JPH07206419A - Casting mold for casting silicon ingot - Google Patents

Abstract

PURPOSE:To prevent intrusion of a release agent and impurity so as to improve a yield and to reduce a cost by applying yttrium oxide on the inside surfaces of a casting mold. CONSTITUTION:The casting mold for casting silicon ingots is produced by applying the yttrium oxide having an average grain size of 0.8 to 15mum on the inside surfaces of the graphite casting mold consisting of casting side walls 1, casting mold bottom 2 and casting mold inside surfaces 3 and drying the coating. On the other hand, silicon dioxide powder is applied on the casting mold inside surfaces 3 and a powder mixture composed of the silicon oxide powder and silicon nitride powder is applied thereon; further, silicon nitride powder is applied thereon, by which the casting mold for casting the silicon ingots is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold for ingot casting of semiconductor silicon used in solar cells and the like.

[0002]

2. Description of the Related Art Conventionally, as a casting method relating to the production of polycrystalline silicon, a single oxide or nitride of silicon, or a mixture thereof is applied to the inner surface of a graphite mold which can be divided, Japanese Unexamined Patent Publication (Kokai) No. 62-108515 discloses a technique of injecting molten silicon and taking out a cooled and solidified silicon ingot without adhering to the inner surface of a mold.

On the other hand, silicon nitride is the best as a mold release agent for coating the inner surface. Maeda et al. El
microchem. Soc. , Vol. 133, N
o. 2, Feb. 1986 p440-443 and T.W. Saito et al., 15th IEEE Photov
olatic Specialists Conf.
(1981) p576-580.

However, when the above-disclosed silicon nitride (Si ₃ N ₄ ) is applied to the inner surface of the graphite mold, the adhesion between graphite and Si ₃ N ₄ is poor and S at high temperature.
There was a problem that i ₃ N ₄ was peeled from the mold, and the melted silicon contacted and invaded the surface of the graphite mold, and the silicon adhered to the mold during cooling and could not be removed. In addition, silicon oxide (SiO ₂ ) or carbide (Si
When C) is applied, since the adhesiveness between SiO ₂ or SiC and graphite is good, there is no adhesion of silicon to the mold when the solidified silicon is removed from the mold, while SiO ₂ and SiC do not adhere to silicon. Has good adhesive properties,
The applied SiO ₂ or SiC adheres to the silicon ingot, and it is very difficult to remove them from the silicon ingot, and it is necessary to remove them by cutting or grinding the surface of the silicon ingot, which causes a problem of reducing the yield of silicon. .

Therefore, SiO 2 is formed on the inner surface of the mold.₂Or SiC
And then Si₃N _FourHow to apply
Possible, but SiO₂Or SiC and Si₃N_FourWhen
Has a weak bonding force and easily peels off between the two
Was not suitable as a coating agent for.

[0006]

SUMMARY OF THE INVENTION In view of the above problems of the prior art, an object of the present invention is to prevent the coating layer on the mold from being caught in the silicon melt by peeling, and The object of the present invention is to propose a mold for casting a silicon ingot, which has an appropriate coating layer in which impurities are not mixed into the silicon ingot by the reaction.

[0007]

Means for Solving the Problems That is, the present invention is a mold for silicon ingot casting, characterized in that the inner surface of the mold is coated with yttrium oxide, and preferably the inner surface of the mold is yttrium oxide having an average particle size of 0.8 to 15 μm. To 0.1
A silicon ingot casting mold characterized in that it is applied to a thickness of ~ 0.5 mm, and the present invention is to apply silicon dioxide powder to the inner surface of the mold, and a mixed powder of silicon dioxide powder and silicon nitride powder thereon. And a silicon nitride powder is further applied onto the silicon ingot casting mold.

The template material is not particularly limited as long as it does not melt, evaporate, soften, deform, decompose, or the like at a temperature above the melting temperature of silicon, but high-purity quartz or graphite is advantageously used.

[0009]

The present inventors have made various studies by paying attention to an oxide-based material which is stable above the melting point of silicon as a release agent applied to the inner surface of the mold used for casting a silicon ingot.
It has been found that yttrium oxide (Y ₂ O ₃ ) is suitable. A desirable coating condition of Y ₂ O ₃ as a release agent to be coated on the inner surface of the mold is to coat yttrium oxide having an average particle diameter of 0.8 to 15 μm to a thickness of 0.1 to 0.5 mm. The average particle size of Y ₂ O ₃ is less than 0.8 μm or 15
If the thickness exceeds μm, the Y ₂ O ₃ grains are significantly entrained in the silicon melt, impairing the soundness of the silicon ingot. When the coating thickness is less than 0.1 mm, the function as a release agent is lost, and the graphite mold behind and the silicon melt react to produce silicon carbide SiC, which is mixed as an impurity in the silicon melt. On the other hand, when the coating thickness is 0.6 mm or more, peeling of this layer occurs remarkably and scatters in the silicon ingot, impairing the soundness.

Further, the present inventors applied silicon dioxide powder as a mold release agent to be applied to the inner surface of a mold used for casting a silicon ingot, and then applied a mixed powder of silicon dioxide powder and silicon nitride powder thereon. By further coating the silicon nitride powder on it, the coating layer on the mold is not caught in the silicon melt by peeling, and impurities are mixed into the silicon ingot by the reaction with the silicon melt. It was found that no suitable coating layer was formed.

