JP2002239682A - Method for forming film onto mold, and method for producing mold and polycrystalline silicon ingot - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a film onto a mold for polycrystalline silicon ingot with which a Si3N4 film having no crack and difficult to exfoliate till obtaining the ingot and by which the ingot can be taken out without damaging the product even in the case of being the thin film thickness, can easily and simply be formed, a mold on which the Si3N4 film is formed and a method for producing the polycrystalline silicon ingot. SOLUTION: The film is formed on the inner surface of the mold by using slurry containing Si3N4 powder having >=7 m2/g specific surface and dispersant as releasing agent slurry.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a mold for producing a polycrystalline silicon ingot, a method for producing the mold, and a method for producing a polycrystalline silicon ingot using the mold.

[0002]

2. Description of the Related Art In recent years, photovoltaic power generation has been spotlighted as a method of supplying energy that is safe and capable of generating power at a low price, and research and development of solar cells have been actively conducted to spread the power. Polycrystalline silicon is attracting attention as the most versatile material for this solar cell, and is also regarded as important as a power supply material for power. Polycrystalline silicon is manufactured by pouring molten silicon (molten metal) into a mold and cooling or solidifying, or melting silicon in the mold and cooling and solidifying it to produce an ingot. At this time, the silicon ingot adheres to the mold, There is a problem that the silicon ingot cracks during cooling. For this reason, conventionally, a mold release agent layer is provided on the inner surface of a quartz or graphite mold.

[0003] As the releasing agent, silicon oxide (S
i O _2: JP-A-11-244988), carbide (Si C), nitride (Si ₃ N _4), yttrium oxide (JP-A-7-206419) and the like, these powders usually polyvinyl A water-based slurry containing a binder such as alcohol (PVA) is applied to the inside of the mold, dried, and fired to form the binder. Among the silicon release agents as described above, it has been reported that silicon nitride (Si ₃ N ₄ ) can provide the highest quality polycrystalline silicon (Y. Maeda et al .: J. Electochem. Soc., vol.133,
No.2, Feb. p440-443 (1986), T. Saito et al .: 15th IEEE Ph
otovoltaic Specialists. Conf., p576-580 (1981)).

However the release agent layer formed of Si ₃ N ₄ slurry, peeling easily, also thick film has problems such as an expensive and therefore conventional, together with Si ₃ N ₄ Si O _2, etc. or coating, or the improvement idea of such a multilayer structure of Si O ₂ have been proposed.

For example, Japanese Patent Application Laid-Open No. 9-175809 discloses that
As a release agent, silicon nitride (Si ₃
When N ₄ ) powder is used as a slurry of an aqueous 8.7% polyvinyl alcohol solution, although there is no adhesion between the mold release agent and the ingot, adhesion between the silicon mold and the mold occurs. Describes that it is necessary to use silicon nitride and silicon dioxide at a specific ratio. Further, for example, a multilayer structure in which an SiO ₂ layer is provided between a Si ₃ N ₄ layer and a mold (Japanese Patent Laid-Open No. 6-144824), an SiO ₂ layer,
i ₃ mixed powder layer of N ₄ and Si O _2, then Si ₃ a multilayer structure of N ₄ layer (JP-A-7-206419) are known.

With the above-mentioned multilayer structure, a mold capable of producing a sound polycrystalline silicon ingot without cracks can be obtained, but it takes time and cost to form a film. In addition, when SiO ₂ is used, the SiO ₂ and the mold may partially adhere to each other, making it impossible to reuse the mold, or there may be a problem of contamination by using SiC.

