JPH08111035A - Optical disk medium and reproducing method therefor - Google Patents

Abstract

PURPOSE: To obtain an optical disk medium capable of obtaining a sufficient reproducing signal level even when recording mark length is decreased. CONSTITUTION: This optical disk medium is provided with at least one of a base layer 2 and an interference layer 5, which is composed of a dielectric layer having the extinction coefficient (k) of 0.01<=k<=0.1 in the wavelength of reproducing light at 20 deg.C. A nitrogen deficiency silicon nitride film, a nitrogen deficiency aluminum nitride film, a silicon carbide film, a hydrogenated silicon carbide film, an oxygen deficiency silicon oxide film and the like are used as the dielectric layer. The reproduction is executed by setting the reproducing power so that a part increased in the extinction coefficient of the dielectric layer by the temp. rising exists within the diameter of reproducing light beam.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk medium such as a read-only CD-ROM, a rewritable magneto-optical disk, a phase change disk and the like, which uses laser light for reading information, and more particularly to an optical disk. The present invention relates to a super-resolution optical disc using a functional film whose optical characteristics change depending on the amount of reproducing light, which is a method of obtaining a high density exceeding the diffraction limit of the above.

[0002]

2. Description of the Related Art C using laser light for reading information
In D-ROMs, magneto-optical discs, and phase-change discs, the limit of densification depends on the beam diameter of laser light.
This means that when the recording mark is smaller than the beam diameter,
This is because a plurality of recording marks exist within the beam diameter, causing intersymbol interference and reducing the reproduction signal amplitude. Since the beam diameter that can be narrowed down by the objective lens is inversely proportional to the wavelength, the wavelength of the laser light source is being shortened, but the development of a light source of 600 nm or less is in the research stage.

As a means for obtaining a high density exceeding the light diffraction limit, there is a method using super-resolution using a functional film whose optical characteristics change depending on the amount of reproducing light (1990,
Applied Optics, No. 29, page 3766 (Ap
plied Optics 29 (1990) 376
6)). As an application of this super resolution in magneto-optical recording,
An iristor method using a reproducing layer of a magnetic exchange coupling film composed of a recording layer and a reproducing layer as a mask has been proposed (for example, 1991, Journal of Japan Applied Magnetics, Supplement No. 15, S1, 319 (J. Magn). .Soc.J
pn. , 15 Sppl. S1 (1991) 31
9)). This method utilizes the fact that the temperature distribution within the reproduction beam diameter is different from the light intensity distribution. RAD in which the magnetization information of the recording layer is transferred to the reproducing layer only in the high temperature part
Then, two types of FAD have been proposed in which a high temperature part is erased. By using this method, a high density more than double is achieved without changing the wavelength of the laser light. As the structure of the magnetic recording film, three layers are required: a recording layer for recording and storing information, a reproducing layer acting as a mask, and a control layer for controlling exchange coupling between both layers by temperature. Also, an initializing magnetic field is required in RAD.

As an example of applying super-resolution to a CD-ROM, a method using GeSbTe as a mask has been proposed (1993, Journal of Applied Physics, 32, 5).
210 (Jpn. J. Appl. Phys. 32
(19939) 5210)). This method also utilizes the fact that the temperature distribution within the reproduction beam diameter is different from the light intensity distribution. Since GeSbTe in the high temperature portion becomes liquid and its optical constant is different from that in the solid state, the reflectance of the high temperature portion can be lowered by selecting the medium configuration. Ge
In order to appropriately determine the reflectance when SbTe is in the liquid state and the solid state and to enable repetition between the liquid phase and the solid phase, 4
A combination of more than one protective layer and a reflective layer is required.

