JPH11353708A - Optical recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deformation of a substrate by irradiation of light when information is recorded by using changes in physical characteristics of a recording layer caused by irradiation of light. SOLUTION: At least a heat insulating layer, reflection layer, lower dielectric layer, recording layer and upper dielectric layer are formed in this order on a substrate, and the substrate is made of a plastic material. The temp. to cause thermal deformation of the substrate is lower than the temp. at which physical characteristics of the recording layer relating to recording are caused. The film thickness of the heat insulating layer is >=2 nm. The optical recording medium is preferably a phase transition type optical recording medium in which information is recorded by using changes in the structural characteristics of the recording layer caused by irradiation of light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium on which information is recorded / reproduced by using light, and more particularly to an optical recording type of recording / reproducing by irradiating a laser beam from a recording film surface side without passing through a plastic substrate. Regarding the medium.

[0002]

2. Description of the Related Art At present, most commonly used optical recording media are substrate-incident type optical disks in which a recording film is irradiated with a laser beam through a transparent substrate. In contrast, in recent years, the magazine "Electronics", May 1996, 87
On page 91, a film-surface incident type optical recording that irradiates a laser beam from the film surface side without passing through a substrate has attracted attention.

Research has been started on this film-incidence type medium because it can perform higher-density recording than conventional media, but recording / reproduction at a practical level has not been demonstrated, and will be overcome in the future. It seems that various issues to be solved will occur.

As a medium, a magneto-optical recording medium and a phase change recording medium are considered.

In a magneto-optical recording medium, the temperature of a recording film is increased by light irradiation, mainly laser light irradiation, the coercive force of the film is reduced, and the magnetization direction is reversed by an external magnetic field to perform recording / erasing. Things. The temperature of the recording film is raised to about 200 ° C.

A phase change recording medium uses a reversible structural change (phase change) between an amorphous state and a crystalline state of a substance caused by light irradiation, mainly laser light irradiation, for recording and erasing information. We are using. The temperature of the recording film is about 600 at the time of recording.
° C, and about 170 ° C during erasing. Such a phase change recording medium has a large recording capacity in addition to a high-speed information processing capability. Another advantage is that the drive structure is simpler than that of a magneto-optical recording drive, so that the cost can be reduced.

A typical structure of the magneto-optical recording medium and the phase change recording medium is a laminate of an inorganic thin film formed by vapor deposition or sputtering on a transparent disc made of polycarbonate (PC). Has a structure of PC substrate / first dielectric / recording film / second dielectric / reflective film / organic resin protective layer. The recording layer of the magneto-optical recording medium is Tb
Rare earth / transition metal alloys such as FeCo and chalcogen alloys such as GeSbTe and AgInSbTe are used for the recording layer of the phase change recording medium. The dielectric of the magneto-optical recording medium is a nitride film such as SiN, and the dielectric of the phase change recording medium is Z.
A ZnS-based film such as nS.SiO ₂ is used.

In a phase change recording medium, the erasing state of information is usually set to the crystalline state of the recording film, and the recording state is usually set to the amorphous state generated by melting and quenching of the film with high laser power. Further, the recording layer of the phase change recording medium is in an amorphous state immediately after the sputter deposition, and is used after being subjected to an overall annealing treatment to be in a crystalline state, that is, an erased state.
The entire surface is annealed, for example, at about 1 watt,
This is performed by irradiating a laser beam having a width of 2 μm and a length of about 100 μm. This step is called initialization (initial crystallization).

For these current commercial ordinary media,
It is a common idea to reverse the stacking order of thin films from a substrate in a film-surface incident type medium for which research has begun. That is, research is being conducted on the configuration of a PC substrate / reflective film / lower dielectric / recording film / upper dielectric.

[0010]

SUMMARY OF THE INVENTION The object of the present invention is to develop the above-mentioned film surface incidence type medium, and to solve the following newly discovered problems.