A coating consisting of Si ₃ N ₄ alone has also been proposed. For example, Japanese Patent Application Laid-Open No. 10-316489 discloses a coating using a dispersion of powder having a particle size of about 0.5 to 1.0 μm as a stress relaxation coating. It has been proposed to form According to the publication, when forming a film, the dispersion medium may be heated so that the dispersion medium evaporates. It is described that the temperature is preferably set to 180 ° C. or less at the maximum in order to prevent the relaxation action from becoming insufficient.
Further, it is described that the thickness of the film is preferably 1 to 3 mm because it is difficult to break and remove the film without breaking the product and the main body of the mold if the thickness is less than 1 mm. It is shown. However, when the film is thickened, a large amount of expensive Si ₃ N ₄ is used.
There is a problem that the cost of the mold and the polycrystalline silicon ingot increases.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the situation of the production of a polycrystalline silicon ingot as described above, has no cracks, is hardly peeled off until an ingot is obtained, and has a thin film. A method of forming a film on a polycrystalline silicon ingot mold that can easily and easily form a Si ₃ N ₄ film that can be taken out without damaging the product even if it is thin, and Si that can reduce the cost of the ingot. ₃ N ₄ film mold formed of, and an object thereof is to provide a method for producing polycrystalline silicon ingot using the template.

[0009]

Means for Solving the Problems The present inventor has obtained by eliminating the conventional problem that Si ₃ N ₄ film is easy to peel, less contamination of the silicon ingots, the hard mold cracked into ingots As a release agent, Si ₃
In the course of examining a method of forming a film that is difficult to peel off using only N ₄ , an attempt was made to reduce the size of the Si ₃ N ₄ powder and increase the adhesion to the mold. When the Si ₃ N ₄ powder had a specific surface area of 7 m ² / g or more, it was confirmed that the adhesive force with the mold was large. However, when Si ₃ N ₄ to be formed a film from powder alone, Si ₃ N ₄ fine powder to form large aggregate particles rather in water, and it is difficult to form a homogeneous powder dispersion slurry, Thus Even if this was applied by spraying or the like, a uniform film could not be obtained, and although the adhesion and strength of the film were increased, it was found that the film was liable to cracks and the like. It was also found that when the water ratio of the slurry was increased to avoid the formation of the above-mentioned aggregated particles, drying took a long time, and cracks tended to occur during drying.

The present inventor further studied and found that S
i ₃ N ₄ by including in the powder of an aqueous slurry of organic compound-based dispersing agent, without increasing the slurry water ratio Si ₃ N
₄ It can be a slurry in which fine powder is homogeneously dispersed,
The inventors have found that it is possible to easily and quickly obtain a coating film using only a homogenous and crack-free Si ₃ N ₄ fine powder as a release agent, thereby completing the present invention. The fired film contains substantially no dispersant.

[0011] That is film-forming method of the polycrystalline silicon ingot mold according to the present invention, the release agent slurry applied to the mold inner surface, dried, in forming a film, the ratio as a release agent slurry surface area 7m ² / A slurry containing a dispersant together with at least g of Si ₃ N ₄ powder is used. In the present invention, a slurry containing the Si ₃ N ₄ powder in an amount of 35 to 70% by mass can be used. The concentration of the dispersant in the slurry is typically the Si ₃ N ₄ powder mass to, is preferably about 0.01 to 2 wt%. A binder may be included in the slurry. In the present invention, the use amount of the binder may be extremely small due to the coexistence of the dispersant as described above, and may be generally 3% by mass or less based on the total water amount forming the slurry. It is preferably at most 2% by mass. After drying, the film is dried in air if necessary.
It can be fired at about 0 to 1000 ° C. Since the amount of binder is small, baking can be omitted in some cases.

In the present invention, a specific surface area of 7 m ²
The present invention provides a mold for a polycrystalline silicon ingot having a coating of Si ₃ N ₄ powder of at least 1 g / g. The thickness of the film is desirably 0.3 mm to 1.0 mm. This film can be formed by the film forming method described above, and has no crack.

