JP2004109827A - Method for depositing thin film for terahertz band optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for depositing a thin film for a terahertz band optical element by which the thin film for a terahertz band optical element is easily manufactured and matching of refractive indexes is possible without degrading the performance of a semiconductor detector. <P>SOLUTION: The optical element of light from 100 GHz to 10 THz is inputted to a plasma CVD device, gaseous starting material for CVD is successively switched and a plurality of laminated films including a film having a large refractive index and a film having a refractive index smaller than that are successively deposited on the surface to obtain an optical wave interference film. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a thin film forming method required for forming an optical interference film such as an antireflection film, a reflection film, or a filter formed on an optical element, and particularly to a thin film forming method for a terahertz band optical element. .
[0002]
[Prior art]
In the visible light region, an anti-reflection film, a reflection film, a filter, or the like is formed and used by light wave interference films having various film thickness configurations. Since the thickness of each thin film constituting the light wave interference film is 1 μm or less in a visible light region, a sputtering method or a vacuum deposition method has been mainly used in the manufacturing process.
[0003]
In a terahertz band having a wavelength longer than the visible light region, the thickness of each thin film constituting the light wave interference film is 10 to 750 μm. Therefore, if the conventional method is applied, it takes an extremely long time to manufacture. . Therefore, a manufacturing method suitable for the terahertz band is required.
[0004]
As a thin film for an optical element in the terahertz band, a manufacturing method described below is known.
1) In the CVD method using TEOS (tetraethylorthosilicate), the temperature at the time of the positive film is 200 ° C. or more, and only the silicon oxide film can be formed. Further, when the film is formed at a temperature lower than 200 ° C., graphite remains in the film, and the light transmittance is reduced. However, a Ge: Ga (semiconductor in which a small amount of Ga is added to Ge) detector is used as a photodetector in the terahertz band. However, when the Ge: Ga crystal is kept at 200 ° C. or higher, the distribution of Ga as an impurity is reduced. This method is not used for this photodetector because the photodetection sensitivity is reduced due to changes in the photodetection.
2) There is a method in which a plastic film is attached to a terahertz band optical element and used as an optical interference film. However, the refractive index of plastic is about 1.5, and is 3.0 or more in the case of a semiconductor (Ge, Si, or GaAs) or a non-linear optical element (in the case of LiNbO ₃ or ZnTe) used in the terahertz band. Therefore, it is difficult to match the refractive indexes.
3) A thin film of fused quartz (refractive index = 1.959) is attached to a germanium substrate with an optical adhesive, and then polished to a thickness of about 20 microns to form an antireflection film. . This method has the advantage that the entire process is near room temperature, the optimum material that can easily match the refractive index with the substrate can be arbitrarily selected, and the controllability of the film pressure is good. However, since a polishing step is required, it is troublesome and the yield is low.
[0005]
[Problems to be solved by the invention]
In the terahertz band light wave interference film formed by the conventional method, the performance of the semiconductor detector is deteriorated, it is difficult to match the refractive index, or the production of the film is troublesome.
[0006]
The present invention has been proposed in view of the above, and proposes a method of forming a thin film for a terahertz band optical element that can match refractive indexes without deteriorating the performance of a semiconductor detector and that is easy to manufacture. It is intended to be.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, a plasma CVD method is used so that a high deposition rate can be realized even in a high-temperature environment. Further, in order to realize a light wave interference film, a film having a high refractive index and a film having a low refractive index are alternately laminated. In this case, the larger the ratio between the higher refractive index and the lower refractive index, the better. This lamination is performed only by switching the source gas of the plasma CVD, and is performed without leaving the film formation chamber of the plasma CVD apparatus. In view of the above, the first invention provides a film having a first refractive index by inputting an optical element of light from 100 GHz to 10 THz to a plasma CVD apparatus and sequentially switching the source gas for CVD. A plurality of stacked films including a film having a second refractive index smaller than the above are sequentially formed on the surface to form a light interference film.
[0008]
According to a second aspect of the present invention, in addition to the first aspect, the source gases for CVD are a silane gas and an oxygen gas, and the oxygen gas is cut off to form a silicon film. It is characterized in that a silicon film is formed.
[0009]
Further, the third invention is characterized in that, in addition to the second invention, an argon gas is used as a carrier gas of the above-mentioned source gas.
[0010]
According to a fourth aspect of the present invention, in addition to the second aspect, the pressure during film formation in the above-described plasma CVD apparatus is 1 Pa or less.
[0011]
