JPH05101442A - Optical recording medium - Google Patents

Abstract

PURPOSE:To stabilize recording by providing the protective layers on a light incident side and light transparent side of a low thermal conductivity in contact with this recording layer on the light incident side and light transparent side of the recording layer and providing cooling layers on the outer sides of these protective layers. CONSTITUTION:The cooling layers 8, 11 having the high thermal conductivity are provided via the protective layers 9, 10 having the low thermal conductivity respectively on both sides of the recording layer 3, i.e., on the light incident side and the light transparent side. Then, the diffusion of heat is efficiently accelerated and the trailing of an isothermal curve is suppressed as far as possible while the unnecessary spread of the recording marks is prevented by the respective protective layers. The thermal interference between the recording marks is decreased in this way and jitters can be decreased. Since the trailing of the isothermal curve can be suppressed in such a manner, the stable recording is eventually attained and the recording level of the reproduced signals is stabilized. Further, the CN can be taken at a higher level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention comprises a recording layer having its optical properties reversibly changed by means of light, heat or the like on a substrate, and rewriting information by utilizing the change of the optical properties. The present invention relates to a rewritable optical recording medium for reproduction, and particularly to an optical recording medium which has excellent cooling efficiency, excellent CN ratio and erasing rate, small jitter, and can be rewritten many times.

[0002]

As a rewritable optical recording method for information, a phase change optical recording medium and a magneto-optical recording medium are known depending on the recording method, and a phase separation optical recording medium proposed by the applicant of the present application. (Japanese Patent Application No. 2-169,710) is known.

Here, in the phase change optical recording medium, the recording layer is irradiated with a high-power laser beam, and the irradiated portion is melted and rapidly cooled to change to an amorphous state, thereby recording information. Further, by irradiating the recording layer with an intermediate output laser beam and maintaining the irradiated portion at a crystallization temperature or higher for a predetermined time, the state is changed to a crystalline state, thereby erasing information, and a low output laser beam. Information is reproduced by irradiating the recording layer with light and detecting the difference in optical properties between the crystalline state and the amorphous state as the difference in the amount of reflected light.

Further, the magneto-optical recording medium is irradiated with a light spot in a state where a magnetic field in a predetermined direction is applied to a recording layer made of a magnetic material, and the Kerr rotation angle is changed by reversing the magnetization direction of the irradiated portion. Information is recorded and reproduced by using this change in the Kerr rotation angle, and a light spot is irradiated onto the recording portion of the recording layer in the state of being reversed in the direction of the magnetic field from that during recording. Information is erased by returning the magnetization direction of the part to the state before recording.

Further, the phase-separated optical recording medium is provided with a recording layer whose optical properties are changed by phase separation on the substrate, and binodal decomposition or spinodal decomposition is caused to the recording layer by means of light, heat or the like. , Which selectively causes phase separation, and records, reproduces, and erases information by utilizing the difference in optical properties between the part where phase separation is performed and the part where phase separation is not performed. is there.

However, in each of these optical recording methods, it is necessary to heat to a high temperature at the time of rewriting information, but the heat given at the time of rewriting is difficult to escape, and the escape of this heat is incomplete. The thermal interference between the recording mark and the recording mark causes the recording material that forms the recording layer to become amorphized (recorded), resulting in unstable recording level of the reproduced signal and increased jitter in the reproduced waveform. ,
Further, there is a problem in that the CN ratio cannot be increased, and moreover, the grain size of the crystal is difficult to be uniform, so that the erasing rate is lowered.

