JPH06251422A - Optical disk - Google Patents

Abstract

PURPOSE:To improve the reproduction sensitivity of an optical disk for ultrahigh resolution reproduction system and to exactty execute an ultrahigh resolution reprroduction with good C/N even at the time of exectiting the repraduction with rather high line speed. CONSTITUTION:A phase transfer material layer 4 which shows change in reflectance at least after melting is formed on a transparent substrate 1 in which recording pits 2 optically readable are formed according to information signals. A reflecting layer 6 is formed on the phase transfer material layer 4. When the disk is irradiated with reading light, the phase transfer material layer 4 is partially changed into a liquid phase in the scanned spot of light to change reflectance. The reflecting layer 6 consists of such a material having <=1.0J/cmKs thermal conductivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk for reproducing information by irradiating a laser beam, and more particularly to an optical disk suitable for high density recording.

[0002]

2. Description of the Related Art For optical discs such as digital audio discs (so-called compact discs) and video discs, an aluminum reflective film is formed on a transparent substrate on which recording pits, so-called phase pits are formed in advance according to an information signal. Then, a protective film or the like is formed on it. In such an optical disc,
Signals are read (reproduced) by irradiating the disk surface with read light and detecting a large decrease in the amount of reflected light due to light diffraction at the phase pit formation portion.

By the way, in the above-mentioned optical disk, the resolution of signal reproduction is almost determined by the wavelength λ of the light source of the reproduction optical system and the numerical aperture NA of the objective lens, and the spatial frequency 2NA / λ is the reproduction limit. Therefore, in order to achieve high density in such an optical disc, it is necessary to shorten the wavelength λ of the light source (for example, semiconductor laser) of the reproducing optical system or increase the numerical aperture NA of the objective lens. .

[0004]

However, the wavelength λ of the light source is
There is a limit to the improvement of the numerical aperture NA of the objective lens and the objective lens, and it is difficult to dramatically increase the recording density by this. Therefore, the present applicant has proposed an optical disc that can obtain a resolution higher than the above-mentioned limits by the wavelength λ and the numerical aperture NA by utilizing the reflectance change due to the partial phase change in the scanning spot of the reading light. (See Japanese Patent Application Nos. 2-94452 and 3-249511).

The inventions related to these applications relate to an optical disc or a reproducing system thereof for performing super-resolution reproduction by changing the reflectance by a partial phase change in the laser spot of the reading light.

However, in the inventions related to these applications, the reproduction sensitivity when the phase change material layer is used is not sufficiently taken into consideration. For example, in recent years, a high information transfer rate of about 10 Mbps has been required for a video disc and the like, and it has been desired to realize high frequency recording of about 10 and several MHz, but the linear velocity is, for example, 3.6 m / s or 7.6 m / s.
When the speed is relatively fast, such as about s, there is a disadvantage that a sufficient C / N (carrier / noise ratio) or S / N (sound / noise ratio) cannot be obtained.

The present invention makes it possible to more stably perform super-resolution reproduction with a high C / N by improving the sensitivity of the optical disc for performing the super-resolution reproduction as described above.

[0008]

According to the present invention, as shown in the schematic sectional view of the essential portion of FIG. 1, a transparent substrate 1 having a recording pit 2 which can be optically read according to an information signal is formed. A phase change material layer 4 capable of changing reflectance at least after melting is formed thereon, and a reflection layer 6 is formed on the phase change material layer 4, and when the reading light is irradiated, the phase change material 4 is formed. The layer 4 has a structure in which the reflectance changes due to partial liquid phase in the scanning spot of the reading light, and the reflective layer 6 is made of a material having a thermal conductivity of 1.0 J / cmKs or less. Further, in the present invention, the lanthanoid is used as the material of the reflective layer 6 in the above structure.

[0009]

The optical disk according to the present invention has the recording pit 2
When reading or reproducing the recording by the recording medium, the temperature distribution in the scanning spot of the reading light is used to partially part the phase change material layer 4 in the high temperature region generated in the spot.
A liquid phase state is generated in the optical disk so that, for example, the reflectance here is remarkably increased, and for example, the recording pit in the liquid phase state portion can be read by diffraction or the like.

