JPS5933533B2 - Method for treating the inner surface of a mold for producing high-purity silicon - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating the inner surface of a mold for producing high-purity silicon.

To produce high-purity silicon of semiconductor industry grade, a high-purity silicon rod (hereinafter simply referred to as a silicon rod) placed in a reactor is heated with electricity and a mixed gas of chlorosilane and hydrogen is introduced into the reactor. A method is used in which polycrystalline silicon is precipitated on a silicon rod through the decomposition reaction, which is then taken out and melted in a crucible or the like, and a single crystal seed is added thereto to grow it to obtain a single crystal.

The silicon rods used at this time are manufactured by pouring molten silicon into a quartz tubular mold, cooling it, and then taking out the molded body. At this time, if the silicon and the mold are in close contact, there will be a difference in the coefficient of expansion between the two. This causes cracks in the silicon molded body.

Conventionally, after blasting the inner surface of a mold such as a quartz crucible, it is heated above the boiling point of a volatile liquid such as water, and then a slurry of high-purity silicon nitride powder dispersed in these liquids is sprayed onto the inner surface of the quartz crucible to evaporate the liquid. A method has been proposed in which silicon nitride is deposited on the inner surface of a mold by causing the silicon nitride to adhere to the inner surface of the mold (US Pat. No. 3,041,690).

When molten silicon is poured into a mold coated with silicon nitride powder, silicon nitride and molten silicon do not easily get wet, and since silicon nitride is a powder, it buffers the stress caused by the difference in shrinkage between the mold and silicon. This has the effect of preventing cracks from forming in the silicon molded body.

However, this method is not suitable for blasting the elongated tubular mold or the complex-shaped inner surface of the mold for manufacturing the silicon rods as described above.

Another disadvantage is that the silicon nitride powder on the inner surface of the mold easily falls off.

As a result of intensive studies to solve the above problems, the present inventor found that if silicon nitride powder is fixed with a hydrolyzed product of chlorosilane or alkoxysilane or colloidal silica, it will be difficult to peel off from the inner surface of the mold. The inventors have completed the method of the present invention by discovering that it is possible to obtain a coated surface that does not damage silicon molded bodies.

That is, in the present invention, silicon nitride powder is added to liquid chlorosilane, alkoxysilane, or colloidal silica to form a slurry, and the slurry is coated on the inner surface of a mold for manufacturing high-purity silicon made of a heat-resistant material such as quartz, alumina, or graphite. For manufacturing high-purity silicon, the method comprises hydrolyzing the chlorosilane or alkoxysilane and then drying the coated surface to coat the inner surface of the mold with a hydrolyzed product or silicon nitride powder containing colloidal silica as a binder. This is a method for treating the inner surface of a mold.

If only colloidal silica is used, the hydrolysis step is omitted.

Examples of the liquid chlorosilane used in the present invention include trichlorosilane and tetrachlorosilane, and examples of the alkoxysilane include tetramethoxy 7rane, tetraethoxysilane, and tetraethoxysilane.

A silicon nitride powder having a particle size of preferably 100 mesh or less is added to this to form a slurry and applied to the inner surface of the mold.

The slurry concentration is preferably about 30 to 70% by weight.

In order to obtain a high-purity silicon molded body, silicon nitride,
Chlorosilane, alkoxysilane and colloidal silica must be of high purity.

In order to obtain a thick coating layer, the above operation may be repeated many times.

Next, when chlorosilanes are used, the inner coating layer of the mold is brought into contact with humid air or the like to hydrolyze the chlorosilanes.

In addition, when alkoxysilane is used, hydrolysis is performed with added hydrochloric acid.

The hydrolysis product is presumed to be mainly amorphous silica, which forms a polymer through oxygen atoms, acts as a binder for the silicon nitride powder, and at the same time firmly adheres to the inner surface of the mold.

Furthermore, the mold is heated and dried to remove moisture and hydrogen chloride, and if alkoxysilane is used, alcohol is also removed at the same time.

When colloidal silica is used, similar results can be obtained by simply heating or drying it.

As explained above, according to the method of the present invention, a binder mainly composed of amorphous silica is formed on the inner surface of the mold, so unlike the case where silicon nitride powder is simply physically deposited, there is no need to blast the inner surface of the mold. .

Therefore, silicon nitride powder can be easily deposited on the inner surface of elongated tubular molds or molds of complex shapes.

