JPH035329A - Production of synthetic quartz glass - Google Patents

Abstract

PURPOSE:To increase purity by hydrolyzing methyl silicate in the presence of NH3 and successively subjecting formed SiO2 particles to decarbonizing by heating, sintering under reduced pressure and heating under ordinary pressure or elevated pressure. CONSTITUTION:Methyl silicate is hydrolyzed at 40-50 deg.C in the presence of NH3 as a catalyst and the resulting silica sol is subjected to solid-liq. separation to obtain SiO2 powder of 400-1,000nm particle size. This SiO2 powder is dehydrated and decarbonized by heating at about 800 deg.C in the presence of air, filled into a carbon case and sintered at 1,500-1,700 deg.C under 10<-2>-10<-4>Torr pressure to form quartz glass. This quartz glass is heated to 1,800-2,200 deg.C under 1-10 atm pressure to obtain high purity synthetic quartz glass free from bubbles and having high transparency and a very low rate of diffusion of impurities.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a method for producing synthetic quartz glass, particularly for use in semiconductors such as process tubes, boats, and cantilevers because of its high purity, high viscosity, and low gas emission. The present invention relates to a method for manufacturing synthetic quartz glass suitable for heat-resistant jigs.

[Conventional technology] For the production of quartz glass, natural crystal powder is heated under reduced pressure.
A well-known method is to melt crystal powder at a temperature around 1,000°C, and in this method, crystal powder is placed in a graphite crucible lined with silicon carbide and heated under reduced pressure of 10-2 to 10-4 Torr. The method of melting at 000°C, releasing the reduced pressure during cooling, and applying pressure with carbon dioxide gas to make the bubbles so small that they are invisible to the naked eye is called the Berber gel method. The quartz glass made by this method has extremely low OH group content, high viscosity because it contains -5i-5i- bonds, and slow impurity diffusion, so it can be used as a semiconductor diffusion member. It is often used in lamps, etc.

In addition, regarding the production of this quartz glass, fumed silica is molded using a binder, and then molded under reduced pressure to a diameter of 1.300 to 1.
A method of sintering at 500℃ and continuously drawing a tube,
A method is also known in which finely powdered silica soot produced by flame hydrolysis of silicon tetrachloride is attached to a target and grown, and a porous glass member obtained is sintered under reduced pressure. A sol-gel method is also known, in which the silica sol obtained is mixed with colloidal silica, molded, and then sintered under reduced pressure.

[Problems to be Solved by the Invention] However, the quartz glass obtained by vacuum melting natural quartz has poor purity, for example, AP of 20 ppm,
Fe: 1 ppm, Na, K, Li,
Since the Ca content is 1 to 2 ppm, if it is used as a semiconductor diffusion furnace member, the silicon wafer will be contaminated by the diffusion of impurities, which will pose a major problem in improving the integration degree of silicon devices. Although the method of using fuming silica is cheap, it is also A.
j, Fe is 500 ppb, Na, K, LL,
The disadvantage is that the Ca content is 2 to 300 ppb, the purity is not very good, and the viscosity is not high.

In addition, the above-mentioned soot method using flame hydrolysis of silicon tetrachloride has disadvantages in terms of cost, as the amount of oxyhydrogen used is large compared to the weight of the product, and it is difficult to mass produce and scale up. . In addition, the sol-gel method, in which silica sol and colloidal silica are mixed and molded, cracks when the drying, calcination, and sintering speeds are high, and this tendency is particularly strong in large products. It has the disadvantage that it is difficult to synthesize, and furthermore, in order to increase the purity, not only reagents and tools must be selected, but also drying, calcining, and sintering must be performed in a clean room.

[Means for Solving the Problems] The present invention relates to a method for producing synthetic quartz glass that solves the above disadvantages and disadvantages. After generating silica particles, they were decarburized by heating in an oxidizing atmosphere, and then heated to 1,500 N1,700 under reduced pressure.
sintered at 1,800 °C, then sintered at normal pressure or under pressure.
It is characterized by heating to ~2,200°C.

In other words, the present inventors investigated various methods for obtaining synthetic quartz glass with high purity, high viscosity, no bubbles, and good transparency, and as a result, they identified methyl silicate as the alkoxysilane used as the starting material in the sol-gel method. However, by sufficiently refining this and hydrolyzing it in the presence of ammonia, spherical silica with a large particle size and large pore size and a three-dimensional matrix structure can be easily obtained, which is sintered under reduced pressure. Next, they discovered that by heating at high temperatures, it was possible to obtain synthetic quartz glass with high purity and high viscosity, as well as an extremely slow diffusion rate of impurities.They conducted research on the sintering conditions and subsequent heat treatment conditions, and developed the present invention. completed.

This will be explained in further detail below.

[Function] The method for producing synthetic quartz glass of the present invention involves hydrolyzing methyl silicate in the presence of ammonia to generate silica particles, then decarburizing, sintering, and heat-treating the silica particles to produce silica glass. That is.

The starting material used in the method of the present invention is highly reactive, and in the presence of ammonia, the particle size can easily be reduced to 400 mm without a solvent.
Methyl silicate is selected because it produces spherical silica with a large diameter of ~1,000 nm;
This methyl silicate needs to be sufficiently purified by distillation or the like before being supplied, since the desired synthetic quartz glass is to be of high purity.