In the method for manufacturing a polycrystalline silicon ingot according to the present invention, the molten silicon is cooled and solidified by using the above-mentioned mold to produce a polycrystalline silicon ingot.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below more specifically. In this specification, the term “mold” is used to include not only a mold into which a molten metal is poured but also a crucible, and its shape and form are not particularly limited. The mold material is not particularly limited as long as it does not melt, evaporate, decompose, or the like at a temperature equal to or higher than the melting point of silicon. desirable. Graphite or quartz that has been subjected to a high-purification treatment with a halogen gas or the like is more preferable.

In the present invention, a Si ₃ N ₄ powder is used as a release agent, but the specific surface area of the Si ₃ N ₄ powder is 7 m ² / g.
Or more, and preferably 10 m ² / g or more. The Si ₃ N ₄ powder having such a specific surface area can form a film having high strength and strong adhesion to a mold. Thus, a crack-free ingot can be manufactured. Note coating formed from Si ₃ N ₄ powder of less than a specific surface area of 7m ² / g, the strength is insufficient, for weak adhesion to the mold, the part until the molten silicon is directionally solidified The coating is apt to peel off, which tends to cause cracks from the ingot bonded to the mold during cooling of the ingot. Here, the specific surface area of the Si ₃ N ₄ powder is a value measured according to the BET method (for example, “Ceramic Processing”, p.68 by Mizutani et al., Technical Institute Publishing Co., Ltd.).

The powder having such a specific surface area is 7 m
² / g or more powder is normally fine powder, it is desirable particle size distribution is narrower, the average particle diameter is smaller than 0.5 [mu] m, and preferably be from 0.1 [mu] m 0.5 [mu] m, and 1.0μm or more It is desirable not to include a powder having a particle size of The upper limit of the specific surface area is not particularly limited, but is preferably 20 m ² / g or less from the viewpoint of easy handling of the powder. In the present invention, as the Si ₃ N ₄ powder, a commercial product may be used as it is as long as it has a specific surface area of 7 m ² / g or more. it can.

The release agent (Si ₃ N ₄ powder) is used as an aqueous slurry. Dispersion medium for slurry preparation,
Water alone may be used, or alcohols may be added thereto. In the present invention, this slurry contains a dispersant. Examples of the dispersant include carboxymethylcellulose-Na salt, carboxymethylcellulose-NH
_4- salt, polycarboxylic acid type anionic surfactant, sodium alginate, sodium polyacrylate, condensed naphthalene
Sodium sulfonic acid, condensed naphthalene sulfonic acid-NH ₄
Salt, acrylic acid (or methacrylic acid, or mixture)
Oligomers, organic acids, citric acid, tartaric acid, _L -ascorbic acid, wax emulsion, polyvinylpyrrolidone,
NH ₄ salts or amine acid copolymer of isobutylene or styrene and maleic anhydride, NH ₄ salts of a copolymer of butadiene and maleic acid can be used. These may be used in combination of two or more. Among the above, compounds containing no metal are preferably used. Usually 0.01 to 2 parts by mass of the release agent (Si ₃ N ₄ powder).
% Can be used.

Generally, the slurry concentration is 35 to 70% by mass.
At such a high concentration, the viscosity sharply increases and the dispersibility sharply decreases, which causes various technical obstacles during spray coating and drying. For example, the specific surface area is 7 m ² /
The inventors of the present invention have examined a slurry of Si ₃ N ₄ powder having a mass of not less than 35 g, and found that the concentration which can be practically used is less than 35% when no dispersant is added. In the present invention, a slurry is prepared using the above dispersant, and a homogeneous slurry can be obtained without reducing the slurry concentration even though Si ₃ N ₄ is a fine powder.
Specifically, the slurry concentration (Si ₃ N ₄ powder content) is 35
A slurry of about 70% by mass can be used.

