JPH0558651A - Production of silica glass - Google Patents

Abstract

PURPOSE:To improve optical properties without any holes by hydrolyzing a silicon alkoxide, gelatinizing the resultant silica sol, drying the gel, burning the dried gel and then sintering the resultant glass body or precursor on a specific bottom board or in a crucible. CONSTITUTION:An organic polymer or an organic substance having a higher boiling point than that of water and a lower surface tension than that of water or both are mixed with an organic solvent and a silicon alkoxide and a catalyst are added to hydrolyze the above-mentioned mixture. The resultant silica sol is then kept at <=70 deg.C for several hr, gelatinized and then dried at a temperature above ambient temperature for several days to several weeks to afford a dried gel, which is subsequently heated at 1000-1400 deg.C to afford a glass body or a glass precursor. The resultant glass body or precursor is then placed on a bottom board or a crucible composed of high-purity carbon with <=50ppm metallic content and sintered at 1720-2200 deg.C for 5min to 2hr.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to optical, semiconductor industry,
The present invention relates to a method for producing silica glass used for electronic industries, science and engineering, and the like.

[0002]

BACKGROUND OF THE INVENTION Silica glass is an important material indispensable for semiconductor manufacturing because it is excellent in heat resistance, corrosion resistance and optical properties, and further, it is a photomask substrate for manufacturing optical fibers, IC substrates, and TFT substrates. It is used for various purposes, and its usage is expanding more and more. Conventional methods for producing silica glass include a method of melting natural quartz by an electric furnace or an oxyhydrogen flame, or a method of oxidizing silicon tetrachloride at a high temperature in an oxyhydrogen flame or a plasma flame to dissolve it. Also requires a high temperature of 2000 ° C. or higher in the manufacturing process, consumes a large amount of energy, and requires a material that can withstand such a high temperature during manufacturing, and it is difficult to obtain a higher purity product. There are some problems in terms of quality.

On the other hand, in recent years, a method called a sol-gel method for synthesizing silica glass at a low temperature has attracted attention.
The outline is briefly described. By dispersing the SiO ₂ fine particles produced by vapor phase decomposition of silicon tetrachloride in water, an organic solvent or a water-organic solvent mixture, or by hydrolyzing and polymerizing a silicon alkoxide in an organic solvent together with a catalyst, Use silica sol. At this time,
Appropriate alcohol is usually used as the organic solvent so that the silicon alkoxide and the water become a uniform system. This silica sol is gelated by standing, heating, addition of a gelling agent, and the like. Then, the organic solvent and water are evaporated from the gel, and the gel is dried to obtain a dry gel. The dried gel is usually heated at 1000 to 1400 ° C. in a suitable atmosphere such as helium to obtain silica glass.

However, the above-mentioned method for producing silica glass by the sol-gel method has the following problems. That is,
With the method of dispersing the SiO ₂ fine particles in water, an organic solvent, etc., it is difficult to uniformly disperse the SiO ₂ fine particles,
Large SiO ₂ agglomerates are likely to remain in the silica sol, and these SiO ₂ agglomerates cause defects (holes) in the glass when fired and deteriorate the quality. Further, in the method of hydrolyzing / polymerizing a silicon alkoxide with an catalyst in an organic solvent to form a silica sol, cracks and cracks are easily generated in the gel in the subsequent drying process, and fine glass is obtained by firing it. Vacancies are likely to occur. Therefore, in order to solve these problems, the present inventors hydrolyzed and polymerized silicon alkoxide with an organic solvent in an organic solvent in the presence of an organic polymer such as polyethylene glycol to form a silica sol, which was gelated and dried. , 1000-1
A method of firing at 400 ° C. (JP-A-1-140766, JP-A-1-141483, etc.) or hydrolysis of a silicon alkoxide in an organic solvent together with a catalyst in the presence of an organic substance having a boiling point higher than that of water Polymerization to obtain silica sol, gelation, drying, and firing at 1000 to 1400 ° C (JP-A-2-25435, JP-A-2-25).
No. 438, etc.) has been proposed. Further, as a method which is contrary to energy saving, as another method, a glass body or a glass precursor obtained by the sol-gel method is heated to 1500 to 2200 ° C. and kept for a certain period of time to obtain silica glass (Japanese Patent Laid-Open Publication No. 6-58242).
No. 2-105936) is also proposed.

