JPH0226299B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a laminate for optical memory. More specifically, the present invention relates to a laminate for optical memory in which a chalcogenide compound thin film capable of reversible refractive index change of optical recording/light/thermal erasing type is coated with a silicon thin film.

Conventionally, many studies have been conducted on chalcogenide compound thin films, whose refractive index changes reversibly upon optical recording and erasure with light or heat, as optical memory media. For example, A _s -S thin film and A _s -S-S _e -G _e thin film are A _r
The refractive index changes due to light irradiation from an ion laser or the like. Furthermore, it is known that the refractive index reversibly returns to its original state by heating or irradiating with strong light. Many studies have been conducted using this phenomenon to develop repeatable hologram recording materials, optical memory materials, or ultra-microfilm materials that utilize the simultaneous change in absorption edge. However, what has hindered the practical application of these materials is the instability that exists in chalcogenide compound thin films.

After a chalcogenide compound thin film is formed on a substrate, it is annealed at a temperature below the glass transition point in order to be used as a reversible optical memory. However, in the conventional processing, optical recording
When used repeatedly by light and heat erasing, cracks occur in the film and the recording characteristics deteriorate, which poses a practical problem. Furthermore, the recordability was not good. These problems are caused by material instability, and it has been desired to stabilize the materials for practical use.

In order to improve the above-mentioned instability, the present inventor has
As a result of extensive studies, it has been found that stability can be improved by covering the chalcogenide compound thin film as described above with a silicon thin film.

Furthermore, we have discovered that by coating both sides of the chalcogenide film with a silicon thin film and appropriately selecting the film thickness, it is possible to obtain not only improved stability but also a sharp change in readout signal output, and have thus arrived at the present invention.

The present invention is a laminate for optical memory in which both sides of a chalcogenide compound thin film with a thickness of 1,000 Å to 20,000 Å are coated with silicon thin films of 200 Å to 700 Å, which are capable of reversibly changing the refractive index of the optical recording/light/thermal erasing type.

The optical recording-optical/thermal erasable chalcogenide compound thin film capable of reversibly changing refractive index in the present invention has two states with different refractive indexes, and can reversibly transition between the two states by light irradiation. This is a chalcogenide compound thin film that undergoes reversible transition upon exposure to light or heat. For example, As
and chalcogenide thin films containing As and Ge, i.e., As-Se, As
-S, As-Se-Ge, As-S-Ge, As-Se-S
-Ge-based glass thin film or chalcogenide thin film containing P and Ge, i.e. P-Se-Ge and P
-S-Ge based glass thin film, etc. These thin films may contain additives such as Te, if necessary.

Although the thin film may be formed by any method, it is generally formed on the molded substrate by vacuum evaporation or sputtering. The substrate of the molded product may be ceramics such as glass, plastics such as polymethyl methacrylate or polyethylene terephthalate, or aluminum.
It is a molded product of metal such as, etc., and is not particularly limited.

The thickness of the thin film needs to be 1000 Å or more in order to utilize reversible refractive index change as a phase change and to extract a large signal output, and 2 μm or less in view of film stability.

The silicon thin film in the present invention is a thin film made of crystalline or amorphous Si. A polycrystalline thin film or an amorphous thin film of Si, or a microcrystalline silicon thin film with a grain size of about 100 Å or less is preferable because of ease of manufacturing the thin film. In particular, amorphous thin films or grains with a diameter of approx.
Microcrystalline silicon thin films with a thickness of 100 Å or less can be created at a substrate temperature of approximately 300°C or less during film formation.
This is particularly preferred because it can be created without affecting the chalcogenide compound thin film.

It is better that no impurities exist in the silicon thin film, but for example, impurities such as H, F, P, As, B, Ga, Al, Ge, etc.
The additives such as the like may be included to the extent that the effects of the present invention are not impaired.

The thin film may be formed by any method, including vacuum evaporation method, ion plating method, spat method,
It is formed by a glow discharge method or the like.

The thickness of the thin film is preferably 5000 Å or less in view of transparency to writing and reading light of the optical memory.
Further, in order to have the effect of improving the stability of the chalcogenide compound thin film, it is necessary to have a thickness of about 15 Å or more. In addition, in order to particularly demonstrate the above effects, 100
Particularly preferred is Å or more and 800 Å or less.

The optical memory laminate according to the present invention is formed by laminating a chalcogenide compound thin film B and a silicon thin film C on a substrate A. The configuration order of the laminate is A-C-B-C. The stabilization of the chalcogenide compound thin film in the present invention is achieved by A-C-B-
The effect is great in combination with C. Furthermore, an additional layer that does not impair the basic effects of the above structure, such as a protective layer on the top layer or an undercoat layer between the substrate and the thin film, may be provided.

