JP2003064388A - Releasant and casting method for polycrystalline silicon using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a releasant including a binder capable of reducing the carbon residues in ingot, and to provide a casting method for polycrystalline silicon by using the releasant. SOLUTION: This casting method for polycrystalline silicon comprises the steps of coating the inner wall of a mold with the releasant which is prepared by blending the binder and a solvent with the main material consisting of at least one of silicon nitride, silicon carbide and silicon oxide and then solidifying a silicon melt, wherein the binder comprises a compound of the formula: [Mg5.34 Li0.66 Si8 O20 (OH)4 ]Na<+> 0.66 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold release material and a method for casting polycrystalline silicon using the mold release material, and more particularly to a mold release material suitable for producing a polycrystalline silicon substrate for a solar cell and a polycrystal using the mold release material. The present invention relates to a method for casting silicon.

[0002]

2. Description of the Related Art Polycrystalline silicon is cast by melting a silicon raw material in a mold and solidifying it in the same mold, or by casting a silicon melt melted in a melting crucible into another solidifying mold for solidification. It is roughly divided into the methods to make.

In any case, since it is necessary to solidify the molten silicon in the mold, a method of applying a mold release material to the inner wall of the mold is used. This mold release material is for the purpose of relaxing the thermal stress in the silicon ingot generated during the process of cooling and solidifying silicon, and for preventing the active silicon melt from reacting with the mold material and sticking to the inner wall of the mold. There is a purpose.

Generally, powders of silicon nitride, silicon carbide, silicon oxide, etc. are mixed into a solution composed of a suitable binder and a solvent to form a slurry, which is coated on the inner wall of the mold by means such as coating or spraying. Is known as a known technique. For example, Y. Maeda et. Al, J.
Electrochem. Soc., Vol. 133, No. 2, Feb. 1986 p440
-443 and T. Saito et. Al, 15 ^th IEEE Photovoltai
c Specialists Conf. (1981) p576-580 etc. correspond to this.

Further, the slurry-like release material is usually formed by appropriately mixing a solvent such as water or alcohol, a binder for coating and molding, and an additive material for improving fluidity.

PVA (polyvinyl alcohol) is the most widely used substance among molding binders. PVA is suitable for adhering powder because it has excellent adhesiveness.

After molding (coating), degreasing is performed at a temperature of about 600 ° C. in an oxidizing atmosphere in order to prevent the thermal decomposition products from being mixed into the melt during subsequent heating or contact with the melt. Is usually done.

[0008]

PVA rapidly undergoes thermal decomposition at around 300 ° C. and is gasified into CO and the like.
It can be removed up to 0% rapidly, but the remaining 1
Even if 0% is heated to a temperature of 500 ° C. or higher, it is not easily removed and often remains as a carbon residue.

When a carbon-based mold material is coated with a mold release material, high-temperature degreasing in an oxidizing atmosphere causes the mold material to be consumed due to oxidation, resulting in deterioration of durability and, as a result, the manufacturing cost of the silicon ingot. There is a problem that increases.

On the other hand, when the degreasing is carried out in an inert atmosphere, the thermal decomposition reaction of the organic polymer proceeds rapidly, and as a result, hydrogen atoms are extracted and CHs are arranged in a straight line, which becomes a ring and becomes benzene or other. Becomes a cyclic compound. Further, the dehydrogenation reaction is repeated to cause large condensation and grow into soot having a large amount of carbon. Once stabilized as soot, it is difficult to remove it by thermal decomposition, so it comes into contact with the silicon melt in the release material or while adhering to the surface of the release material. Soot in contact with the melt or carbon dissolved in the melt not only deteriorates the characteristics of the solar cell, but also causes precipitation and cutting of the ingot, and often causes processing defects when slicing. .

There are various organic binders that can replace PVA, but the reality is that none of them have both coating properties and adhesive properties.

The present invention has been made in view of the above circumstances, and provides a mold release material containing a binder capable of reducing the amount of carbon residue in an ingot, and a method for casting polycrystalline silicon using the same. The purpose is to do.

[0013]

In order to achieve the above object, the mold release material according to claim 1 comprises a binder and a solvent with at least one of silicon nitride, silicon carbide and silicon oxide as a main raw material. In the mold release material mixed with [Mg _5.34 Li _0.66 Si ₈ O ₂₀ (O
H) ₄ ] Na ⁺ _0.66 .

The above-mentioned mold release material is the above-mentioned [Mg _5.34 Li _0.66 S
It is desirable that i ₈ O ₂₀ (OH) ₄ ] Na ⁺ _0.66 be contained in the range of 2 to 3.5 wt% with respect to one or more of silicon nitride, silicon carbide, and silicon oxide.

In the method for casting polycrystalline silicon according to the third aspect of the present invention, a mold release material obtained by mixing a binder and a solvent with one or more of silicon nitride, silicon carbide and silicon oxide as a main material is used as the inner wall of the mold. In the method for casting polycrystalline silicon, which is applied to the above to solidify a silicon melt, [Mg _5.34 Li _0.66 Si ₈ O ₂₀ (O
H) ₄ ] Na ⁺ _0.66 is used as a release material.

