JPH11349336A - Production of synthetic silica glass powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain silica glass powder with slight bubbling when melted, useful for ultra-high-purity products, by grinding and drying a silica gel produced by hydrolyzing an alkoxysilane, effecting a specific liquid content of the silica gel whose particle distribution is also regulated so as to remove isolated silanol at low-temperature baking. SOLUTION: This silica glass powder is obtained by the following process: an alkoxysilane such as tetramethoxysilane is hydrolyzed using about 1-10 equivalent times of water; the resulting gel is ground and dried into silica gel powder <=50 wt.% in liquid content; thereafter, the silica gel powder is subjected to ultrasonic sieve or wet classification to reduce particles of <=45 μm size to <=1 wt.% (pref. about <=0.5 wt.%) content; the resulting silica gel powder is then baked, pref. through the following procedure: the silica gel powder is held at 300-600 deg.C or so once to sufficiently reduce the carbon levels therein followed by heat treatment at 1,000-1,300 deg.C or so in a dry gas atmosphere with a dew point of about <=-200 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor manufacturing field and an optical fiber field, in particular, a quartz glass used as a raw material of an ultra-high-purity quartz glass product which is used in a high temperature range of 1000.degree. Provide powder.

[0002]

2. Description of the Related Art In recent years, with respect to jigs and crucibles made of various kinds of glass used in the optical fiber and semiconductor industries, very strict control has been performed on the purity of constituent glass materials as semiconductors become more highly integrated. I have. As a method for producing a high-purity glass product applied to these uses, conventionally, an alkoxysilane is used as a starting material, which is hydrolyzed, and a silica gel powder is obtained by a process called a sol-gel method. There is known a method in which a desired glass product is produced by baking a glass powder to obtain a desired glass product after melting the glass into a glass powder. According to this method, the alkoxysilane as a starting material can be easily purified by distillation, and thus a high-purity glass product can be obtained.

However, in this method, hydrolysis and
Since the silanol groups existing at the time of gelation remain as residual silanol groups in the particles even after firing, they have a higher silanol content than natural quartz powder. If the silanol content is high, the viscosity at high temperatures is low, so that it becomes difficult to use it in a crucible for pulling a silicon single crystal, a semiconductor diffusion furnace, and the like (Japanese Patent Application Laid-Open No. 1-320232). In fact, for use in this application, a silanol content of 100 pp
m, preferably 50 ppm or less. For this reason, most of the internal silanol groups (infrared absorption wavelength around 3670 cm ^-1 ) are removed at 600 to 1000 ° C. in the first step in an atmosphere with a low water vapor partial pressure in order to lower the silanol concentration. At a temperature of 1200 ° C or more, an isolated silanol group (infrared absorption wavelength 3740 cm
⁽ Near ^-1 ) is known (JP-A-2-289416).

[0004] In addition, quartz glass members used for optical fibers and semiconductors have problems not only with high-temperature strength but also with the presence of bubbles. This is because, for example, in the case of a crucible for pulling a silicon single crystal, if bubbles are ruptured on the inner surface of the crucible during pulling of the single crystal, a defect is generated due to fluctuation of the liquid surface, leading to a decrease in yield. In addition, bubbles expand when heated, and there is a problem in terms of dimensional stability. Further, in the optical fiber, the problem of attenuation of optical information and disconnection of the fiber occurs due to the presence of bubbles. It is known that as a cause of bubbles when the quartz glass powder is melt-molded, if the silanol concentration is too high, it becomes steam gas and becomes bubbles. In addition, among the silanol groups, it is known that even the above-mentioned isolated silanol groups, even in a small amount, can cause foaming (Japanese Patent Application Laid-Open No. Hei 8-26742).

