JP2004006045A - Phase change optical disk and method for playing phase change optical disk - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a phase change optical disk from which recorded information can be stably reproduced without erasing the recorded information and to provide a method for playing the phase change optical disk using a blue (wavelength 400-450 nm) laser beam. <P>SOLUTION: On a substrate 1, a first dielectric layer 2, a second dielectric layer 3, a third dielectric layer 4, a recording layer 5, a fourth dielectric layer 6 and a reflection layer 7 are sequentially laminated. When the recording layer 5 is in an amorphous state, the absorption index of the recording layer is defined as ≤70%, when the recording layer 5 is in a crystal state, its reflectivity is defined as ≥3% and ≤15%, and its reproducing power is defined as 0.5-0.9 mW. Because the reflectivity of the recording layer 5 is low, the amount of temperature rising when the blue laser beam having high energy density is emitted as a reproducing laser beam is suppressed to prevent an amorphous part in the recording layer 5 from being cystalized, which stably reproduces the recorded information without erasing the recorded information. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to an optical information recording medium for recording and reproducing information by irradiating a laser beam, and more particularly to a phase change optical disk and a method for reproducing a phase change optical disk.

A phase-change optical disk has been proposed as an optical disk capable of recording, erasing, and reproducing information. When recording information, a laser beam is focused on the optical disk to make it amorphous. When reproducing recorded information, a laser beam is focused on the optical disk, and information is read out based on the difference in the reflectance or transmittance between the amorphous portion and the crystal portion. Further, in erasing information, an amorphous portion is crystallized by laser light. By the way, narrowing the focused spot size of a laser beam used for recording, erasing, and reproducing such information is a powerful technique for increasing the density of an optical disc, and narrowing the spot size requires a laser light source. It is essential to shorten the wavelength. Currently, red semiconductor lasers having a wavelength of about 650 nm are used in optical discs such as DVD-ROMs and DVD-RAMs. Research on blue semiconductor lasers having a wavelength of about 410 nm has been steadily progressing, and stable at room temperature. Lasers capable of continuous oscillation have also been developed. It is very likely that blue semiconductor lasers will be mass-produced in a few years and will be available on the market.
JP-A-8-63781

However, in the blue laser light source, as described above, the condensed spot size becomes smaller than that of the red light source as the wavelength becomes shorter, so that the amount of light energy per unit area of the condensed spot, that is, the energy density is higher. As a result, the amount of temperature rise of the optical disk when the laser light is irradiated on the optical disk increases. For example, assuming that the absorption rate of light energy in the recording layer is the same for red light and blue light, and when the temperature rise amount of the recording film when irradiating the laser light of the red light source of 1 mW is 100 ° C. With a blue light source, a temperature rise of about 200 ° C. will occur. Since the crystallization temperature of the recording layer used in the phase change optical disk is about 150 to 300 ° C., when the recording layer on which information is recorded is irradiated with laser light from a blue light source to reproduce the information, Due to the temperature rise of 200 ° C., the possibility that the recorded information in the amorphous state is crystallized and the recorded information is erased is very likely to occur.

In order to prevent such crystallization of the recording layer due to the reproduction laser beam, the laser power at the time of reproducing the information should be reduced as much as possible. However, when the laser power is lowered, the laser noise increases and the reproduction signal amplitude decreases. And signal quality is degraded. In particular, in a conventional phase change optical disk, the reflectance in the amorphous state is designed to be close to 0%, and as a result, the absorption of the recording layer in the amorphous state is 90% or more. Many. The amount of temperature rise in the recording layer when irradiating the reproduction laser beam is determined by the power of the laser and the absorptivity in the recording layer. It is difficult to prevent crystallization of the amorphous part.

An object of the present invention is to provide a phase change optical disk capable of stably reproducing recorded information recorded on an optical disk without erasing even when an optical disk device using a laser beam of a blue light source having a wavelength of 400 to 450 nm is configured. An object of the present invention is to provide a reproducing method.

