JPH09115179A - Optical disk and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to reproduce an optical disk with light of shorter wavelength by forming a VO2 film having an excellent surface smoothness sufficient for executing high-density recording with good reproducibility. SOLUTION: This optical disk is constituted by laminating a transparent substrate 1, a ground surface layer 6 consisting of tantalum (Ta) formed on the surface of this transparent substrate 1, a vanadium oxide thin film 2 exhibiting metal-nonmetal transition reversibly with respect to temp. and a reflection film 3 in this order. In such a case, the thicknesses of the ground surface layer 6 consisting of the tantalum and the vanadium oxide thin film 2 exhibiting the metal-nonmetal transition are specified respectively to 25 to 30nm and 60 to 75nm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disc capable of optically reproducing information, and more particularly to a high density type optical disc.

[0002]

2. Description of the Related Art In recent years, an increase in the capacity of an optical disk has been studied.
Various proposals have been made. In high-density recording on optical discs, it is possible to form recording marks smaller than the light spot diameter by controlling the laser light power during recording, but the limit is the crystal grain size of the medium. It depends on the dependent resolution, the threshold value of the light intensity of the medium, the response speed of the medium, and the like. However, the minimum limit of a record mark that can be recorded is much smaller than the minimum limit of a reproducible record mark.

[0003] This is because when reproducing an optical disk, a reproducing laser beam (hereinafter, the reproducing laser beam is also simply referred to as a reproducing beam).
This is because the light spot diameter when condensing light with a lens has a limit value determined by optical theory. Therefore, increasing the density of an optical disc depends on how to reduce the spot diameter of the reproduction laser light. ing. The limit value of the repetitive wavelength of a reproducible recording mark (pit) is given by λ / 2NA, where λ is the wavelength of the laser beam for reproduction and NA is the numerical aperture of a condenser lens for condensing the laser beam.

Accordingly, in order to identify and reproduce a recording mark having a shorter recording wavelength, it is sufficient to reproduce the light with light having a shorter wavelength λ or use a lens having a larger numerical aperture NA. However, it is technically difficult to shorten the wavelength of a semiconductor laser used for reproduction, and it is not easy to incorporate a lens having a large numerical aperture NA into an optical disk device. In addition to these methods, the method of effectively reducing the light spot during reproduction is described in "Solid State Physics" VOL.26 NO.6 1991 P393-P398.
It is described in. This utilizes the fact that when a laser beam is condensed and irradiated on a magneto-optical disk medium, a low-temperature region and a high-temperature region are generated in this light spot, and only the high-temperature portion or the low-temperature portion is read. However, it will be possible. As a result, the effect of substantially reducing the diameter of the light spot is obtained.

However, this technique can be applied to a rewritable optical disk by a magneto-optical system (hereinafter, the rewritable optical disk is also referred to as a RAM optical disk).
There is a problem that it cannot be applied to a read-only optical disc (hereinafter, the read-only optical disc is also referred to as a ROM optical disc). In order to solve this, a metal-nonmetal transition layer made of a substance exhibiting a metal-nonmetal transition is provided on a transparent substrate on which information is previously formed in a circular shape, and the diameter of a reproduction light spot is substantially reduced. The present invention has been applied for by the present applicant (Japanese Patent Application No. 5-29).
900). The optical disk of this prior application has a structure as shown in FIG. 2, and VO ₂ is used as a substance exhibiting a metal-nonmetal transition of ₂ .

FIG. 2 is a sectional view showing the structure of the optical disc of the prior application. The ROM type (read-only type) optical disc 20 shown in FIG. 2 includes a transparent substrate 1 and a metal-nonmetal transition layer 2.
An optical disc having a configuration in which a reflective layer 3, a protective layer 4, and a protective layer 4 are laminated in this order. As the metal-nonmetal transition layer 2, for example, a thin film made of vanadium dioxide VO ₂ is used, and as the reflective layer 3, an Al thin film having a thickness of 200 nm is used.