In the above-mentioned method for casting polycrystalline silicon, the release agent containing [Mg _5.34 Li _0.66 Si ₈ O ₂₀ (OH) ₄ ] Na ⁺ _0.66 in the range of 2 to 3.5 wt% with respect to the main raw material. Is preferably applied.

In the method of casting polycrystalline silicon, the mold is graphite, carbon composite material, quartz,
Alternatively, it is desirable to be made of any one of heat resistant ceramics.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have diligently investigated a substance which is not an organic binder but an inorganic substance and has both coatability and adhesiveness, and as a result, [Mg _5.34 Li _0.66 Si ₈ O ₂₀
Focusing on the properties of natural hectorite represented by (OH) ₄ ] Na ⁺ _0.66 , attention was paid to a high-purity synthetic product having the same structure.

[Mg _5.34 Li _0.66 Si ₈ O ₂₀ (OH) ₄ ]
Na ⁺ _0.66 is an aggregate composed of fine disc-shaped primary particles having a radius of 10 to 20 nm and a height of 1 nm, and is white and odorless and easily gels when added to water. The plate surface of the primary particles is −
In addition, the end faces are positively charged, and Na ⁺ exists above and below the plate surface and electrically neutralizes, forming a layered overlap and aggregating.

[0020] are suspended the aggregates in water, water intrudes between layers of the primary particles, eventually spreads out layers of plate-like particles by Na ⁺ hydrates are dissociated Na ^+. As a result, the plate surface becomes negatively charged, and the primary particles repel each other and disperse in water (dispersibility).
After that, the − of the plate surface of the primary particles and the + of the end surface attract each other, so that the structure of the primary particles in water (card house)
To increase the viscosity (thickening) to increase the adhesiveness.

[Mg _5.34 Li _0.66 Si ₈ O ₂₀ (OH) ₄ ]
Na ⁺ _0.66 and PVA were mixed at various mixing ratios and mixed with silicon nitride, and the experiment was repeated for the coating property on the mold material and the releasability from silicon. As a result, [Mg _5.34 Li _0.66 S
i at ₈ O ₂₀ (OH) _4] The use of binder containing Na ⁺ _{0.6 6,} although it is not possible to eliminate the PVA _{completely, [Mg 5.34 Li 0.66 Si 8} O 20 (OH) 4] Na
⁺ _0.66 by mixing in a proportion ranging pair nitride silicon concentration 2～3Wt% and found that can reduce by about half the PVA in the Taikonatai concentration, by the use of this release material, silicon We have succeeded in greatly reducing the amount of precipitates such as silicon carbide and foreign substances that precipitate in the ingot.

The binder in the mold release material is originally used to impart adhesiveness and fluidity to the powder, but if it is completely degreased at a temperature of about 1000 ° C., the mold release material is completely degreased. Since the silicon nitride powder, which is the main component of the above, cannot be sintered, the powder is completely returned to the powder or loosely bonded. This state is
Subsequent contact with the melt causes easy breakage and induces peeling of the release material or drop into the melt in a partial or large area. Therefore, it is desired that the release material has some strength even after degreasing of carbon.

[Mg _5.34 Li _0.66 Si ₈ O ₂₀ (OH) ₄ ]
Since the primary particles of Na ⁺ _0.66 are extremely small, if they are sufficiently stirred in a slurry state, they will be uniformly distributed on the surface of the powder particles. Further, by utilizing the structural feature that contains a suitable amount of SiO ₂ component, it is possible to obtain SiO at a relatively low temperature.
_By liquefying the _two components and dissolving and re-precipitating the silicon nitride powder in the liquid phase, the bond between silicon nitrides is strengthened, and it is possible to give the mold release material sufficient strength. . That is, [Mg _5.34 Li _0.66 Si ₈ O ₂₀ (O
When H) ₄ ] Na ⁺ _0.66 is used, the strength of the release material is maintained during and after degreasing even if the amount of PVA added is small.

PVA and [Mg _5.34 Li _0.66 Si ₈ O
₂₀ (OH) ₄ ] Na ⁺ _0.66 is dissolved in pure water, silicon nitride powder is mixed with this to prepare a mold release material slurry, and this is applied to a mold material to obtain coatability and adhesiveness.

The results of the coatability test of the release material are shown in FIG. From FIG. 1, the compounding ratio and [Mg
It was found that there was an optimum blending ratio at each ratio on a straight line connecting the blending ratios in the case of only _5.34 Li _0.66 Si ₈ O ₂₀ (OH) ₄ ] Na ⁺ _0.66 .

Next, the mold release material prepared at each mixing ratio was applied to the mold material and used for actual silicon casting to perform a mold release test. As a result of the experiment, [Mg _5.34 Li _0.66 S
i ₈ O ₂₀ (OH) ₄ ] Na ⁺ _0.66 vs. silicon nitride concentration 3.5
It has been found that when the mixture is mixed at a ratio of wt% or more, the mold release material cracks during the casting of silicon and the melt exudes. In addition, [Mg _5.34 Li _0.66 Si ₈ O ₂₀ (OH) ₄ ] Na ⁺
_When the ratio of _0.66 to the silicon nitride concentration is less than 2.0 wt%, it is difficult to maintain the strength only with this binder.
Since it is necessary to increase the VA content, the amount of carbon in the organic binder, which was the original purpose, cannot be sufficiently reduced.

