JPS6210410B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical image device that performs incoherent-coherent conversion of images, temporary storage of images, etc. using a single crystal plate having an electro-optic effect and a photoconductive effect.

FIG. 1 is a diagram showing the structure of an optical image element. It is composed of a single crystal plate 1, insulating layers 2, 2', transparent electrodes 3, 3', and a power source 4. The single crystal plate 1 is a single crystal such as bismuth silicon oxide (Bi ₁₂ SiO ₂₀ ) or bismuth germanium oxide (Bi ₁₂ GeO ₂₀ ) that has an electro-optic effect and a wavelength-dependent photoconductive effect, and has a transparent electrode. 3, 3' apply an electric field through the insulating layers 2, 2'. 7 is a polarizer, and 9 is an analyzer. This optical image element is used, for example, to convert an image of incoherent light into an image of coherent light, or to temporarily store an image. For example, it operates as follows. .

When the voltage V of the power source 4 is applied to the single crystal plate 1 and the insulating layers 2, 2' through the transparent electrodes 3, 3', in a steady state, the thickness of the single crystal plate 1 and the insulating layers 2, 2', respectively A constant potential distribution determined by the dielectric constant of is generated inside the optical image element. In this state, when an incoherent written image 5 of a wavelength at which the single crystal plate 1 exhibits a photoconductive effect is focused on the single crystal plate 1, a potential distribution corresponding to the written image 5 is formed in the irradiated area. That is, due to the photoconductive effect of the single crystal plate 1, electron/hole pairs are generated according to the spatial light intensity distribution of the image, and these electron/hole pairs are transferred to each transparent electrode 3, depending on the polarity of each. 3', and is trapped at the interface between the insulating layers 2, 2' and the single crystal plate 1, forming a potential distribution.

Since the single crystal plate 1 has an electro-optical effect, its refractive index changes according to the above-mentioned potential distribution, and eventually a refractive index distribution corresponding to the written image 5 is formed.
When using the above-mentioned bismuth silicon oxide etc. as the front crystal plate 1, this crystal plate has a high resistance (approximately 10 ¹⁷ Ωcm), so the electrons and
The hole pairs remain trapped and the written image 5 is accumulated in the optical image element.

After the written image 5 is formed and stored as a refractive index distribution on the single crystal plate 1, coherent light of a wavelength that causes almost no photoconductive effect in the single crystal plate 1 is read out as read light 6, and is linearly polarized by a polarizer 7 to be converted into a single crystal. When the light 8 is incident on the crystal plate 1, the light 8 transmitted through the single crystal plate 1 is subjected to an optical phase difference Γ corresponding to the refractive index distribution of the single crystal plate 1. Therefore, the output light 10 obtained by detecting the transmitted light 8 with the analyzer 9 becomes light having an optical intensity distribution again, and in the end, the incoherent written image is converted into a coherent image.

In the optical image element, the insulating layers 2 and 2' are used to prevent carriers generated inside the single crystal plate 1 from flowing out to the power source 4, so they need to have a high insulation resistance value and are also transparent to light. Therefore, it is naturally required that the film be an optically homogeneous transparent thin film. Therefore, the material for the insulating layers 2, 2' must have a high insulation resistance value and be able to easily form an optically homogeneous transparent thin film. Furthermore, for the reasons described below, it is necessary that the breakdown voltage value and dielectric constant be large.

Generally, there is a relationship between the output light intensity I of the optical image element and the applied voltage _V1 of the single crystal plate 1 as shown in the following equation.

I∝sin ² (π/2·V ₁ /V〓) ……(1) However, V〓 is a half-wave voltage and is a constant determined by the following formula.

V〓=λ _p /2n _p ³ γ ₄₁ …(2) λ _p ; Light wavelength in free space n _p ; Refractive index γ ₄₁ ; Electro-optic coefficient As is clear from equation (1), the output image The contrast ratio is determined by the applied voltage V ₁ , and the maximum contrast ratio is obtained when V ₁ =V〓. Therefore, when using the optical image device, the power source 4 is usually set so that an applied voltage of V is applied to the single crystal plate 1.