FIG. 4 shows an example of characteristics of the VO ₂ thin film obtained in the experiment. FIG. 4 is a diagram showing the relationship between temperature and reflectance in a VO ₂ thin film. As shown in FIG. 4, the phase transition point Tch at the time of temperature rise is about 65 ° C., the phase transition point Tcl at the time of temperature decrease is about 55 ° C., and the reflectance is 1 at the low temperature phase.
It changed reversibly at 6% and 3.5% in the high temperature phase. Next, a method for manufacturing the ROM type optical disk 20 having the VO ₂ thin film as a constituent element will be briefly described. Here, a VO ₂ film was formed by a DC magnetron sputtering device on a quartz glass substrate on which information pits were previously formed by dry etching.
A film is formed, an Al reflective film is formed thereon by a sputtering method, and an ultraviolet-curable protective film is further formed thereon by a spin coating method.

Then, a laser beam having a wavelength of 780 nm is radiated from the substrate side of the optical disc 20, and the optical disc 2
The reflectance of the optical disk was measured by detecting the reflected light from the land portion (mirror portion) of 0. As a result, the respective reflectances in the low temperature phase and the high temperature phase when the thickness of the VO ₂ film was changed were
It was as shown in FIG. FIG. 5 is a diagram showing the relationship between the film thickness of the VO ₂ film and the reflectance in the optical disc of the present invention.

In FIG. 5, the horizontal axis represents the thickness of the VO ₂ thin film,
The vertical axis represents the reflectance of the mirror portion when laser light having a wavelength of 780 nm is irradiated from the transparent substrate side of the optical disc. The curve A shown by the solid line shows the characteristics in the low temperature phase measured at an ambient temperature of 20 ° C., and the curve B shown by the dotted line shows the characteristics in the high temperature phase measured at an ambient temperature of 100 ° C. FIG.
When the VO ₂ film thickness is 50 nm, the reflectance is
It was 5% in the low temperature phase and 42% in the high temperature phase.

When a high density disc was reproduced by utilizing this difference in reflectance, crosstalk from recording marks on adjacent tracks and intersymbol interference in the same track were reduced. Further, when the VO ₂ film thickness was 130 nm, the reflectance was 20% in the low temperature phase and 3% in the high temperature phase. When reproducing a high-density disc by utilizing this difference in reflectance, similarly, crosstalk from recording marks on adjacent tracks and intersymbol interference in the same track were reduced.

A method of reproducing the optical disc shown in FIG. 2 will be described below. In reproduction of the ROM type optical disk shown in FIG. 2, reproduction laser light (reproduction light) is focused on the rotating optical disk and irradiated from the substrate side, and return light from the optical disk is detected. Utilizing the return light, focus control and tracking control for irradiating a laser beam to a predetermined position on the optical disk in a spot shape are performed, and reading of minute recording marks (pits) is performed.

When the metal-nonmetal transition layer 2 of the optical disk 20 is irradiated with reproducing light, the light is absorbed and the temperature rises. Since the light intensity distribution of the laser light spot has a Gaussian distribution, when the optical disk is stopped, the temperature at the center of the laser light spot becomes high according to the light intensity. However, since the optical disk is rotating during reproduction, a temperature distribution occurs in the track direction around the laser light spot. That is, in front of the laser beam spot with respect to the rotation direction of the optical disc,
That is, the temperature of the rear side of the laser light spot, that is, the side of the laser beam irradiated later is higher than the temperature of the side irradiated with the light beam first.

When the output of the laser light is set so that the temperature in front of the laser light spot becomes lower than the phase transition point Tc and the temperature in the rear becomes higher than Tc, the refractive index at a part inside the laser light spot. Difference occurs. As a result of this, FIG.
As shown in, when the reflectance R decreases at a temperature higher than the phase transition point Tc, for example, when the film thickness of VO ₂ is 130 nm, only the reflected light from the front of the laser light spot becomes strong and the reproduced signal Is obtained. As a result, an effect substantially equivalent to a reduction in the diameter of the reproduction light spot can be obtained.

