JP2002321037A - Casting method for silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a casting method for silicon which can reduce the amount of carbon residue remaining in a parting agent without wasting of a carbon mold. SOLUTION: In the silicon casting method to pour and solidify melted silicon in a casting mold, the inner surface of the mold is coated with the parting agent containing one or more kinds among a silicon nitride, a silicon carbide and a silicon oxide as main components, and a binder, then degreased at 450-600 deg.C in the oxidizing atmosphere. After that, the melted silicon is poured into the mold and solidified.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for casting silicon, and more particularly to a method for casting silicon using a mold made of a carbon-based material.

[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 mold and solidifying it. The method is roughly divided into In any case, since it is necessary to solidify the silicon in a molten state in the mold, a mold release material is applied to the inner surface of the mold. The purpose of the release material is to alleviate the thermal stress in the silicon ingot generated during the process of solidifying and cooling the silicon, and to prevent the active silicon melt from reacting with the mold and fusing.

[0003] Generally, powder such as silicon nitride, silicon carbide or silicon oxide is mixed into a solution composed of an appropriate binder and a solvent and stirred to form a slurry, which is applied to the inner surface of a mold or sprayed. (For example, Y. Maed et.
al, J. Electrochem. Soc., vol. 133, No. 2, Feb. 198
6p440-443 and ^{T. Saito et. Al, 15 th} IEEE Photov
oltaic SpecialistsConf. (1981) p576-580).

[0004] Further, in order to prepare a slurry-like release material and apply it to a mold, a solvent such as water or alcohol, a binder for coating and forming, and an additive material for improving fluidity are appropriately mixed. Generally, it is made by stirring.

[0005] PVA (polyvinyl alcohol) is one of the most utilized materials among molding binders.
PVA is suitable for bonding and bonding powder because of its excellent adhesiveness. This molding binder is heated in a subsequent step, or becomes a thermal decomposition product in contact with the melt, but in order to prevent such a thermal decomposition product of the molding binder from being mixed into the melt, It is a common practice to apply a molding binder and then remove the binder at a temperature of about 600 ° C. to 900 ° C. in an oxidizing atmosphere. The above-mentioned T. Sato et al. Show that the release material becomes a strong release material layer by heat-treating the release material at 600 ° C. That is, such a release material does not return to a powder state by heat treatment at about 600 ° C. The term “return to powder” means that only the powder remains when the PVA in the release material is removed by the binder removal treatment.
If the degree is small, the bonding force between the powders due to sintering is small, so that the strength of the release material after the binder is removed is small, and the powder layer does not become a sintered layer but is broken. Returning to a powdery state refers to such a state, but the above-mentioned document indicates that the material has sufficient strength even after the debinding treatment.

When the crucible or the mold is made of a ceramic such as silicon nitride, silicon carbide, or silicon oxide such as quartz,
Even if the high temperature debinding treatment was performed in an oxidizing atmosphere as described above, there was no problem in that the crucible or the mold was consumed by oxidation or the like. That is, the degree of oxidation consumption of the ceramic material in a relatively low temperature range of about 600 ° C. to 900 ° C. can be almost ignored.

[0007]

However, when a low-cost carbon-based material is used as a mold, the carbon-based material generally consumes a large amount from about 600 ° C. due to oxidation with an increase in temperature. For this reason, when a release material is applied to a mold made of a carbon-based material, if high-temperature debinding is performed in an oxidizing atmosphere, the oxidation of the mold will be oxidized and the durability will be reduced. As a result, the production cost of the silicon ingot will be reduced. There was a problem that it would increase.

On the other hand, when the binder is removed in a vacuum or in an inert atmosphere, the thermal decomposition reaction of the organic polymer constituting the binder proceeds rapidly, so that hydrogen atoms are extracted and CH is arranged in a straight line. It becomes cyclic and becomes benzene and other cyclic compounds. Further, the dehydrogenation reaction is further repeated to greatly condense and grow into carbon-rich soot. In other words, an organic polymer has a form in which hydrogen atoms are usually arranged outside the polymer around the carbon atom skeleton, and the polymer is usually small and does not aggregate due to the repulsive force of the hydrogen atoms. Although it has a shape, the supply of thermal energy increases the vibrational energy of the hydrogen atom to carbon atom bond, breaking the bond and extracting the hydrogen atom. Once stabilized as soot, it is difficult to remove it by thermal decomposition, and soot comes into contact with the silicon melt while adhering to the mold release material or on the surface. Soot in contact with the melt and carbon dissolved in the melt not only deteriorate the characteristics of the solar cell, but also precipitate as foreign matter and lower the yield.

Further, if the release material returns to a powdery state again by the binder removal treatment, the release material cannot form a release material layer having strength enough to withstand the casting of silicon.
In a process such as dissolution or solidification of silicon, a high-temperature silicon melt and a mold are fused, and the original function of the release material cannot be fulfilled. Therefore, in order to temporarily sinter the powder in the release material so that the release material forms a release material layer having strength enough to withstand the casting of silicon, the binder is generally removed at a temperature of about 600 ° C. to 900 ° C. It was thought that processing was necessary. That is, after the binder removal treatment, the adhesive (binder) connecting the powders is lost, so that the bonding force between the powders is lost and the release material has no layer strength. In general, the interaction between powders starts at a temperature of about 1/3 to 1/2 of the melting point. In such a low temperature range, the surface diffusion between the particles is dominant, so the bonding is weak, but the interaction between the particles is weak. Occurs, and the layer has the strength.