To apply a voltage of V to the single crystal plate 1, the first
In the structure shown in the figure, the voltage of the power supply 4 must be greater than V〓. Now, if the thickness and dielectric constant of the single crystal plate 1 are d ₁ and ε ₁ and the thickness and dielectric constant of the insulating layers 2 and 2' are d ₂ and ε ₂ , then the voltage V ₁ applied to the single crystal plate 1 is as follows. It becomes like the formula.

That is, 1/(1+
2d ₂・ε ₁ /d ₁・ε ₂ ). Therefore, in order to keep the voltage V of the power supply 4 low and apply a predetermined voltage to the single crystal plate 1, assuming that the thickness d ₁ and the dielectric constant ε ₁ of the single crystal plate 1 are constant, the insulating layer 2, It is desirable that the thickness d ₂ of 2' be small and its dielectric constant ε ₂ be large. The thickness d ₂ is limited by the breakdown voltage value of the insulating layers 2, 2', and if it is made too small, electrical breakdown will occur, so the larger the dielectric constant ε ₂ , the more suitable the insulating layers 2, 2' are. .

In this way, the insulating layers 2 and 2' must satisfy the above conditions, and conventionally polyparaxylylene,
Various materials have been proposed, such as mica plates and silicone insulating oil. However, although these conventional insulating materials can provide fairly satisfactory results in terms of insulation resistance, breakdown voltage, and the ability to easily form optically homogeneous and transparent thin films, they have relatively low dielectric constants. There was a drawback that the voltage V of the power supply 4 had to be increased. For example, when polyparaxylylene is used, its dielectric constant ε ₂ is 3.0, so ε ₁ = 56,
Forming the insulating layers 2, _{2' to a thickness of d 2 =} 5 μm on bismuth silicon oxide with d 1 =200 μm results in approximately 1/2 of the voltage V of the power source 4 being applied to the insulating layer. Therefore, the half-wavelength voltage V = 3.9KV of bismuth silicon oxide single crystal (however, in equation (2), λ _p = 633 nm, n _p = 2.54, γ
₄₁ = 5×10 ^-12 m/V),
It was necessary to apply a voltage of 7.5KV, about twice that amount. By repeatedly applying such a high voltage, the polymer film deteriorates and the electrical characteristics sometimes deteriorate significantly. Another drawback was that the polymer film absorbed moisture from the atmosphere, resulting in lower insulation properties and deterioration of device characteristics.

An object of the present invention is to provide an optical image element that solves the above-mentioned drawbacks and has significantly improved characteristics.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of the optical image device of the present invention. The optical image element of the present invention differs from conventional optical image elements in that silicon nitride, which has excellent properties, is used as the insulating layers 2 and 2', and that it is made from bismuth silicon oxide single crystal 1 and silicon nitride. By applying a thin film of an oxide such as silicon dioxide, zirconia, alumina, or yttrium oxide, which has a high specific resistance and dielectric constant, as a protective film for the single crystal 1 on one or both of the insulating layers 2 and 2'. be.

Silicon nitride film has been actively researched in recent years as a protective film for semiconductors, and its excellent electrical properties have been recognized.
It has also been found to be a good protective membrane against water or Na ⁺ ions. This silicon nitride film has an insulation resistance value (volume resistivity) of 10 ¹⁵ to 10 ²⁰ Ω・cm, a dielectric constant of 5 to 7, which is higher than conventional polyparaxylylene or silicone insulating oil, and a breakdown voltage. It was confirmed that the film also has a pressure resistance of 300KV/mm, which is equivalent to that of polyparaxylylene film. Silicon nitride is processed by atmospheric pressure CVD, low pressure CVD, and plasma.
Thin films can be formed by CVD and sputtering, but in normal pressure CVD or low pressure CVD, the substrate temperature is over 700°C, so the oxygen vacancies and thermal expansion coefficient of the bismuth silicon oxide single crystal surface that becomes the substrate are reduced. Due to differences, peeling or cracking of the membrane is likely to occur.

In the present invention, silicon nitride films are formed as insulating layers 2 and 2' on both sides of bismuth silicon oxide single crystal 1 by plasma CVD, which can produce optically uniform thick silicon nitride films at relatively low temperatures. The membrane created by this method is
Compared to films made by CVD, the optical uniformity is higher and the decrease in light transmittance due to oxygen vacancies in bismuth silicon oxide is significantly less.