[0005]

As described above, in order to suppress foaming during melting of quartz glass powder, it is important to reduce the concentration of silanol as much as possible, particularly to reduce the concentration of isolated silanol. Removal of isolated silanol groups requires a higher firing temperature (1200 ° C. or higher) than removal of internal silanol groups, which lowers the durability of a heat-resistant container when firing quartz gel powder, leading to an increase in production costs. It will be. In order to maintain the synthetic quartz glass powder at a high purity, it is considered preferable to use a quartz glass crucible which is a common material as a firing vessel. In particular, regarding a quartz glass crucible, a necessary firing temperature is around 1200 ° C. In this case, even if the difference is only about 10 to 20 ° C., a great difference occurs in the service life. Therefore, it is an issue how to lower the concentration of isolated silanol, which causes foaming, at a lower temperature.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that a silica gel having a liquid content of 50% by weight or less on a dry basis at 160 ° C. before firing. At the powder stage, by setting the content of fine particles having a particle size of 45 μm or less to 1% by weight or less, preferably 0.5% by weight or less, it is possible to lower the firing temperature required for removing isolated silanol. Heading, the present invention has been reached. That is, the present invention provides a method for producing a synthetic quartz glass powder, comprising firing silica gel powder having a liquid content of 50% by weight or less and having a particle size of 45 μm or less having a content of 1% by weight or less. A silica gel powder having a liquid content of 50% by weight or less and a content of particles of 45 μm or less of 1% by weight or less, and a quartz glass molded body characterized by melt-molding the synthetic quartz glass powder. Manufacturing method.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The synthetic quartz glass powder obtained by the present invention is obtained by firing silica gel powder to make it nonporous and vitrified. As described above, silica gel powder generally contains a large amount of silanol groups, and it is necessary to perform calcination at a high temperature of 1000 ° C. or more for removal thereof and 1200 ° C. or more for isolated silanol for a long time.

Here, the silica gel powder to be calcined is not particularly limited, but in order to obtain high purity, it can be obtained by a method called a sol-gel method using, for example, alkoxysilane as a raw material. As described above, it is relatively easy to achieve high purification of the silica gel powder derived from alkoxysilane. The hydrolysis of alkoxysilane by the sol-gel method is carried out by reacting alkoxysilane with water according to a well-known method. As the alkoxysilane used as a raw material,
4 such as tetramethoxysilane and tetraethoxysilane
Functional, lower alkoxysilanes having 1 to 4 carbon atoms or oligomers thereof are preferred because hydrolysis is performed quickly.

The amount of water used for the hydrolysis is usually selected from the range of 1 to 10 equivalents of the alkoxy group in the alkoxysilane. At this time, if necessary, an organic solvent such as alcohols and ethers compatible with water may be mixed. Representative 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, since the alkoxy group bonded to the alkoxysilane is liberated as alcohol as the hydrolysis reaction proceeds, if the reaction solution is in a uniform state before gelation, it is not necessary to add alcohol or the like. The reaction can be performed without any practical problems.

In this hydrolysis reaction, hydrochloric acid as a catalyst,
An acid such as acetic acid or an alkali such as ammonia may be used. Naturally, in order to obtain a high-purity silica gel powder, all the substances to be introduced into this reaction system such as the raw material alkoxysilane, water, catalyst and the like used here are of high purity. In order to gel the hydrolysis product, the gel can be obtained faster by heating, but it gels in a few hours even if left at room temperature, so by adjusting the degree of heating, the gelation You can adjust the time. The obtained gel may be dried after being subdivided, or may be subdivided after being dried.

In the present invention, drying is performed at least until the liquid content becomes 50% by weight or less. Here, the liquid content refers to the liquid content on a 160 ° C. absolute dry basis. The average particle size of the dried silica gel is usually 50 to
The thickness is 1000 μm, preferably 90 to 600 μm. However, since this silica gel powder contains unnecessary fine particles and coarse particles as a final product, in order to increase the productivity in the calcination step described below, a classification operation is performed at the gel stage, and It is considered preferable to cut off particles that do not become the size. Although the particle size range required as the size of the synthetic quartz glass powder, which is the product, varies depending on the application, semiconductor manufacturing related members,
In optical fiber tube applications, usually 50 to 6
A particle size of 00 μm is required, and after performing a classification operation so as to be within this particle size range, it is made nonporous and vitrified in a firing step to obtain a synthetic quartz glass powder.