That is, even when a plurality of, for example, two recording pits 2 enter the read-out light spot, for example, an area where one recording pit 2 is optically disappeared is formed, and a single recording pit is formed in the spot. It can read only, and can perform super-resolution reproduction that is not limited to λ / 2NA.

In particular, in the present invention, as the material of the reflective layer 6, a material having a thermal conductivity of 1.0 J / cmKs or less is selected. Conventionally, a material having a high thermal conductivity such as Al is selected as the material for the reflective layer 6. In this case, for example, a semiconductor laser may not be able to obtain a sufficient temperature gradient and reproduction may not be possible. It was However, according to the present invention, since the thermal conductivity of the reflective layer 6 is appropriately selected, a sufficient temperature gradient can be obtained and good reproduction can be performed even when the linear velocity of the optical disk is increased.

In particular, by using lanthanoid as the material, the sensitivity required for various applications can be obtained, and super-resolution reproduction can be performed more stably with a high C / N.

[0013]

Embodiments of the present invention will now be described in detail with reference to the drawings. As shown in the basic structure of the cross-sectional view of FIG. 1, the present invention, at least on a transparent substrate 1 on which recording pits 2 formed of unevenness corresponding to information, so-called phase pits (or embossed pits) are formed. The phase change material layer 4 that can return to the initial state after melting is formed, and the reflective layer 6 is formed on the phase change material layer.

When the phase change material layer 4 is irradiated with read-out light, for example, a laser beam, the phase-change material layer 4 partially becomes in a liquid phase in the scanning spot of the read-out light and the reflectance is reduced. At the same time, the reflectance is returned to the initial state in the normal state after reading.

In the example shown in FIG. 1, the recording pit 2
The case where the phase change material layer 4 and the reflective layer 6 are directly formed on the transparent substrate 1 having
As shown in the schematic enlarged cross-sectional view of the main part thereof, the phase change material layer 4 is formed on the transparent substrate 1 having the recording pits 2 via the first dielectric layer 3, and further on this. A reflective film 6 is formed via the second dielectric layer 5, and a third film is formed on the reflective film 6.
Dielectric layer 7 is formed, and a protective film (not shown) is further formed on the dielectric layer 7 if any, and optical characteristics such as reflectance are set by the first and second dielectric layers 5 and 6. Can be configured. Further, by providing the third dielectric layer 7, the mechanical strength of the laminated film is improved,
Repeated reading durability is improved.

Example 1 In this example, a glass 2P substrate was used as the transparent substrate 1 when the configuration described in FIG. 2 was adopted. The 2P referred to here is a photopolymer method. In this embodiment, the track pitch P is 1.6 μm, the pit depth is about 120 nm, the pit width, that is, the width in the disk radial direction is about 0.4 μm, and the pit length is 0.3 to 2.0 μm.
The recording pit 2 was formed under the setting conditions.

Then, on the one main surface of the transparent substrate 1 having the recording pits 2, a first dielectric layer 3 made of, for example, a mixture of ZnS 80 atomic% and SiO ₂ 20 atomic% having a thickness of 110 nm is adhered and formed. On top of this, for example a thickness of 23.4
A phase change material layer 4 made of Ge ₂ Sb ₂ Te ₅ ternary alloy or the like having a thickness of 1 nm was deposited. Further on top of this a thickness of eg 70
second dielectric layer 5 made of a mixture of 80 nm% ZnS and 20 at% SiO ₂ is deposited, and a reflection film 4 made of Gd or the like is deposited on the second dielectric layer 5 to a thickness of 200 nm, for example. On top of this, for example 400 nm thick Zn
S80 atomic%, and the third dielectric layer 7 made of SiO ₂ 20 atomic% mixtures is deposited and formed.

With respect to the optical disk thus formed, the numerical aperture of the lens in the optical system is 0.50,
The linear velocity of the laser spot with respect to the optical disk was changed to 7.6 by using the reproducing device whose semiconductor laser wavelength is 780 nm.
Set the playback power to 9 mW by setting m / s and the pit length L
= 0.3, 0.4, 0.5, 0.8, 1.2, 2.0μ
The C / N at each pit length of m was measured. The result is shown in FIG.