This problem will be described below by taking a phase change optical recording medium as an example. As shown in FIG. 4, the structure of a typical conventional phase change optical recording medium is an inorganic material such as glass, or polycarbonate, polymethylmethacrylate (P
The substrate 1 formed of a resin material such as an acrylic resin such as MMA) has relatively low thermal conductivity in order to prevent the diffusion of heat generated by the irradiated laser beam L and concentrate the heat. A light incident side protective layer 2 formed of an inorganic dielectric material such as ZnS-SiO ₂ and laminated on the substrate 1.
And Ge-Sb-Te, In-Sb-Te, In-Ge
A recording layer 3 formed of a recording material such as -Sb-Te and laminated on the light incident side protective layer 2, and an inorganic dielectric material similar to the light incident side protective layer 2, and the recording layer 3 is laminated on the light transmission side protection layer 4, a reflection layer 5 formed of a material such as an aluminum alloy and laminated on the light transmission side protection layer 4, and is laminated on the reflection layer 5. The ultraviolet curable resin 6 and the reflective layer 5 by the ultraviolet curable resin 6
And a protective plate 7 formed of a resin material such as polycarbonate that is adhered to the upper surface (see, for example, Japanese Unexamined Patent Application Publication No. Hei 2
-270,145). Then, in such a phase change optical recording medium, the position of dust adhering to the medium surface on the light incident side is set as far as possible from the recording layer 3 on which the laser beam L is focused, and is introduced into the medium. In order to reduce the influence of dust that inevitably adheres to the medium surface on the laser light L as much as possible, the laser light L is usually irradiated from the substrate 1 side.

When the phase change optical recording medium of FIG. 4 is irradiated with a light spot for rewriting, the recording layer 3
Since the melting point of the recording material constituting the
The part of the recording mark irradiated with this light spot and formed on the recording layer 3 exceeds 600 ° C., and when the moving direction of this light spot is the direction of arrow A, the longitudinal section of the medium along this moving direction A is taken. 600 ℃, 400 ℃ on the surface
And the isothermal curve C at 200 ° C. are schematically shown in FIG. 5, and in particular, the so-called isothermal curve spread broadly behind the moving direction A of the light spot in the substrate 1 and the light incident side protective layer 2. The trailing phenomenon of C occurs.

The tailing phenomenon of the isothermal curve C is a cause of the thermal interference between the recording marks described above, which is one of the serious causes of various problems in the conventional medium structure. Is becoming

Therefore, it is conceivable that a material having a high thermal conductivity is selected as a material for forming the periphery of the recording layer and the heat applied to the recording layer at the time of rewriting is designed to escape easily. Is that the light spot of the isothermal curve required for rewriting when the light spot is irradiated at the time of rewriting spreads more than necessary due to the influence of the high thermal conductivity of the surrounding material, resulting in the widening of the recording mark and the lowering of the recording sensitivity. Cause another problem.

[0011]

Therefore, as a result of intensive studies to solve this problem, the present inventors have found that a reflection layer (cooling layer) having a large thermal conductivity only on the light transmission side of laser light or the like. ) Cannot solve this problem, and it is necessary to form cooling layers with high thermal conductivity on both sides of the recording layer, which are the light incident side and the light transmitting side, for efficient cooling. At the same time, they have found that it is necessary to provide a protective layer having a small thermal conductivity, which is in direct contact with the recording layer, on both sides of the recording layer at the same time to prevent the deterioration of the recording sensitivity due to the spread of the recording mark, and the present invention has been accomplished.

Therefore, an object of the present invention is to reduce the trailing phenomenon of the isothermal curve which occurs at the time of rewriting information without causing a new problem, and thereby to minimize the thermal interference between recording marks. To eliminate it, stabilize the record,
An object of the present invention is to provide an optical recording medium which stabilizes the recording level of a reproduction signal, has a small reproduction waveform jitter, is suitable for edge recording, has a large CN ratio, and has a high erasing rate.

Another object of the present invention is to reduce the trailing phenomenon of the isothermal curve that occurs at the time of rewriting information without causing a new problem, and thereby to reduce the recording sensitivity a large number of times. An object is to provide a rewritable optical recording medium.