In the present invention, the slurry may contain a binder such as polyvinyl alcohol (PVA).
The binder is usually used after being dissolved in water, and can be used in an amount of 3% by mass or less based on the total amount of water forming the slurry. The above-mentioned conventional dispersant-free Si ₃ N
₄ Powder slurry has low slurry concentration (large amount of water)
Then, the Si ₃ N ₄ powder is dispersed. For this reason, when spraying the slurry on the mold, a large amount of binder (PVA) (10% or more, usually 20%
Degree). In the present invention, since a slurry having a high concentration can be obtained, the amount of the binder can be significantly reduced. Therefore, in the present invention, baking usually performed for removing the binder can be omitted in some cases.

According to the present invention, even when the slurry has such a high concentration, uniform application by spraying is possible.
Moreover, since the slurry has a high concentration, the drying time can be significantly reduced. For example, when a dilute slurry is used, it is necessary to evaporate a large amount of water, the drying time is essentially long, and even if the coating is dried under mild conditions over a long period of time, fine cracks are likely to occur in the coating, The use of the slurry as described above can greatly reduce the rate of occurrence of cracks in the film that is likely to occur during drying, and can provide a mold to which the ingot is unlikely to adhere. If the slurry concentration exceeds 70% by mass, even if a dispersant is contained, Si ₃ N
₄ It becomes difficult to uniformly disperse the powder.

The above slurry can be applied by a general method such as spraying or brushing. Spray coating is preferable in terms of workability, work time, uniform coatability, and the like. After applying the slurry to the inner surface of the mold, the slurry is dried to obtain a film. Drying is desirably performed at 60 to 120 ° C, and is usually performed for 1 to 2 hours. The baking of the film after drying,
Usually, it is performed in air at 500 to 1000 ° C. for about 30 to 120 minutes. The fired film consists essentially of the release agent. In the present invention, it is also possible to omit the firing step after drying, and the omission of the firing step simplifies the working steps and eliminates the need for a firing apparatus, so that the manufacturing process can be significantly reduced in cost. . That is, even if the slurry of the present invention contains a binder, the amount may be as small as 3% or less as described above. Therefore, there is almost no carbon amount expected to be mixed in when the firing is omitted. In the present invention, it is possible to form a uniform film on the inner surface of the mold,
For example, a thin film having a thickness of 1.0 mm or less can be formed.

The coating obtained above has a specific surface area of 7 m ² /
g or more of Si ₃ N ₄ powder, and has high film strength and strong adhesion to a mold. In the present invention, the film thickness is 0.3
If it is not less than mm, the function as a release agent can be sufficiently exerted without increasing the thickness as in the prior art. Expensive S
From the viewpoint of reducing the amount of i ₃ N ₄ powder used, it is desirable that the film thickness be smaller than 1.0 mm. The thickness of the film formed in the present invention is usually about 0.3 mm to 1.0 mm, preferably about 0.3 to 0.9 mm, and more preferably about 0.3 to 0.6 mm. Even such a thin film can be taken out without damaging the ingot.

In the present invention, a mold having such a coating on the inner surface of the mold is also provided, and a method for producing a polycrystalline silicon ingot using the mold is also provided. The polycrystalline silicon ingot can be manufactured in the same manner as in the production of a general polycrystalline silicon ingot except for using the above-mentioned mold. Since the mold having the coating is hardly peeled off when the molten silicon is cooled and solidified, adhesion between the silicon and the mold does not easily occur, and cracking of the silicon ingot caused by the adhesion hardly occurs. In addition, according to the above-described film forming method, the film is thin, the amount of Si ₃ N ₄ powder used is small, and the film can be formed in a simple operation process. At the same time, the cost of the polycrystalline silicon ingot can be reduced by improving the yield of sound ingots.