The classification operation of the gel is usually carried out with a vibrating or low-tapped mesh screen in many cases. When such a method is used and the classification operation is carried out industrially by a continuous method, it is possible to reduce the undersized particle content from the required particle size to about several percent, and to a product size of this level. Is usually not at a level that is particularly problematic during melt molding.

However, surprisingly, according to the study of the present inventors, in the process of sintering the dried silica gel powder to obtain the synthetic quartz glass powder, the fine powder of a few percent greatly reduces the cost of the sintering process. It turned out to be. In other words, the content of isolated silanol groups, which is a cause of foaming particularly when the synthetic quartz glass powder is melt-molded into a quartz glass molded body, is more present as the silica gel is finer, and even when fired at the same temperature. It has been found that the smaller the particle size of silica gel, the longer it takes to reduce the level of isolated silanol groups to a level that does not affect foaming. Therefore, the present inventors have conducted intensive studies and, as a result, have found that, in particular, fine particles having a size of 45 μm or less greatly contribute to the residual silanol. And
Prior to the firing step, fine particles having a particle size of 45 μm or less as a content of 1% by weight or less as dry silica gel powder having a liquid content of 50% by weight or less on an absolutely dry basis at 160 ° C.
It has been found that the firing step can be carried out extremely efficiently by cutting to preferably 0.5% by weight or less. In addition, the liquid content on a 160 ° C. absolutely dry basis refers to a weight reduction rate when a constant weight is reached by heating to 160 ° C. with an infrared heater or the like.

It is to be noted that the fine particles are cut before the firing step, instead of being classified after the synthetic quartz glass powder has been fired. This is after firing
This is because some of these fine particles are hardened by sintering and fixing with, for example, particles having a size of synthetic quartz glass powder as a product, and it is difficult to separate them. On the other hand, a method of removing fine particles by suspending and washing in an aqueous medium at the stage of a wet gel before drying was also devised (Japanese Patent Application Laid-Open No. 5-201718), but when the present inventors tried such a method, When the suspension is suspended in an aqueous medium in the wet gel stage, when the suspension is baked after being converted into a dry gel powder, the fineness of the gel during the calcination is smaller than when the suspension is suspended without suspension in an aqueous medium. It was found that the pore sealing temperature was greatly reduced. As a result, if calcination is performed without taking into account, for example, extremely slowing down the rate of temperature rise, the sealing of the gel is completed while the desorption of residual carbon is insufficient, and the unburned carbon content is included in the glass. It has been found that this causes foaming during melt molding. For these reasons, it is extremely desirable to remove the fine particles in the state of a dried silica gel powder having a liquid content of 50% by weight or less on a 160 ° C. absolute dry basis.

As a method of cutting fine particles having a size of 45 μm or less to the above range, a dry sieve method using an ultrasonic sieve, a wet sieve method using water or alcohol as a solvent, and the like can be considered.
It is not particularly limited to these methods. Although the reaction mechanism is not sufficiently clear,
After the liquid content on a dry basis is reduced to 50 wt% or less,
Even when suspended in water, the phenomenon of remarkably lowering the pore-sealing temperature during firing as seen in the above-described method of suspending at the stage of wet gel is not observed, and processing is performed under normal firing conditions. It is possible to do.

The thus obtained dried silica gel powder from which fine powder has been removed is calcined. At this time, the dried silica gel powder can be directly heated in a temperature range of 1000 to 1300 ° C. to make it nonporous, and can be made into a synthetic quartz glass powder, but once heated and held in a temperature range of 300 to 600 ° C. It is better to perform the heat treatment at 1000 to 1300 ° C. after sufficiently taking the time and sufficiently reducing the carbon concentration, when the obtained synthetic quartz glass powder is melt-molded, foaming derived from carbon is suppressed. It is preferable because it is possible.

The firing atmosphere is at least 60
It is desirable to flow an oxygen-containing gas up to around 0 ° C. and a dry gas having a low water content at temperatures thereafter. The reason is that up to around 600 ° C., an oxygen-containing gas is required to promote the oxidative removal of residual carbon,
If the temperature after that is not a dry atmosphere to some extent, the silanol concentration in the quartz glass powder finally obtained will not drop to the target concentration,
In order to obtain a product having a dew point of pm or less, it is preferable to use a dry gas having a dew point of -20 ° C or less.

