JPH0798667B2 - Method for producing synthetic quartz glass - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a method for producing synthetic quartz glass, particularly high purity, high viscosity, no bubbles, and less stria The present invention relates to a method for producing synthetic quartz glass useful as a substrate and a base material for a semiconductor jig.

(Prior Art) For the production of synthetic quartz glass, various methods have been conventionally tried by sintering colloidal silica powder because a high-purity product can be obtained, and it was produced by hydrolysis of ethyl silicate. A method of producing quartz glass by adding a binder to silica powder to form a molded product by a wet rubber press, and then treating at 1,300 to 1,800 ° C. in an atmosphere containing halogen (see Japanese Patent Laid-Open No. 50-9609). , And recently made dry silica gel from alkoxysilane,
After heat treatment at 0 to 1,300 ℃, wet pulverize and dry after 1.00
There has been proposed a method for producing quartz glass by sintering at 0 to 1,300 ° C. and further at 1,600 ° C. or higher in a vacuum (see JP-A-2-34258 and JP-A-2-133329).

(Problems to be Solved by the Invention) However, in the method disclosed in Japanese Patent Laid-Open No. 50-9609, the purity of quartz glass obtained by adding a binder is lowered, and the cost for firing in a halogen atmosphere is low. However, the method disclosed in JP-A-2-34528 and JP-A-2-13329 has a disadvantage that silica is contaminated during wet milling. There is a drawback that it is difficult to make a process without it.

(Means for Solving the Problems) The present invention relates to a method for producing synthetic quartz glass which solves the above disadvantages and drawbacks, and it relates colloidal silica prepared by a sol-gel method in an oxidizing atmosphere.
After firing at 1,200 ℃ or less, sieve to 80 mesh or less,
Next, it is characterized in that the carbon crucible is filled, weighted from above and heat-treated under reduced pressure at 1,700 or less, and then heat-treated at 1,700 or more under normal pressure or pressure in the same furnace.

That is, the present inventors have various studies on a method for producing a synthetic quartz glass having high purity, high viscosity, no bubbles, and less striae, and the colloidal silica obtained by the sol-gel method in an oxidizing atmosphere at 1,200 Organic matter can be effectively removed by baking at ℃ or less, and if this is crushed to 80 mesh or less, gaps between particles will not be too open, and baking in a carbon crucible will not generate bubbles. It was found that it is possible to increase the bulk specific gravity, and it is possible to easily obtain a synthetic quartz glass having the desired physical properties by firing it at 1,700 ° C. or higher, and the present inventors proceeded with research on these various conditions and proceeded with the present invention. Was completed.

This will be described in more detail below.

(Operation) The present invention relates to an improvement in a synthetic quartz glass manufacturing method by a sol-gel method.

For the production of synthetic quartz glass, methyl silicate,
A method is known in which ethyl silicate or the like is hydrolyzed under acidic or alkaline conditions to form silica sol, which is dried to give silica gel, and then calcined at high temperature to give synthetic quartz glass, which is called the sol-gel method. Has been done.

Colloidal silica used as the starting material in the present invention is also prepared by this known sol-gel method, for example, methyl silicate, ethyl silicate is hydrolyzed with aqueous ammonia in the presence of an alcohol solvent. The obtained product, or the product obtained by continuously hydrolyzing methyl silicate with aqueous ammonia, preferably has a particle size of about 200 to 1,000 nm.

In the present invention, this colloidal silica is first fired at a temperature of 1,200 ° C. or lower in an oxidizing atmosphere. According to this, organic substances derived from methyl silicate, ethyl silicate, etc. are completely and effectively removed. However, this calcination cannot completely remove the organic matter at 800 ° C or lower, and if it is 1,200 ° C or higher, the pores of the particles are promoted and bubbles are easily formed in the particles.
It is preferable that the temperature range is 200 ° C, particularly 1,000 ° C ± 100 ° C.

The colloidal silica thus calcined is then sieved, but the silica powder prepared by the above-mentioned sol-gel method and treated in the above-mentioned calcining step is usually a fine powder having an average particle size of 80 to 100 μm. Therefore, it is not necessary to pulverize it. However, if this is 80 mesh or more, there is too much space between the particles, and bubbles will be generated at the time of firing in the next step or less, so this is sieved to 80
The mesh size is required to be not more than 150 mesh, which is preferably not more than 150 mesh, more preferably not more than 200 mesh.

The sieved silica powder is then fired to obtain synthetic quartz glass. This firing is carried out in a carbon crucible because the desired synthetic quartz glass is made into a high-purity product. As this carbon crucible, when gas is generated from the carbon mold during vacuum firing, bubbles may be generated in the silica powder in contact with the carbon mold,
It is preferable that the halogen treatment is performed at a high temperature or the pressure reduction treatment is performed at a high temperature.

