JPH08315417A - Optical information medium - Google Patents

Abstract

PURPOSE: To improve the erasing rate and to decrease errors by eliminating dispersion of the shape and size of recording pit depending on the state of a recording layer before recording whether it is in a crystal state or amorphous state. CONSTITUTION: A phase transition type optical recording medium in which information can be recorded, erased and reproduced by the irradiation with light is produced by successively forming a dielectric layer 2, recording layer 3, carbon layer 4, and reflection layer 5 on a transparent substrate 1. The thickness of the carbon layer is between 2nm and 40nm. By forming the carbon layer between the recording layer and the reflecting layer, the absorptivity of the recording film in a crystal state is made larger than the absorptivity of the recording film in an amorphous state, and thereby, dispersion of the shape and size of recording pits can be decreased.

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 capable of recording, erasing and reproducing information by irradiating light, and recording and erasing signals by phase change.

[0002]

2. Description of the Related Art In recent years, optical recording media have been attracting attention as playing a central role of recording media in a highly information-oriented society, and research is being actively conducted. Among optical recording media, optical discs are attracting attention as the most promising recording media with the spread of multimedia.

There are three types of such optical discs, a read-only type represented by a compact disc and a laser disc, a write-once type in which information can be written by a user, and a rewritable type in which information can be rewritten. Among them, the rewritable type includes a magneto-optical method and a phase change method. In the phase change method, a material that causes a phase change between crystal and amorphous is used as a recording layer.

In the phase change system, overwriting can be easily performed by irradiating the recording layer with laser light whose intensity is modulated according to a recording signal as shown in FIG. That is, at a position irradiated with a laser beam having a strong power for a short time, an amorphous state (recording pit) is formed by being rapidly heated to a temperature equal to or higher than the melting point and melted, and then rapidly cooled. The position irradiated with is heated to a crystallizable temperature range lower than the melting point and a crystalline state is formed.

Conventionally, in order to increase the signal amplitude and improve the repetitive characteristics, the phase change type optical disk is often used in a laminated structure having a multilayer film of four layers on a substrate as shown in FIG. . That is, it has a four-layer structure in which a transparent dielectric layer 12, a recording layer 13, a transparent dielectric layer 14, and a reflective layer 15 are provided on a transparent substrate 11 such as a polycarbonate resin and the dielectric layer 14 is an interference layer.

A dielectric layer is used as a material used for each layer.
Has excellent recording layer protection characteristics, and recording and erasing can be repeated.
Since it has sufficient mechanical properties for
Mixtures of compounds and oxides or nitrides are generally
Widely used. Also, the crystallization rate of the recording layer
Fast one is preferable, Sb-Se alloy, Sb ₂Te₃Combined
Gold and Te-Ge-Sb alloys are well known.

As the reflective layer, an alloy containing Al as a main component is widely used because it is inexpensive and has excellent environmental resistance.

[0008]

In a conventional phase change type optical recording medium having a four-layer structure, upon overwriting, the absorption of laser light in the recording layer is generally more crystalline in the amorphous state. Since the size of the recording pit is larger than that of the recording pit, the shape and size of the recording pit formed when the recording pit is formed depends on whether the state before recording is a crystalline state or an amorphous state. There is a problem that the margin is reduced and the erase ratio is reduced.

Particularly in pit edge recording, since data is placed on both ends of the pit, variations in the shape and size of the recording pits directly cause an error, which becomes a factor that hinders the improvement of the recording density of the phase change optical disk. Was there.
The present invention is intended to solve such a problem, and an object thereof is to form pits having a uniform size and shape regardless of the state of the recording layer before recording.

[0010]

The present inventors have found that the erasing ratio is high and the erasing ratio does not depend on the state of the recording layer before overwriting.
As a result of intensive research to develop a phase change type optical recording medium capable of forming recording pits with uniform shape and size by overwriting, use of a carbon layer as a protective layer between the recording layer and the reflective layer As a result, they have found that the problem can be achieved, and have completed the present invention based on this finding.

That is, according to the present invention, in a phase change type optical recording medium capable of recording, erasing and reproducing information by irradiating light, a dielectric layer, a recording layer, a carbon layer and a reflective layer are formed on a transparent substrate. An optical recording medium having a structure in which layers are sequentially stacked. In general, a recording layer in a crystalline state has a higher thermal conductivity than that in an amorphous state, and a heat amount corresponding to the latent heat is required for melting, and it is necessary to form a recording pit more than an amorphous state. Requires a large amount of heat.