That is, the inventor of the present invention has been developing a film-incidence type medium having the structure of the above-mentioned PC substrate / reflective film / lower dielectric / recording film / upper dielectric, and the PC substrate is irradiated with laser light from the film surface. We discovered the problem of heat deformation and presenting citron skin.
In particular, when a short wavelength (red) laser beam is used for the purpose of high recording density, the thickness of the lower dielectric becomes thin in order to obtain an appropriate reflectance, and this problem has become apparent. In particular, in a phase change recording medium, this problem became apparent during the initial crystallization process.

SUMMARY OF THE INVENTION The present invention solves such a problem that has become apparent in the course of research and development of high-density optical recording, and provides a film surface incidence type optical recording medium that can withstand practical use.

[0013]

The optical recording medium of the present invention comprises:
In an optical recording medium that records information by utilizing a change in physical characteristics of a recording layer caused by light irradiation, a 2n
The heat insulating layer, the reflective layer, the lower dielectric layer, the recording layer, and the upper dielectric layer having a thickness of at least m are provided in this order, the substrate is made of plastic, and the thermal deformation temperature of the substrate is a physical property related to recording in the recording layer. It is characterized in that the temperature is lower than the temperature at which the characteristic changes.

That is, as a result of intensive studies to solve the problems found in the conventional film-incidence type medium having the structure of PC substrate / reflective film / lower dielectric / recording film / upper dielectric, the results of laser beam irradiation This is considered to be because heat is conducted to the plastic substrate when the temperature of the recording film rises, and the temperature of the plastic substrate rises above its thermal deformation temperature.

Therefore, a remarkable improvement effect was found by inserting a heat insulating layer of a dielectric thin film between the plastic substrate and the reflection film, and the prospect of practical use of the medium was obtained.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The optical recording medium of the present invention comprises a heat insulating layer on one side or both sides of a plastic substrate in order from the substrate surface.
This is a film-incidence type optical recording medium having a basic configuration including a reflective layer, a lower dielectric layer, a recording layer, and an upper dielectric layer.

The heat-insulating layer of the present invention has low thermal conductivity and 2n
m is required. It is considered that the mode of heat conduction from the recording film whose temperature has increased is as follows. In other words, the heat of the recording film passes through the lower dielectric film in the thickness direction, and partially diffuses to the plastic substrate while diffusing the reflection layer (usually an Al alloy film having good thermal conductivity) in the film surface direction. It is considered to be. That is, it is considered that such a problem that the temperature of the plastic substrate rises does not occur if the heat is diffused only in the film surface direction of the reflection film. Therefore, the thermal conductivity of the heat-insulating layer is defined by the relative relationship with the thermal conductivity of the reflective layer. When the thermal conductivity of the reflective layer is high, heat is not easily transferred to the plastic substrate, and the heat-insulating performance of the heat-insulating layer is low. Although not so required, the thermal conductivity of the heat insulating layer when the thermal conductivity of the reflective layer is low needs to be sufficiently low. The thermal conductivity of the heat insulating layer needs to be about one-tenth or less of the thermal conductivity of the reflective layer. Also,
There is no effect when the heat insulating layer is too thin, and a film thickness of 2 nm or more is required.

As described in the section of "Problems to be Solved by the Invention", the thickness of the heat insulating layer depends on the thickness of the lower dielectric, so it cannot be said unconditionally. 3
When the thickness is as thin as about 0 nm, the thickness is preferably 20 nm or more. When the lower dielectric layer is thicker than about 50 nm and has a thickness that substantially prevents the transfer of the temperature of the recording film to the reflective film, the thickness of the heat insulating layer may be relatively thin. The present invention is particularly intended for high-density recording, and is most effective when the wavelength of the laser beam used is short and the thickness of the lower dielectric film is relatively small.

