JP2621491B2 - Method for producing silica glass - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing silica glass used for optics, semiconductor industry, electronics industry, science and engineering, and the like.

[Prior art] Silica glass is an important material for semiconductor production due to its excellent heat resistance, corrosion resistance and optical properties. In addition, photomask substrates for optical fiber and IC production, and TFT substrates It is used for such purposes, and its use is being further expanded.

Conventional methods for producing silica glass include a method in which natural quartz is melted by an electric furnace or an oxyhydrogen flame, and a method in which silicon tetrachloride is oxidized at a high temperature in an oxyhydrogen flame or a plasma flame and dissolved. Requires a high temperature of 2000 ° C or more in the manufacturing process, consumes a large amount of energy, requires materials that can withstand such high temperatures during manufacturing, and is economical because it is difficult to obtain high-purity products. However, it has some problems in quality.

On the other hand, in recent years, a method called sol-gel method for synthesizing silica glass at a low temperature has attracted attention. The outline is briefly described.

Si prepared by hydrolysis and polymerization of silicon alkoxide or by gas phase hydrolysis of silicon tetrachloride
The silica sol of water, an organic solvent, or a mixed solution of water and an organic solvent in which O ₂ fine particles are dispersed is gelled by standing, heating, and adding a gelling agent. Thereafter, the gel is evaporated and dried to obtain a silica dry gel. The dried gel is sintered in an appropriate atmosphere to obtain silica glass.

The method for producing silica glass by the sol-gel method has the following problems. That is, it is extremely difficult to uniformly disperse the SiO ₂ fine particles in water, an organic solvent, or the like, and large aggregates of the SiO ₂ fine particles tend to remain in the silica sol. Such agglomerates cause defects (voids) in the glass produced by sintering the gel, and cause deterioration in quality. Therefore, to improve the dispersion of the SiO ₂ fine particles, a method of irradiating the silica sol with ultrasonic waves, and further sintering the sintered glass under pressure in order to eliminate defects (voids) in the glass caused by the aggregates Methods have been proposed, but all of them cause an increase in the number of steps and an increase in opportunities for impurity contamination.

To eliminate the step of dispersing fine SiO ₂ particles into the liquid phase, by hydrolyzing a silicon alkoxide in the presence of a base catalyst SiO ₂
A method has been proposed in which a sol containing fine particles is produced, and this is directly gelled.

[Problems to be Solved by the Invention] However, in this method, cracks and cracks are easily generated in the gel during the drying process. In order to prevent such gel destruction, it is necessary to increase the diameter of the SiO ₂ fine particles contained in the sol.Therefore, the water used in the hydrolysis of the alkoxide has a large excess and a large concentration with respect to the alkoxide. It must contain a base. Therefore, before gelation of the sol, a concentration step for removing a large excess of water was required, causing an increase in the number of steps. In addition, it is necessary to add an acid in order to bring the sol to a pH suitable for gelation, but this causes a large amount of salt formed in the sol to precipitate in the gel during the drying process and generate porosity when vitrified. Caused a problem.

In order to solve these problems, the present inventors have proposed a method for producing silica glass that does not require a step of dispersing SiO ₂ fine particles and a step of concentrating a sol before gelation, that is, the presence of polyethylene glycol and / or a derivative thereof. The silicon alkoxide is hydrolyzed using a basic catalyst below to obtain a silica sol containing fine particles of SiO ₂ , water is added to the silica sol together with an acid, and then the silicon alkoxide is added, gelled, dried and sintered. A production method of the characteristic silica glass was proposed. However, even with this method, in some batches, cracks and cracks sometimes occur in the gel during the drying step, and the yield of the dried gel is sometimes reduced. An object of the present invention is to provide a method for producing a large-sized silica glass which prevents cracks and cracks in a gel in a drying step and has few defects such as voids.

Means for Solving the Problems In order to achieve the above object, the present inventors have studied various causes of cracks and cracks in the gel during the drying step and methods for preventing the cracks. What happens in the process and such cracking of the gel can be relatively suppressed by increasing the amount of silicon alkoxide added to the silica sol containing SiO ₂ fine particles, but the gel densifies and gel cracking occurs at the end of the drying process And that the gel densification that occurs when the amount of silicon alkoxide is increased can be suppressed by using a mixture of two or more types of silicon alkoxides (Si (OR) ₄ ) having different alkyl groups (R). Heading, the present invention has been completed.

That is, the present invention provides polyethylene glycol and / or
Alternatively, a silicon alkoxide is hydrolyzed using a basic catalyst in the presence of a derivative thereof to obtain a silica sol containing SiO ₂ fine particles, water is added to the silica sol together with an acid, then a silicon alkoxide is added, gelled, dried, In a method for producing silica glass to be sintered, a method for producing silica glass is characterized in that the silicon alkoxide (Si (OR) ₄ ) to be added is a mixture of two or more types of silicon alkoxides having different alkyl groups (R). To provide.

Hydrochloric acid, nitric acid, formic acid, acetic acid, propionic acid and the like can be used as the acid added to the silica sol containing the SiO ₂ fine particles.
The amount of the acid to be added is such that the pH of the silica sol containing the SiO ₂ fine particles is 3
５5.