Therefore, in order to form recording pits of the same size regardless of whether the recording layer before recording is in the crystalline state or the amorphous state, the absorption amount of the crystalline state is in the amorphous state. It is desirable that it is larger than the absorption amount. However, in the conventional phase change type optical recording medium, from the viewpoint of increasing the signal amplitude and improving the repeating characteristics, multiple interference occurs due to the dielectric layer or the reflective layer provided on the recording layer, and The absorptance of the state was larger than that of the crystalline state. Therefore, as shown in FIG. 5, the pits recorded on the region that was in the amorphous state before recording have a different shape from the pits recorded on the region that was in the crystalline state.

On the other hand, according to the phase-change optical recording medium of the present invention, by using the carbon layer as the protective layer between the recording layer and the reflective layer, absorption of laser light into the amorphous recording layer is prevented. It is possible to reduce the amount of laser light to be equal to or less than the absorption of the laser light of the recording layer in the crystalline state. Therefore, as shown in FIG. 4, variations in the shape and size of the recording pits can be reduced regardless of the state of the recording layer before recording.

Regarding the structure in which the carbon layer is laminated on the recording layer, a proposal (Japanese Patent Laid-Open No. 3-100936) aimed at improving the recording sensitivity has been made. Japanese Patent Laid-Open No. 3-10
The proposal of No. 0936 is a structure in which a carbon layer is laminated on the recording layer in a thickness of 50 nm or more and 500 nm or less. However,
In the case of the structure in which the reflective layer is provided on the carbon layer as in the present invention, when the thickness of the carbon layer is 50 nm or more and 500 nm or less, the optical contrast decreases and C /
The N ratio cannot be taken sufficiently. Moreover, since the carbon layer has a very thick and gradually cooled structure of 50 nm or more and 500 nm or less, mass transfer due to repeated recording becomes extremely large.

Further, even if the carbon layer thickness is 50 nm or more and 500 nm or less, the absorptance in the crystalline state can be made higher than or equal to the absorptance in the amorphous state. It becomes steep, and the unerased portion of the pit end is liable to occur, and the erasing ratio cannot be improved. Therefore, in the present invention, the range of the carbon layer film thickness of 50 nm or more and 500 nm or less cannot be used.

The layer structure of the phase change type optical recording medium of the present invention is as follows:
As shown in FIG. 1, it has a structure in which a dielectric layer 2, a recording layer 3, a carbon layer 4, and a reflective layer 5 are sequentially laminated on a transparent substrate 1. The thickness of the carbon layer of the phase-change optical recording medium of the present invention needs to be 2 nm or more and 40 nm or less. When the thickness of the carbon layer is less than 2 nm, the absorption in the carbon layer is insufficient, the absorption in the amorphous recording layer is large, and the absorption in the crystalline recording layer is larger than that in the amorphous recording layer. Can't do more than absorb in.

When the thickness of the carbon layer is thicker than 40 nm, the optical contrast becomes small, and the signal strength is lowered. Further, if the thickness of the carbon layer is thicker than 40 nm, it becomes difficult to maintain the rapid cooling conditions, and the repetitive characteristics and the erasing ratio deteriorate. As described above, the thickness of the carbon layer is appropriately in the range of 2 nm to 40 nm, and more preferably in the range of 5 nm to 30 nm.

The extinction coefficient of the carbon layer of the phase change optical recording medium of the present invention is preferably 0.05 or more. If the extinction coefficient is less than 0.05, the carbon layer hardly absorbs the laser light, and the multiple interference causes absorption in the amorphous recording layer. Therefore, it is appropriate that the extinction coefficient of the carbon layer is 0.05 or more,
Furthermore, 0.1 or more is more desirable. As a film forming method, a known method such as a sputtering method or a vapor deposition method can be used, but a forming method having a transparent diamond structure is not preferable.

The film thickness of the dielectric layer provided between the substrate and the recording layer in the phase change optical recording medium of the present invention is 100 nm.
It is preferably not less than 400 nm and not more than 400 nm. If the film thickness of the dielectric layer is less than 100 nm, the thermal damage to the substrate will be great and the signal quality will be degraded when recording is repeated.
Further, if the film thickness of the dielectric layer is thicker than 400 nm, it takes time to form the film, and further, the substrate may be deformed due to the influence of heat generated during the film formation.

The effect of the present invention becomes greater as the amount of light passing through the amorphous recording layer increases. Therefore, it is desirable that the maximum effect is exhibited in the thickness of the dielectric layer where the reflectance is minimum, and the thickness is in the vicinity thereof. Therefore, the film thickness of the dielectric layer is more preferably 200 nm or more and 300 nm or less. The film thickness of the recording layer in the phase change optical recording medium of the present invention is preferably 10 nm or more and 50 nm or less. When the film thickness of the recording layer is less than 10 nm, the difference in reflectance between the crystalline state and the amorphous state becomes small and the signal quality deteriorates. Further, if the recording layer thickness is thicker than 50 nm, the absorption in the amorphous recording layer becomes large, and the absorption in the crystalline recording layer cannot exceed the absorption in the amorphous recording layer. . The thickness of the recording layer has an optimum range for the above-mentioned reason, and a range of 10 nm or more and 50 nm or less is suitable, but a range of 20 nm or more and 35 nm or less is more preferable.