Further, when the reflectance R rises at a temperature higher than the phase transition point Tc, for example, when the VO ₂ film thickness is 50 nm, only the reflected light from the rear of the laser light spot becomes strong and the reproduced signal is reproduced. can get. As a result, an effect substantially equivalent to a reduction in the diameter of the reproduction light spot can be obtained. Then, crosstalk from recording marks on adjacent tracks and intersymbol interference in the same track can be reduced.

By the way, as a conventionally known method for producing a VO2 thin film, a reactive sputtering method in which vanadium metal is sputtered in a mixed gas of argon and oxygen and vanadium is oxidized, or vanadium is evaporated by an electron beam in oxygen gas. There is a reactive vapor deposition method in which it is oxidized. It has been reported in various literatures that the thin films obtained by these methods do not show a metal-nonmetal transition as they are, and that they show a metal-nonmetal transition only after being subjected to a heat treatment in an oxygen atmosphere or air. There is. Vanadium ion is up to 5
The valences up to the valence are obtained, and VO, V ₂ O ₃ , VO ₂ , and V ₂ O ₅ are known as typical vanadium oxides.

In addition to this, the general formula V _n O _2n-1 (n =
The intermediate compounds represented by 3, 4, 6) exist in a very narrow composition range. Since they are controlled by the equilibrium oxygen partial pressure, it is difficult to selectively obtain only the VO2 thin film from many oxides, and the reproducibility of the film formation is poor.
On the other hand, an example of applying VO ₂ to a rewritable optical disc is "Preparation of VO2 thin film and its direct op
tical bit recording characteristics. "(APPLIEDOPTIC
S Vol.22 No.2 265 (1983)). According to it, by RF (radio frequency) sputtering a vanadium metal target in a nitrogen-oxygen plasma,
VO ₂ is obtained without heat treatment after film formation.

However, when prepared by mixing oxygen into argon, which is a generally used inert gas, the surface property of the thin film is rough, and V ₂ O _{5 which} does not show a metal-nonmetal transition. It is said that a yellow film similar to is obtained. (This document is intended to use VO ₂ as the recording film, when the present invention is characteristics required for the VO ₂ layer is used disparate .vo ₂ in the recording film for use as the auxiliary reproducing layer, VO ₂ The recording state (metal state) cannot be maintained unless the temperature (68 ° C.), which originally has a metal-nonmetal transition, is lowered to room temperature or lower.
The reproduction assisting layer (vanadium oxide thin film) in the present invention positively utilizes the fact that the transition temperature is higher than room temperature, and utilizes the fact that the transition occurs only when the reproduction light is applied. )

Therefore, the present inventors provided a vanadium underlayer between the VO ₂ film and the transparent substrate to form a VO ₂ thin film having a predetermined thickness and a uniform refractive index over a large area with good reproducibility. , An optical disc in which the light spot diameter is substantially reduced is proposed. The optical disc will be described below. FIG. 3 is a sectional view showing the structure of a conventional optical disc. In FIG. 3, the ROM type optical disk 30 is a transparent substrate 1 and a vanadium oxide layer 2 of the optical disk 20.
And a vanadium base layer 5 are formed between them. The vanadium underlayer 5 has a thickness of 3 to 30 nm, and the vanadium oxide thin film layer 2 has a thickness of 10 nm.
It is a thin film composed of vanadium dioxide VO ₂ having a thickness of nm to 5 μm.