As can be seen from FIG. 3, in this case, the C / N ratio was as good as 50 dB at a pit length of 0.3 μm below the diffraction limit.
As described above, it can be seen that excellent super-resolution reproduction can be performed with a high C / N even at a relatively high linear velocity of 7.6 m / s. That is, as described above, even when the present invention is applied to a video disc or the like and high transfer rate of about 10 Mbps and high frequency recording of about 10 MHz are performed, good reproduction is performed with a high linear velocity as described above. be able to.

Example 2 In this example, the reflective layer had a thermal conductivity of 0.214.
The structure was the same as that of the above-described Example 1 except that an Al 65 wt% and Ti 35 wt% alloy having J / cmKs was used.

With respect to the optical disc thus formed, the linear velocity is set to 3.6 m / s and the reproducing power is set to 1
0 mW and each pit length L = 0.
The respective C / N at 3, 0.4, 0.5, 0.8, 1.2 and 2.0 μm were measured. The result is shown in FIG. Even in this case, the pit length of 0.
Super-resolution reproduction can be performed with a good C / N of 50 dB at 3 μm, which is suitable for application to a disc having a high rotational speed and a high recording frequency.

Comparative Example 1 In this example, the reflective layer had a thermal conductivity of 1.0 J /
For example, Al having a viscosity exceeding 2.14 J / cmKs exceeding cmKs was used, and the other structures were the same as those of the above-described first embodiment. For the optical disc thus formed, the linear velocity is set to 3.6 m / s and the reproducing power is set to 1
0 mW, pit length L = 0.3, 0.4, 0.
The results of measuring the relationship between C / N and the pit length at 5, 0.8, 1.2 and 2.0 μm are shown in FIG.

In this case, 20d with a pit length of 0.3 μm
It can be seen that only a low C / N of about 35 dB can be obtained at B and 0.4 μm, which is not suitable for practical use when performing super-resolution reproduction.

Similarly, with respect to an optical disc formed by omitting the reflection layer 6, a C / N having a pit length of 0.3 μm when the linear velocity is set to 7.6 m / s and the reproduction power is set to 9 mW. Was 20 dB, which was not suitable for practical use as in Comparative Example 1 described above.

Therefore, particularly when reproducing at a relatively high linear velocity exceeding 1.8 m / s, according to the present invention, the thermal conductivity of the reflective layer 6 should be 1.0 J / cmKs or less. From the above, it is understood that a sufficient temperature gradient can be obtained and excellent super-resolution reproduction can be performed even when a light source having a relatively low power such as a semiconductor laser is used.

As the material of the transparent substrate 1, acrylic resin, polyolefin resin, glass or the like can be used. Further, as the phase change material layer 4, Se, Te
Any chalcogenide and its compound, chalcogen, can be applied to the optical disk of the present invention.

Further, as the reflective layer 6, the above-mentioned Gd, A
Other lanthanoids, not limited to 1Ti alloys,
A metalloid element such as Sb and Bi, a semiconductor element such as Ge and Si, or a compound or mixture can be used, but a material having an optimum thermal conductivity depending on the transfer rate, linear velocity, etc. used for the optical disk. By selecting, good reproduction sensitivity can be obtained.

Further, in the above-mentioned structure shown in FIG. 2, the first to third dielectric layers 3, 5 and 7 are made of Al or Si.
Examples thereof include nitrides, oxides, and sulfides of the same metal and semiconductor elements, and any compound may be used as long as it has no absorption in the semiconductor laser wavelength region.

The optical disk according to the present invention utilizes the temperature distribution of the reading light in the scanning spot upon reproduction thereof to make the phase change material layer 4 partially in the liquid phase state in the high temperature region generated in the spot. For example, the phase pits in this liquid phase state portion can be read out by diffraction or the like so that the reflectance in this portion is remarkably increased and super resolution reproduction is enabled. You can

Hereinafter, a reproducing mode when a laser spot is irradiated on the optical disk according to the present invention will be described with reference to FIGS. 6A and 6B. In FIG. 6A, the horizontal axis represents the position in the scanning direction of the spot. Looking now at the state where the optical disk is irradiated with the laser beam spot by the laser irradiation, in this case, the light intensity is shown by the solid line L in the figure. The distribution is shown. On the other hand, the temperature distribution corresponding to the temperature distribution in the phase change material layer 4 has a temperature peak slightly on the scanning direction side of the disk corresponding to the scanning speed of the laser spot as shown by the solid line T.