As the material of the recording layer in the phase change type optical recording medium of the present invention, known phase change materials can be used.
For example, a GeTe system, a GeTeSb system, an InSb system, or a system to which a small amount of elements of these systems are added may be used. Examples of the substrate material used in the phase change type optical recording medium of the present invention include transparent materials such as glass, polypropylene, acrylic resin, polycarbonate resin, styrene resin, vinyl chloride resin epoxy resin, and polyolefin resin. Therefore, polycarbonate resin and acrylic resin are preferable in terms of optical characteristics.

As the material of the dielectric layer of the phase change type optical recording medium of the present invention, known dielectric materials can be used.
S, SiO ₂ , SiN, AlN, Al ₂ O ₃ , Ta ₂ O
Examples thereof include metal sulfides such as _{5 and the} like, metal oxides, metal nitrides, metal carbides, metal selenides, and mixtures thereof. As the material of the reflective layer of the phase change type optical recording medium of the present invention, known materials can be used, and Al, Au, Ni, C are used.
Examples thereof include metals such as r, alloys thereof, and those obtained by adding a small amount of element to these metals and alloys.

The method for forming the recording film in the phase change type optical recording medium of the present invention is not particularly limited, and a known vapor deposition method, sputtering method or the like can be used.

[0024]

Next, the present invention will be described in more detail by way of examples.

[0025]

Example 1 A protective layer made of a ZnS—SiO ₂ thin film having a thickness of 200 nm, a recording layer made of a SbTeGe alloy thin film having a thickness of 25 nm, a carbon layer having a thickness of 20 nm was formed on a clean polycarbonate substrate having guide grooves. 150 nm thickness
A phase-change optical disk in which a reflective layer composed of the Al alloy thin film was sequentially formed by sputtering, and the surface of the recording film was covered with an ultraviolet curable resin.

The two single-sided discs thus prepared were adhered with a hot melt adhesive so that the recording surface was on the inside, to obtain an optical disc having a full-face contact structure. The optical disc thus created was rotated at a linear velocity of 15 m / sec and a pulse width of 20
nsec, frequencies 16 MHz and 6 MHz were alternately overwritten with a recording waveform as shown in FIG. 2, and the erasing ratio of each frequency was measured. At this time, the optical system has a wavelength λ =
680 nm, NA of the objective lens is NA = 0.6, bias power is 5 mW, and peak power is 13 mW.

As a result, when the signal of 6 MHz is recorded and the signal of 16 MHz is overwritten, 1
The C / N ratio of 6 MHz is 50 dB or more, and 6 MHz
The erasing ratio of the signal is 28 dB, and the erasing ratio is improved by 8 dB as compared with the optical disc of Comparative Example 1 described later.
When the waveform at this time was observed, the waveform was as shown in FIG. 6, and no amplitude modulation was observed.

When recording was repeated 100,000 times, the jitter was the same as at the beginning and the bit error rate did not deteriorate.

[0029]

Example 2 A protective layer made of a ZnS—SiO ₂ thin film having a thickness of 200 nm, a recording layer made of a SbTeGe alloy thin film having a thickness of 25 nm, a carbon layer having a thickness of 10 nm, was formed on a clean polycarbonate substrate having guide grooves. Thickness 100nm
A phase-change optical disk in which a reflective layer composed of the Al alloy thin film was sequentially formed by sputtering, and the surface of the recording film was covered with an ultraviolet curable resin.

The two single-sided discs thus prepared were adhered to each other with a hot melt adhesive so that the recording surface was on the inside, to obtain an optical disc having a full-face contact structure. The optical disc thus created was rotated at a linear velocity of 15 m / sec and a pulse width of 20
nsec, frequencies 16 MHz and 6 MHz were alternately overwritten with a recording waveform as shown in FIG. 2, and the erasing ratio of each frequency was measured. At this time, the optical system has a wavelength λ =
680 nm, NA of the objective lens is NA = 0.6, bias power is 5 mW, and peak power is 13 mW.

As a result, when the signal of 6 MHz was recorded and the signal of 16 MHz was overwritten, 1
The C / N ratio of 6 MHz is 50 dB or more, and 6 MHz
The erasing ratio of the signal is 27 dB, which is 7 dB better than that of the optical disc of Comparative Example 1 described later.
When the waveform at this time was observed, the waveform was as shown in FIG. 6, and no amplitude modulation was observed.