Examples of the heat insulating layer include a ZnS.SiO ₂ film, a ZnS film, a SiN film, and an AlS film formed by a sputtering method.
SiN film, Al ₂ O ₃ film, SiO ₂ film, Ta ₂ O ₅ film, Ti
A layer formed of a chalcogenide, a nitride, an oxide, or a mixture thereof such as an O ₂ film or a Y ₂ O ₃ film can be used. The reflective film is usually made of Ti, Ta, Cr, Au on Al.
An Al alloy film to which several atomic% is added is used. Al
The thermal conductivity of the alloy film differs depending on the content of the additional element. Generally, the higher the content of the additional element, the lower the thermal conductivity, and the value is estimated to be about 50 to 120 w / (m · K). I have. The thermal conductivity of the heat insulating layer needs to be 5 to 12 w / (m · K) or less, and preferably 2 w / (m · K) or less.

Further, as a heat insulating layer, ZnS.S, which is a stable amorphous which has a particularly low thermal conductivity and is hardly crystallized by heat, is used.
iO ₂ film is preferable. Since the ZnS.SiO ₂ film is used as an upper dielectric and a lower dielectric when a film-incident type phase change recording medium is intended, it is preferable to use the same material as a heat insulating layer in terms of production management. . ZnS / SiO
_{The two} films are obtained by sputtering a mixture of about 8: 2 of ZnS and SiO ₂ .

In particular, the present invention has a large effect when applied to a phase change recording medium, and is suitably used. This is because the temperature of the recording film at the time of recording on the phase change recording medium rises above the temperature of the recording film at the time of recording and erasing on the magneto-optical recording medium.
The recording film of the magneto-optical recording medium rises to about 200 ° C. during recording and erasing, but the recording film of the phase change recording medium (GeSbTe)
The melting temperature of the film at the time of recording was about 600 ° C. Also, in the initial crystallization, a laser beam much larger than the diameter of the recording laser beam (about 1 μm) is used, and a wide range is simultaneously heated and annealed. The effect of the invention is great.

As described above, the present invention relates to a recording medium for recording using a change in physical properties such as a crystal morphology and magnetic properties of a recording layer, which is manifested by a rise in temperature due to light irradiation. This prevents thermal deformation of the plastic substrate due to propagation of (heat) to the plastic substrate. The present invention is effective only when the thermal deformation temperature of the plastic substrate is lower than the physical property change temperature of the recording layer. The thermal deformation temperature of the plastic substrate is the temperature at which the plastic substrate starts to thermally deform under stress. When deformation starts at the glass transition temperature of the thermoplastic resin or at a temperature lower by about 10 ° C. than the glass transition temperature, the deformation start temperature corresponds to the temperature. A typical plastic substrate is a polycarbonate resin, and its glass transition temperature varies depending on the molecular weight and molecular weight distribution, but is typically about 135 ° C.

As is apparent from the above description, the physical property change temperature of the recording layer is the temperature of the recording film at the time of recording and / or erasing. At that temperature, the physical properties of the recording film change. In a magneto-optical recording film, the coercive force is a temperature at which the coercive force drops to a value sufficient to cause the magnetization to be reversed by an external magnetic field. In a phase change recording medium, the recording film changes from a crystalline state to an amorphous state (melting). The temperature and the temperature at which the amorphous state changes to the crystalline state (annealing). The heat distortion temperature of the plastic substrate needs to be lower than any of the physical property change temperatures described above.

[0024]

Examples 1-4, Comparative Example 1 and Reference Examples 1, 2 1.2
On one side of a plastic substrate having a thickness of mm, a film incident type phase-change optical recording medium having a structure including a heat insulating layer, a reflective layer, a lower dielectric layer, a recording layer, and an upper dielectric layer in order from the substrate surface is manufactured. did. As a comparative example, a medium having a normal configuration without a heat insulating layer (a configuration including a reflective layer, a lower dielectric layer, a recording layer, and an upper dielectric layer in this order from the substrate surface) was simultaneously manufactured.