In the present invention, firing of silica powder in the carbon crucible is performed in two steps. In this first step, the silica crucible obtained above is filled in a carbon crucible, and then the crucible is weighted and heat-treated at a temperature of 1,700 ° C. or less under reduced pressure. Since bubbles are likely to be formed during firing when the specific gravity is 0.6 or less, it is for making the bulk specific gravity of 0.6 or more, and it is necessary that the weight is 7.5 g / m ² or more. However, since it is invariable in a high temperature and vacuum atmosphere, and it is cheap and easy to process, it is preferable to use a carbonaceous material, and if it is necessary to make it heavier, use molybdenum or tungsten. Good. The silica loaded with this weight may be formed under a reduced pressure, for example, under a reduced pressure such as ^{2 to} 4 × 10 ^-2 Torr, at a temperature of 1,700 ° C. or lower. Therefore, it is characterized by having very few bubbles. This heating may be performed at a temperature of 1,200 to 1,250 ° C for 2 to 6 hours and then reheating at a temperature of 1,600 to 1,700 ° C for 1 to 2 hours. If it is done slowly, the silica powder will be sequentially sintered from the outside to form bubbles inside, so it is necessary to perform rapid heating at 30 ° C./min or more.

The firing in the second stage is performed in the same furnace at a temperature of 1,700 ° C or higher under atmospheric pressure or pressure following the first stage, but the silica powder fired at a temperature of 1,700 ° C or lower is cooled. When taken out, β-cristobalite changes to α-cristobalite and has fine cracks, which changes into fine bubbles when reheated,
This second-stage firing needs to be performed successively without cooling the above-mentioned first-stage fired product in the same furnace. Since this calcination uses silica powder as synthetic quartz glass, it is necessary to raise the temperature to 1,700 ° C or higher, for example, 1,900 to 2,000 ° C, and perform it for 0 to 0.5 hours, but this is to reduce bubbles. It is necessary to carry out under atmospheric pressure or under pressure, for example under pressure of 1 to 2 atm.

Colloidal silica as the starting material is made into colorless and transparent synthetic quartz glass by the firing in the second stage, but since this step is performed as described above, the obtained synthetic quartz glass has high purity, high viscosity, and foam. And the advantage of being less striae.

(Example) Next, the Example of this invention and a comparative example are given.

Example, Comparative Example 5 Methyl silicate 27 / hr and 20 wt% ammonia water 24.5 / hr were dropped into the quartz glass flask, and the methyl silicate was continuously hydrolyzed to produce silica powder, and this silica powder was centrifuged. After dehydration with a dehydrator, the temperature was raised to 1,100 ° C. in the air for 10 hours and the organic matter was removed by heat treatment for 2 hours to obtain colloidal silica having an average particle size of 700 nm.

Then, add this colloidal silica to polypropylene 20
After sieving with a 0 mesh sieving machine, 500 g of 200 mesh or less was obtained with a yield of 50%, so this was filled in a carbon cylinder with a diameter of 5 inches x 300, and about 3
Place 50 g of carbon lumps, place it in a vacuum heat treatment furnace, depressurize to 5 × 10 ^-2 Torr at room temperature, and
The temperature was raised to 250 ° C. and kept at 2.5 × 10 ⁻² Torr for 3 hours, further raised to 1,650 ° C. in 10 minutes and kept for 1 hour, and then nitrogen gas was introduced to return to normal pressure.

Next, this was heated to 1,950 ° C. for 30 minutes, held for 30 minutes, and then cooled to room temperature to obtain a synthetic quartz glass, but the synthetic quartz glass thus obtained had no bubbles. It had a low striae, high OH content, which was not observed in the infrared absorption spectrum, and a high viscosity at 1,400 ° C of 3.5 × 10 ¹⁰ poise.

However, for the purpose of comparison, silica-leoroseal (trade name of Tokuyama Soda Co., Ltd.), which was produced by the flame hydrolysis method instead of the sol-gel method, was used, and the same treatment as above was applied to this. When glass was prepared, the obtained quartz glass had one side that contracted in the axial direction as well as in the radial direction, and this had many bubbles.

(Effects of the Invention) The present invention relates to a method for producing a synthetic quartz glass having high purity, high viscosity, no bubbles, and less striae, which is the colloidal silica prepared by the sol-gel method as described above. 80 after firing at 1,200 ° C or lower in oxidizing atmosphere
After sieving into mesh or less, then filling in a carbon crucible, applying weight from the top and heat-treating under reduced pressure at 1,700 ° C or less, and then in the same furnace at 1,700 ° C under normal pressure or pressure.
The above is characterized by heat treatment, and according to this, colloidal silica as a starting material is made by the sol-gel method, and since it has no additives, it is a high-purity synthetic quartz glass. Since this synthetic quartz glass is produced by the method described above, it is free of bubbles, has less striae, has a high bulk specific gravity, and is highly viscous. In addition to being useful as a base material, it is industrially advantageous because it is cheap and is also useful for general consumers.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-2-80329 (JP, A) JP-A-62-176931 (JP, A) JP-A-3-141129 (JP, A) JP-A-2- 180723 (JP, A) JP-A-3-228839 (JP, A) JP-A-3-5329 (JP, A)

Claims (1)

[Claims] 1. Colloidal silica prepared by the sol-gel method is fired at 1,200 ° C. or lower in an oxidizing atmosphere, and then 8
After sieving to 0 mesh or less, then filling in a carbon crucible, heat it from the top under a reduced pressure of 1,700 ° C or less, and then heat treat it at 1,700 ° C or more under normal pressure or pressure in the same furnace. A method for producing a synthetic quartz glass characterized.