In addition, since silicon nitride powder can be applied in multiple layers by the action of a binder, when a silicon molten material is poured into a heat-resistant mold and molded, stress due to the difference in shrinkage between the mold and the silicon raw material is applied to each layer of the laminated silicon nitride particles. The rate of buffering due to misalignment is high, and phenomena such as cracking of the molded product hardly occur.

Furthermore, conventionally, when manufacturing high-purity silicon, the mold material was limited to homogeneous quartz, etc., but according to the method of the present invention, silicon nitride powder is bound by a siliceous binder to form a coating layer on the inner surface of the mold. Therefore, since the mold material does not come into direct contact with high-purity silicon, molds made of different materials such as graphite and alumina can be used, which has various industrial advantages.

As mentioned above, the method of the present invention is particularly useful as a method for treating the inner surface of a mold for molding a silicon rod, which is necessary when producing high-purity silicon, but it can also be applied to any mold such as a crucible.

Example 1, Comparative Example High-purity silicon nitride powder (average particle size of about 150
A slurry consisting of 2 parts by weight of mesh and 3 parts by weight of tetrachlorosilane was poured to coat the inner surface of the quartz tube.

Next, moist air is blown into one end of the quartz tube to hydrolyze the tetrachlorosilane, forming a rough coating film to which the silicon nitride powder is fixed by the amorphous silica binder produced.

This operation was repeated three times to obtain a coating film with a thickness of about 0.4 mm.

Next, this quartz tube is heated to about 100° C. to remove moisture and hydrogen chloride.

One end of the quartz tube with the inner surface treated in this way was heated to a temperature of about 14
It was immersed in a molten silicon bath at 50° C. and the molten silicon was sucked out by vacuum operation.

When this was lifted from the bath and left to cool, it was separated from the quartz tube to obtain a high purity silicon rod with a diameter of 2.7 mm and a length of 600 m111, with no cracks observed at all.

For comparison, a quartz tube without a slurry of high-purity silicon nitride 1" powder and tetrachlorosilane was used, and approximately 1450
When molten silicon at ℃ was sucked in, the quartz tube cracked.
At the same time, the silicon rod developed many cracks and broke, and only a silicon rod with a length of about 10 mm was obtained.

Example 2 High purity silicon nitride powder (average particle size approximately 150 mesh) 2
A slurry consisting of 3 parts by weight of trichlorosilane to parts by weight is applied to the inner surface of a quartz crucible having an internal volume of about 100 m1.

Moist air is then sent through the coating to hydrolyze the trichlorosilane, forming a rough coating film in which the silicon nitride powder is fixed by the binder of the hydrolyzed product.

Repeat this operation three times until the thickness is approximately 0.4. A coating of mrrt was obtained.

Next, this quartz crucible is heated to about 100° C. to remove water and hydrogen chloride.

The same molten silicon as in Example 1 was poured into the quartz crucible whose inner surface had been treated in this way, and allowed to cool and solidify.

The high-purity silicon ingot thus obtained was easily taken out without damaging the quartz crucible, and no cracks were observed.

Example 3 Add 2.5 ml of tetraethoxysilane and 5 ml of K ethyl alcohol and add 5 ml of hydrochloric acid with a concentration of 2.5% by weight while stirring.
Add.

Furthermore, 40 pieces of silicon nitride powder with an average particle size of about 150 mesh
2 was added to form a slurry and applied to the inner wall of the quartz tube used in Example 1.

Hydrolysis with hydrochloric acid is completed within 20 minutes.

Next, this quartz tube was dried at 100° C., and the molten silicon was sucked and allowed to cool in the same manner as in Example 1. A high-purity silicon rod of the same type without any cracks was obtained.

Example 4 15 g of high-purity silicon nitride powder with an average particle size of about 150 mesh was added to 10 g of colloidal silica with a solid content of 20% by weight to form a slurry, which was coated on the inner wall of an alumina crucible with a volume of 100 m7 and kept at room temperature. Dry with.

After performing this operation twice, the same molten silicon as in Example 1 was poured and cooled and solidified. The resulting high-purity silicon block was easily taken out and no cracks were observed.

Claims (1)

[Claims] 1. Add silicon nitride powder to liquid chlorosilane, alkoxysilane or colloidal silica to form a slurry, apply it to the inner surface of a mold for producing high purity silicon made of a heat-resistant material such as quartz, alumina, graphite, etc., and then add the above chlorosilane or colloidal silica. Inner surface treatment of a mold for manufacturing high-purity silicon, characterized in that after hydrolyzing alkoxysilane, the coating is dried to coat the inner surface of the mold with a hydrolysis product or silicon nitride powder with colloidal silica as a binder. Method.