Here, the plastic substrate has a length of 120 m.
A substrate made of polycarbonate having a center hole having a diameter of 15 mm and an inner diameter of 15 mm was used. Spiral groove for continuous servo is 24mm in radius by injection molding of substrate
It is formed in a range of up to 58 mm. Groove depth is 70nm
And the track pitch is 1.10 μm. The groove width and the land width are both about 0.55 μm. On the recording surface (groove surface) of the substrate, a film incident type phase change medium having heat insulating layers of various thicknesses described below was produced.
ZnS-S as heat insulating layer, lower dielectric layer and upper dielectric layer
An iO ₂ film (a film obtained by sputtering a target of ZnS: SiO ₂ = 80: 20 mol%) was used. The recording layer is a GeSbTe alloy film (Ge: Sb: Te = 2: 2:
5 atomic%, film thickness 20 nm). The reflective layer is made of AlCr
Alloy film (Al: Cr = 97: 3 atomic%, thickness 150n)
m). These inorganic thin films were formed on a transparent substrate by magnetron sputtering. The sputtering apparatus used was ANELVA Corp. Inline Sputtering System (ILC3102 type) with a target of 8
With an inch diameter, the substrate is formed while revolving around its axis. The film thickness was adjusted by the sputtering time. The reflectance (reflectance after initial crystallization) when light of 680 nm is irradiated from the film surface is about 30.
%, The film thicknesses of the lower dielectric and the upper dielectric were adjusted. Specifically, the refractive index of the ZnS—SiO ₂ film is 2.
13, the thickness of the lower dielectric is 15 nm, and the thickness of the upper dielectric is 105 nm. Table 1 shows the thickness of each layer of the samples manufactured as Examples 1 to 4, Comparative Example 1, and Reference Examples 1 and 2 by changing the thickness of the heat insulating layer.

Initial crystallization was performed on the seven samples prepared as Examples 1 to 4, Comparative Example 1, and Reference Examples 1 and 2. For initial crystallization, a bulk eraser device (model LK101A) manufactured by Shibasoku Co., Ltd. was used. However, the optical head has a laser beam intensity of about 1 watt on the disk surface, a wavelength of 810 nm, an NA (objective lens numerical aperture) of 0.34, and a spot size of 125 μm × 1.
A disk having a diameter of 27 μm was used by being inclined by 30 degrees from the disk radial direction. Initialization is performed by rotating the disk at a constant linear speed of 5 m / sec while moving the optical head at a feed speed of 8
The optical head was fed at 6 μm / rotation (the optical head advances 86 μm in the radial direction during one rotation of the disk). 5m / se
Initialization was performed by changing the laser beam intensity while maintaining constant conditions of c and 86 μm / rotation, and the reflectance from the film surface after the initialization and the presence or absence of abnormality of the plastic substrate were examined. The following laser light intensity is the maximum output power (about 1 watt on the board)
It is displayed as a percentage (%) with respect to.

The reflectivity was measured by a fiber type spectrophotometer at a flat portion having no groove on the inner peripheral side from a radius of 24 mm. The reflectance shown below is the reflectance at a wavelength of 680 nm. When the recording layer (GeSbTe layer) immediately after sputtering was amorphous, the reflectance was about 5%. The crystallization increases the reflectance. When a sufficient reflectance (about 30%) could not be obtained, it was judged that crystallization was insufficient.

The presence or absence of abnormalities in the plastic substrate (citrus skin phenomenon) was determined by visual observation of the back side of the plastic substrate and microscopic observation of the recording film surface at a magnification of 200 to 400 times. When there is no abnormality in the plastic substrate, it cannot be observed from the back surface of the substrate (a flat surface on which no grooves are formed) whether or not the initial crystallization has occurred. This is because light does not sufficiently pass through the relatively thick AlCr alloy film of 150 nm. However, the present inventor has found that a disk without a heat insulating layer can observe the initialized area from the back side of the plastic substrate when sufficiently initialized. When this part was observed in detail with a microscope, it was found that the plastic substrate was deformed (yuzu skin, and when it was remarkable, wavy). Hereinafter, the degree of deformation of the plastic substrate is referred to as small deformation, medium deformation, and large deformation based on the results of visual observation and microscopic observation.
The small deformation refers to a slight change in the surface of the substrate, which has been observed with a microscope, and the large deformation refers to a case in which the deformation can be visually observed from the back side of the plastic substrate. The middle of both is described as being deformed. Table 2 shows the evaluation results after the initial crystallization.