[0024]

EXAMPLES Next, the present invention will be described specifically with reference to examples, but the present invention is not limited to these examples. (Examples 1 to 11, Comparative Examples 1 to 3 and Reference Examples 1 and 2) <Formation of Mold Film> Inner Dimension 550 mm × 550 mm × 40
A release agent was applied to the inner surface of a 0 mm quartz mold under various conditions as shown in Table 1 to form a film. As a release agent, Si ₃ N ₄ having a specific surface area of 3 m ² / g, 5 m ² / g or 10 m ² / g
The powder (commercially available) is pulverized as it is or by a ball mill if necessary, and the specific surface area is 6 m ² / g, 7 m ² / g, 13 m ²
/ Using Si ₃ N ₄ powder adjusted to g. In water (pure water) containing 2% of polyvinyl alcohol PVA, the above Si ₃ N
₄ powder was added in such an amount as to give a slurry concentration shown in Table 1, and an acrylic acid oligomer-based dispersant (excluding Comparative Example 3) of 0.2% by mass with respect to the Si ₃ N ₄ powder was further added thereto. For 4 hours to prepare a slurry. The obtained slurry was applied to a predetermined thickness on the inner surface of the mold by spraying, and then dried at 120 ° C. for 60 minutes to form a film. Then, it was baked at 800 ° C. for 60 minutes. Apply each of the above slurries,
The approximate time required to obtain a film without cracking when dried was measured using a small piece of mold material (quartz). It is shown in Table 1 as “coating / drying time”.

<Preparation of Polycrystalline Silicon Ingot> Each mold on which the coating was formed as described above was placed in a furnace in an Ar atmosphere using a graphite heater as a heat source, and the raw material silicon 150 was prepared.
kg, and melted at 1500 ° C. From the bottom of the mold,
Unidirectional solidification was performed at a speed of 0.5 mm / min. The cooled and solidified silicon ingot was removed from each mold, and the presence or absence of cracks in the ingot was observed. Table 1 shows the results. In the table, cracks in the ingot are indicated as “present” when the depth from the surface is 20 mm or more, and “absent” when the depth is less than 20 mm.

[0026] As apparent from Table 1, according to the present invention, although the use of Si ₃ N ₄ fine powder alone, the application of the slurry, with drying time short and release by simple work It has been found that an agent film can be formed, and that a polycrystalline silicon ingot without cracks can be produced. Furthermore, in the present invention, the thickness of the release agent film may be thin, and the amount of expensive Si ₃ N ₄ fine powder used can be reduced. Further, the product was not damaged when the polycrystalline silicon ingot was removed from the mold.

[0027]

[Table 1]

[0028]

According to the present invention, a Si ₃ N ₄ powder having a large specific surface area is mixed with water together with a dispersing agent and used as a slurry having a concentration suitable for application and drying. A film that does not crack even if it is not a multilayer film structure,
It can be easily formed by a simple operation. Also this Si ₃
If a mold having an N ₄ film is used, a polycrystalline silicon ingot without cracks can be obtained, so that a sound polycrystalline silicon ingot can be manufactured quickly and inexpensively.

Continuing on the front page (72) Inventor Narutoshi Nakagawa 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. No address) Inside Mizushima Works of Kawasaki Steel Corporation (72) Inventor Shoichi Hiwasa 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture NA10 NB10

Claims (5)

[Claims] A slurry containing a dispersant together with a Si ₃ N ₄ powder having a specific surface area of 7 m ² / g or more is used as a mold release agent slurry when a mold release agent slurry is applied to the inner surface of a mold and dried to form a film. For forming a film on a mold for a polycrystalline silicon ingot. 2. The method according to claim 1, wherein a slurry containing the Si ₃ N ₄ powder in an amount of 35 to 70% by mass is used. 3. The method according to claim 1, wherein a slurry containing the dispersant in an amount of 0.01 to 2% by mass based on the mass of the Si ₃ N ₄ powder is used. 4. An inner surface of a mold having a specific surface area of 7 m ² / g or more.
i ₃ N ₄ powder, 0.3mm to 1.0m thick
m for a polycrystalline silicon ingot having a coating of m. 5. A method for producing a polycrystalline silicon ingot, wherein the molten silicon is cooled and solidified by using the mold according to claim 4 to produce a polycrystalline silicon ingot.