mechanical strength, the recording layer, the protective layer, and the substrate may be deformed or cracked or peeled during repeated recording and erasing. However, there is a problem that defects and noise increase with the number of times recording / erasing is repeated.

Whether or not the film has excellent physical properties as the protective layer depends largely on the film forming conditions in addition to the material.
For example, the present inventors have already shown that if tantalum oxide having a density of 7.25 g / cm ³ or more is used as the dielectric protective film, a medium having excellent repeating characteristics can be obtained. Other dielectric materials used for the protective layer include Si, A
Oxides and nitrides such as l, ZnS, ZnO, and mixtures thereof have been proposed (JP-A-62-167090,
63-102048, 63-276724).

Generally, these dielectric materials are formed by a sputtering method, but with a single oxide, nitride, sulfide, etc., the so-called atomic peening effect (J. Vac. Sci. Technol., A7 (1989), 1105) tends to generate compressive stress, and this tendency becomes more remarkable as the pressure of the inert gas during sputtering is lowered. According to the study of the present inventors, it was found that the dielectric film formed at a low pressure was less deteriorated by repeated overwrite, but these films had a very high compressive stress, and thus the swelling and Peeling is likely to occur and there is often a problem with stability over time.

In particular, peeling easily occurs between the upper dielectric protective layer and the recording layer, it is necessary to reduce the compressive stress of the upper dielectric protective layer and improve the adhesive strength between the recording layer and the upper dielectric protective layer. there were. If the pressure during sputtering is increased to reduce the compressive stress, blistering and peeling are less likely to occur,
Mechanical strength is not sufficient and deterioration due to repeated overwriting is rapid. A film made of a mixture of ZnS and a metal oxide has low compressive stress and good adhesion to the recording layer, but is inferior in repeatability to a tantalum oxide film having high compressive stress.

Therefore, it is generally in a trade-off relationship with each other to obtain a dielectric protective layer excellent in both repeated overwrite characteristics and stability over time.

[0012]

In order to solve the above-mentioned problems, the inventors of the present invention sandwich the recording layer with a dielectric protective layer having a graded composition (step composition) to obtain characteristics of the interface with the recording layer. The present invention has been achieved as a result of extensive studies on a recording medium that can satisfy both the mechanical strength of the protective layer itself. The present invention is
A first dielectric protective layer, a phase change recording layer, and
In an optical information recording medium in which a second dielectric protective layer and a reflective layer are arranged in this order, the dielectric protective layer is made of a mixture of ZnS and a metal oxide, and the metal on the side facing the recording layer. The optical information recording medium is characterized in that the oxide content is less than 50 mol% and the metal oxide content on the surface of the dielectric protective layer farthest from the recording layer is 90 mol% or more.

The contents of the present invention will be described in detail below. The layer structure of the recording medium in the present invention is basically composed of a substrate, a first dielectric protective layer, a recording layer, a second dielectric protective layer, a metal reflective layer, and a hard coat layer made of a thermosetting or ultraviolet curable resin. Become. Each layer other than the hard coat layer can be formed by a sputtering method or a vapor deposition method, but it is desirable to use a sputtering method which excels in mass productivity and to form a film in an in-line apparatus for consistent film formation in a vacuum.

In addition to the reasons described below, 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. . Examples of the substrate include polycarbonate, acrylic such as polymethylmethacrylate, transparent resin such as amorphous polyolefin, and glass. In the present invention, the recording layer material is not particularly limited, but a chalcogen-based alloy thin film is often used as the recording layer.

For example, InSbTe, GeSbTe, Ge
Ternary alloys such as SnTe, and Ta, Co,
Examples include those to which Ag or the like has been added. In particular, GeSbT
The e-ternary alloy system has a high crystallization rate, excellent stability of an amorphous bit over time, and practically sufficient characteristics. The thickness of the recording layer is usually selected in the range of 100Å to 1000Å. If the thickness of the recording layer is less than 100 Å, a sufficient reflectance difference between the crystalline state and the amorphous state cannot be obtained, while 1
If it exceeds 000Å, cracks are likely to occur.

The total thickness of each dielectric protective layer is preferably in the range of 100Å to 5000Å. Thickness 10
If it is less than 0 Å, the effect of preventing deformation of the substrate or recording film is insufficient, and if it is 5000 Å or more, cracks are likely to occur.
The first and second dielectric protection layers are ZnS having a graded composition.
And a metal oxide mixed layer.