[0014]

That is, according to the present invention, a recording layer whose optical properties are reversibly changed by means of light, heat or the like is provided on a substrate, and the change of the optical properties is utilized. In an optical recording medium for rewriting and reproducing information, the light incident side and the light transmitting side of the recording layer are respectively provided with protective layers on the light incident side and the light transmitting side having a small thermal conductivity which are in contact with the recording layer, The optical recording medium is provided with a cooling layer on the light incident side and a cooling layer on the light transmitting side, which has high thermal conductivity, outside the protective layers on the light incident side and the light transmitting side, respectively.

In the present invention, as a recording material constituting the central recording layer of the optical recording medium, for example, a phase change optical recording material will be described. TeO _x (Ge, Se added), In-Se, In -Sb, In-Te, Sb-T
e, Sb-Se, Sn-Te, Bi-Te, Bi-S
e, Te-N, Ge- Te, Ag-Zn, and a binary system material such as _{As 2 S 3, Ge-Sn} -Te, In-Se-T
e, In-Sb-Se, In-Se-Tl, Ge-Sb
-Te, Ge-Te-Tl, Ge-Te-Au, Ge-
Te-Cu, Ge-Te-Co, Ge-Te-Ni, S
b-Se-Bi, Sb-Se-Te, Sn-Se-Te
Ternary materials such as Ge-Te-Sb-Se, Ge-T
e-Sn-Au, Ge-Te-Sb-Cu, Ge-Te
-Sn-Su, In-Se-Tl-Co, In-Ge-
Sb-Te, Ge-Sb-Te-Co, Ge-Sb-T
Examples thereof include quaternary materials such as e-Pb. Of course, other than these phase change optical recording materials, for example, Li ₂ O
Oxides -SiO ₂ mixed system or the like - oxide mixture-based material and, Z
Oxide-oxide mixed material such as rO ₂ —ThO ₂ mixed system, halide-halide mixed material such as LiCl—NaCl mixed system, oxide or halogen such as Bi—Bi ₂ O ₃ mixed system In addition to non-stoichiometric compounds such as compounds, polymer blends that cause binodal decomposition or spinodal decomposition, and phase-separated recording materials such as organic materials that cause micro-phase separation, as well as magneto-optical recording media. it can. In applying the present invention, the most effective of these recording materials is a phase change optical recording material that needs to be heated to a high temperature of 600 ° C. or higher,
More preferably Ge-Sb-Te or In-Ge-Sb-
It is a phase change optical recording material such as Te.

The layer thickness of the recording layer formed of these recording materials is usually 5 to 200 nm, preferably 1
It is 0 to 40 nm. When the layer thickness of the recording layer becomes thicker than 200 nm, it becomes difficult to remarkably achieve the operational effects of the present invention of improving the number of times of rewriting and recording sensitivity. If the recording layer has a thickness of 5 nm or less, it is difficult to form a uniform film. Therefore, it is desirable that the thickness is 5 nm or more in the production methods such as vapor deposition and sputtering.

In the present invention, each layer on the light incident side (substrate,
As for the material forming the protective layer, the cooling layer, etc.), it is necessary for light to pass therethrough. Therefore, the extinction coefficient of light in the material is small, preferably 0.2 or less. The materials of the protective layers (light incident side and light transmitting side) provided in direct contact with both sides of the recording layer have high melting points because they are exposed to high temperatures, preferably 1,500 ° C. or higher. It is desirable that the thermal conductivity is small so that the recording mark does not spread and the recording sensitivity is not lowered. And
Regarding the materials of the cooling layers (light incident side and light transmitting side) provided on both sides of the recording layer, at least in order to eliminate the tailing phenomenon of the isothermal curve during irradiation of the light spot during rewriting, It is desirable that the thermal conductivity of the protective layer is higher than that of the protective layer, and preferably the thermal conductivity of the protective layer is higher by one digit or more. Further, it is desirable that the difference between the coefficients of thermal expansion of the materials forming the protective layer and the cooling layer located on the light incident side or the light transmitting side is as small as possible.

