JPH06340411A - Production of powdery synthetic quartz glass - Google Patents

Abstract

(57) [Summary] [Purpose] To obtain quartz glass powder suitable as a raw material for ultra-high purity quartz glass products. [Structure] When the wet silica gel obtained by hydrolyzing tetraalkoxysilane is dried in a drying container, the one whose inner wall is previously washed with an alkaline aqueous solution of an alkali metal is used as the drying container.

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 1,000 ° C. or higher.

[0002]

2. Description of the Related Art Conventionally, crucibles and jigs for manufacturing semiconductor single crystals have been manufactured by melting natural quartz powder obtained by crushing natural quartz, but natural quartz is of high quality. Also contains various metal impurities, and was not sufficiently satisfactory in terms of purity. In particular, in a high-purity single crystal that is required as the semiconductor industry becomes more sophisticated, if metal impurities are mixed, the performance of the semiconductor will be adversely affected. Cannot be used. For this reason, recently, synthetic silica glass powder of high purity has been required.

In recent years, as a highly pure silica source,
Silica glass produced by sol-gel method using alkoxysilane as a raw material has been introduced. For example, JP-A-62-17
Japanese Patent No. 6928 discloses a method of producing a silica glass powder by hydrolyzing an alkoxysilane in the presence of an acid or an alkali to prepare a gel, crushing and drying the gel, and then firing the silica glass powder.

[0004]

SUMMARY OF THE INVENTION Synthetic quartz glass powder is
It is obtained by baking a dry gel obtained by hydrolyzing, drying and crushing an alkoxysilane to 1,000 to 1,300 ° C. However, in the drying container used in this gel drying step, film-like deposits may be generated on the inner wall portion as the usage frequency increases. If this is left as it is, and the number of times of drying is repeated, the thickness of the film-like adhered substance increases, and during the drying, peeling occurs, and the film-like adhered substance is mixed into the product as a flaky foreign substance.
There is no problem as long as the flaky foreign matter does not affect the product quality in the subsequent processing, but the flaky foreign matter contains unburned carbon in the inside thereof in the firing step and is likely to become a black foreign matter. I understood. If carbon is contained in the glass layer and becomes a black foreign substance as described above, it is very problematic because it melts and gasifies to form bubbles when it is molded into a crucible or an ingot.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies in order to obtain synthetic quartz glass powder free from the above-mentioned inconvenience, and as a result, in the production of synthetic quartz glass powder by the sol-gel method, By periodically dissolving and washing the flaky foreign substances adhering to the inside of the drying container used in the drying process with an alkali metal-based alkaline aqueous solution, there is no contamination of the flaky foreign substances in the product powder, and by extension, the crucible or ingot. The inventors have found that a synthetic quartz glass powder with less foaming during melt-molding can be provided, and have reached the present invention.

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

The amount of water used is usually selected from 1 to 10 times the equivalent of the alkoxy groups in the alkoxysilane. At this time, an organic solvent such as alcohols may be mixed if necessary. Examples of alcohols include methanol, ethanol, propanol, butanol and the like.
As the catalyst, an acid such as hydrochloric acid or acetic acid or an alkali such as ammonia may be used.

As a matter of course, in order to obtain quartz glass powder of high purity, the raw material alkoxysilane used here,
It is preferable that all substances introduced into this reaction system such as water and solvent have high purity. In order to gel the hydrolysis product, you can obtain the gel immediately by heating, but it will gel in several hours even if left at room temperature, so the gelling time can be adjusted by adjusting the degree of heating. It can be adjusted. The obtained gel may be subdivided and then dried, or may be dried and subdivided. The particle size of the dried silica gel powder is 60 to 900 μm, preferably 80 to 900 μm.
It is 800 μm. This drying is usually carried out by heating the gel to 100 to 200 ° C. in vacuum or in an inert gas.

In the method of the present invention, the film-like deposit (having a component of SiO ₂ ) is dissolved and removed as an alkali silicate salt by immersion cleaning with an alkali metal-based alkaline aqueous solution. The types of alkali metal-based alkalis used at this time include NaOH, KOH, Na ₂ CO ₃ , and K.
_{Although 2} CO _{3 and the} like are suitable, the alkali metal-based alkali that forms a salt of alkali silicate is not particularly limited thereto. The concentration of the aqueous solution to be used varies depending on the type of alkali and the temperature at the time of washing, but is preferably 0.01% by weight to 10% by weight. If this concentration is too low, the dissolution rate will be slow and the amount of film-like foreign matter that can be dissolved will be small, such being undesirable. The temperature during the alkali immersion cleaning is usually 20 to 150 ° C., preferably 60 to 120 ° C. When the washing temperature is low, the dissolution rate is slow, so that a long immersion time is required.
For example, when boiling cleaning is performed using a 1% NaOH aqueous solution, 90 to 1 in a 100 to 500 liter container is used.
A short time of 20 minutes is sufficient. Further, the frequency of dissolution and washing varies depending on the form of the drying container, the drying method, the amount of dry charge per time, and the like, so it is generally determined while observing the growth state of the film-like deposit. Usually 10
It is preferable to carry out once for every 100 batches, and even if the cleaning is carried out in an unnecessarily short cycle, it takes time and is not industrial.