Next, a single crystal protective film will be explained. Bismuth silicon oxide single crystal 1 is plasma
At the temperature at which CVD is performed (250°C), no oxygen vacancies are observed even in vacuum, so if oxygen vacancies occur, other causes such as those described below can be considered. The silicon nitride film is formed by plasma CVD by flowing silane gas, ammonia gas, and nitrogen gas into plasma and depositing them on a single crystal substrate heated to 250°C. This silane gas has high reactivity with oxygen and easily reacts with a trace amount of oxygen as shown by SiH ₄ +2O ₂ →SiO ₂ +2H ₂ O, producing silicon dioxide. Therefore plasma CVD
is carried out under low pressure with careful removal of oxygen.
However, when the substrate is bismuth silicon oxide single crystal, the oxygen escape ability is high under conditions of 250℃ and 0.1 torr, and it easily reacts with silane gas, producing silicon dioxide on the surface of the crystal. is in a state of oxygen deficiency. However, once this silicon dioxide is formed to a certain extent, the reaction does not proceed any further and it becomes a protective film for the bismuth silicon oxide single crystal against silane gas. In the present invention, in order to solve the problem of oxygen vacancies in the bismuth silicon oxide single crystal 1, silicon dioxide, which has relatively high resistance and high dielectric constant and is often used as an insulating film, is first applied to the surface of the single crystal 1. The optical image element is constructed by applying a thin film of oxide such as zirconia, alumina, or yttrium oxide as a protective film, and then depositing a silicon nitride film as insulating layers 2 and 2'.

Examples of the present invention in which silicon dioxide SiO ₂ is used as the protective film will be described in more detail below. Approximately 300μm thick bismuth silicon oxide single crystal plate is coated on both sides with evaporation, CVD, sputtering or spinner method.
After forming a 1000A silicon dioxide film, approximately 500A
The film is densified by heat treatment at ℃. Thereafter, a silicon nitride film is formed by plasma CVD. A silicon nitride film of about 1 μm was obtained by a gas phase reaction for about 20 minutes under conditions of a substrate temperature of 250°C, an ammonia gas to silane gas ratio of 6:1, and an RF power supply output of 200 W. This silicon nitride film had a film thickness uniformity of ±5% or less, and was a good film without cracking or peeling. When a transparent electrode was provided on the surface of this thin film and a voltage V was applied from a power source 4, 94% of the applied voltage V was applied to the single crystal plate, and the image contrast ratio was approximately 40 _dB , which was achieved using a conventional polyparaxylylene film. This is a significant improvement compared to the approximately _20dB of the case. In addition, zirconia, alumina,
Similar characteristics to those described above were confirmed when yttrium oxide was applied.

As explained above, in the optical image device of the present invention, the insulating films 2 and 2' are made of silicon nitride film formed by plasma CVD, and silicon dioxide, zirconia, alumina, and yttrium oxide as a protective film of bismuth silicon oxide single crystal. Since this inorganic film has a relatively high dielectric constant, it has the advantage that the power supply voltage can be set lower than the conventional voltage value. Therefore, when used with the same power supply voltage value as in the past, the voltage applied to the single crystal plate 1 in the present invention is higher than in the past, so the output image contrast ratio of the optical image element is improved. There are advantages.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of an optical image element. 1: Single crystal plate, 2, 2': Insulating layer, 3, 3': Transparent electrode, 4: Power supply, 5: Writing light, 6: Reading light (incident light), 7: Polarizer, 8: Reading light ( 9: analyzer, 10: output light, V: power supply voltage, d ₁ : single crystal plate thickness, d ₂ : insulating layer thickness.

Claims (1)

[Claims] 1. A single crystal plate having an electro-optical effect and a photoconductive effect, a protective layer provided on one or both sides of the single crystal plate, and silicon nitride provided on both sides outside the protective layer. 1. An optical image device comprising: a composite insulating layer made of a film; and a transparent electrode that applies an electric field to the composite insulating layer and the single crystal plate. 2. The optical image element according to claim 1, wherein the protective layer comprises silicon dioxide, zirconia,
An optical image element characterized by being made of an oxide such as alumina or yttrium oxide.