In particular, regarding the respective materials forming the protective layer and the cooling layer on the light incident side, the smaller the thermal conductivity of the protective layer is, the better, from the viewpoint of thermal conductivity, and the material forming the cooling layer. The larger the thermal conductivity, the better, and the larger the difference between the two, the better. Specifically, the thermal conductivity of the material of the protective layer is 10 W · m ⁻¹ · K ⁻¹ or less, preferably 3 W · m ⁻¹ · K ⁻¹ or less, and the material of the cooling layer is good. The thermal conductivity of 10 W · m ⁻¹ · K ⁻¹ or more, preferably 20 W · m ⁻¹ · K ⁻¹ or more, the difference between the two is 10 W · m ⁻¹ · K ⁻¹ or more, Preferably 15 Wm
^It is preferably ^-1 · K ^-1 or more. From the viewpoint of the coefficient of thermal expansion, the smaller the difference between the material of the protective layer and the material of the cooling layer is, the better, and preferably, both the materials themselves have a small coefficient of thermal expansion. Specifically, the difference in the coefficient of thermal expansion is 10 × 10 ⁻⁶ / d, although it depends on the material and layer thickness.
EG or less, preferably 7 × 10 ^-6 / deg or less, more preferably 3 × 10 ^-6 / deg or less, and the thermal expansion coefficient of both materials themselves is 10 × 10 ^-6 / deg or less, preferably 5 × 10 6. ^-6 / deg or less, more preferably 2 x 10 ^-6 /
It is preferably deg or less.

Considering the combination of the respective materials forming the protective layer and the cooling layer located on the light incident side from such a viewpoint, the protective layer materials are ZnS, SiO ₂ and SiO.
And a material containing a mixture of two or more selected from ZrO ₂ as a main component, and the material of the cooling layer is AlN, S
iC, BeO, Al ₂ O ₃ , Si ₃ N ₄ , In ₂ O ₃ ,
SnO ₂ , MgO, TiC, TiN, BN, Ta
₂ O ₅ , ZnO: one or two selected from Al and Si
A material containing a mixture of at least one kind as a main component is preferable. Si is a semiconductor rather than a dielectric, and although light absorption is a problem, the extinction coefficient at 800 nm is 0.
It is as small as about 2 and can be used as a cooling layer on the light incident side.

In most materials, it is believed that phonons (lattice vibrations) are responsible for the thermal conductivity. Therefore, for example, in the case of a crystalline material, a material having large crystal grains and few crystal grain boundaries has higher thermal conductivity.
Therefore, even if the same material is used, the thermal conductivity changes depending on the layer manufacturing method and layer manufacturing conditions. For example, when a layer of AlN is formed, the layer may be formed by a sputtering method using an AlN target, but it is more crystalline when formed by an ion plating method in which Al is ion plated in a nitrogen atmosphere. A good film can be formed and the thermal conductivity becomes large. Further, when a suitable sintering aid is added to fill the crystal grain boundaries, phonon scattering at the grain boundaries is reduced and thermal conductivity is increased. When the thermal conductivity is changed by the sintering aid, the protective layer and the cooling layer on the light incident side can be formed without changing the base material, depending on whether or not the sintering aid is used.

On the other hand, it is considered that the carriers responsible for heat conduction in In ₂ O ₃ and SnO ₂ are conduction electrons generated due to defects. Therefore, these In ₂ O
Rather than using ₃ or SnO ₂ individually, these 2
The thermal conductivity is higher when the seeds are mixed or other elements such as WO ₃ and MoO ₃ are added and used as a mixture.

Further, as for the materials of the protective layer and the cooling layer located on the light transmitting side, the materials can be selected and used in combination from the same viewpoint as in the case of the light incident side.
Further, the material of the cooling layer on the light transmitting side does not need to be particularly transparent, unlike the light incident side, and Al, Au, Ag, Cu can be used similarly to the reflective material forming the conventional reflective layer.
It is possible to use a metal material such as, or a mixed material thereof, or a material containing these materials as a main component, and also for the material forming the protective layer, ZnS having a smaller thermal conductivity than the material of the cooling layer, SiO ₂ , SiO, T
a ₂ O ₅ , ZrO ₂ , AlN, In ₂ O ₃ , S
A material containing, as a main component, one kind or a mixture of two or more kinds selected from inorganic dielectric materials such as nO ₂ , Al ₂ O ₃ and Si ₃ N ₄ can be used.

