JPH0826741A - Production of synthetic quartz glass powder - Google Patents

Abstract

PURPOSE:To obtain synthetic quartz glass powder having a low silanol group content. CONSTITUTION:When powder of silica gel obtd. by hydrolyzing tetraalkoxysilane is filled into a heat resistant vessel and fired to produce synthetic quartz glass powder, the silica gel powder filled into the heat resistant vessel is fired while introducing dehumidified gas having <=-30 deg.C dew point into the interior of the powder.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a silica glass powder suitable as a raw material for an ultra-high purity silica glass product used in the semiconductor manufacturing field, particularly in a high temperature range of 1000 ° C. or higher.

[0002]

2. Description of the Related Art In recent years, with respect to various glass jigs and crucibles used in the semiconductor industry and the like, with the high integration of semiconductors, the purity of the constituent glass materials has been extremely strictly controlled. . As a method for producing a high-purity glass product applied to these applications, conventionally, alkoxysilane is used as a starting material, which is hydrolyzed to obtain a sol-
A method is known in which silica powder is obtained by a process called a gel method, which is then fired to give a glass powder, which is then melted and vitrified to produce a desired desired glass product. According to this method, since the starting alkoxysilane can be easily purified by distillation, a high-purity glass product can be obtained.

[0003]

However, although the above-mentioned conventional method can obtain the metal impurities having a very small content, the silanol groups which were present at the time of hydrolysis / gelation are not suitable for the particles even after firing. Since it remains as a residual silanol group, it has a higher silanol content than natural quartz powder. If the silanol content is high, the viscosity at high temperature is low, so that it cannot be used in a crucible for pulling a silicon single crystal, a semiconductor diffusion furnace, etc.
Issue). In fact, it is preferable that the silanol content is 100 to 50 ppm or less for use in this application.
Therefore, in order to reduce the concentration of silanol, there is known a method for producing low silanol silica by firing in an atmosphere having a low water vapor partial pressure (JP-A-2-289416). However, a 550 mmφ × 600 mmH heat-resistant cylindrical container was filled with silica gel powder up to a powder layer height of 500 mm, and calcined at 1200 ° C. for 50 hours while flowing dehumidified air with a dew point of −45 ° C. to the silanol at the bottom of the container. It was found that the content was 50 ppm in the surface layer and 73 ppm in the bottom, about 1.5 times.
Even when the powder bed height is set high, the silanol concentration can be lowered by prolonging the firing time or raising the firing temperature. Post-processing such as crushing after calcination becomes necessary after firing. Therefore, in order to increase the production capacity, it has been a subject to increase the charging amount per firing and to shorten the firing time.

[0004]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the inventors have found that a difference in silanol concentration occurs between the surface layer and the bottom of a container as the height of the stock layer increases. Is because it takes time to disperse the steam generated from the powder during firing into the surface layer and replace it with dehumidified gas,
It was clarified that the water vapor pressure around the powder increases from the surface layer to the bottom, which seems to suppress diffusion and desorption of silanol groups from inside the particle. From this, the supply of dehumidifying gas is not supplied to the surface layer part of the dry gel powder charged in the heat-resistant container, but inside the powder layer, preferably, supplied from the lowermost layer part, and the gas flow toward the powder surface layer part. By making the, the steam generated during firing is smoothly purged, and individual powder particles are constantly in contact with the dehumidifying gas, resulting in the promotion of diffusion and elimination of silanol groups, and the firing time can be greatly shortened. They have found the present invention and reached the present invention. That is, the gist of the present invention is to produce a synthetic quartz glass powder by putting a silica gel powder obtained by hydrolysis of tetraalkoxysilane in a heat-resistant container and firing it, and the dew point is inside the silica gel powder filled in the heat-resistant container. A method for producing a synthetic quartz glass powder, which comprises firing while introducing a dehumidifying gas at −30 ° C. or lower.