In a magneto-optical recording medium or a phase change medium, a structure having a dielectric film is general. In the medium, the dielectric
Two types of functions are required: a protective effect for the recording film and an effect of enhancing the optical interference by enhancing the optical interference. In order to enhance the interference effect, it is necessary that the real part of the complex index be high and the imaginary part be small in extinction coefficient. Therefore, a silicon nitride film, an aluminum nitride film, a silicon carbide film or the like having a large refractive index is used as the dielectric film. The complex refractive index of each dielectric can be changed by changing the chemical composition, but it is common to use a stoichiometric composition with an extinction coefficient of 0 in order to enhance optical interference and enhance the reproduction signal. Is.

[0006]

In the conventional method as described above, a new functional layer is required to add the super-resolution mechanism. Therefore, the medium structure becomes complicated,
There is a drawback that the medium is expensive.

An object of the present invention is to provide an optical disk medium which realizes a super-resolution mechanism with a simple structure and a reproducing method thereof.

[0008]

According to the present invention, when the extinction coefficient k at the wavelength of reproducing light is 20 ° C., 0.01 ≦ k ≦
It is an optical disk medium having at least one of a base layer and an interference layer made of a dielectric layer within a range of 0.1. Examples of substances that can be used for the dielectric layer include a nitrogen-deficient silicon nitride film, a nitrogen-deficient aluminum nitride film, a silicon carbide film, a hydrogenated silicon carbide film, and an oxygen-deficient silicon oxide film. The present invention, the reproduction light beam diameter inner setting the reproducing power such that the portion of extinction coefficient of the dielectric layer increases there by increase in temperature, ie k ₀ extinction coefficient at 20 ° C., the reproduction light The reproducing light power is supplied so that the relationship of k ₁ / k ₀ > 1 is established, where k ₁ is the extinction coefficient of the dielectric layer at the rear end of the reproducing light beam that has changed due to temperature rise due to absorption of And a method for reproducing an optical disk medium.

[0009]

In the reproducing light of which the energy is slightly higher than the light absorption edge, the dielectric material absorbs light. The band gap that determines the light absorption edge becomes slightly narrower as the temperature rises. When the energy of the reproducing light is largely deviated from the energy of the bandgap, the optical constant of the dielectric of the bandgap change due to temperature change is not affected,
In regenerated light with a slightly higher energy than the light absorption edge,
The optical absorption of the dielectric increases with increasing temperature. Therefore, the extinction coefficient of the high temperature portion within the beam diameter changes, and the reflectance or Kerr rotation angle proportional to the reproduction output changes.
As a result, it becomes possible to achieve super-resolution that is equal to or larger than the beam diameter of the reproduction light. By using such a dielectric as the interference layer or the underlayer, the super-resolution function can be realized without adding a new functional layer.

As reproduction light having energy slightly higher than that at the light absorption edge, a combination with a dielectric material in which k at the wavelength of reproduction light is 0.01≤k is suitable. 0 ≦ k <0.
In 01, even if the temperature rises, the value of k does not change and cannot be used. On the other hand, when k ≧ 0.1, the optical absorption of the interference layer is too large and the reflectance or the Kerr rotation angle is reduced, and the reproduction output is reduced, which is not suitable. With light having a wavelength of 600 nm or more, which is currently used for reproducing optical discs, nitrogen deficient silicon nitride film, nitrogen deficient aluminum nitride film, silicon carbide film, hydrogenated silicon carbide film, oxygen deficient oxidation are used as materials satisfying these optical conditions. There is a silicon film. Further, the light absorption edge of these dielectric films can be changed by changing the composition ratio, and it is possible to cope with the case where the wavelength of the reproduction light is changed.

[0011]

Embodiments of the present invention will now be described in detail with reference to the drawings.