The substrate material for forming the above-mentioned substrate may be any conventionally known one and is not particularly limited. For example, an inorganic material such as glass, polycarbonate,
Resin materials such as acrylic resin, polyolefins, polydichloropentadiene, polyimide, and epoxy resin can be mentioned. The refractive index of these substrate materials is usually in the range of 1.45 to 1.6. The thickness of this substrate may be a thickness that does not cause deformation and can prevent the adverse effect of dust, and is, for example, 1.2 mm. Particularly, in the present invention, since the cooling layer is also provided on the light incident side, the cooling layer also exerts the function of protecting the substrate from the heat load. Therefore, it is suitable to use a resin material which is relatively weak against heat as a substrate material.

A protective plate is preferably provided on the light transmitting side via an ultraviolet curable resin, and examples of the ultraviolet curable resin include acrylate-based materials. Further, the protective plate is a substrate material. Similarly, there may be mentioned inorganic materials such as glass and resin materials such as polycarbonate, acrylic resin, polyolefins, polydichloropentadiene, polyimide and epoxy resin. In the present invention, the protective layer is in direct contact with each of the light-incident side and the light-transmissive side of the recording layer, and the cooling layers on the light-incident side and the light-transmissive side may be present outside thereof, for example, A layer for other purposes may be present between these protective layer and cooling layer.

The method for producing the recording layer, the protective layer on the light incident side and the protective layer on the light transmitting side, and the cooling layer in the optical recording medium of the present invention is not particularly limited, and it is conventional according to the characteristics of each layer. Various known methods such as vacuum deposition method, sputtering method, ion plating method, molecular beam epitaxy method (MBE), chemical vapor deposition method (CVD)
Etc. can be adopted.

[0026]

According to the present invention, since the cooling layer having high thermal conductivity is provided on both sides of the recording layer, that is, the light incident side and the light transmitting side, through the protective layers having low thermal conductivity, respectively.
Each protective layer prevents unnecessary spread of recording marks, efficiently promotes heat diffusion, and suppresses the tailing phenomenon of isothermal curves as much as possible, which results in thermal interference between recording marks. It is thought that it is possible to reduce the jitter and the jitter. In addition, the ability to suppress the tailing phenomenon of the isothermal curve in this way can achieve stable recording, stabilize the recording level of the reproduction signal, and
The ratio can be made large, and moreover, the crystal grain size of the crystal phase becomes uniform and the erasing rate can be increased. In addition, the fact that the tailing phenomenon of the isothermal curve can be suppressed as much as possible means that the heat distribution during rewriting approaches a symmetrical shape above and below the recording layer, and the occurrence of distortion during rewriting is suppressed. The durability is increased and the number of times of rewriting is improved.

[0027]

EXAMPLES The present invention will be specifically described below based on Examples and Comparative Examples.

Example 1 As shown in FIG. 1, 80 nm thick AlN [containing a sintering aid,
Thermal conductivity (bulk value): 170 W · m ⁻¹ · K ⁻¹ , thermal expansion coefficient (bulk value): 5 × 10 ⁻⁶ / deg] The light incident side cooling layer 8 having a thickness of 80 nm ( ZnS) ₈₀ (Si
O ₂ ) ₂₀ [heat conductivity: 0.7 W · m ⁻¹ · K ⁻¹ , coefficient of thermal expansion (bulk value): 6 × 10 ⁻⁶ / deg] light incident side protective layer 9 and thickness 20 nm Recording layer 3 made of Ge-Sb-Te, a light-transmitting side protective layer 10 made of (ZnS) ₈₀ (SiO ₂ ) ₂₀ having a thickness of 40 nm, and an Al alloy having a thickness of 100 nm [thermal conductivity (bulk value ): 230 W · m ⁻¹ · K ⁻¹ , coefficient of thermal expansion (bulk value): 20 × 10 ⁻⁶ / deg] and a light transmission side cooling layer 11 formed by sputtering, respectively.
Next, a 1.2 mm-thick polycarbonate protection plate 7 was adhered using an ultraviolet curable resin 6 to prepare an optical recording medium of Example 1.

