JPH0624993B2 - Quartz glass manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing quartz glass, and in particular, to a crucible for pulling a silicon single crystal and a heat treatment furnace, which has high purity and heat resistance equivalent to that of natural quartz glass. The present invention relates to an improvement in a method for producing quartz glass, which is most suitable for the semiconductor industry such as a furnace core tube.

(Prior Art) For the production of high-purity quartz glass, a method of purifying natural quartz by washing or screening or electrolyzing the obtained natural quartz glass (see International Patent Publication W086 / 02919), or silicon tetrachloride A method is known in which a silicon compound such as silicon is flame-hydrolyzed with an oxyhydrogen flame to deposit silica on a carrier, and the silica is fused and vitrified.

However, the one using this natural quartz as a starting material has insufficient purity for the semiconductor industry, and synthetic quartz glass obtained by melting silica obtained by flame hydrolysis of silicon tetrachloride is included in this. Since the amount of OH groups is as large as 800 ppm or more, it has the disadvantage of being inferior in heat resistance, and this has the disadvantage of being expensive.

For this reason, the present inventors previously heat-processed silica obtained by flame hydrolysis of a silicon compound in an oxyhydrogen flame to form cristobalite crystals, and then melted to obtain high-purity quartz glass with high heat resistance. In addition to the method (see Japanese Patent Laid-Open No. 62-113729) and the so-called expensive synthetic quartz glass as a raw material, an inexpensive silica gel prepared by a sol-gel method was used as a starting raw material to crystallize this. A method for producing a high-purity silica glass that is economically highly heat-resistant by subsequent vitrification (Japanese Patent Application No. 61-312).
770) was proposed.

In these methods, it is advantageous to perform crystallization (cristobalite formation) in the presence of an alkaline component from the viewpoint of carrying out the production with high productivity.
It is necessary to carry out dealkalization prior to molten vitrification. However, at present, no technique has been developed to carry out this dealkalization more simply, cheaply and surely, and this solution is an urgent problem.

(Structure of the Invention) The present invention relates to a method for producing quartz glass which solves the above problems, in which an amorphous silicon dioxide produced by a sol-gel method using a metal alkoxide or an inorganic silicate as a raw material is BET. Specific surface area by the method is 50 m ² / g
It is characterized in that it is made smaller, and after adding an alkali component to this to heat it to β-cristobalite, it is cooled and transformed into α-cristobalite powder, and then it is melted into vitrified silica glass. Is.

That is, the present inventors conducted various tests to find the optimum conditions in the dealkalization step after the silica obtained by the sol-gel method was used as cristobalite in the presence of alkali, and the results were obtained by the sol-gel method. It was found that the specific surface area of the produced amorphous silicon dioxide is greatly related to the ease of subsequent dealkalization, and if the specific surface area by the BET method is set to 50 m ² / g or less, dealkalization by heating will be extremely difficult. The present invention has been completed by finding that it becomes easy and the residual amount can be minimized, and that it is possible to obtain a quartz glass having high purity and high heat resistance by converting this into a molten glass.

Hereinafter, the present invention will be described for each step.

The method of the present invention firstly produces amorphous silicon dioxide from a metal alkoxide or an inorganic silicate, which may be performed by a known sol-gel method. In this sol-gel method, for example, a mixed solution of water and ethyl alcohol of a metal alkoxide such as ethyl silicate [Si (OC ₂ H ₅ ) ₄ ] or an aqueous solution of an inorganic silicate such as water glass is heated under agar. After sol-formation, it is heated to drive out alcohol and water to obtain silicon dioxide as a dry gel. According to this method, silicon dioxide is obtained as an amorphous substance.

According to the method of the present invention, the amorphous silicon dioxide obtained by the known sol-gel method has a specific surface area of 50 m ² / BET method.
The specific surface area should be less than 5
As a method for adjusting the amount to 0 m ² / g or less, for example, there is a method of heat-treating this. The heating temperature may be 1,000 ° C., but considering the productivity, for example, to satisfy the condition that the heating time is within 2 hours, 1,100 ° C. or higher is preferable. It is preferable to set the temperature to 1,200 ° C. or lower, which is a boundary at which the specifications of the furnace become significantly expensive.

As described above, in the temperature range of 1,100 to 1,200 ° C., the heating time of 2 to 3 hours is sufficient. On the other hand, in order to reduce the specific surface area to 50 m ² / g or less, amorphous silicon dioxide prepared by the sol-gel method may be treated by a chemical method, for example, a metal alkoxide or an inorganic material by the sol-gel method. Method for allowing hydrolysis reaction of silicate in the presence of formamide (Mat. Res. Soc. Syp. Pro
c. Vo173, 1986, P35-47 acid). In addition, you may heat-process what was processed by this method.