If the disk is moved here as shown by arrow d, that is, the laser spot is scanned in the opposite direction, the temperature of the optical disk gradually rises from the front end side in the traveling direction of the laser spot. Next, the temperature becomes equal to or higher than the melting point mp of the phase change material layer 4. At this stage, the phase change material layer 4 changes from the initial crystalline state to the molten state, and the reflectance decreases due to the transition to the molten state.

Therefore, in the laser beam spot, there are a region having a relatively low reflectance, that is, a mask 13, which is shown with a dotted pattern in FIG. 6B, and a region aperture 12 capable of reading the recording pit 2. In this case, even if, for example, two recording pits 2 are present in the same spot as shown in the figure, the reading is performed only for one recording pit 2 present in the aperture 12 which is an area having a high reflectance. The other recording pits 2 cannot be read because they are in the region of the mask 13 having extremely low reflectance.

Even if a plurality of recording pits 2 are present in the same spot as described above, it is possible to read out only a single recording pit 2, and the numerical aperture NA of the lens system and the wavelength λ of the reading light are read. Super resolution reproduction is possible without being limited to.

At this time, the reflectance is specified by the optical constant and the film thickness of the constituent material of each layer. Therefore, the sensitivity can be adjusted by using a reflective layer having different thermal conductivity. Here, in the super-resolution reproduction in the present invention, since the phase change material layer flows due to the melting, repeated reproduction becomes difficult. Therefore, it does not mean that sensitivity is good, but it is necessary to strike a balance between sensitivity and repeated durability.

Further, in each of the above-mentioned embodiments, the recording pits 2 are formed by the unevenness on the transparent substrate 1, but the present invention is also applicable to forming other optically readable recording pits. Needless to say, it can be applied and various other modifications can be made.

[0036]

As described above, according to the present invention, the difference in the reflectance is caused by the difference in the temperature distribution in the read light spot so that the read can be performed only with respect to a specific phase pit in the light spot. In addition to performing super-resolution reproduction, it is possible to perform reproduction with a high C / N (S / N) especially by using a material having a thermal conductivity of 1.0 J / cmKs or less for the reflective layer. Thus, it is possible to obtain an optical disc having a high recording density.

Particularly, for example, in a video disc or the like, 1
To achieve a high transfer rate of 0 Mbps, 10
Even when high-frequency recording of about several MHz is performed and reading is performed at a relatively high linear velocity of, for example, 3.6 m / s or 7.6 m / s, which exceeds the linear velocity of 1.8 m / s, the laser spot is within the laser spot. A sufficient temperature distribution can be configured, and super-resolution reproduction can be performed more stably with a high C / N.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a main part showing a configuration of an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view of an essential part showing the configuration of another embodiment of the present invention.

FIG. 3 is a diagram showing a relationship between a pit length and C / N in Example 1.

FIG. 4 is a diagram showing a relationship between a pit length and C / N in Example 2.

5 is a diagram showing a relationship between a pit length and C / N in Comparative Example 1. FIG.

FIG. 6 is a diagram showing a relationship between a light intensity distribution of a laser spot and a temperature distribution of an optical disc.

[Explanation of symbols]

 1 Transparent Substrate 2 Recording Pit 3 First Dielectric Layer 4 Phase Change Material Layer 5 Second Dielectric Layer 6 Reflective Layer 7 Third Dielectric Layer

Claims (2)

[Claims] 1. A transparent substrate on which a recording pit that can be optically read according to an information signal is formed, at least a phase change material layer is formed, and a reflection layer is formed on the phase change material layer. When the reading light is irradiated, the phase change material layer is partially liquefied in the scanning spot of the reading light to change the reflectance, and the reflection layer has a thermal conductivity of An optical disc comprising a material having a value of 1.0 J / cmKs or less. 2. The optical disk according to claim 1, wherein a lanthanoid is used as a material of the reflective layer.