Here, A, which is the material of the light incident side cooling layer 8,
The refractive index of 1N is about 1.9, which is substantially the same as the refractive index of about 2.0 of (ZnS) ₈₀ (SiO ₂ ) ₂₀ which is the material of the light incident side protective layer 9. As a result, when viewed optically, the (ZnS) ₈₀ (SiO ₂ ) ₂₀ layer and the AlN layer function almost equivalently, and the film thickness (total 170 nm) is about the same as the conventional one (comparative example below). Light reflectance comparable to conventional
The light absorption rate was secured. Although the thermal conductivity of AlN, which is the material of the light-incident-side cooling layer 8, is reduced by making it thinner, it is still the material of the light-incident-side protective layer 9 (Zn.
It is expected that the thermal conductivity of S) ₈₀ (SiO ₂ ) ₂₀ is several tens of times or more, and the thermal energy reaching the light incident side cooling layer 8 is instantaneously diffused in the light incident side cooling layer 8 and The thermal energy density drops sharply.

On the other hand, Al forming the light transmitting side cooling layer 11
The thermal conductivity of the alloy is expected to be several thousand times higher than the thermal conductivity of (ZnS) ₈₀ (SiO ₂ ) ₂₀ which is the material of the light transmitting side protective layer 10, and the light transmitting side has the same light transmittance. Side cooling layer 1
The thermal energy that has reached 1 is instantaneously diffused in the light transmission side cooling layer 11, and the thermal energy density thereof is drastically reduced.

As a result, when the temperature distribution after the elapse of a certain time when the light spot of the laser light was irradiated at the time of rewriting was measured, 600 ° C. and 400 ° C. in the longitudinal section of the medium along the moving direction A of the light spot. 2 and the isothermal curve C at 200 ° C. are schematically shown in FIG. 2, and a slight spread is formed in the light incident side protective layer 9 behind the recording layer 3 in the moving direction A of the light spot. Although it can be seen, it has a substantially symmetrical shape, and the so-called isothermal curve C tailing phenomenon is suppressed as much as possible.

The heat energy reaching the light incident side cooling layer 8 is instantly diffused in the light incident side cooling layer 8 and the heat energy density thereof is rapidly reduced, so that the heat reaching the upper surface of the substrate 1 is reduced. The energy density will be lower, and as a result,
The temperature of the upper surface of the substrate 1 can be suppressed to a temperature lower than the softening point of the polycarbonate substrate, and thermal adverse effects on the substrate 1 can be reduced. Further, since the thermal conductivity of the light-incident-side cooling layer 8 is high, there is concern that the recording sensitivity may be deteriorated. However, it is possible to increase the film thickness of the light-transmissive-side protective layer 10 as compared with the conventional method. By reducing the amount of thermal energy flowing into the cooling layer 11, it is possible to easily secure the same recording sensitivity as in the conventional case. Further, Al which is a material for forming the light incident side cooling layer 8 and the light incident side protective layer 9
Since the thermal expansion coefficients of N and (ZnS) ₈₀ (SiO ₂ ) ₂₀ are about the same, there is no risk of problems such as separation of this interface due to thermal expansion.

Next, the optical recording medium thus obtained is rotated at a rotation speed of 1800 rpm, and a semiconductor laser beam having a wavelength of 830 nm is recorded on the recording layer 3 by using an objective lens having a numerical aperture of 0.5. Rewriting is performed by so-called 1-beam overwrite method, in which the laser output is focused and biased by the two values of peak power and bias power and recorded, and the distortion rate, CN ratio, erasing rate and number of times of rewriting occurring in the reproduced waveform are measured. Evaluated. The results are shown in Table 1.