As a method for introducing the dry gas into the silica gel powder, the gas is circulated through the surface of the silica gel powder layer filled in the container or supplied from an insertion tube inserted in the powder layer. It is simple. Among these methods, the method of supplying from an insertion tube inserted into the powder layer is advantageous because a synthetic quartz glass powder having uniform characteristics can be obtained even when firing is performed using a large-capacity container. However, when the gas is supplied from the insertion tube in the powder layer, it is desirable to select the gas flow rate from a region where the powder does not exhibit the bubbling phenomenon. When the bubbling phenomenon occurs, although the reason is not clear, the reduction rate of the silanol is reduced, and the powder is spilled out of the container.

The synthetic quartz glass powder obtained in this manner has a low generation of bubbles at the time of melt-molding, and has a high purity which is resistant to generation of bubbles, such as a crucible for pulling a silicon single crystal, a diffusion furnace tube for a semiconductor, and a tube for an optical fiber. It can be suitably used as a raw material for silica glass products.

[0020]

The present invention will be described more specifically with reference to the following examples. (Example 1) 15 kg of ultrapure water was charged into a jacketed horizontal cylindrical reactor having a ribbon-type stirring blade, and then charged for 20 r.
Stirring was started at pm. Thereafter, 25 kg of tetramethoxysilane was charged over 3 minutes. The reactor used was of a jacket type, and the temperature of hot water passing through the jacket was 45 ° C. Thereafter, when a uniform sol was obtained, stirring was stopped, and the contents were allowed to stand for 30 minutes. When observing the change in the contents from the glass window of the reaction vessel,
It gelled 40 minutes after the completion of the charging. Then again
Start rotation of the stirring blade, until the size is smaller than the fist size,
After the coarse pulverization of the gel was performed, the valve provided at the bottom of the reactor was opened, and the bulk gel was removed from the reactor.