As a result of repeating the experiment, [Mg _5.34 Li
_When a mold release material prepared by mixing _0.66 Si ₈ O ₂₀ (OH) ₄ ] Na ⁺ _0.66 with respect to the silicon nitride concentration in the range of 2 to 3.5 wt% is applied to the mold material, the mold release of the silicon ingot is performed. It has been found that it has good properties and can significantly reduce the amount of deposits such as silicon carbide and foreign matters deposited in the ingot.

[0028]

EXAMPLES Next, examples will be described. 300 mm x 300
Silicon nitride: P on the inner wall of a graphite mold of mm × 300 mm ^t
VA: [Mg _5.34 Li _0.66 Si ₈ O ₂₀ (OH) ₄ ] Na ⁺
A mold coated with a mold release material prepared in a weight ratio of _0.66 = 1: 0.05: 0.025 and a mold coated with a conventional mold release material were prepared, and 70 kg of silicon was melted. After the silicon was completely melted, a cooling plate was placed on the bottom of the mold and the mold was rotated to produce a silicon ingot by the unidirectional solidification method from the bottom.

Foreign matter in the ingot due to the difference in the release material
Table 1 shows the difference in the amount of precipitate generated. Table 2 shows the difference in conversion efficiency of the solar cells.

[0030]

[Table 1]

[0031]

[Table 2]

From Table 1 and Table 2, in the solar cell formed of polycrystalline silicon using the mold release material of the present invention, the foreign matter / precipitate in the ingot is greatly reduced so as not to lower the conversion efficiency of the solar cell. It turns out to get.

[0033]

As described above, according to the mold release material of the first aspect, the mold release material in which the binder and the solvent are mixed with at least one of silicon nitride, silicon carbide and silicon oxide as a main material. As a binder for [Mg _5.34 Li
Since containing _{0.66 Si 8 O 20 (OH)} 4] Na + 0 .66, when used as a release agent for casting silicon, it is possible to suppress foreign matter and deposition amount in the ingot.

Further, according to the method for casting polycrystalline silicon according to the third aspect, a mold release material obtained by mixing a binder and a solvent with at least one of silicon nitride, silicon carbide and silicon oxide as a main material is used as a mold. As a binder when the silicon melt is solidified by applying [Mg _5.34
Since the mold release material containing Li _0.66 Si ₈ O ₂₀ (OH) ₄ ] Na ⁺ _0.66 is applied, the amount of foreign matters and precipitates in the ingot can be suppressed.

[Brief description of drawings]

FIG. 1 is a diagram showing a test result of a mold release material coatability according to a method for producing polycrystalline silicon according to the present invention.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C10M 103/00 C10M 103/00 A 103/02 103/02 Z // C10N 40:36 C10N 40:36 F Term (reference) 4G072 AA01 BB12 GG01 GG03 HH01 NN01 UU02 4H104 AA13C AA21A AA22A FA01 FA02 PA48 RA01 4J038 AA011 AA012 CE021 CE022 HA436 HA446 HA451 HA452 KA06 NA10 PC01 PC03 PC04

Claims (5)

[Claims] 1. A mold release material in which a binder and a solvent are mixed with at least one of silicon nitride, silicon carbide, and silicon oxide as a main raw material, and the binder is [Mg _5.34 Li _0.66 Si ₈ O ₂₀ (OH ) ₄ ] Na ⁺ _0.66 is contained, The mold release material characterized by the above-mentioned. 2. The [Mg _5.34 Li _0.66 Si ₈ O ₂₀ (O
H) ₄ ] Na ⁺ _0.66 in an amount of 2 to 3.5 w with respect to any one or more of silicon nitride, silicon carbide, and silicon oxide.
The release material according to claim 1, wherein the release agent is contained in a range of t%. 3. A polycrystal for solidifying a silicon melt by applying a mold release material obtained by mixing a binder and a solvent using at least one of silicon nitride, silicon carbide and silicon oxide as a main raw material to the inner wall of a mold. In the method of casting silicon,
As the binder, [Mg _5.34 Li _0.66 Si ₈ O
_A method for casting polycrystalline silicon, which comprises applying a mold release material containing ₂₀ (OH) ₄ ] Na ⁺ _0.66 . 4. The [Mg _5.34 Li _0.66 Si ₈ O ₂₀ (O
H) ₄ ] Na ⁺ _0.66 in an amount of 2 to 3.5 wt% with respect to the main raw material
The method for casting polycrystalline silicon according to claim 3, wherein a mold release material contained in the range of 10% is applied. 5. The method for casting polycrystalline silicon according to claim 3, wherein the mold is made of any one of graphite, a carbon composite material, quartz, and heat resistant ceramics.