In order to solve the above problems, a phase change optical disk according to the present invention has a light absorption rate of 70% or less in a recording layer formed on the phase change optical disk when the recording layer is in an amorphous state. It is characterized by the following. Further, in this case, preferably, the reflectance when the recording layer is in a crystalline state is 3% or more and 15% or less. Further, the reflectance when the recording layer is in an amorphous state is higher than the reflectance when the recording layer is in a crystalline state. The thickness of the dielectric layer provided immediately above the recording layer is set to 50 nm or less.

Further, in the optical disk reproducing method of the present invention, a laser beam having a wavelength of 400 to 450 nm is used as reproducing light for the optical disk of the present invention, and the laser power of the laser light is 0.5 mW or more and 0.9 mW or less. It is characterized by the following.

According to the present invention, since the absorption rate of the recording layer with respect to the blue light source is set to 70% or less, it is possible to suppress an increase in the temperature of the recording layer during irradiation with the reproduction laser beam. In addition, since the reflectivity when the recording layer is in a crystalline state is 3% or more and 15% or less, a sufficient reproduction signal amplitude is obtained, and the focus and tracking servo operations of the optical head are stably maintained. Is possible. Further, by setting the thickness of the fourth dielectric layer on the recording layer to 50 nm or less, it is possible to enhance the heat dissipation from the recording layer to the reflection layer. Further, at the time of reproducing information, by setting the power of the laser beam to 0.5 to 0.9 mW, it is possible to prevent an increase in laser noise and a decrease in the amplitude of a reproduction signal, and to surely prevent crystallization in the recording layer. It becomes possible.

As described above, according to the present invention, even when information is reproduced using a blue laser beam, the recording layer is made to have an absorptivity of light of 70% or less when the recording layer is in an amorphous state. , The crystallization of the amorphous portion can be prevented, and information can be reproduced stably using blue laser light. In addition, since the reflectivity when the recording layer is in a crystalline state is 3% or more and 15% or less, a sufficient reproduction signal amplitude is obtained, and the focus and tracking servo operations of the optical head are stably maintained. Becomes possible. Further, by setting the thickness of the dielectric layer immediately above the recording layer to 50 nm or less, it is possible to enhance the heat dissipation from the recording layer to the reflective layer. Further, at the time of reproducing information, by setting the power of the laser beam to 0.5 to 0.9 mW, it is possible to prevent an increase in laser noise and a decrease in the amplitude of a reproduced signal, and to reliably prevent crystallization in the recording layer. It is also possible.

Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a view for explaining a phase change optical disk of the present invention. As shown in an enlarged view of a sectional structure of an optical disk 11, a concentric or spiral guide groove 12 is formed on the surface of a transparent substrate 1. On the surface of the transparent substrate 1, a first dielectric layer 2, a second reflective layer 3, a third dielectric layer 4, a recording layer 5, a fourth dielectric layer 6, and a reflective layer 7 are sequentially laminated. The substrate 1 is made of polycarbonate, and a pitch (track pitch) of the guide grooves in the radial direction is set to 1.0 μm. Further, ZnS-SiO2 is 30 nm as the first dielectric layer 2, SiO2 is 90 nm as the second dielectric layer 3, ZnS-SiO2 is 20 nm as the third dielectric layer 4, Ge2 Sb2 Te5 is 13 nm as the recording layer 5, and The fourth dielectric layer 6 is made of ZnS-SiO2 of 40 nm, and the reflective layer 7 is made of Al (aluminum) of 120 nm. These are sequentially laminated by sputtering.