(Embodiment 1) FIG. 1 is a partial sectional view of a magneto-optical recording medium according to an embodiment of the present invention. In this magneto-optical recording medium, a nitrogen-deficient silicon nitride underlayer 2 having a thickness of 59 nm is formed on a plastic substrate 1 by using GdFeC.
o 30 nm of reproducing layer 3 and 30 nm of TbFeCo recording layer 4
, A nitrogen-deficient silicon nitride interference layer 5 having a thickness of 114 nm and an aluminum reflecting layer 6 having a thickness of 40 nm are sequentially formed. The nitrogen-deficient silicon nitride film (SiNx) was formed by rf sputtering using a silicon target.
Sputtering gas pressure is 0.2 Pa, argon gas flow rate is 50 s
ccm, nitrogen gas flow rate 13 sccm, input power 800
W. GdFeCo and TbFeCo were formed by dc magnetron sputtering using a composite target. The sputtering gas pressure is 0.08 Pa, the argon gas flow rate is 50 sccm, and the input power is 100 W. For comparison, a disk using a stoichiometric silicon nitride film (SiN) was prepared by changing the film forming conditions of the nitrogen-deficient silicon nitride film. The film forming conditions at this time are that the sputtering gas pressure is
2 Pa having a flow rate of 0.2 Pa, an argon gas flow rate of 50 sccm, a nitrogen gas flow rate of 50 sccm, and an input power of 800 W.
It is a temperature change of transmittance and reflectance at a wavelength of 690 nm of the SiNx film and the SiN film. The extinction coefficient k at 20 ° C. is 0.02 for the SiNx film and 0.0 for the SiN film. In the SiNx film, both the transmittance and the reflectance decrease as the temperature rises. Therefore, the optical absorption will increase with temperature. On the other hand, in the SiN film, there is no temperature change in both the transmittance and the reflectance.

FIG. 3 shows changes in the reproduction light amount of carriers and noise when a recording signal with a mark length of 0.3 μm is reproduced at a wavelength of 690 nm for two types of discs using a SiNx film and a SiN film. The linear velocity is 8 m / s and the reproduction wavelength is 690 nm. Noise increases monotonically as the amount of reproducing light increases for both discs. On the other hand, the carrier is SiN in the dielectric.
In the disk using, the noise increases monotonically like the increase in noise, but in the disk using SiNx, the reproducing power sharply increases at a reproducing power of 1.5 mW or more, and becomes 20 dB or more larger than that of the SiN disk. This is because the super-resolution occurred due to the temperature change of the optical absorption of the SiNx film.

FIG. 4 shows the recording frequency dependency of C / N. The reproduction power is 2.5 mW. In the disk using the SiN film, the C / N greatly decreases at 9 MHz,
C / N up to 12MHz for discs using SiNx film
Is 45 dB or more.

Example 2 Next, an example of a magneto-optical disk using a nitrogen-deficient aluminum nitride film (AlNx) will be described. The disk structure is the same as that of FIG. 1 except that the nitrogen-deficient silicon nitride layer is replaced with a nitrogen-defective aluminum nitride layer.
The AlNx film was formed by rf sputtering using an aluminum target. The sputtering gas pressure is 0.2 Pa, the argon gas flow rate is 50 sccm, and the nitrogen gas flow rate is 13 sccc.
m, input power 800W. Wavelength 6 at 20 ° C
The refractive index n at 90 nm is 2.2 and the extinction coefficient k is 0.03.

Also in this disc, the frequency characteristic is improved by increasing the reproduction power, and C
/ N was 45 dB or more.

(Embodiment 3) Next, an embodiment of a magneto-optical disk using a silicon carbide film (SiC) will be described. The disk structure is the same as that shown in FIG. 1 except that the nitrogen-deficient silicon nitride layer is replaced with a SiC layer. The SiC film was formed by rf sputtering using a SiC target. The sputtering gas pressure is 0.2 Pa, the argon gas flow rate is 50 sccm, and the input power is 800 W. Wavelength 690nm at 20 ℃
Has a refractive index n of 2.3 and an extinction coefficient k of 0.02.

Also in this disc, the frequency characteristic is improved by increasing the reproducing power, and C / 12 up to 12 MHz.
N was 45 dB or more.