[0005]

However, in the presence of an organic polymer or an organic substance having a higher boiling point than water, a silicon alkoxide is hydrolyzed and polymerized with a catalyst in an organic solvent to give a silica sol, which is gelled and dried, 1000-1
Even by the method of firing at 400 ° C., it is difficult to obtain silica glass having no defects (voids). It is an object of the present invention to provide a method for producing large-sized silica glass at a low cost, which has fewer defects (pores), excellent optical properties, and moreover.

[0006]

In order to achieve the above object, the inventors of the present invention have conducted in the presence or absence of an organic polymer and / or an organic substance having a boiling point higher than that of water and a surface tension lower than that of water. In addition, silicon alkoxide is hydrolyzed with a catalyst in an organic solvent to give a silica sol, which is gelled and dried to give a dried gel, which is then calcined in a calcining furnace at a temperature lower than 1720 ° C or higher than 1720 ° C. When we were examining a method for producing silica glass,
When fired at a temperature of 20 ° C. or higher, the obtained silica glass has very few pores, but it is observed that the optical properties, especially the light transmittance at a wavelength of 300 nm or less, are inferior to those of conventional silica glass. did. Therefore, as a result of diligent examination of the cause by the present inventors, the purity of carbon in the material used for the floorboard and the crucible, particularly the content of alkali and alkaline earth metals in the carbon has a great influence on the above light transmittance. Has a metal content of 50 as a floorboard and crucible
If it is fired by using carbon of high purity of less than ppm, the light transmittance is as high as that of the conventional silica glass even at a wavelength of 300 nm or less, and there is no hole, so that a large silica glass can be obtained. They have found that they can be obtained and have completed the present invention.

That is, the present invention relates to the following (1) and (2). (1) Silicon alkoxide is hydrolyzed in an organic solvent together with a catalyst into a silica sol, which is then gelled and dried into a dry gel, which is heated at 1000 to 1400 ° C. to form a glass body or a glass precursor. , Placing it on a floor plate made of high-purity carbon having a metal content of 50 ppm or less, or placing it in a crucible made of high-purity carbon having a metal content of 50 ppm or less, and firing at a temperature of 1720 ° C. or higher in a firing furnace. Characteristic silica glass manufacturing method. (2) The boiling point is higher than that of the organic polymer and / or water,
And, in the presence of an organic substance having a lower surface tension than water, silicon alkoxide is hydrolyzed with a catalyst in an organic solvent to give a silica sol, which is then gelled and dried to give a dry gel, which is heated at 1000 to 1400 ° C. To obtain a glass body or a glass precursor, and then use this for a metal content of 50 p
A silica glass characterized by being placed on a floor plate made of high-purity carbon of pm or less, or put in a crucible made of high-purity carbon of 50 ppm or less in metal content, and fired at a temperature of 1720 ° C. or higher in a firing furnace. Manufacturing method.

The silicon alkoxide in the present invention may be partially polycondensed. As the alkyl group of silicon alkoxide, from the viewpoint of ease of hydrolysis and gelation time, a methyl group, an ethyl group, a propyl group,
Examples thereof include a butyl group.

As the organic solvent of the present invention, methyl alcohol, ethyl alcohol, n-propyl alcohol, i-
Alcohols such as propyl alcohol and n-butyl alcohol, ethers such as dimethyl ether, acetone,
Examples thereof include ketones such as methyl ethyl ketone, esters such as ethyl acetate, mixed solvents thereof, etc., preferably alcohols such as methyl alcohol and ethyl alcohol, and more preferably methyl alcohol.

As the catalyst, basic catalysts such as ammonia, choline, trimethylammonium hydroxide, triethylammonium hydroxide, tripropylammonium hydroxide, tributylammonium hydroxide and various amines,
Acid catalysts such as hydrochloric acid and nitric acid are mentioned, and these are used together with water.

As the organic polymer, polyethylene glycol, polymethylene glycol, polypropylene glycol, polyalkylene glycol such as polybutylene glycol, polyethylene glycol methyl ether,
Polyalkylene glycol ether such as polyethylene glycol ethyl ether, polyethylene glycol propyl ether and polyethylene glycol butyl ether, hydroxyalkyl cellulose such as hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and hydroxybutyl cellulose,
There are polyvinyl acetate and the like. Organic substances having a boiling point higher than that of water and a surface tension lower than that of water include N, N-dimethylformamide, N, N-dimethylacetamide, tetrahydrofurfuryl alcohol, 1-methyl-2-pyrrolidone and ethylene glycol monomethyl ether. , Ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, and the like.