In FIG. 1, an optical head 20 is opposed to the back surface of the substrate 1 with respect to the optical disk, and the light from a laser light source 21 is condensed on the recording layer 5 by a lens optical system 22 to store information. Recording and erasing. Also, information is reproduced by separating the reflected light from the recording layer 5 by the beam splitter 23 and receiving the light by the photodiode 24. As the laser light source 21, a laser element capable of emitting blue laser light of 400 to 450 nm is used. The optical head 20 is provided with a control system for performing tracking control on the guide groove 12 of the optical disc 11 and focusing control on the recording layer 5. The configuration of this type of optical head is already widely known. Therefore, detailed description is omitted here.

In the optical disk having the above configuration, a three-layer dielectric layer having the first dielectric layer 2, the second dielectric layer 3, and the third dielectric layer 4 interposed between the recording layer 5 and the substrate 1. When the refractive index of the first dielectric layer 2 is n1, the refractive index of the second dielectric layer 3 is n2, and the refractive index of the third dielectric layer 4 is n3, n1> n2, and By satisfying the relationship of n3> n2, the reflectance of the recording layer 5 is increased and the absorption of the recording layer 5 is decreased from the relationship with the refractive index when the recording layer 5 is in the amorphous state. I have. Therefore, when a blue laser light source is used as the laser light source of the optical head 20, the condensed spot size becomes smaller and the condensed spot energy density increases as the wavelength of the blue laser light becomes shorter as compared with the red light source. Even when the information is recorded on the recording layer 5, the reflectivity at the amorphous portion is increased and the absorptance is reduced, so that blue laser light is applied to the recording layer 5 to reproduce information. The temperature rise upon irradiation is suppressed, and the problem that the recorded information is crystallized and the recorded information is erased is solved.

Here, since the reflectance when the recording layer 5 is in the amorphous state is high, in order to obtain a large reproduction signal amplitude, the reflectance when the recording layer 5 is in the crystalline state is as low as possible. % Is desirable. However, if the reflectance before recording is too low, the focus and tracking servo operations of the optical head 20 become unstable. Therefore, the reflectance when the recording layer 5 is in a crystalline state is at least 3% or more. Is desirable. Although it is possible to suppress the temperature rise of the recording layer 5 due to the reproduction laser beam by lowering the absorptivity when the recording layer 5 is in the amorphous state, the fourth layer adjacent to the upper layer of the recording layer 5 can be suppressed. If the thickness of the dielectric layer is too large, the escape of heat from the recording layer 5 to the reflective layer 7 becomes worse, which causes an increase in the temperature rise of the recording layer 4. Is desirably 50 nm or less. Further, at the time of reproducing information, the power of the laser beam is preferably set to 0.5 to 0.9 mW. The reason why the power of the laser light is set to 0.5 mW or more is to prevent an increase in laser noise and a decrease in reproduction signal amplitude. On the other hand, when the reproduction laser light is too high, crystallization occurs in the recording layer 5 and information is lost. Since the laser light is lost, it is desirable that the laser light be 0.9 mW or less.

Incidentally, in the optical disk 11, when the recording layer 5 was in the amorphous state, the absorption Aa of the recording layer 5 was 55%, and when the recording layer 5 was in the crystalline state, the reflectance Rc was 10%. . The optical disk 11 is rotated at a linear velocity of 5 m / s, recording is performed using an optical head having a wavelength of 660 nm and a numerical aperture NA of the objective lens of the lens optical system 22 of 0.6, and a wavelength of 430 nm and a numerical aperture NA of the objective lens. Was reproduced using an optical head having a 0.6. After a signal of 1 MHz and a duty (duty) of 50% was recorded on a land portion formed between the guide grooves 12 of the optical disk 11, the signal was repeatedly reproduced at a reproduction power of 0.6 mW, and changes in carrier and noise were examined. . As a result, as shown in FIG. 2, there was no decrease in carrier and no increase in noise even after reproduction 100,000 times, and no crystallization of the recording film occurred.