The present invention will be described in more detail below. The synthetic quartz glass powder targeted by the present invention is silica glass powder obtained by drying and firing silica gel obtained by hydrolyzing alkoxysilane. Hydrolysis of alkoxysilane by the sol-gel method is performed by reacting alkoxysilane with water according to a well-known method. As the alkoxysilane used as a raw material, a lower alkoxysilane having 1 to 4 carbon atoms such as tetramethoxysilane and tetraethoxysilane or an oligomer thereof is preferable.

The amount of water used is usually selected from 1 to 10 equivalents of the alkoxy groups in the alkoxysilane. At this time, organic solvents such as alcohols and ethers which are compatible with water may be mixed if necessary. Specific examples of the alcohol used include lower aliphatic alcohols such as methanol and ethanol, and the use of these organic solvents can make the reaction system uniform and stable. However, as the hydrolysis reaction progresses, the alkoxy groups bound to the alkoxysilane are liberated as alcohols, so if the reaction solution is in a uniform state before gelation, it may be necessary to add alcohol. The reaction can be performed without any trouble.

For this hydrolysis reaction, hydrochloric acid as a catalyst,
An acid such as acetic acid or an alkali such as ammonia may be used. In addition, as a matter of course, in order to obtain a high-purity silica gel powder, it is necessary that all the substances to be introduced into this reaction system such as the raw material alkoxysilane, water, and solvent used here are of high purity. is there.

In order to gelate the hydrolysis product, the gel can be obtained faster by heating, but even if it is left at room temperature, it gels in several hours, so by adjusting the degree of heating. The gel time can be adjusted.
The obtained gel may be subdivided and then dried, or may be dried and subdivided. The average particle size of dried silica gel is usually 100 to 1000 μm.

The degree of drying of the gel is usually 1 to 30% by weight based on the content of H ₂ O, and the gel is usually dried under reduced pressure or in an inert gas atmosphere at 100 to 200 ° C.
By heating to. The dried silica gel powder produced as described above is fired under the specific firing conditions described below. That is, the firing is performed while supplying the dehumidifying gas from below the center of the charged powder layer height of the dry silica gel powder, preferably from the bottommost layer, during the firing in the heat-resistant container. Here, as a method of supplying gas to the powder lower layer portion, as shown in FIG. 1, a method of inserting a heat-resistant cylindrical ceramic filter 2 into the center portion of the heat-resistant container 1 (3: dry gel powder), As shown in FIG. 2, a method of providing a heat resistant porous plate 4 in the lower portion of the heat resistant container 1 and supplying dehumidified gas from the heat resistant tube 5 through the heat resistant porous plate 4 can be considered.
It is not limited to this, just a heat resistant single tube,
The effect can be expected by just inserting it in the center of the powder. By such a dehumidifying gas supply method, the water vapor generated from the gel during firing can be diffused and removed toward the surface layer portion without staying inside the powder layer.
The atmosphere around the powder particles can always be maintained at a low water vapor partial pressure, and synthetic quartz glass powder having a target silanol amount can be obtained in a short time. As the kind of dehumidifying gas used at this time, at least up to around 600 ° C. at which removal of carbon remaining in the gel is completed,
An oxygen-containing atmosphere is required, but after that,
It may be an inert gas atmosphere. The dew point of the dehumidifying gas used varies depending on the required silanol concentration, but by using a gas having a dew point of −30 ° C. or lower, a silanol concentration of 50 ppm or lower can be obtained. This is because the residual silanol in the quartz glass is in equilibrium with the water vapor around the powder, so using a gas with a dew point higher than this temperature not only requires a long time for firing, but in some cases, as a high temperature application. It will not be possible to reduce to the required target silanol concentration. The firing temperature is 900 ° C or higher at which the gel becomes completely non-porous, 14
It is performed at 00 ° C or lower. If the temperature is higher than this, sintering between particles occurs and crushing is required after firing, which is not preferable.