Transfer the obtained silica sol to a suitable container,
The mixture is kept at room temperature to 70 ° C for several hours for gelation, and then heated and dried at a temperature higher than room temperature for several days to several weeks to obtain porous silica gel.

The high-purity carbon used for the floor plate or crucible is a metal, that is, an alkali metal such as Li, Na or K, or C.
Alkaline earth metals such as a, Mg, Ba, Fe, Ni, C
High-purity carbon having a total content of transition metals such as o and other metals such as B, Al, and Cu of 50 ppm or less. When low-purity carbon having a metal content of more than 50 ppm is used, it is difficult to obtain a high light transmittance at a wavelength of 300 nm or less as expected. Metal content 50p
High-purity carbon of pm or less has a metal content of 50 pp
It is possible to produce a block of carbon having a low purity exceeding m by high temperature treatment in a gas containing a halogen compound such as carbon tetrachloride and freon. In addition, commercially available high-purity carbon (for example, isotropic carbon manufactured by Hitachi Chemical Co., Ltd.) can be obtained. In addition to the floor plate or crucible, it is desirable to use carbon having a metal content of 50 ppm or less for the furnace material of the firing furnace at the portion in contact with the atmosphere in the furnace.

Calcination at a temperature of 1720 ° C. or higher means 1
720 to 2200 ° C., more preferably 1730 to 200
It means holding at 0 ° C. for a certain period of time, for example, 5 minutes to 2 hours. At temperatures below 1720 ° C, it is difficult to reduce the number of pores in the silica glass, and at temperatures above 2200 ° C, energy is wasted.

The floor plate is made of high-purity carbon having a metal content of 50 ppm or less, and the crucible has a metal content of 50 ppm.
When firing in a firing furnace consisting of the following high-purity carbon,
The reason why a silica glass having a high light transmittance and a small amount of fluorescence generated when irradiated with high energy light is obtained is that the metal content is 5
If low purity carbon exceeding 0 ppm is used, 1
Impurities from floor boards and furnace materials at high temperatures of 720 to 2200 ° C,
In particular, Na ₂ O, K ₂ O, MgO, which has a large vapor pressure at high temperatures,
It is considered that CaO or the like is released into the firing furnace, which contaminates the silica glass and reduces the light transmittance at a wavelength of 300 nm or less. Therefore, by using high-purity carbon, such contamination can be prevented.

[0016]

【Example】

Example 1 50 wt% polyvinyl acetate / methyl alcohol solution 6
45 g and 3500 g of methyl alcohol were mixed, to which 4300 g of a partial polycondensate of silicon tetramethoxide (Si (OCH ₃ ) ₄ ) was added and mixed. To this mixed solution, 1676 g of a 15 mM choline aqueous solution was added dropwise and sufficiently mixed to obtain a silica sol. 2500 of the obtained sols
360 mm coated with g of polyfluoroethylene
Place in a square stainless steel container, cover with a polyvinyl chloride film, gel at 30 ° C, and then 6 at 60 ° C.
It was dried for 1 day at 120 ° C. for 1 day to obtain a dry gel. The dried gel was heated to 700 ° C. in an air stream and then heated to 1300 ° C. in a helium atmosphere to obtain silica glass or a silica glass precursor. This was placed on a plate of high-purity carbon (made by Hitachi Chemical Co., Ltd., isotropic carbon, PD-610S) having a metal content of 20 ppm, which was also made of high-purity carbon (made by Hitachi Chemical Co., Ltd.). , Isotropic carbon, PD-610S) in a crucible, and heat-treated at 1740 ° C. for 30 minutes,
A silica glass was obtained. Polishing the obtained silica glass,
The test piece 1 was used.

Comparative Example 1 Silica glass or a silica glass precursor obtained by the same program as in Example 1 was used to obtain a metal content of 320 p.
It was placed on a pm carbon plate, put in a crucible made of carbon of the same purity, and heat-treated at 1740 ° C. for 30 minutes to obtain silica glass. The obtained silica glass was polished to obtain a test piece 2.