In order to improve the adhesion to the metal recording layer, the content of the metal oxide on the surface in contact with the recording layer is less than 50% mol. In that case, the mechanical strength is slightly inferior to that of the metal oxide itself. The film thickness of the portion of less than 50 mol% of the metal oxide is desirably less than 50Å, and it is not necessary to be more than 50Å to improve the adhesion at the interface. On the other hand, in order to maintain the mechanical strength of the protective layer as a whole, it is desirable that 90 mol% or more, usually substantially 100%, of the metal oxide is on the side of each protective layer that is not in contact with the recording layer. In order to form a mixed layer of ZnS and a metal oxide having such a gradient composition, a film having a metal oxide content of less than 50% mol is formed by sputtering, and then 90 mol% or more (substantially 100% %) Metal oxide film may be formed by sputtering or the like.

The thickness of the layer containing 90 mol% or more of metal oxide is preferably 50% or more of the thickness of each protective layer. Particularly, the interface of the first dielectric protective layer on the side in contact with the substrate is preferably a substantially 100% metal oxide dielectric having excellent heat resistance and mechanical strength and good adhesion to the glass or plastic substrate. According to the study by the present inventors, since the heat dissipation effect of the metal reflective layer cannot be expected in the first protective layer, a dielectric having higher heat resistance and mechanical strength than the upper protective layer is desirable. It is preferable that the ratio of the film thickness of the layer containing 90 mol% or more of the metal oxide be 90% or more. It is desirable to use a high-density film as the metal oxide in the portion which becomes a layer containing 90 mol% or more of metal oxide, and the density thereof is preferably 80% or more of the bulk density.

On the other hand, since the second dielectric protective layer is more likely to swell than the first dielectric protective layer, it is preferable that the second dielectric protective layer has a low compressive stress, and the compressive stress is 2 × 10 ⁹ dyn / c.
It is preferably less than m ² . In the gradient composition medium of the present invention, the composition ratio of the metal oxide and ZnS does not necessarily have to change continuously, and may change in steps.

For example, a layer of 20 mol% of metal oxide 50Å
A layer of 100% metal oxide and 1000Å may be laminated. However, ZnS 100% and metal oxide 1
A dielectric protective film consisting of two layers of 00% is not preferable because there is a problem in adhesion at the interface between the two layers, but a 50/50 mixture layer of ZnS and a metal oxide is interposed between the two layers. If so, the problem of adhesion is solved.

The gradient composition film of ZnS and metal oxide in the present invention is obtained, for example, as follows. (1) A target made of a metal oxide and a target having a desired mixed composition are prepared, and these targets are sequentially sputtered to obtain a dielectric film having a composition change in steps (gradient composition, step composition). To be

(2) A mixed dielectric film is formed by performing co-sputtering on two targets of metal oxide and ZnS while revolving the substrate. The composition ratio can be adjusted by changing the RF discharge power of each target. 90 mol
% Of the metal oxide is contained in the layer obtained by stopping the discharge of ZnS. Regarding the film thickness composition of each layer, an arbitrary gradient composition can be obtained by continuously changing the discharge power of each target with time or by changing it stepwise.

The same thing can be obtained by an electron beam evaporation method and an ion beam sputtering method using two targets of a metal oxide and ZnS.

[0024]

According to the present invention, by using the dielectric protective layer having the above graded composition, it is possible to provide a phase change medium having excellent stability over time and excellent repetitive overwrite characteristics.

[0025]

EXAMPLES The present invention will be described below with reference to examples. The layer structure of the examples described below is a structure of substrate / first dielectric protective layer / phase change recording layer / second dielectric protective layer / reflection layer / hard coat layer. As the substrate, a 1.2 mm-thick polycarbonate resin substrate having a groove on its surface was used. As the recording layer, a ternary alloy having a composition of Ge ₁₄ Sb ₃₄ Te ₅₂ (numerical values are component ratios, atomic%) was formed by the DC sputtering method. As the hard coat layer, an ultraviolet curable resin having a thickness of 4 μm was used.

Example 1 A target made of tantalum oxide (TaOx) was used as a first dielectric protective layer, and DC reactive sputtering was performed to obtain 100%.
A film was formed to a thickness of 0Å.

Next, Z containing 20 mol% of TaOx
A target made of a mixture of nS and TaOx (ZnS ₈₀ TaOx ₂₀ ) was used to form a film with a thickness of 30 Å by RF sputtering. After the recording layer was formed to a thickness of 700 Å, ZnS ₈₀ TaOx ₂₀ was used as a second dielectric protection layer.
Using a target consisting of 30 Å, followed by Ta
A film of 1500 Å was formed using a target made of Ox.