The synthetic quartz glass powder thus obtained is used in the semiconductor manufacturing field such as a crucible for pulling a silicon single crystal, a tube of a diffusion furnace, a jig, etc. It can be suitably used as a raw material for silica glass products.

[0011]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded.

[0012]

Example 1 Tetramethoxysilane and water in an amount of 5 times the amount of tetramethoxysilane were charged into a stirring tank and stirred at a temperature of 30 ° C. for 1 hour to obtain a uniform sol solution by hydrolysis reaction. Further, this was transferred to a vinyl chloride vat and left for 5 hours for gelation. After drying this gel for 12 hours using a vacuum dryer at 140 ° C., particle size was adjusted to a particle size of 100 to 500 μm. 100 kg of the dried silica gel powder thus obtained was placed in a 550 mmφ × 600 mmH quartz glass crucible having a heat resistant porous plate (4) at the bottom as shown in FIG. The height of the charged layer before firing was 50 cm. And dew point -4 through the perforated plate
While supplying dehumidified air at 5 ° C at 40 l / min, 1
The temperature was raised to 220 ° C. over 5 hours, and the temperature was maintained for 50 hours. After the firing, the powder was collected by cooling to room temperature. The layer height after firing is 36 cm, which is 0 to 12 cm (upper part), 12 to 24 cm (middle part) from the surface layer part,
It was collected in three places of 24 to 36 cm (lower part). When the silanol concentration of each part was measured by the infrared absorption method, it was 28 ppm in the upper part, 34 ppm in the middle part and 29 ppm in the lower part.

[0013]

Example 2 100 kg of dried silica gel powder produced by the same method as in Example 1 was 550 mmφ × 600 mmH.
It was placed in a quartz glass crucible. 1 in the center of this powder
Insert a 0 mmφ quartz glass tube to a position 30 cm from the powder surface layer, and supply dehumidified air with a dew point of −45 ° C. at 40 l / min through the quartz tube at 1220 ° C.
Up to 5 hours and kept at that temperature for 50 hours. After firing, the powder was recovered in the same manner as in Example 1 and the silanol concentration was measured by an infrared absorption method.
ppm, 45 ppm in the middle part and 55 ppm in the lower part.

[0014]

Comparative Example 1 100 kg of dried silica gel powder prepared by the same method as in Example 1 was 550 mmφ × 600 mm
It was placed in a quartz glass crucible of H. Make a cut in the upper part of the periphery of this crucible, insert a 10 mmφ quartz glass tube from the lateral direction, and supply dehumidified air with a dew point of −45 ° C. to the powder surface layer at 40 l / min for 5 hours up to 1220 ° C. And the temperature was maintained for 50 hours. After the firing, the powder was recovered in the same manner as in Example 1 and the silanol concentration was measured by the infrared absorption method.
It was 64 ppm in the middle part and 73 ppm in the lower part.

[0015]

As described above, according to the method of the present invention, the synthetic silica glass powder having the target silanol concentration is
Good productivity and can be obtained in a short time.

[Brief description of drawings]

FIG. 1 shows an example of a firing method used in the present invention.

FIG. 2 is an example of a firing method used in the present invention

[Explanation of symbols]

 1 Heat-resistant container 2 Ceramic filter 3 Dry gel powder 4 Heat-resistant porous plate 5 Heat-resistant tube

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Haruo Asaya 1000, Kamoshida-cho, Midori-ku, Yokohama-shi, Kanagawa Sanryo Kasei Co., Ltd.

Claims (1)

[Claims] 1. When producing a synthetic quartz glass powder by placing a silica gel powder obtained by hydrolysis of tetraalkoxysilane in a heat-resistant container and firing it, a dew point inside the silica-gel powder filled in the heat-resistant container is- Thirty
A method for producing synthetic quartz glass powder, which comprises firing while introducing a dehumidifying gas at a temperature of not higher than 0 ° C.