Here, the distortion rate occurring in the reproduced waveform was measured as follows. That is, the thermal interference of the recording mark becomes larger in a medium in which the heat storage during recording is larger, and when this heat storage is large, the front of the recording mark swells when a long recording mark is recorded, and the reproduced waveform is distorted. This is illustrated in FIG. 3, and the shape of the amorphized recording mark 13 formed in the crystal 12 of the recording layer is in front of the moving direction A of the light spot as shown in FIG. Swells, and the reproduced waveform 14 corresponding to this bulge is a difference (d) between the rear side and the front side with respect to the moving direction A of the light spot at the amorphous level (so-called reproduced waveform) as shown in FIG. 3B. Distortion occurs, and the width of the entire reproduced waveform 14 becomes D
And the ratio of the magnitude of this distortion d (d / D × 10
0) was defined as the distortion rate (%) generated in the reproduced waveform.

The CN ratio is a recording frequency of 3.7 MHz.
Was calculated as the value for the signal. Erasure rate is 3.7MH
3.7M when 1.39MHz is recorded on the z signal
The erase rate was obtained by calculating the amount of decrease in the carrier level at Hz.
The number of rewrites is 1.39 MHz after the 3.7 MHz signal is overwritten in a certain sector a certain number of times.
The operation of rewriting at 7 MHz and measuring the CN ratio with respect to 3.7 MHz was repeated, and the number of repetitions when the CN ratio started to decrease at this time was determined.

As is clear from the results shown in Table 1, the phase-change optical recording medium of Example 1 has a low distortion rate and a small effect of heat storage. As a result, the thermal interference between recording marks is small and the edge is small. It was found that it is suitable for high-density recording such as recording. In addition, the CN ratio is also relatively large, which is due to the increase of the carrier level due to the decrease of the amorphous level, which corresponds to the fact that the cooling rate has increased and the amorphization has progressed more completely. The erasing rate is also high, which is thought to be due to the increase in carrier level and the increase in cooling rate and the homogenization of crystal grains. It was confirmed by microscopic observation. Further, regarding the number of times of rewriting, when the number of rewriting was 3 × 10 ⁶ , no deterioration of the CN ratio was observed, and at this time, neither deformation nor peeling of the recording film surface was observed.

Example 2 In the structure shown in FIG. 1, the light incident side cooling layer 8 has a thickness of 70 nm.
(In ₂ O ₃ ) ₈₀ (SnO ₂ ) ₂₀ (thermal conductivity: 27 W
.M ⁻¹ · K ⁻¹ ) and the thickness of the light incident side protective layer 9 is 80 nm.
SiO ₂ [thermal conductivity (bulk value): 1 W · m ⁻¹ · K
^-1 , Thermal expansion coefficient (bulk value): 0.6 x 10 ^-6 / de
g], and the light-transmitting side protective layer 10 is made of SiO 2 with a thickness of 40 nm.
An optical recording medium was prepared in the same manner as in the first embodiment except that the number ₂ was changed. Also for the optical recording medium of Example 2,
In the same manner as in the first embodiment, the distortion rate of the reproduced waveform, C
The N ratio, the erase rate and the number of rewritings were measured. The results are shown in Table 1.

Example 3 In the configuration of FIG. 1, the light incident side cooling layer 8 has a thickness of 50 nm.
SiC [containing sintering aid BeO, thermal conductivity (bulk value): 270 W · m ⁻¹ · K ⁻¹ , thermal expansion coefficient (bulk value): 4 × 10 ⁻⁶ / deg], light transmission side Protective layer 10
An optical recording medium was prepared in the same manner as in Example 1 except that (ZnS) ₈₀ (SiO ₂ ) ₂₀ having a thickness of 70 nm was used. With respect to the optical recording medium of Example 3, the distortion rate, the CN ratio, the erasing rate, and the number of times of rewriting generated in the reproduced waveform were measured in the same manner as in Example 1. The results are shown in Table 1.