From the evaluation results shown in Table 2, the relationship between the initialization laser power and the reflectance is not significantly different between the samples. From the fact that the reflectivity is maximized at about 60% power, it can be seen that crystallization is sufficiently performed under the initialization conditions in the vicinity. 6
It is considered that the reason why the reflectivity slightly decreases at a laser power of 0% or more is that amorphization slightly progresses with excessive power. It is considered that the reason why the decrease in the reflectance with the excessive power in the comparative example is slightly large is that the surface roughness of the plastic substrate such as yellowing is caused.

In any case, although 60% laser power is required for sufficient initial crystallization,
In the sample of Comparative Example 1, a substrate abnormality occurred, and a practically usable recording medium could not be obtained. Even if it is not complete initial crystallization, there is a possibility that it can be used with a 50% power initialization in practical use. In the first embodiment, the power can be barely practically used with an initialization of 50 to 60%. Other Examples 2
In No. 4, sufficient initial crystallization could be performed without deformation of the substrate, and the prospect of practical use of the phase change recording medium of the film surface incidence type could be obtained. In this experimental example, the thickness of the lower dielectric layer was as thin as 15 nm, and the thickness of the heat insulating layer was 20 nm.
In Reference Examples 1 and 2 of nm or less, sufficient initialization could not be performed.

[0031]

Examples 5, 6, and 7 In the same manner as in Examples 1-4, Table 3
A sample having a film thickness of Initial crystallization of the three samples described in Table 3 was performed in the same manner as in Examples 1 to 4. Table 4 shows the evaluation results of the initial crystallization. In this experimental example, the reflectance of the medium immediately after sputtering was larger than that of the samples of Examples 1 to 4, and was about 15%. The reflectivity of the fully crystallized sample was about 38%.

From the evaluation results in Table 4, it can be seen that even when the heat insulating layer is 20 nm or less, when the thickness of the lower dielectric layer is as thick as 50 nm, sufficient initial crystallization can be performed within the range of the initializing power at which no deformation occurs. It was possible.

The above embodiment relates to the initialization of a phase change recording medium of the film surface incidence type, and is performed at the time of recording on the phase change recording medium (the recording film is melted and becomes amorphous by quenching) or magneto-optical. The present invention is also effective when recording and erasing a recording medium because the same effect of heat on the substrate causes the same phenomenon.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

[0038]

As described above, according to the optical recording medium of the present invention, even when a plastic having a heat deformation temperature equal to or lower than the physical property change temperature of the recording layer is used as a substrate, a heat insulating layer is provided between the plastic substrate and the reflective layer. By providing the optical recording medium, thermal deformation of the plastic substrate can be prevented, and the film-incident type optical recording medium can be put to practical use.

Claims (3)

[Claims] 1. An optical recording medium for recording information by utilizing a change in physical characteristics of a recording layer caused by light irradiation, a heat insulating layer, a reflective layer, a lower dielectric layer, a recording layer,
When the upper dielectric layer is provided at least in this order, the substrate is made of plastic, and the thermal deformation temperature of the substrate is lower than the temperature at which physical characteristics related to recording change in the recording layer, and the thickness of the heat insulating layer is 2 nm or more. An optical recording medium, comprising: 2. The optical recording according to claim 1, wherein the heat conductivity of the heat insulating layer is one-tenth or less of the heat conductivity of the reflective layer, and the film thickness of the heat insulating layer is 20 nm or more. Medium. 3. The optical recording medium according to claim 1, wherein the optical recording medium is a phase-change optical recording medium for recording information by utilizing a change in the structural characteristics of a recording layer caused by light irradiation. The optical recording medium according to any one of the above.