In addition to material stability, especially A-C-B-
In the C structure, the C film thickness is 700 Å from 200 Å.
By appropriately selecting the film thicknesses of Å and B within the range of 1000 Å or more and 2 μm or less, an optical memory with a large readout signal output can be obtained. In the case of a transmissive optical memory, the signal output is detected as a change in transmittance with respect to read light, and in the case of a reflective optical memory, it is detected as a change in reflectance. In particular, the film thickness of C is λ/4ns (450nm<λ<900nm, ns is the refractive index of C at wavelength λ), and the film thickness of B is mλ/2nc (nc is the refractive index of B at wavelength λ, m is an integer). The laminated body for an optical memory of the present invention having the above structure is preferable because it becomes an extremely sensitive optical memory by appropriately selecting the wavelength of the readout light. Further, in order to obtain a large change in signal output with respect to the readout light, m is preferably 3 or more, and particularly preferably 10 or less for ease of film formation.

The optical memory laminate of the present invention is similar to conventional optical recording.
This greatly improves the instability of chalcogenide compound thin films that are capable of reversible refractive index changes through light and heat erasure. Although the cause of this effect is not clear, the interface between the silicon thin film and the chalcogenide compound thin film is exposed during film annealing and optical recording.
It is thought that there is a function to prevent instability such as the occurrence of cracks in the film due to reversible changes due to thermal extinction. It is also possible that some chemical bonding occurs at the interface. In addition, in a structure in which the upper layer of an amorphous thin film of chalcogenide compound is covered with a thin silicon film,
The silicon thin film is also considered to have the effect of a passivation film.

With the optical memory laminate of the present invention, an optical memory with very large and sensitive signal output changes can be created. This is a silicon thin film and a chalcogenide compound thin film.
This is thought to be due to the extremely excellent optical matching characteristics.

That is, regarding the optical memory of the present invention, the typical refractive index of each layer is silicon (3.0~
3.5)/chalcogenide compound (2.0-2.7)/silicon (3.0-3.5)/substrate (approximately 1.5).
Light transmitted through the silicon layer and incident on the chalcogenide compound layer is reflected at the interface between both silicon layers and the chalcogenide compound layer, causing multiple interference.

In multiple interference, the phase difference δ is calculated by the following formula () δ = 2π / λ ₀ ΔL = 4π / λ ₀ n ₂ h cosα ₂ ... () (In the formula, λ ₀ is the wavelength, h is the thickness, and n ₂ is The refractive index, _α2 , indicates the angle of refracted light.

In a particular thin film, since the laser wavelength λ ₀ , the thickness h, and the angle α ₂ of the refracted light are constant, δ=Kn ₂ .

Therefore, after recording with laser light, the refractive index is different from before recording, so the phase difference is also different.
From Figure 1 showing the relationship between δ and transmittance [Figures 2 and 20 in ``Lightwave Electron Optics'' co-authored by Jiro Koyama and Hiroshi Nishihara (May 15, 1980, Corona Publishing, p. 41), it is clear that the phase difference before recording is If (a) and the phase difference after recording is (b), a case may occur in which the transmittance decreases (or conversely, the reflectance increases) before and after recording. Furthermore, by setting the film thickness h so that it is C before recording and D after recording, the reflectance can be greatly reduced.

The laminate for optical memory of the present invention has excellent stability as a repeatable optical memory medium of the optical recording/light/thermal erasing type. Moreover, it is extremely useful as a sensitive optical memory.

The effects of the optical memory laminate of the present invention will be specifically explained below using examples.

Example 1 Semiconductor-grade silicon was deposited on a slide glass plate placed in a vacuum deposition apparatus by an electron beam method to form a silicon film with a thickness of 500 Å. According to X-ray diffraction, the film was amorphous.
As ₄₀ Se ₂₅ S ₂₅ Ge ₁₀ Chalcogenide glass (However,
The composition ratio is atomic percent) was deposited by electron beam method to form an amorphous chalcogenide compound film with a thickness of 8000 Å. The degree of vacuum during the deposition was 2×10 ^-5 torr, and the deposition rate was about 20 Å/S.

Furthermore, semiconductor-grade silicon was deposited again by the electron beam method to form an amorphous silicon film with a thickness of 500 Å, thereby obtaining a laminate for an optical memory consisting of three layers on a glass plate. Subsequent annealing in air at 200°C for 10 minutes resulted in a homogeneous, crack-free memory layer.