The optical disk 30 is manufactured by a DC magnetron device as shown in FIG. The DC magnetron device 40 shown in FIG. 6 is a DC magnetron device including a pair of cathodes (targets) and anodes (substrate holders). In FIG. 6, a substrate 12 and a target 22 are arranged to face each other in a vacuum chamber 11, and a film thickness sensor 14 is provided between them. Several targets 22 are arranged in the vacuum chamber 11, and the targets are moved to a position facing the substrate so that a voltage is applied from the DC power supply 16 to the metal to be sputtered to form a film on the substrate. It has become. Cooling water is flowing through the water cooling pipe 24 to the lower portion 23 of the target 22, so that the temperature of the target 22 rises and the sputter rate does not change. A glass substrate is used for the substrate and vanadium metal is used for the target. The film thickness sensor 14 is of a crystal type and utilizes that the resonance frequency changes when a thin film is attached to the surface of the crystal unit. The controller 19 controls the voltage of the DC power supply 16 so that the sputter rate becomes constant by changing the resonance frequency of the film thickness sensor 14. During film formation, the substrate is heater 13 embedded in the substrate holder.
It can be heated to 300-500 ° C.

The vacuum chamber 11 is evacuated in advance to 8 × 10 ^-4 Pa or less by a vacuum pump 18. Gas pipe 17
After the Ar gas is introduced through the exhaust gas and the exhaust amount is adjusted by the exhaust control valve 25, a predetermined degree of vacuum is obtained, and vanadium metal is sputtered before the main sputtering. This removes the oxide layer on the target surface. Then shutter 15
Is opened and the vanadium underlayer 5 is formed to a predetermined thickness. The vanadium underlayer 5 has a film thickness of 3 to 30 nm, and when light is incident through the substrate, it is preferable to set the film thickness so that the amount of incident light is not attenuated.

Then, reactive sputtering is performed in a mixed gas of Ar and O ₂ . After the shutter 15 is closed and pre-sputtering is performed until the oxidation state of the target surface becomes steady, the shutter 15 is opened and VO ₂ is deposited to a predetermined film thickness.
At this time, the film-forming speed is monitored by the film thickness sensor 14, and the electric power supplied to the DC power supply 16 is adjusted by a current through the controller 19 so that the film-forming speed becomes constant. If the film formation rate is high, VO ₂ is insufficiently oxidized (excess of vanadium), and if it is slow, it is excessively oxidized. The oxidation state of vanadium is sensitive to the partial pressure of oxygen, and even a slight fluctuation significantly changes. Also,
Even if the oxygen partial pressure and the applied sputtering power are fixed, the film formation rate varies, so it is necessary to directly monitor and adjust the film formation rate corresponding to the vanadium oxide production rate.
By this method, as-depo (as deposition) metal-
Polycrystalline VO ₂ exhibiting a non-metal transition can be obtained uniformly and with good reproducibility.

[0022]

By the way, since there is a large difference in lattice constant between vanadium oxide VO ₂ and vanadium V, when VO ₂ is laminated on vanadium underlayer 5, VO ₂ becomes irregular. There is a problem in that the VO ₂ crystal grains are stacked and become a size that cannot be ignored as compared with the size of minute recording marks (pits) formed below the diffraction limit. In such an optical disc that detects information using light, such a large crystal grain
This causes noise in the reproduction signal and increases the reproduction error occurrence rate.

Therefore, the present inventors have found that vanadium and VO
To alleviate the lattice constant between ₂ and the vanadium underlayer 5, between the vanadium oxide thin film 2, vanadium by vanadium oxide VO _x was gradually with different content of oxygen in the depth direction Oxide intermediate layer (hereinafter referred to as VO _x
The intermediate layer or the intermediate layer) is provided.
As a result, the smoothness of the VO ₂ thin film surface was improved as compared with the case where VO ₂ was directly laminated on the vanadium underlayer. However, even if the VO _x intermediate layer is provided, the size of the crystal grains on the surface of the VO ₂ thin film does not become so small that it can be completely ignored with respect to the size of the recording mark (pit). You will be limited. Therefore, in order to record the optical disc at a higher density, VO ₂
It is necessary to further improve the smoothness of the thin film surface.