This methyl silicate is made into silica sol by hydrolysis, but if this hydrolysis is carried out in the presence of a known acid catalyst such as hydrochloric acid, the resulting silica will have small particles, so in the presence of an ammonia catalyst It is necessary to do so. The silica obtained by hydrolyzing this methyl silicate in the presence of an ammonia catalyst has a particle size of 400~
It becomes a large spherical object with a diameter of 1,000 nm, and this object has a very regular three-dimensional matrix structure with large pores on its surface.

This hydrolysis reaction may be carried out at 40 to 50°C,
The silica sol thus obtained may be subjected to solid-liquid separation using, for example, a filter press to obtain silica.

The silica powder obtained in this way is then packed into a carbon case and sintered under reduced pressure.
The sintering temperature may be i,
If the temperature is lower than 1,700°C, sufficient sintering will not take place and the next heating step will result in an opaque appearance; if the temperature is higher than 1,700°C, the bubbles contained in the silica particles will grow and become larger. , this bubble will remain until the end, so this temperature is 1,500 to 1,700℃.
It needs to be within the range of

Silica is made into synthetic glass by this sintering, but in the method of the present invention, the silica glass obtained in this way is further
, 800-2,200°C.

In other words, the silica glass as a sintered product obtained in this way is heated again after being taken out of the furnace, but if the heating temperature is 1,800 to 2,200°C, the viscosity of the quartz glass is low. This gives the advantage that the bubbles generated in the previous stage of vacuum sintering are easily crushed by the pressure difference, and there are no visible bubbles.
If the temperature is above 0℃, SIO steam will be generated violently.
This is preferably in the range of 1.800 to 2,000°C. Note that this heating may be carried out at normal pressure, but it may also be carried out under pressure of 1 to 10 atmospheres, which has the advantage that the bubbles are further crushed and become completely invisible to the naked eye. It will be done.

[Example] Next, examples of the present invention and comparative examples will be given.

Example 1 20 wt% NH in a 1rn' glass-lined reactor
300 u of 4OH was added, 265 ft of sufficiently purified methyl silicate was added dropwise thereto, a hydrolysis reaction was carried out at 40 to 50 t, and the reaction liquid was separated into solid and liquid using a filter press to produce 135 kg of silica powder.

Next, this silica was placed in a quartz container and heated at 800°C for 20 hours in the presence of clean air to dehydrate and decarburize the silica powder, which weighed 95 kg.
Pack 0 kg into a carbon case with an outer diameter of 300 mmφ and an inner diameter of 280 mmφ and a volume of 1.000 Jl, and
1,600 at a heating rate of 50°C/hour under a vacuum of Torr.
The temperature was raised to .degree. C. and sintered for 2 hours, and when the temperature was lowered and taken out, an opaque quartz ingot with a diameter of 278 mm and a volume of 75 J2 was obtained.

Next, this ingot was heated to 2,0°C under an argon gas atmosphere.
When heated at 00℃ for 1 hour, the diameter was 470 mm.
44 kg of transparent ingot with a volume of 2i was obtained at φ,
When we investigated the amount of impurities (chemical analysis values), viscosity, and diffusion rate of impurities (diffusion coefficient of Na'' ions), we found that
The results shown in Table 1 were obtained.

However, for comparison, quartz made by melting natural crystal powder at 2,000°C under a reduced pressure of 10-3 Torr (Comparative Example 1) and fuming silica were molded into a cylinder shape using polyvinyl alcohol. , 1.500 under a vacuum of 10-' Torr.
Quartz made by sintering at ℃ (Comparative Example 2), silica sol obtained by hydrolyzing tetraethyl silicate in the presence of hydrochloric acid was mixed with colloidal silica, and after molding, 10-'
Quartz obtained by sintering at 1,800℃ under Torr vacuum (
Comparative Example 3) was examined for its chemical analysis values, viscosity, and diffusion rate of impurities, and the results shown in Table 1 were obtained, indicating that the quartz obtained by the method of the present invention has excellent physical properties. This was confirmed.

Because it is possible to obtain synthetic quartz glass with extremely slow
The advantage is that synthetic quartz glass, which is useful as heat-resistant jigs for semiconductors such as process tubes, boats, and cantilevers, and as sealing materials for halogen lamps and infrared heaters, can be obtained easily and at low cost. .

Example 2 The opaque quartz ingot obtained in Example 1 was circularly polished to produce an ingot of 250 rnmφx 701.
A hole of 50 mmφ was made in the center and stretched in an electric furnace at 1,950°C to make a vibrator with an outer diameter of 70 mm and an inner diameter of 80 mm.The above opaque ingot was also circularly polished to a diameter of 100 mmφ×701 mm. After making it into an ingot, a 10 mm diameter bar was prototyped using an oxyhydrogen flame burner.

Next, when the physical properties of the quartz vibrator and rod thus produced were examined, they showed the same characteristics as in Table 1 of Example 1.

[Effect of the invention]

Claims (1)

[Claims] 1. Methyl silicate is hydrolyzed in the presence of ammonia and subjected to condensation polymerization to produce silica particles, which are then heated in an oxidizing atmosphere to decarburize, and then heated to 1,50 ml under reduced pressure.
A method for producing synthetic quartz glass, which comprises sintering at 0 to 1,700°C, and then heating to 1,800 to 2,200°C under normal pressure or pressure.