The memory layer was irradiated with a light beam of Ar ion laser having a wavelength of 514.5 nm. Beam diameter is 2mm, output is
It was irradiated at 5 mW for 10 seconds and recorded in bits.

The light from a GaAlAs semiconductor laser oscillating at 830 nm was focused on the recording section using a 40x objective lens as read light, and the intensity of the reflected light was measured. The reflectance before and after recording changed significantly from 65% to 5%, and optical recording was performed. Further, when heated in air at 200°C for 1 minute, the records were completely erased. Recording and erasing was repeated 20 times, but no deterioration or cracking of the memory layer was observed.

Example 2 A thoroughly cleaned slide glass plate was exposed to high frequency 2
Installed in a pole sputtering device, ^and
A semiconductor-grade silicon target was sputtered in a gas atmosphere to form a silicon film with a thickness of 500 Å.
According to X-ray diffraction, the film was amorphous. Subsequently, a target made of As ₄₀ Se ₅₀ Ge ₁₀ chalcogenide glass to which 1% by weight of Te was added was sputtered to form a chalcogenide film with a thickness of 4800 Å.
Furthermore, sputter the silicon target again,
A silicon film with a thickness of 500 Å was formed. continuation,
Annealing was performed in air at 180°C for 10 minutes to obtain a homogeneous, crack-free optical memory laminate.

After uniformly irradiating the memory layer with a 500W high-pressure mercury lamp, a GaAlAs semiconductor laser beam oscillating at a wavelength of 780nm was focused to perform bit-like recording. However, the recording beam intensity was adjusted using an optical attenuator to prevent the sample from melting. Similar to Example 1
When the recorded area was read out using reflected light using a GaAlAs semiconductor laser that oscillated at a wavelength of 830 nm, the reflectance changed from 30% to 65% before and after recording, and recording was performed.
Furthermore, the recorded portion was erased by irradiation with a mercury lamp, and repeated recording was confirmed.

Reference Example 1 After thoroughly cleaning a glass slide, it was attached to a substrate holder in a vacuum evaporation apparatus. Using semiconductor-grade silicon as an evaporation source, an amorphous silicon thin film was formed by electronic beer evaporation at a vacuum level of 2×10 ^-5 Torr. The film thickness was 200 Å.
As ₄₀ Se ₂₅ S ₂₅ Ge ₁₀ chalcogenide glass was placed in an alumina-coated tungsten crucible and deposited by resistance heating to form an amorphous thin film of a chalcogenide compound on a silicon thin film. The deposition rate is approximately 30
The film thickness was 9800 Å.

After the film was formed, it was annealed in air at 200°C for 10 minutes. The obtained film was homogeneous and no cracks were observed on the film surface. The stability of the membrane was not lost even after being stored in air for an additional 3 months.

Reference Example 2 As in Reference Example 1, As ₄₀ Se ₂₅ S ₂₅ Ge ₁₀ chalcogenide glass was vapor-deposited by resistance heating on a slide glass plate placed in a vacuum evaporation apparatus. The film thickness was 9800 Å. Semiconductor-grade silicon was vacuum-deposited on the film by electron beam method in the same manner as in Example 3 to form an amorphous silicon film. Film thickness is 120
It was Å. Subsequently, it was annealed in air at 200°C for 10 minutes. As in Example 3, the obtained film was homogeneous and no cracks were observed within the film.

Reference Example 3 As in Reference Example 1, As ₄₀ Se ₂₅ S ₂₅ Ge ₁₀ chalcogenide glass was vapor-deposited by resistance heating on a slide glass plate placed in a vacuum evaporation apparatus. The film thickness was 9800 Å. Continue in air at 200℃
After annealing for 10 minutes, cracks appeared in the film. When the film was left for one day after treatment, cracks in the film increased.

[Brief explanation of drawings]

FIG. 1 shows the transmitted light intensity characteristics of a parallel plate. In the figure, I _t /I _i indicates the ratio of transmitted light intensity (I _t ) to incident light intensity (I _i ). R is the reflection coefficient at the layer interface, and F is the index finesse that indicates the sharpness of the selectivity of the transmitted wavelength. shows.

Claims (1)

[Claims] 1. Optical recording - A laminate for optical memory in which both sides of a chalcogenide compound thin film with a thickness of 1,000 Å to 20,000 Å which is capable of reversible refractive index change of the optical/thermal erasable type are coated with a silicon thin film with a thickness of 200 Å to 700 Å.