Further, if the change in reflectance can be made large with respect to light of a short wavelength, it becomes possible to reproduce a finer area,
It becomes possible to sufficiently cope with an optical disc recorded at high density. The present invention has been made in view of the above-mentioned problems, and an optical disc capable of forming a VO ₂ thin film having excellent surface smoothness sufficient for high density recording and reproducing by light of a shorter wavelength, and the same. It is intended to provide a manufacturing method.

[0025]

The invention according to claim 1 for achieving the above-mentioned object is "at least a transparent substrate,
In the optical disk provided with a vanadium oxide thin film showing a metal-nonmetal transition, an underlayer made of tantalum, and a reflective film in this order, the vanadium oxide thin film showing the metal-nonmetal transition and the thickness of the underlayer 60 to each
An optical disk having a thickness of 75 nm and 25 to 30 nm. "

Further, the invention according to claim 2 is "an optical disk provided at least with a transparent substrate, a vanadium oxide thin film showing a metal-nonmetal transition, an underlayer made of tantalum, and a reflective film. In the manufacturing method, the thickness of the underlying layer made of tantalum is 25 on the surface of the transparent substrate by sputtering tantalum metal with an inert gas.
To 30 nm, the temperature of the transparent substrate on which the underlayer is formed is kept at 300 to 500 ° C., and the film formation rate is 2 × 10 ^{−2 to} 6 × in an inert gas having an oxygen partial pressure of 3.5% or less. 10 ^-2 n
A method for producing an optical disc, wherein the thickness of the vanadium oxide thin film exhibiting a metal-nonmetal transition on the underlayer is controlled to 60 to 75 nm by controlling the m / s to reactively sputter vanadium metal. . ”.

When the metal-nonmetal transition layer of the optical disk is irradiated with reproducing light, the light is absorbed and the temperature rises. The optical intensity distribution of the reproduction light spot has a Gaussian distribution, so that the center of the reproduction light spot has a high temperature in an optical disc which is stopped, but the temperature behind the reproduction light spot becomes higher during rotation. When the laser light output is set such that the temperature in front of the reproduction light spot is lower than the phase transition point and the temperature in the rear is higher than the phase transition point, a change (distribution) in the refractive index occurs in the reproduction light spot. . As a result, when the reflectance decreases at a temperature higher than the phase transition point Tc, only the reflected light from the front of the reproduction light spot becomes strong and a reproduction signal is obtained, and the reflectance increases at a temperature higher than the phase transition point Tc. In such a case, only the reflected light from the rear of the laser light spot becomes strong and a reproduction signal is obtained. As a result, an effect substantially equivalent to that of reducing the diameter of the reproduction light spot is obtained, and the information from the minute pit smaller than the light spot is optically read. As a result, crosstalk from recording marks on adjacent tracks and intersymbol interference in the same track can be reduced. When a tantalum underlayer having a thickness of 25 to 30 nm is provided on a transparent substrate, vanadium oxide laminated with the thin film of this underlayer as a nucleus causes crystal growth, and polycrystalline vanadium showing a metal-nonmetal transition. As the oxide thin film is formed, a vanadium oxide thin film having a smooth surface can be obtained. Further, the thickness of the polycrystalline vanadium oxide thin film exhibiting a metal-nonmetal transition is set to 60 to 75.
When the wavelength is set to nm, reproduction can be performed with shorter wavelength light, so that an optical disc of higher density recording can be obtained.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The optical disk of the present invention is an optical disk in which a layer made of a vanadium oxide thin film substance which causes a metal-nonmetal transition in response to a temperature change is provided between a transparent substrate and a reflective layer. At the time of reproduction, it is possible to read a minute recording mark below the diffraction limit of light by utilizing the change in optical characteristics accompanying the temperature distribution generated in the light spot of the reproduction laser light (reproduction light) irradiated on the optical disc. It was done. Therefore, this optical disk is particularly suitable for a high density type optical disk. First, a substance showing a metal-nonmetal transition used in the optical disc of the present invention will be described.