Good results are obtained if the amount of water added with the acid is 0.1 to 4 times the theoretical amount required for the hydrolysis of the silicon alkoxide to be subsequently added.

After adding water together with the acid, the mixture of silicon alkoxides to be added is at least two kinds selected from tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, or a partially condensed polymer thereof. Can be used. For suitable combinations and compositions of mixtures of two or more silicon alkoxides, see Si
Composition of silica sol containing O ₂ fine particles, especially alcohol (derived from hydrolysis product of silicon alkoxide and solvent)
Is not particularly limited since it varies depending on the composition of

Silica glass is prepared by transferring the silica sol prepared as described above to a container such as a Petri dish, gelling while maintaining the temperature at room temperature to 70 ° C., and then drying at a temperature equal to or higher than room temperature to obtain a dried gel. It is obtained by raising the temperature to 1000 to 1400 ° C in air and sintering.

[Action] In the presence of polyethylene glycol and / or a derivative thereof, water is added together with an acid to silica sol containing SiO ₂ fine particles produced by hydrolyzing silicon alkoxide using a basic catalyst, and then the alkali group (R) is different. When a mixture of two or more types of silicon alkoxides (Si (OR) ₄ ) is added and gelled, the gel does not densify even if a large amount of silicon alkoxides are added, and as a result, the yield of dry gel is significantly improved. . Although the details of this reason are unknown, after adding water to the silica sol containing the SiO ₂ fine particles together with the acid and then adding a large amount of silicon alkoxide (for example, silicon butoxide) containing a large amount of R, aggregation of the SiO ₂ fine particles occurs. For example, when two types of silicon alkoxides having different R (R ₁ > R ₂ ) are added to a silica sol, the larger silicon alkoxide itself containing R ₁ or the alcohol as a hydrolysis product is converted into fine particles of SiO ₂ in the silica sol and the same. It is presumed that this promotes the growth of the secondary particles and consequently suppresses the densification of the gel. It should be noted that the present invention is not limited to the above assumption.

[Example] (Example 1) 284 g of 2-propyl alcohol and 160 M aqueous solution of 0.01M choline
g of polyethylene glycol (molecular weight 200
00) 16.8 g was added and dissolved. The obtained solution was slowly added to 340 g of tetramethoxysilane (Si (OCH ₃ ) ₄ ), and further sufficiently mixed to obtain a silica sol. This was allowed to stand at room temperature overnight to ripen. Then, with vigorous stirring, 48.3 g of water was added to the silica sol together with 0.8 ml of a 1M aqueous hydrochloric acid solution, and then a mixture of 153 g of tetramethoxysilane and 23.3 g of tetraethoxysilane was added. Put this in a 300 mm diameter stainless steel petri dish coated with Teflon,
It was covered with aluminum foil and gelled at 30 ° C. Drill a hole in the lid,
It was dried in a thermostat at 60 ° C. for 2 weeks, then heated to 120 ° C. and dried at that temperature for 1 day to obtain a dried gel. No cracks or cracks were found in the obtained dried gel. The obtained dried gel was heated to 1300 ° C. in the air and sintered to obtain silica glass. The obtained silica glass had no defects such as voids.

(Example 2) 48.3 g of water was added together with 0.8 ml of a 1M aqueous hydrochloric acid solution to a silica sol prepared in the same manner as in Example 1 while stirring vigorously.
Then, a mixture of 136 g of tetramethoxysilane and 46.5 g of tetraethoxysilane was added. This was gelled, dried, and sintered vitrified in the same manner as in Example 1. For dried gels,
No cracks or cracks were observed, and the obtained silica glass had no defects such as voids.

(Example 3) To a silica sol prepared in the same manner as in Example 1, 48.3 g of water was added together with 0.8 ml of a 1 M aqueous hydrochloric acid solution while vigorously stirring, and then 102 g of tetramethoxysilane and tetraethoxysilane were added.
93.1 g of the mixture was added. This was gelled, dried, and sintered vitrified in the same manner as in Example 1. The dried gel had no cracks or cracks, and the obtained silica glass had no defects such as voids.

[Effects of the Invention] According to the present invention, large-sized silica glass free from cracks and cracks and having few defects such as voids can be easily produced by a gel-sol method. The size is basically not limited, and it can be manufactured in any shape such as a plate shape, a rod shape, and a tubular shape.

Further, according to the present invention, silica glass can be manufactured at a lower cost than in the past, so that its application can be expanded not only in the field of photomask base materials for IC manufacturing and the like, but also in liquid crystal display base materials and the like, which have been conventionally used. .

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Toshikatsu Shimazaki 48, Wadai, Tsukuba-shi, Ibaraki Hitachi Chemical Co., Ltd. Tsukuba Development Laboratory (56) References JP-A-3-40927 (JP, A)

Claims (1)

(57) [Claims] A silica sol containing SiO ₂ fine particles is obtained by hydrolyzing a silicon alkoxide using a basic catalyst in the presence of polyethylene glycol and / or a derivative thereof, and water is added to the silica sol together with an acid. Is added, gelled, dried and sintered, the silicon alkoxide (Si (OR) ₄ ) to be added is a mixture of two or more silicon alkoxides having different alkyl groups (R). Characteristic method of producing silica glass.