Using a phase change optical disk having the same configuration as the optical disk, the disk is rotated at a linear velocity of 5 m / s, recording is performed using an optical head having a wavelength of 660 nm and a numerical aperture NA of the objective lens of 0.6, and a wavelength of 430 nm. Reproduction was performed using an optical head having a numerical aperture NA of an objective lens of 0.6. After recording a signal of 1 MHz and duty = 50% on the land, the signals were reproduced with different reproduction powers and the relationship between the reproduction power and C / N was examined. As shown in FIG. 3, it was confirmed that the C / N was reduced when the reproducing power was 0.5 mW or less.

As another embodiment of the present invention, polycarbonate is used as the substrate 1 of the optical disk 11, ZnS-SiO2 is 30 nm as the first dielectric layer 2, SiO2 is 80 nm as the second dielectric layer 3, and the third dielectric layer 4 is 140 nm of Zn-SiO2, 14 nm of Ge2 Sb2 Te5 as the recording layer 5, 50 nm of ZnS-SiO2 as the fourth dielectric layer 6, and 100 nm of Al as the reflective layer 7 were sequentially laminated by sputtering. The track pitch was 1.0 μm as in the above embodiment. In this optical disc, the absorption Aa when the recording layer 5 was in the amorphous state was 70%, and the reflectance Rc when the recording layer 5 was in the crystalline state was 7%. In addition, the relationship between the difference between the carrier C1 in the first reproduction and the carrier C2 in the 100,000th reproduction and the reproduction power was examined on this optical disc. As shown in FIG. 4, when the reproduction power exceeded 0.9 mW, It was confirmed that the carrier had decreased.

According to the studies of the present inventors including the above embodiments, the object can be achieved by setting the absorptivity when the recording layer is in an amorphous state to 70% or less, and preferably, the recording layer is It was confirmed that it is preferable to set the absorptance in the crystalline state to 3% or more and 15% or less.

FIG. 2 is a diagram showing a configuration of a phase change optical disc according to the present invention. FIG. 4 is a diagram showing the relationship between the number of times of reproduction, carrier, and noise in the phase change optical disk according to the present invention. FIG. 3 is a diagram showing the relationship between reproduction power, carrier, and noise in the phase change optical disk according to the present invention. FIG. 4 is a diagram illustrating a relationship between a reproduction power and a change in a carrier in the phase change optical disk according to the present invention.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Substrate 2 1st dielectric layer 3 2nd dielectric layer 4 3rd dielectric layer 5 Recording layer 6 4th dielectric layer 7 Reflective layer 11 Optical disk 12 Guide groove 20 Optical head 21 Laser light source 22 Lens optical system 23 Beam splitter 24 Photodiode

Claims (6)

A phase change optical disc for recording, erasing, and reproducing information by using laser light having a wavelength of 400 to 450 nm,
A first dielectric layer, a second dielectric layer, a third dielectric layer, the recording layer, a fourth dielectric layer, and a reflective layer are sequentially formed on a substrate;
The refractive index n1 of the first dielectric layer, the refractive index n2 of the second dielectric layer, and the refractive index n3 of the third dielectric layer satisfy a relationship of n1> n2 and n3> n2. Phase change optical disk. 2. The phase change optical disk according to claim 1, wherein the light absorption of the recording layer formed on the phase change optical disk is 70% or less when the recording layer is in an amorphous state. 3. The phase change optical disk according to claim 1, wherein the reflectance when the recording layer is in a crystalline state is 3% or more and 15% or less. 4. The phase change optical disk according to claim 1, wherein the reflectance when the recording layer is in an amorphous state is higher than the reflectance when the recording layer is in a crystalline state. 6. The phase change optical disk according to claim 1, wherein the thickness of the fourth dielectric layer is 50 nm or less. 6. A reproducing method for reproducing information recorded on the phase change optical disk according to claim 1, wherein a laser beam having a wavelength of 400 to 450 nm is used as the reproducing beam, and the laser power of the laser beam is set to 0.5 mW. A method for reproducing a phase change optical disk, wherein the power is not less than 0.9 mW or less.

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