A substance exhibiting a metal-nonmetal transition is one in which the property of the substance changes from metal to nonmetal or vice versa when the temperature of the substance is changed. The metal-nonmetal transition is generally known as a Mott transition. The non-metal refers to a semiconductor and an insulator. With the metal-nonmetal transition, the electrical and optical properties of the material change significantly, but in the optical disc of the present invention, the complex refractive index (m-ik) represented by the real part m and the imaginary part k.
The reproduction is performed by using the change in the reflectance R or the phase of the light due to the change in the light intensity.

Examples of the substance exhibiting a metal-nonmetal transition include crystalline thin films such as VO ₂ , VO, V ₂ O ₃ , and Ti ₂ O ₃ , and the transition temperature Tc in the optical disc of the present invention.
Uses VO ₂ which is higher than room temperature. This VO ₂ thin film has a semiconducting property at a temperature lower than the transition temperature Tc and a metallic property at a temperature higher than the transition temperature Tc, but a change in this property is reversible with a temperature change, that is, the temperature decreases even when the temperature rises. Sometimes it happens. A method for manufacturing an optical disc according to the present invention will be described below with reference to FIG.
This will be described based on FIG. FIG. 1 is a cross-sectional view showing the structure of the optical disc of the present invention. Referring to FIG. 1, a ROM type optical disk 10 includes an underlayer 6, a vanadium oxide thin film layer 2, a reflective layer 3, and a protective film 4 on the transparent substrate 1 on which information pits are formed in advance. They are stacked in order. The information pits are provided concentrically or spirally to form an information track.

As the substrate 1, a general transparent substrate material used for ordinary optical disks, for example, plastics such as polycarbonate, acryl, epoxy and polyolefin, and glass are selected and used. On this substrate, there are provided a pit portion in which information is formed in advance in the form of pits having concavities and convexities, and a land portion (mirror portion) having no pit. The pits are manufactured by a dry etching method, a photopolymer method, an injection molding method or the like. The reflective layer 3 is made of Au, Ag, Cu, Al, I.
One of metals such as n, Pt, Cr and Ni or an alloy thereof is used, and the film thickness is 10 by vacuum deposition or sputtering.
To 500 nm, preferably 50 to 200 n
m.

The underlayer is made of tantalum (Ta) and has a film thickness of 5 to 100 nm, and when reproducing light is incident through the substrate, it is desirable that the amount of incident light is not attenuated. . However, on the contrary, if the underlayer 6 is too thin, the underlayer 6 is entirely oxidized by heating in an oxygen atmosphere at the time of its production, and the effect of promoting the subsequent VO ₂ formation is lost. Therefore, the thickness of the underlayer 6 is 25 to
30 nm is more preferable. The underlying layer 6, as well to promote the crystal growth of the VO _2, it is provided in order to improve the surface smoothness of the VO ₂ thin film.

Next, a method of manufacturing the optical disk of the present invention will be described with reference to FIGS. To manufacture the above-mentioned optical disk, the DC magnetron device 40 shown in FIG. 6 is used similarly to the above-mentioned conventional optical disk 30. In FIG. 6, the substrate 12 and the target 22 are placed in the vacuum chamber 11.
Are opposed to each other, and the film thickness sensor 14 is provided between them. Several targets 22 are arranged in the vacuum chamber 11, and the targets are moved to a position facing the substrate so that a voltage is applied from the DC power supply 16 to the metal to be sputtered to form a film on the substrate. It has become. Target 2
Cooling water is flowing through the water cooling pipe 24 to the lower portion 23 of the No. 2 so that the temperature of the target 22 rises and the spatter rate is increased.
It does not change. A glass substrate is used for the substrate, and tantalum metal is used for the target. The film thickness sensor 14 is of a crystal type and utilizes that the resonance frequency changes when a thin film is attached to the surface of the crystal unit.
The controller 19 controls the voltage of the DC power supply 16 so that the sputter rate becomes constant by changing the resonance frequency of the film thickness sensor 14. During film formation, the substrate can be heated by the heater 13 embedded in the substrate holder. First, the film formation of the underlayer 6 made of tantalum will be described. The vacuum chamber 11 is evacuated in advance to 3 × 10 ⁻⁴ Pa or less by the vacuum pump 18. After Ar gas is introduced through the gas pipe 17 and the exhaust amount is adjusted by the exhaust control valve 25, the degree of vacuum is set to 4 × 10 ⁻¹ Pa and tantalum metal is sputtered before the main sputtering. This removes the oxide layer on the target surface.