Example 4 Next, a hydrogenated silicon carbide film (S
An example of a magneto-optical disk using iCHx) will be described. The disk structure is a nitrogen-deficient silicon nitride layer made of Si.
The same as FIG. 1 except that the CHx layer is used. SiCHx
The film was formed by rf sputtering using a SiC target. The sputtering gas pressure is 0.2 Pa, the argon gas flow rate is 50 sccm, the hydrogen gas flow rate is 2 sccm, and the input power is 800 W. The refractive index n at a wavelength of 690 nm at 20 ° C. is 2.2 and the extinction coefficient k is 0.01.

Also in this disc, the frequency characteristic is improved by increasing the reproduction power, and C /
N was 45 dB or more.

(Embodiment 5) Next, an embodiment of a magneto-optical disk using an oxygen-deficient silicon oxide film (SiOx) will be described. The disk structure is nitrogen-deficient silicon nitride layer S
The same as FIG. 1 except that the iOx layer is used. The SiOx film was formed by rf sputtering using a Si target. Sputtering gas pressure is 0.2 Pa, argon gas flow rate 50 sccm, oxygen gas flow rate 5 sccm, input power 8
It is 00W. Refractive index n at a wavelength of 690 nm at 20 ° C
Is 2.3 and the extinction coefficient k is 0.03.

Also in this disc, the frequency characteristic is improved by increasing the reproduction power, and C / C up to 12 MHz is obtained.
N was 45 dB or more.

(Embodiment 6) Next, an embodiment of a magneto-optical disk in which the film forming conditions of the nitrogen-deficient silicon nitride film (SiNx) are changed from those of Embodiment 1 will be described. The disk configuration is shown in Figure 1.
Is the same as The sputtering gas pressure is 0.2 Pa, the argon gas flow rate is 50 sccm, and the nitrogen gas flow rate is 10.5 scc.
There are two types, m and 10 sccm. Input power 800W
Is. The optical characteristics at a wavelength of 690 nm at 20 ° C. are n = 2.9 and k = 0.1 at a nitrogen gas flow rate of 10.5 sccm.
At 0 and 10 sccm, n = 3.1 and k = 0.11.

Also in this disc, the frequency characteristic is improved by increasing the reproduction power. Nitrogen gas flow rate 1
At 0.5 sccm, C / N of 45 dB was obtained up to 12 MHz. However, the C / N was 42 dB at 12 MHz in the disk having an extinction coefficient of 0.11 and a nitrogen gas flow rate of 10.5 sccm. This is because the Kerr rotation angle was reduced due to the large optical absorption of the dielectric layer.

(Embodiment 7) An embodiment in which a nitrogen-deficient silicon nitride film (SiNx) is used as a phase change recording medium will be described. A nitrogen-deficient silicon nitride underlayer having a thickness of 100 nm, a GeSbTe recording layer having a thickness of 20 nm, a nitrogen-deficient silicon nitride interference layer having a thickness of 15 nm, and an aluminum reflective layer having a thickness of 40 nm are sequentially formed on a plastic substrate. It was made. The nitrogen-deficient silicon nitride film (SiNx) was formed by rf sputtering using a silicon target. Sputtering gas pressure is 0.2 Pa, argon gas flow rate is 50 sc
cm, nitrogen gas flow rate 13 sccm, input power 800 W
Is. GeSbTe membrane is a dc with composite target
The film was formed by magnetron sputtering. The sputtering gas pressure is 0.08 Pa, the argon gas flow rate is 50 sccm, and the input power is 100 W. For comparison, a disk using a stoichiometric silicon nitride film (SiN) was prepared by changing the film forming conditions of the nitrogen-deficient silicon nitride film. The film forming conditions at this time are as follows: sputtering gas pressure 0.2 Pa, argon gas flow rate 50 sccm, nitrogen gas flow rate 50 sccm,
C for two types of disks with input power of 800 W
The recording frequency dependence of / N was measured. Playback power is 2.
It is 5 mW. 9MH for discs using SiN film
Although the C / N greatly decreases with z, the C / N is 45 dB or more up to 13 MHz in the disk using the SiNx film.