The mass gel is screened with a screen diameter of 1140 μm.
The powdery gel was vacuum-dried to a liquid content of 20% by weight on a dry basis at 160 ° C., and then wet-classified in an aqueous system to obtain a powdery gel having a particle size of 10%.
The resultant was cut in a range of 6 to 500 μm and dried again in vacuum to obtain a dry gel powder. This dry gel powder was analyzed using a laser diffraction / scattering type particle size distribution analyzer (SK-LMSPRO-7000S manufactured by Seishin Enterprise Co., Ltd.).
When the particle size distribution was measured, the content of fine particles having a particle size of 45 μm or less was 0.2 wt%. This dry gel powder 7
5 kg was charged into a quartz glass crucible of 550 mmφ × 600 mmH, covered, and set in an electric furnace.
A 30mmφ hole is made in the center of this lid,
Through this hole, a quartz glass tube having an inner diameter of 10 mm was inserted into the powder and inserted from the lower part of the crucible to a position about 1 cm. Then, through this quartz tube, the dew point is -50 ° C
The calcination was performed while supplying dehumidified air of the above. The sintering temperature pattern is indicated by the indicated value of the thermocouple position for controlling the electric furnace.
The temperature was raised to 00 ° C. in 3 hours, kept at 500 ° C. for 10 hours, further raised to 1150 ° C. in 5 hours, and then raised to 1150 ° C. for 3 hours.
Hold for 0 hours. After firing, near the center of the surface in the crucible,
The fired powder near the center of the side wall, near the center of the bottom, and at the center of the powder was sampled, and the silanol concentration was measured by an infrared absorption method.
As a result, the internal silanol group concentration was 103, respectively.
It was 109, 112, and 122 ppm, and no isolated silanol group was found. The whole amount of the fired powder was charged into a conical type blender, and tumbled and mixed at 3 rpm for 30 minutes, followed by an oxyhydrogen flame melting method (Bernui method).
When a quartz glass ingot of 15 mmφ × 50 mmH was produced, there was no foaming. (Example 2) In Example 1, the powdery gel obtained after the pulverization was vacuum-dried to a liquid content of 2 at 160 ° C. on a dry basis.
0 wt%, and then dry-processed using an ultrasonic sieve.
A synthetic quartz glass powder was obtained in the same manner as in Example 1 except that a dry gel powder was obtained by cutting to a particle size of ~ 500 µm. Incidentally, when the particle size distribution of the dry gel powder before firing was measured by the same method as in Example 1, the content of fine particles of 45 μm or less was 0.8 wt%.
Met. The silanol concentration of the obtained synthetic quartz glass powder at the same position as in Example 1 was measured. The internal silanol concentration was 103 and 11 respectively.
It was 0, 110, and 120 ppm. On the other hand, traces of isolated silanol groups were observed on the measurement chart of the infrared absorption spectrum.
ppm or less. Here, in calculating the isolated silanol concentration, since the extinction coefficient value is unknown, it was determined by the area ratio of the absorption peak measured by the infrared absorption method, assuming that the extinction coefficient was the same as the internal silanol group. Value. The whole amount of the obtained quartz glass powder was mixed in the same manner as in Example 1, and mixed with an oxyhydrogen flame method (Bernui method).
When a quartz glass ingot of mmφ × 50 mmH was produced, no remarkable foaming was observed. (Comparative Example 1) In Example 1, the powdery gel obtained after the pulverization was vacuum-dried to a liquid content of 2 at 160 ° C. on a dry basis.
0 wt%, and then dry-processed using a vibrating sieve.
A synthetic quartz glass powder was obtained in the same manner as in Example 1 except that a dry gel powder was obtained by cutting to a particle size of ~ 500 µm. Incidentally, when the particle size distribution of the dry gel powder before firing was measured by the same method as in Example 1, the content of fine particles having a size of 45 μm or less was 2.8 wt%.
Met. The silanol concentration of the obtained synthetic quartz glass powder at the same position as in Example 1 was measured.
0, 110, and 119 ppm, which were almost the same as those in Example 1, except that isolated silanol groups were 5, 7, 6, and 8, respectively.
ppm was confirmed to be contained. The whole amount of the obtained quartz glass powder was mixed in the same manner as in Example 1 to produce a quartz glass ingot of 15 mmφ × 50 mmH by an oxyhydrogen flame method (Bernui method). Many occurred.

[0022]

According to the present invention, a synthetic quartz glass powder capable of removing silanol groups by firing at a lower temperature can be obtained.

Claims (6)

[Claims] 1. A method for producing a synthetic quartz glass powder, comprising firing a silica gel powder having a liquid content of 50% by weight or less and having a content of particles of 45 μm or less of 1% by weight or less. . 2. The method according to claim 1, wherein the silica gel powder is obtained by pulverizing and drying a gel obtained by hydrolyzing alkoxysilane. 3. A silica gel powder having a liquid content of not more than 50% by weight and having a content of particles of not more than 45 μm of not more than 1% by weight is obtained by pulverizing and drying a gel obtained by hydrolyzing alkoxysilane. The synthetic quartz glass according to claim 1 or 2, obtained by reducing the liquid content of the gel to 50% by weight or less and then removing particles of 45 µm or less to 1% by weight or less using an ultrasonic sieve. Powder manufacturing method. 4. A silica gel powder having a liquid content of not more than 50% by weight and having a particle content of not more than 45 μm of not more than 1% by weight is obtained by pulverizing and drying a gel obtained by hydrolyzing alkoxysilane. 3. The production of a synthetic quartz glass powder according to claim 1, wherein the liquid content of the gel is reduced to 50% by weight or less, and particles having a size of 45 μm or less are removed to 1% by weight or less by wet classification. Method. 5. The method according to claim 1, wherein the liquid content is 50% by weight or less,
A silica gel powder having a content of particles of m or less of 1% by weight or less. 6. The silica gel powder according to claim 5, which is obtained by hydrolyzing an alkoxysilane, pulverizing and drying the obtained gel.