The shutter 15 is opened, and the tantalum (Ta) underlayer 6 is formed to a predetermined film thickness. After the predetermined film formation, the shutter 15 is closed. During the film formation, the substrate 12 is 300 to
Heat to 500 ° C. The thickness of the underlayer 6 is 5 to 100 nm as described above, and preferably,
25 to 30 nm.

Next, VO on the underlayer 6 made of tantalum
₂ The formation of a thin film will be described. The target 22 is changed to vanadium metal, and the vacuum degree in the vacuum chamber 11 is evacuated to 3 × 10 ⁻⁴ Pa or less by the vacuum pump 18 in the same manner as in the formation of the tantalum base layer 6, and then Ar gas is introduced. , To a predetermined degree of vacuum. The substrate on which the underlayer 6 is formed is heated to 300 to 500 ° C., and reactive sputtering of vanadium is performed in a mixed gas of Ar and O ₂ . After that, as in the case of the optical disc 30, the shutter 15 is closed and pre-sputtering is performed, and then the film formation rate is set to 2 × 10 ^{−2 to} 6 × 10 ⁻² nm.
And the oxygen partial pressure is controlled to 3.5% or less to form VO2 film. The thickness of VO2 thin film is 60 ~ 75
nmn, preferably 65-70. As a result of the above VO ₂ film forming experiment, the heating temperature of the substrate is preferably in the range of 300 to 380 ° C. Within this range, the VO ₂ thin film has a large film thickness and crystallinity over a large area. It was clarified that the state and the like were formed uniformly. In the range other than this, although VO ₂ can be formed, there are some cases in which a thin film that cannot be used as an optical disk is formed, for example, a film is formed with a non-uniform film thickness.

By the above method, a polycrystalline VO ₂ thin film exhibiting a metal-nonmetal transition in as-depo can be obtained with excellent surface smoothness and with good reproducibility. Further, by forming the underlayer 6 of tantalum, the optical disc 3 having the vanadium underlayer 5 is formed.
It is possible to stack VO ₂ even if the substrate heating temperature is lower than 0.