According to a fifth aspect of the present invention, in addition to the second aspect, the material of the optical element is a semiconductor obtained by adding a shallow impurity whose impurity level is 40 meV or less to germanium, silicon, or gallium arsenide. Is not more than 160 ° C.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described below with reference to FIG. FIG. 1 shows a plasma CVD apparatus for forming an antireflection film, a reflection film, or an interference film serving as an optical filter on an optical element used in the terahertz band. This device supplies a raw material gas from a gas introduction pipe 1, causes a chemical reaction promoted by plasma generated by an electromagnetic wave from an induction electrode 9 in a reaction chamber 2, and puts molecules generated by the chemical reaction into a main chamber 5. This is to adhere to the surface of a certain optical element 8. The exhaust from the main chamber 5 is performed by an exhaust pipe 6. The source gases (10a, 10b,..., 10n) are, for example, silane gas 11a and oxygen gas 11b, and the carrier gas is argon gas 11n. The flow rates of these gases are controlled by respective flow control valves 10a, 10b,... 10n. The source gas used here is not limited to these three types, and it is apparent that the source gas needs to be switched according to the film to be formed. As for the film thickness during the film forming process, a well-known optical measurement in which the surface of the optical element 8 is irradiated with the light beam 3 for measuring the film thickness from the observation window 4 can be performed. The temperature of the optical element to be processed can be overheated by the heater 7 for heating. For example, when the optical element is made of a Ge: Ga crystal, the temperature can be set to 200 ° C. or lower to prevent the performance of the element from being deteriorated.
[0013]
As described above, when the source gas is the silane gas 11a and the oxygen gas 11b, and the carrier gas is the argon gas 11n, it is desirable that the pressure during film formation be 1 Pascal or less. Further, since the supply of oxygen gas is stopped to form a silicon film, a multilayer film including a silicon film and a silicon oxide film can be easily formed by sequentially turning on and off supply of oxygen gas.
[0014]
Further, the film formed by the CVD method, it is possible to form a stoichiometrically different variety of film, for example, it is possible to form the SiO _x film (x = 1.94) as a silicon oxide film. In this case, the refractive index of germanium is equal to the refractive index of the silicon oxide film, and the refractive indexes can be matched. Incidentally, in the case of the fused silica film, since the refractive index is 1.96, at least 0.04% reflection occurs. By using the silicon oxide curtain, an antireflection film having lower reflection can be realized.
[0015]
Further, when a multilayer film is formed using silicon Si (refractive index = 3.42) and the above-mentioned silicon oxide SiO ₂ film (refractive index = 1.96), by changing the film thickness configuration, As in the case of the visible light region, for a substrate material having a refractive index in a region (3.84 to 11.68) between the squares of the respective refractive indices, a light wave interference film having consistency with the substrate is obtained. Can be easily made.
[0016]
【The invention's effect】
Since the present invention includes the above-described means, the following effects can be obtained.
[0017]
In the first invention, a high-refractive-index film and a low-refractive-index film are alternately laminated using a plasma CVD method to form an optical interference film, and this lamination is performed only by switching the source gas. Therefore, a high film forming rate can be realized even in a non-high temperature environment, so that a light wave interference film applicable to a terahertz band optical element can be easily realized.
[0018]
In the second, third, fourth, and fifth aspects of the invention, the source gases for CVD are silane gas and oxygen gas. The oxygen gas is cut off to form a silicon film, and the oxygen gas is introduced to oxidize. Since the silicon film is formed, a light wave interference film having a refractive index matched to germanium crystal or the like often used in a terahertz band can be formed.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Gas introduction pipe 2 Reaction chamber 3 Light beam 4 Observation window 5 Main chamber 6 Exhaust pipe 7 Heater 8 Optical element 9 Induction electrodes 10 a, 10 b, 10 n Flow control valves 11 a, 11 b, 11 n Source gas

Claims (5)

An optical element of light from 100 GHz to 10 THz is input to a plasma CVD apparatus, and a source gas for CVD is sequentially switched to form a film having a first refractive index and a second refractive index smaller than the film. A thin film forming method for a terahertz band optical element, wherein a plurality of laminated films including a film and a film are sequentially formed on the surface to form a light wave interference film. 2. The method according to claim 1, wherein the source gases for the CVD are a silane gas and an oxygen gas, wherein the silicon gas is formed by blocking the oxygen gas, and the oxygen gas is introduced to form the silicon oxide film. Thin film forming method for a terahertz band optical element. 3. The method for forming a thin film for a terahertz band optical element according to claim 2, wherein an argon gas is used as a carrier gas of the raw material gas. 3. The method for forming a thin film for a terahertz band optical element according to claim 2, wherein the pressure at the time of film formation in the plasma CVD apparatus is 1 Pascal or less. The material of the optical element is a semiconductor in which germanium, silicon, or gallium arsenide is added with a shallow impurity whose impurity level is 40 meV or less, and the film forming temperature is 160 ° C. or less. Item 3. A method for forming a thin film for a terahertz band optical element according to Item 2.

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