When silicon dioxide powder is applied to a mold, for example, a graphite mold, the mold is well compatible with the inner surface of the mold and almost no peeling occurs. On the other hand, since silicon dioxide easily adheres to silicon,
It is necessary to apply a silicon nitride powder having good releasability to silicon on the above-mentioned silicon dioxide powder coating layer.
Silicon dioxide and silicon nitride have poor adhesion and are easily peeled off. Therefore, in the present invention, a mixed powder of a silicon dioxide powder and a silicon nitride powder, which are well compatible with both, is applied between both layers. By coating three layers in this manner, the peeling of each coating layer is eliminated. Further, in this case, even if the coating layer peels off and adheres to the silicon ingot, it can be easily removed.

[0012]

【Example】

Examples 1 to 10 and Comparative Examples 1 to 4 Various oxide-based refractory powders shown in Table 1 (all having a purity of 99.9% or more) and 4% polyvinyl alcohol on the inner surface of the graphite mold shown in FIG. A slurry prepared by using an aqueous solution as a binder is applied as a release agent,
It was dried at ° C. This mold uses a graphite heater as a heat source A
It is placed in an r atmosphere furnace, melted at 1500 ° C with 15 kg of silicon charged, and held for 1 hour, then 0.8 mm
It was solidified at a solidification rate of / min.

After cooling, the solidified silicon ingot was taken out of the graphite mold, and the oxygen analysis of the silicon ingot was carried out to observe the peeling condition of various mold release agents from the mold and to observe the oxide decomposition condition. The case where peeling is not observed and the oxygen concentration is 10 ppmw or less is regarded as good, and a mark “○” is slightly peeled, but the oxygen concentration is 10 ppm.
The results are also shown in Table 1 in which the case of w or less is marked with Δ, and the case of not satisfying both is marked with x.

From these results, Y ₂ O ₃ is more practical as a release agent than other oxide refractories, and especially Y ₂ O.
It can be seen that when _{3 has} an average particle size of 0.8 to 15 μm and is applied to a thickness of 0.1 to 0.5 mm, the effect as a release agent is remarkable.

[0015]

[Table 1]

Examples 11 to 13 and Comparative Examples 5 to 8 Silicon dioxide SiO ₂ (purity 99.9% or more, average particle size 1) was formed on the inner surface of the graphite mold shown in FIG.
00 μm), silicon carbide SiC (purity 99.9% or more, average particle size 1 μm) and silicon nitride Si ₃ N ₄ (purity 99.9%)
As described above, each powder having an average particle size of 1 μm) was slurried using a 4% polyvinyl alcohol aqueous solution as a binder, and 1 to 3 layers were applied as a release agent according to the combinations shown in Table 2 and dried at 80 to 120 ° C. This mold is placed in an Ar atmosphere furnace using a graphite heater as a heat source, and silicon 1
With 5 kg charged, it was melted at 1500 ° C., held for 1 hour and then solidified at a solidification rate of 0.8 mm / min.

After cooling, the solidified silicon ingot was taken out of the graphite mold, and the state of adhesion of various mold release agents to the silicon ingot was observed and the ingot yield after cutting or grinding and removing the deposit on the surface of the ingot was examined. The results are also shown in Table 2. From this, Si
When O ₂ , SiC and Si ₃ N ₄ are applied individually, or when SiO ₂ is applied first and then Si ₃ is applied
Even when N ₄ was applied, the release agent adhered to the silicon ingot, so that it was found that the amount of cutting and grinding was large and the silicon yield was low.

On the other hand, when SiO ₂ is applied first, a mixture of SiO ₂ and Si ₃ N ₄ is applied thereon, and then Si ₃ N ₄ is applied thereon, the release agent of silicon is used. Almost no adhesion to the ingot was observed, so grinding of the ingot was sufficient to remove the surface irregularities, and a silicon yield of 97% or more was obtained. The mixture of SiO ₂ and Si ₃ N ₄ , which is applied as the second layer, is made of Si.
The weight ratio of O ₂ : Si ₃ N ₄ is preferably in the range of 1: 9 to 9: 1.

[0019]

[Table 2]

[0020]

EFFECTS OF THE INVENTION According to the present invention, it is possible to obtain a silicon ingot which can prevent adhesion of silicon to a mold and a mold release agent, and which does not contain a mold release agent or impurities in silicon.
Since the manufacturing yield of silicon is also improved, the manufacturing cost can be reduced.

[Brief description of drawings]

FIG. 1 is an explanatory view of a mold used when carrying out the present invention.

[Explanation of symbols]

 1 Mold side wall 2 Mold bottom 3 Mold inner surface

Claims (4)

[Claims] 1. A mold for casting a silicon ingot, characterized in that the inner surface of the mold is coated with yttrium oxide. 2. A mold for casting a silicon ingot, wherein the inner surface of the mold is coated with yttrium oxide having an average particle diameter of 0.8 to 15 μm to a thickness of 0.1 to 0.5 mm. 3. A silicon ingot casting characterized in that silicon dioxide powder is applied to the inner surface of the mold, a mixed powder of silicon dioxide powder and silicon nitride powder is applied onto it, and silicon nitride powder is applied onto it. Mold. 4. The mold for casting a silicon ingot according to claim 1, 2 or 3, wherein the mold body is graphite.