<Experimental Example> The atmosphere in the vacuum chamber 11 was evacuated to 3 × 10 ^-4 Pa, and then argon was introduced.
It was set to 4.0 × 10 ⁻¹ Pa. The target 22 is a tantalum metal. First, tantalum metal is sputtered prior to the main sputtering. Sputtering of tantalum metal is performed for 5 minutes at a sputtering current of 0.5 A, a voltage of 360 V supplied from the DC power source 16 and a film forming rate of 0.1 nm / s.
After that, the shutter 15 is opened to form a film of tantalum metal on the substrate 12 with a thickness of 25 to 30 nm, and then the shutter 15 is closed to stop the film formation. Then, the target 22 was changed to vanadium metal, oxygen and argon were introduced, and the total pressure was 4.2 × 10 ^-1.
Pre-sputtering is performed at a sputtering current of 0.1 A and a voltage of 410 V for 30 minutes with Pa at an oxygen partial pressure of 1.4 × 10 ^-2 Pa. After that, main sputtering was performed on the substrate 12 with a Ta underlayer having a surface temperature of 360 ° C. while controlling the deposition rate. A VO ₂ film was formed with good reproducibility in the film formation rate range of ^{2 to} 6 × 10 ⁻² nm / s. At this time, by controlling the sputtering time, samples were produced in which the film thickness on the substrate 12 was changed from 40 nm to 90 nm. When the reflectance change of this VO ₂ film thickness with respect to temperature was measured with light having a wavelength of 600 nm, a metal-nonmetal transition was observed at 68 ° C. FIG. 7 shows the relationship between the reflectance and the VO ₂ film thickness with respect to the temperature of the VO ₂ film thickness when light having a wavelength of 600 nm is irradiated from the transparent substrate side of the optical disk. The open circles show the characteristics in the low temperature phase measured at an ambient temperature of 20 ° C, and the open circles show the characteristics in the high temperature phase measured at an ambient temperature of 100 ° C. As shown in FIG. 7, the VO ₂ film thickness is 60 to 7
Between 5 nm, the reflectance in the low temperature phase (20 ° C) is 22-2.
The reflectance at 5% and the high temperature phase (100 ° C.) was 5 to 10%. Further, when the VO ₂ film thickness is 65 to 70 nm, the reflectance in the low temperature phase (20 ° C.) is 23 to 24%, and the reflectance in the high temperature phase (100 ° C.).
The reflectance at (° C.) was 5 to 6%. The larger the difference in reflectance between the low temperature phase and the high temperature phase is, the smaller the reproduction error generation rate of the optical disk can be made. Therefore, it is practically preferable that the VO ₂ film thickness having a large reflectance difference is 65 to 70 nm. The crystal grain size was so fine that it could not be observed with a scanning electron microscope.

[0038]

As described above, according to the optical disk of the present invention and the method for manufacturing the same, a high density recording optical disk which can be reproduced with shorter wavelength light can be obtained by forming a vanadium oxide thin film layer having a thickness of 60 to 75 nm. It becomes possible, and further 2
By providing an underlayer of tantalum having a thickness of 5 to 30 nm, the smoothness of the vanadium oxide thin film layer surface is improved,
It becomes possible to sufficiently adapt to an optical disc recorded at high density.

[Brief description of the drawings]

FIG. 1 is a sectional view showing the configuration of a first embodiment of an optical disc of the present invention.

FIG. 2 is a cross-sectional view showing a configuration of an optical disc of a prior application.

FIG. 3 is a cross-sectional view showing a configuration of an optical disc of a prior application.

FIG. 4 is a diagram showing the relationship between the temperature and reflectance of a VO ₂ film.

FIG. 5 is a diagram showing the relationship between the film thickness of a VO ₂ film and the reflectance in a conventional disc.

FIG. 6 is a diagram showing an example of a DC magnetron device.

FIG. 7 is a diagram showing the relationship between the film thickness of the VO ₂ film and the reflectance in the disk of the present invention.

[Explanation of symbols]

 10, 20, 30 Optical disc 1 Transparent substrate 2 Vanadium oxide thin film 3 Reflective layer 4 Protective layer 5 Underlayer

Claims (2)

[Claims] 1. An optical disk provided with at least a transparent substrate, a vanadium oxide thin film exhibiting a metal-nonmetal transition, an underlayer made of tantalum, and a reflective film in this order, wherein the metal-nonmetal transition is The thicknesses of the vanadium oxide thin film and the underlayer shown are 60 to 75 nm and 25 to 30 n, respectively.
An optical disk characterized by having m. 2. An optical disk manufacturing method comprising at least a transparent substrate, a vanadium oxide thin film exhibiting a metal-nonmetal transition, an underlayer made of tantalum, and a reflective film, which are provided in this order. The thickness of the underlying layer made of tantalum is 25 to 30 nm on the surface of the transparent substrate by sputtering metal, and the temperature of the transparent substrate on which the underlying layer is formed is 300 to 500.
Kept at ℃, in an inert gas with an oxygen partial pressure of 3.5% or less,
Thickness of vanadium oxide thin film showing metal-nonmetal transition on the underlayer by reactive sputtering of vanadium metal by controlling the film formation rate to 2 × 10 ^{-2 to} 6 × 10 ^-2 nm / s. Of 60 to 75 nm.