Finally, a 1000 Å film was formed using a target made of an Al alloy to form a reflective layer, and the ultraviolet curable resin layer was provided thereon. These dielectrics were created by the high frequency sputtering (frequency 13.56 MHz) method. The density of the layer using the tantalum oxide target was 85% of the bulk density.

The repetitive overwrite characteristic has a linear velocity of 10
m / s, recording power 15 mw, erasing power 8 mw and single frequency (4 MHz, duty 50%). After performing a predetermined number of overwrites, the C / N ratio was measured, only the erase power was irradiated once, and the erase ratio was obtained from the decrease in the carrier level. In the recording medium of the above embodiment, an initial C / N ratio of 55 dB and an erasing ratio of 26 dB can be obtained. C / N ratio after repeated overwrite 1 × 10 ⁶ times is 50
The erasing ratio was 26 dB and the erasing ratio was 26 dB. Burst defects did not occur and very good repeating characteristics were obtained. Further, when this medium was subjected to an acceleration test under the conditions of 85 ° C. and 85% relative humidity, no increase in defects was observed even after 1000 hours, and the C / N ratio, the erasing ratio, and the repetition ratio were repeated. No deterioration was observed in the characteristics.

Example 2 The thickness of the recording layer of Example 1 was set to 200Å, and the first dielectric protection was performed.
The layer was formed in the same manner as in Example 1. The second dielectric layer is first Z
nS₈₀TaOx₂₀After forming a film of 30Å, TaOx film 1
A film of 50 Å was formed. Initial C / N 55dB, erase ratio 28d
10 for B ⁶C / N after repeated overwriting
Is 50 dB, the erase ratio is 28 dB, and burst defects are also
It did not occur and had good characteristics. In addition, the acceleration test 1000
After time, increase of defects, C / N, erase ratio,
No deterioration was seen.

Example 3 Except that silicon oxide (SiOx) was used as the metal oxide of the mixture layer of the dielectric protection layer, the composition ratio with ZnS and the thickness of each mixed layer were exactly the same as in Example 1. It was configured. As initial characteristics, a C / N ratio of 56 dB and an erase ratio of 27 dB were obtained, and after 10 ⁶ times, the C / N ratio was 50 dB and the erase ratio was 25 d.
B and good characteristics were obtained. Also, acceleration test 1000
No increase in defects, deterioration of C / N, erasing ratio, and repetitive characteristics were observed even after the elapse of time.

Comparative Example 1 A recording medium was manufactured in the same manner as in Example 1 except that the second dielectric protective layer was a tantalum oxide film having a thickness of 1500Å only. The repeatability was good up to 10 ⁶ times, but in the accelerated test, peeling and blistering occurred between the recording layer and the second dielectric protective layer after 100 hours.

Comparative Example 2 A recording medium was prepared in the same manner as in Example 1 except that the second dielectric protective layer was a 200 Å-thick ZnS / SiO ₂ mixed film. In the accelerated test, no deterioration was observed even after 1000 hours. In repeated overwriting, the initial C / N ratio of 55 dB dropped to 46 dB after 1 × 10 ⁶ times.

Comparative Example 3 The second dielectric protective layer was formed of ZnS ₈₀ SiO _{2 20} having a thickness of 100 Å.
And a 50 Å SiO ₂ film, and having the same layer structure as in Example 3 except for the above, C / after repeated overwrite
The decrease in N ratio was remarkable.

[0035]

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

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Hidemi Yoshida Inventor Hidemi Yoshida 1000 Kamoshida-cho, Midori-ku, Yokohama, Kanagawa Sanryo Kasei Co., Ltd.

Claims (4)

[Claims] 1. An optical information recording medium comprising a transparent substrate on which a first dielectric protective layer, a phase change recording layer, a second dielectric protective layer, and a reflective layer are arranged in this order. The dielectric protective layer is composed of a mixture of ZnS and a metal oxide,
An optical characteristic in which the metal oxide content on the side facing the recording layer is less than 50 mol% and the metal oxide content on the surface of the dielectric protective layer farthest from the recording layer is 90 mol% or more. Information recording medium. 2. Each of the dielectric protective layers has a film thickness of less than 50 Å in a portion having a metal oxide content of less than 50 mol% and 100 Å or more in a portion having a metal oxide content of 90 mol% or more. And the total thickness of the dielectric protection layer is 5
The optical information recording medium according to claim 1, which is 0% or more. 3. The dielectric protective layer has a metal oxide content of 90 m.
The optical information recording medium according to claim 1, wherein the density of the portion of ol% or more is 80% or more of the bulk density. 4. The optical information recording medium according to claim 1, wherein the metal oxide is tantalum oxide.