Example 4 In the structure shown in FIG. 1, the cooling layer 8 on the light incident side has a thickness of 50 nm.
Of SiC, and the light-incident-side protective layer 9 having a thickness of 70 nm (Z
nS) ₈₀ (SiO ₂ ) _20, and a 10 nm-thickness (ZnS) ₈₀ (Si) between the light incident side cooling layer 8 and the substrate 1.
An optical recording medium was prepared in the same manner as in the first embodiment except that a protective layer of O ₂ ) ₂₀ was provided. With respect to the optical recording medium of Example 4, the distortion rate, the CN ratio, the erasing rate, and the number of times of rewriting generated in the reproduced waveform were measured in the same manner as in Example 1. The results are shown in Table 1.

Comparative Example In the structure of FIG. 1, the cooling layer 8 on the light incident side is not provided, and the protective layer 9 on the light incident side is made of (ZnS) ₈₀ having a thickness of 160 nm.
(SiO ₂ ) ₂₀ and the thickness of the light transmitting side cooling layer 10 is 30 n
An optical recording medium was prepared in the same manner as in the first embodiment except that (ZnS) ₈₀ (SiO ₂ ) _{20 of} m was used. With respect to the optical recording medium of this comparative example, the distortion rate, the CN ratio, the erasing rate and the number of times of rewriting generated in the reproduced waveform were measured in the same manner as in Example 1 above. The results are shown in Table 1.

Regarding the optical recording medium of this comparative example, the temperature distribution after a certain period of time when the light spot of the laser beam was irradiated at the time of rewriting, the longitudinal distribution of the medium along the moving direction A of the light spot was measured. 600 in the plane
The isothermal curve C at 400 ° C., 400 ° C. and 200 ° C. has a shape similar to that of FIG.
A large spread was seen in the light incident side protective layer 9 in the rear of the moving direction A of the light spot with respect to the center, and a so-called tailing phenomenon of the isothermal curve C was observed. Further, as a result of observing the recording film surface of the medium after measuring the distortion rate, the CN ratio, the erasing rate and the number of times of rewriting generated in the reproduced waveform with a transmission electron microscope, deformation of the recording film surface was observed around the recording layer 3. It was

[0042]

[Table 1]

[0043]

According to the present invention, since the cooling efficiency of the medium is improved, the thermal interference between the recording marks is reduced, the jitter is reduced, and the high density recording such as the edge recording can be performed. , Erasure rate, and reliability of reproduced data are improved.
In addition, since the thermal strain at the time of rewriting is reduced, the number of rewritings is increased, and even if a resin material is used as the substrate, the thermal load on this substrate is reduced, and from this point also the number of rewritings is increased. Further, since the protective layer having a small thermal conductivity is provided with the recording layer sandwiched, the recording sensitivity does not decrease.

[Brief description of drawings]

FIG. 1 is a cross-sectional explanatory view showing a configuration of a phase change optical recording medium according to an example of the present invention.

FIG. 2 is an explanatory diagram schematically showing isothermal curves during rewriting in the optical recording medium shown in FIG.

FIG. 3 is an explanatory diagram showing a relationship between a shape of a recording mark and a shape of a reproduced waveform for explaining a distortion rate of the reproduced waveform.

FIG. 4 is an explanatory sectional view showing the structure of a conventional phase change optical recording medium.

5 is an explanatory diagram schematically showing isothermal curves during rewriting in the optical recording medium shown in FIG.

Claims (1)

[Claims] 1. An optical recording medium comprising a recording layer on a substrate, the optical properties of which are reversibly changed by means of light, heat, etc., and information is rewritten and reproduced by utilizing the change of the optical properties. In the above, the light incident side and the light transmitting side of the recording layer are provided with protective layers on the light incident side and the light transmitting side, respectively, which are in contact with the recording layer and have a small thermal conductivity. An optical recording medium, characterized in that a cooling layer on the light incident side and a cooling layer on the light transmitting side, each having a large thermal conductivity, is provided outside the layer.