The gel is dried while periodically washing the inner wall of the drying container by using the method as described above to obtain a dried silica gel powder. The dried silica gel powder thus obtained is fired and made non-porous in the temperature range of 1000 to 1400 ° C. to obtain a synthetic quartz glass powder. In the product powder, almost no black foreign matter containing unburned carbon caused by flaky foreign matter was found, and when this powder was used to melt-mold an ingot, a transparent ingot with very few bubbles could be obtained. Can be done.

[0011]

EXAMPLES The present invention will be described in detail with reference to the following examples, but the present invention is not limited to the following examples unless it exceeds the gist.

Reference Example A conical dryer (conical dryer) having an internal volume of 250 liters was charged with 50 kg of tetramethoxysilane and 30 kg of pure water at room temperature and the conical dryer was 6 rp / min.
The mixture was tumbled for 20 minutes at m to mix both solutions and carry out a hydrolysis reaction. When the temperature of the gas phase portion of the dryer reached 65 ° C. due to the heat of reaction, the rolling was stopped and gelation was performed. 3 at normal pressure
When left for 0 minutes, gelation was completed and a wet gel body was obtained. Next, add 3 kg to the jacket of the conical dryer.
/ Cm ² G of steam is flowed to heat, the internal pressure is further reduced to 100 Torr, and while rolling at 6 rpm,
Drying was performed until the temperature of the internal vapor phase portion reached 125 ° C. The dried silica gel powder that has been dried is extracted from the conical dryer, the particle size is adjusted to 100 to 500 μm, the above operation is repeated, and the same procedure is followed.
Samples of the 0th, 40th, 60th and 80th batches were collected.
At this time, when the inner wall of the dryer was observed for each sampling, it was confirmed visually that a film-like deposit was generated in the vicinity of the 40th batch. It was confirmed that some of them had peeled off. Next, 1 kg of each sampled sample is heated to 1200 ° C. at a rate of 200 ° C./Hr, and
Hold at 00 ° C. for 5 hours. After cooling, each sample was taken out and the number of black foreign matters was checked. As a result, it can be visually confirmed that the first batch is 0, the 20th batch is 0, the 40th batch is 2 and the 60th batch is 5 7 in 80th batch
It was an individual. Furthermore, each sample was melted using a Bernoulli-type melting device by heating an oxygen-hydrogen flame, and 12 mmφ
A × 50 mm ingot was prepared and the foaming state was examined.
The results are shown in Table 1, and the passing level ingot is 40
It was up to the batch, and the 60th and 80th batches failed.

Example 1 In Reference Example 1, when 40 batches had been completed, 130 liters of a 1% NaOH aqueous solution was charged in a conical dryer, and 3 kg / cm ² G of steam was flowed into the jacket to heat the mixture, and the mixture was heated to 6 rpm. While rolling at speed 1
The inner wall was boiled and washed for a certain period of time. After that, the NaOH aqueous solution in the container was taken out, and 130 liters of ultrapure water was charged to completely remove the NaOH in the container.
After rolling for 5 minutes at m, the operation of withdrawing was repeated 5 times. After washing, the gel was hydrolyzed and dried again in the same manner as in Reference Example 1, and the operation was continued while performing alkali washing for every 40th batch. Observation of the inner wall of the container after the alkali cleaning revealed that the film-like deposit was completely dissolved and removed. Furthermore, when the Na concentration in the silica gel dried after alkali cleaning was measured, it was 0.01 pp.
There was no problem when it was less than m. Also, sampling is 4
The 0th, 41st, 80th, and 81st batches were performed, and the same evaluation as in Reference Example 1 was performed. The results are shown in Table 1.

Comparative Example 1 In Example 1, the dryer was operated while performing the same washing operation every 40 batches with ultrapure water without using the NaOH aqueous solution, and 40, 41, 80, 81 batches were performed. Each eye was sampled. Each sample was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0015]

[Table 1]

[0016]

Industrial Applicability According to the present invention, it is possible to provide a synthetic quartz glass powder suitable for producing a transparent quartz glass product without foaming.

Claims (2)

[Claims] 1. When the wet silica gel obtained by hydrolyzing tetraalkoxysilane is dried in a drying container, a container whose inner wall is previously washed with an alkali metal-based alkaline aqueous solution is used as the drying container. A method for producing a synthetic quartz glass powder characterized. 2. The method according to claim 1, wherein the aqueous alkaline solution is a hydroxide or carbonate of an alkali metal.