Example 2 50% by weight of polyvinyl acetate / methyl alcohol solution 2
15 g, 1000 g of N, N-dimethylformamide and 2450 g of methyl alcohol were mixed, to which 4300 g of a partial polycondensate of silicon tetramethoxide (Si (OCH ₃ ) ₄ ) was added and mixed. 15 mM in this mixed solution
1676 g of the aqueous choline solution was added dropwise and sufficiently mixed to obtain silica sol. Out of the obtained sol, 2500 g was put in a 360 mm square stainless steel container coated with polyfluoroethylene, covered with a polyvinyl chloride film, and gelated at 30 ° C., then at 60 ° C. for 6 days, Subsequently, it was dried at 120 ° C. for 1 day to obtain a dry gel. The dried gel was heated to 700 ° C. in an air stream and then heated to 1300 ° C. in a helium atmosphere to obtain silica glass or a silica glass precursor. It has a metal content of 2
It was placed on a plate of 0 ppm high-purity carbon (manufactured by Hitachi Chemical Co., Ltd., isotropic carbon, PD-610S), and this was also put on high-purity carbon (manufactured by Hitachi Chemical Co., Ltd., isotropic carbon, It was put in a crucible made of PD-610S) and heat-treated at 1740 ° C. for 30 minutes to obtain silica glass. The obtained silica glass was polished to obtain a test piece 3.

Comparative Example 2 Silica glass or a silica glass precursor obtained by the same program as in Example 2 was used, and the metal content was 320 p.
It was placed on a pm carbon plate, put in a crucible made of carbon of the same purity, and heat-treated at 1740 ° C. for 30 minutes to obtain silica glass. The obtained silica glass was polished to obtain a test piece 4.

Example of analysis of light transmittance of silica glass Silica glass test pieces 1 and 3 obtained in Examples 1 and 2
The light transmittance at a wavelength of 200 to 500 nm was compared with the silica glass test pieces 2 and 4 obtained in Comparative Examples 1 and 2 and the conventional silica glass test piece 5. The thickness of the glass test piece is 2.35 mm, and the spectrophotometer (Hitachi,
U-3400 type). FIG. 1 shows the light transmittance patterns of these test pieces. From the results in Figure 1,
The silica glass test pieces 2 and 4 of the comparative example have a wavelength of 300 nm.
Although the light transmittance starts to drop sharply below, it can be seen that the silica glass test pieces 1 and 3 maintain the same high light transmittance as that of the conventional silica glass even at a wavelength of 300 nm or less.

[0021]

Industrial Applicability According to the present invention, it is possible to provide a method for inexpensively producing a large-sized silica glass having excellent optical properties, no voids.

[Brief description of drawings]

FIG. 1 is a spectrum showing the light transmittance of silica glass according to the present invention, silica glass of a comparative example, and conventional silica glass.

[Explanation of symbols]

 1: Silica glass test piece 1 of the present invention 1: Silica glass test piece 2 of Comparative Example 1 3: Silica glass test piece 3 of the present invention 4: Silica glass test piece 4 of Comparative Example 2 5: Conventional silica glass

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshikatsu Shimazaki 48, Wadai, Tsukuba City, Ibaraki Prefecture Hitachi Chemical Co., Ltd. Tsukuba Development Laboratory

Claims (2)

[Claims] 1. A silicon alkoxide is hydrolyzed with a catalyst in an organic solvent to give a silica sol, which is then gelled and dried to give a dry gel, which is 1000 to 1400.
After heating to a glass body or a glass precursor by heating at 0 ° C, this is placed on a floor plate made of high-purity carbon having a metal content of 50 ppm or less, or put in a crucible made of high-purity carbon having a metal content of 50 ppm or less, 17 in the firing furnace
A method for producing silica glass, which comprises firing at a temperature of 20 ° C. or higher. 2. In the presence of an organic polymer and / or an organic substance having a boiling point higher than that of water and a surface tension lower than that of water,
Silicon alkoxide is hydrolyzed in an organic solvent together with a catalyst to give a silica sol, which is then gelled and dried to give a dry gel, which is heated at 1000 to 1400 ° C. to obtain a glass body or a glass precursor, which is then prepared. Place it on a floor plate made of high-purity carbon with a metal content of 50 ppm or less,
Alternatively, a method for producing silica glass is characterized in that the silica glass is put in a crucible made of high-purity carbon having a metal content of 50 ppm or less and fired at a temperature of 1720 ° C. or higher in a firing furnace.