(Embodiment 8) Next, an embodiment in which a nitrogen-deficient silicon nitride film (SiNx) is used for a CD-ROM will be described. A nitrogen-deficient silicon nitride underlayer having a thickness of 100 nm and an aluminum reflecting layer having a thickness of 40 nm were sequentially formed on a plastic substrate having recording signals as prepits. Nitrogen-deficient silicon nitride film (SiN
x) was formed by rf sputtering using a silicon target. The sputtering gas pressure is 0.2 Pa, the argon gas flow rate is 50 sccm, the nitrogen gas flow rate is 13 sccm, and the input power is 800 W. For comparison, a disk using a stoichiometric silicon nitride film (SiN) was prepared by changing the film forming conditions of the nitrogen-deficient silicon nitride film. The film forming conditions at this time are as follows: sputtering gas pressure 0.2 Pa, argon gas flow rate 50 sccm, nitrogen gas flow rate 50 sccm,
The recording frequency dependence of C / N was measured for two types of disks having an input power of 800 W. The reproduction power is 2.5 mW. In the disk using the SiN film, the C / N greatly decreases at 9 MHz, but in the disk using the SiNx film, the C / N is 45 dB or more up to 13 MHz.

[0027]

As described above, according to the present invention, when the recording mark length is shorter than the laser beam diameter by adding the super-resolution function to the dielectric layer used as the interference layer or the underlayer. In this way, a sufficient reproduction signal level was obtained, and it became possible to provide a high-performance and inexpensive optical disk medium.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view of a magneto-optical recording medium of the present invention.

FIG. 2 is a diagram showing changes in transmittance and reflectance with temperature of a nitrogen-deficient silicon nitride film and a stoichiometric silicon nitride film at a wavelength of 690 nm.

[Fig. 3] Wavelength of 690 nm for a recording signal with a mark length of 0.3 μm
FIG. 6 is a diagram showing a change in reproduction light amount of carrier and noise when reproduction is performed in FIG.

FIG. 4 is a diagram showing recording frequency dependence of C / N.

[Explanation of symbols]

1 Plastic Substrate 2 Nitrogen-deficient Silicon Nitride Underlayer 3 GdFeCo Reproducing Layer 4 TbFeCo Recording Layer 5 Nitrogen-deficient Silicon Nitride Interference Layer 6 Al Reflective Film 21 Stoichiometry Silicon Nitride Film Transmittance and Reflectance 22 Nitrogen-deficient Silicon Nitride Film Transmission Ratio 23 Reflectivity of nitrogen-deficient silicon nitride film 31 Noise and carrier of disk using stoichiometric silicon nitride film 32 Noise and carrier of disk using nitrogen-deficient silicon nitride film 41 Disk using stoichiometric silicon nitride film C /
C / of a disk using N 42 nitrogen-deficient silicon nitride film
N

Claims (3)

[Claims] 1. An optical disk medium having at least one of a base layer and an interference layer made of a dielectric layer, wherein the extinction coefficient k of the dielectric with respect to the wavelength of reproduction light at 20 ° C. is 0.01 ≦ k. An optical disk medium having a range of ≦ 0.1. 2. The dielectric layer is selected from a nitrogen-deficient silicon nitride film, a nitrogen-deficient aluminum nitride film, a silicon carbide film, a hydrogenated silicon carbide film, and an oxygen-deficient silicon oxide film. Optical disc media. 3. When the extinction coefficient at 20 ° C. is k ₀ and the extinction coefficient of the dielectric layer at the rear end of the reproduction light beam changed due to temperature rise due to absorption of reproduction light is k ₁ , k ₁ / k
A reproducing method of an optical disc medium, characterized in that the power of reproducing light is supplied so that the relation of ₀ > 1 is always established.