When recording was repeated 100,000 times, the jitter was the same as at the beginning and the bit error rate did not deteriorate.

[0033]

Comparative Example 1 A protective layer made of a ZnS—SiO ₂ thin film having a thickness of 200 nm, a recording layer made of a SbTeGe alloy thin film having a thickness of 25 nm, and a ZnS—SiO having a thickness of 20 nm were formed on a clean polycarbonate substrate having guide grooves. A phase-change optical disk in which an interference layer composed of _two thin films and a reflection layer composed of an Al alloy thin film having a thickness of 150 nm were sequentially laminated was formed by a sputtering method, and then the surface of the recording film was covered with an ultraviolet curable resin. The two single-sided discs thus produced were adhered with a hot melt adhesive so that the recording surface was on the inside, to obtain an optical disc having a full-face contact structure.

As a result, when the signal of 6 MHz is recorded and the signal of 16 MHz is overwritten, 6
The cancellation ratio of the MHz signal was 20 dB. When the waveform at this time was observed, amplitude modulation at a frequency of 6 MHz as shown in FIG. 7 was observed, and clearly the state of the recording layer before overwriting (the unrecorded portion in the crystalline state and the recording pits in the amorphous state) was observed. It can be seen that the size and shape of the pit formed by) have changed.

When recording was repeated 100,000 times, the jitter increased and the bit error rate significantly deteriorated.

[0036]

Comparative Example 2 On a clean polycarbonate substrate provided with guide grooves, a protective layer made of a ZnS-SiO ₂ thin film having a thickness of 200 nm, a recording layer made of a SbTeGe alloy thin film having a thickness of 25 nm, a carbon layer having a thickness of 100 nm, Thickness 150n
A phase-change type optical disk in which a reflective layer composed of an Al alloy thin film of m was sequentially laminated was formed by a sputtering method, and then the surface of the recording film was covered with an ultraviolet curable resin.

The two single-sided discs thus prepared were adhered with a hot-melt adhesive so that the recording surface was on the inside, to obtain an optical disc having a full-face contact structure. As a result, when the signal of 6 MHz was recorded and the signal of 16 MHz was overwritten, the C / N ratio of 16 MHz was only 44 dB. This is because the optical modulation degree is lowered by increasing the thickness of the carbon layer.

[0038]

According to the present invention, in a rewritable phase change type optical recording medium, a dielectric layer, a recording layer,
By having a structure in which a carbon layer and a reflective layer are sequentially laminated,
Regardless of whether the state of the recording layer before recording is the crystalline state or the amorphous state, it is possible to reduce the variation in the shape and size of the recording pit.

As a result, it is possible to provide an optical recording medium in which the jitter is reduced, the erasing ratio is improved, the window margin is increased, and the error is reduced.

[Brief description of drawings]

FIG. 1 is a sectional view showing the structure of an optical recording medium of the present invention.

FIG. 2 is a waveform diagram of laser beam intensity during overwriting.

FIG. 3 is a cross-sectional view showing the structure of a conventional four-layer optical recording medium.

FIG. 4 is a conceptual diagram of a pit shape formed when recording is performed on an amorphous state and a crystalline state in the optical recording medium of the present invention.

FIG. 5 is a conceptual diagram of a pit shape formed when recording is performed on an amorphous state and a crystalline state in a conventional optical recording medium.

FIG. 6 is a signal waveform after overwriting in the optical recording medium of the present invention.

FIG. 7 is a signal waveform after overwriting in the conventional optical recording medium.

[Explanation of symbols]

 1 Transparent Substrate 2 Dielectric Layer 3 Recording Layer 4 Carbon Layer 5 Reflective Layer 11 Transparent Substrate 12 First Dielectric Layer 13 Recording Layer 14 Second Dielectric Layer 15 Reflective Layer

Claims (4)

[Claims] 1. Recording of information by irradiating with light,
In a phase change type optical recording medium that can be erased and reproduced,
It has a structure in which a dielectric layer, a recording layer, a carbon layer, and a reflective layer are sequentially stacked on a transparent substrate, and the thickness of the carbon layer is 2 nm.
An optical recording medium having a thickness of 40 nm or more. 2. The optical recording medium according to claim 1, wherein the extinction coefficient of the carbon layer is 0.05 or more. 3. The optical recording medium according to claim 1, wherein the dielectric layer provided between the transparent substrate and the recording layer has a film thickness of 100 nm or more and 400 nm or less. 4. The optical recording medium according to claim 1, 2 or 3, wherein the thickness of the recording layer is 10 nm or more and 50 nm or less.