According to the method of the present invention, the amorphous silicon dioxide having a specific surface area of 50 m ² / g or less according to the BET method is then heat-treated to give β-cristobalite, which is heated by β-cristobalite. It is necessary to carry out at a melting point of 1723 ° C. or lower, but when this cristobaliteization is carried out in the presence of a crystallization accelerator such as an alkali compound, a temperature of 1,100 to 1,200 ° C. which is easily obtained in an industrial furnace. It proceeds rapidly in the presence of a Na-based alkalizing agent such as an aqueous NaOH solution.
It is preferable to heat to 1,200 ° C. Further, the β-cristobalite thus obtained needs to be α-cristobalite, which can be easily converted to α-cristobalite by cooling the heated product to obtain β-cristobalite at room temperature. it can.

Next, in the method of the present invention, the α-cristobalite thus obtained is heated and melted to form a quartz glass. This α-cristobalite converts silicon dioxide into β-
An alkali compound is added to obtain cristobalite, and this is mixed in the obtained quartz glass. Therefore, it is necessary to remove this alkali component prior to the molten vitrification.

The dealkalization of cristobalite is performed, for example, by heating at about 1,300 ° C. in a halogen gas atmosphere or by acid cleaning. However, it is preferable to carry out dealkalization only by heat treatment, in consideration of process safety and equipment for countermeasures.

In the method of the present invention, for example, when the amount of alkali added is 50 ppm, the purity achieved at 1,500 ° C. for 5 hours is 1 ppm or less when the original specific surface area of silica is around 50 m ² / g. , 0.3 m ² / g, the amount becomes 0.2 ppm or less.

The method of the present invention is to obtain high-purity quartz glass excellent in heat resistance by heating and melting the high-purity α-cristobalite powder dealkalized by such a method according to a conventional method. It was found that the purity values (Na content) shown in Table 2 were inherited almost as they were.

On the other hand, when a quartz glass ingot was prototyped by the Bernoulli method, the Na content tended to decrease, but the trend of purity values (rank) in Table 2 did not change, and therefore the purity of the raw material is important. It can be seen that

Next, examples of the present invention will be given.

Example 1 To 1 mol of silicon ethoxide (Si (OC ₂ H ₅ ) ₄ ), 3 mol of 0.01N ammonia water and 4 mol of methanol were mixed and stirred to prepare a sol liquid. In a plastic beaker and in a constant temperature bath at 60 ° C for 1
After leaving it for 0 hours, it is further heated in an electric furnace at 200 ° C for 2 hours.
When heated and held for a period of time, powdery amorphous silicon dioxide (hereinafter abbreviated as sample 1) was obtained.
This was housed in a quartz glass boat and placed in a horizontal tubular furnace equipped with a silicon carbide heater at 900 ° C, 1,000 ° C, 1,
Samples 2, 3, 4, 5, and 6 were prepared by heating at 100 ° C., 1,200 ° C., and 1,300 ° C. for 2 hours. When measured by the BET method, the results shown in Table 1 below were obtained.

Then, to the samples 1 to 6 thus obtained, a 50 ppm NaOH aqueous solution in terms of Na was added and dried, then 1,20
When heated at 0 ° C. for 5 hours, they were determined to be β-cristobalite, so they were cooled to room temperature to obtain α-cristobalite, which were then placed in a fused alumina boat,
It was heated at 1,500 ° C. for 5 hours in an electric furnace equipped with a molybdenum silicide heater, and after cooling, the Na content contained in this α-cristobalite was examined by an atomic absorption photometry method. The results shown in the table were obtained.

Next, the dealkalized α obtained in this way
-Cristobalite was put into a rotary mold, arc-melted while being rotated to make a crucible having a diameter of 8 inches, and the obtained crucible made of quartz glass was examined, and the purity values shown in Table 2 ( It was found that the Na content) was inherited almost as it was.

On the other hand, when a quartz glass ingot was prototyped by the Bernoulli method, the Na content tended to decrease, but the trend of purity values (rank) in Table 2 did not change, and therefore the purity of the raw material is important. Therefore, it can be seen that the specific surface area of the silica powder, which is an intermediate product before addition of alkali, is important for the purity of the product.

Claims (3)

[Claims] 1. An amorphous silicon dioxide produced by a sol-gel method using a metal alkoxide or an inorganic silicate as a raw material having a specific surface area of less than 50 m ² / g by the BET method,
Production of quartz glass characterized by adding an alkali component to this and heating it to obtain β-cristobalite, cooling this to transform it into α-cristobalite powder, subjecting it to dealkalization, and then subjecting it to molten vitrification Method. 2. Amorphous silicon dioxide produced by the sol-gel method is 1,000 ° C. or higher, preferably 1,100-1,200 ° C.
The specific surface area by BET method by heating to 50 m ²
The method for producing quartz glass according to claim 1, wherein the value is smaller than / g. 3. The quartz glass according to claim 1, wherein the dealkalizing treatment of the α-cristobalite powder is carried out by any one of heat treatment, heat treatment in the presence of a halogen component, acid washing treatment, or a combination thereof. Manufacturing method.