To remove PVA, it is necessary to remove the PVA in an oxidizing atmosphere. Conventionally, it has been considered that the higher the processing temperature, the higher the strength of the release material layer.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a silicon casting method capable of suppressing the amount of carbon residue remaining in a release material without consuming a mold.

[0012]

In order to achieve the above object, a silicon casting method according to the present invention is directed to a silicon casting method in which a silicon melt is poured into a mold and solidified.
A mold release material containing a binder and a main constituent material of at least one of silicon nitride, silicon carbide, and silicon oxide is applied to the inner surface of the mold, and the mold is exposed to an atmosphere containing 4% or less of an oxide atmosphere.
After degreasing at 50 to 600 ° C., the silicon melt is poured into the mold and solidified.

In the above-mentioned silicon casting method, it is desirable that the mold is made of a carbon-based material such as graphite or a graphite composite material.

In the above-mentioned silicon casting method, it is desirable that the release agent contains polyvinyl alcohol as a binder.

After the release material is coated and degreased at 450 to 600 ° C. in an oxidizing atmosphere as described above, a silicon melt is poured into the mold, whereby the amount of carbon residues remaining in the release material can be suppressed. Since the release material forms a release material layer that does not return to powder and sufficiently withstands casting of silicon, silicon can be cast without consuming the carbon-based mold.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
FIG. 1 is a view showing an example of a mold used in the silicon casting method according to the present invention. The mold 1 is made of, for example, graphite or a graphite composite material, and has one bottom member 1a and four side members 1b.
And a split mold that can be divided and assembled. The bottom member 1a and the side member 1b can be divided and assembled by fixing them with bolts (not shown), or can be divided by fixing with the frame member (not shown) where the bottom member 1a and the side member 1b just fit. Assembled.

Usually, an isotropic graphite material is used. However, a graphite composite material, for example, a material having better heat insulation and strength than graphite, such as a C / C composite material, is used as a mold material. Can be used as

On the inner surface of the mold 1, a bottom member 1a and side members 1a
The release material 2 is applied so that b can be used repeatedly. As such a release material 2, a release material having silicon nitride, silicon carbide, silicon oxide, or the like as a main constituent substance is used. For example, silicon nitride (Si ₃ N ₄ ), silicon carbide (SiC), silicon dioxide (SiO ₂ ), or a mixed powder thereof is applied. At the time of coating, this silicon nitride, silicon carbide, silicon dioxide or a mixed powder thereof is mixed with a PVA (polyvinyl alcohol) aqueous solution having a concentration of about 5 to 15% by weight to form a slurry, and the inner surface of the mold 1 is spatula or brush. Apply to. In this state, it is dried naturally or placed on a hot plate and dried.

The silicon melt has a very active reactivity and reacts with most substances. Even a so-called high-melting substance having a melting point higher than the melting point of silicon generates binary silicide and reacts with it. It could not be used as a mold.
Various release materials have been tried so far, but no substance that does not react with silicon in a solid state has been found. So the powder and silicon fuse together, but do not penetrate the silicon melt,
In addition, a method is employed in which the powder layer is easily broken after casting to have a role of a release material. A for solar cells
Since l and B work as carriers for impurity semiconductors,
Powders containing these elements cannot be used. Further, as metal impurities, it is necessary to use highly purified powder in order to form a deep impurity level in the semiconductor. SiN, SiC, and SiO ₂ powders are used at a level that satisfies these conditions and can be used at a low cost.

When a slurry obtained by mixing a powder and a solvent is applied to the inner surface of a mold, a binder (binder) is used for the purpose of maintaining the shape of the applied layer and imparting adhesiveness to the applied surface. Is used. For the binder, PVA (polyvinyl alcohol), PVB
(Polyvinyl butyral), PEC (polyethylene glycol), MC (methyl cellulose), CMC (carboxymethyl cellulose), EC (ethyl cellulose),
HPC (hydroxypropylcellulose), wax, etc. are generally used, but the adhesiveness is the most important point for applying the slurry to the mold material, and PVA is the best binder among the commonly used binders. It is.

The thickness of the release material to be applied is several tens μm to several m.
m, and the release material is not limited to one type, and a plurality of release materials may be applied in layers.

Next, the mold thus prepared is placed in a furnace in which the inside is kept in an oxidizing atmosphere, heated to 450 to 600 ° C. over 0 to 30 hours, and kept at 0 to 30 hours for debinding. . Thereafter, it is completed by cooling and taking out from the furnace. At this time, the temperature range is 450 ° C
The reason why the temperature is determined to be less than 600 ° C. is that thermal decomposition of the binder is insufficient at 450 ° C. or less, and that the carbon-based material of the mold is oxidized and consumed greatly at 600 ° C. or more.

The organic polymer is composed of a CC bond as a skeleton, a CC bond in a side chain, and a CH bond bonded to a side chain. , The energy is transmitted to the C-C bond, and at each moment the vibration energy exceeds the bond energy, the bond is broken and released.
However, in general, bond breaking occurs everywhere, and polymers having various molecular weights are generated. In such a state, if the atmosphere is an inert gas or a vacuum, the once released polymers are arranged in a straight line, and they recombine to form a ring to form a benzene or other cyclic compound.
Further, the dehydrogenation reaction is further repeated to greatly condense and grow into carbon-rich soot. Once stabilized as soot, it becomes difficult to remove it by thermal decomposition. However, if oxygen atoms exist around the released polymer,
Carbon is converted into CO gas by reaction with oxygen and finally becomes stable CO ₂ , and hydrogen is converted into water vapor by reaction with oxygen and exhausted. For this purpose, the binder is removed in an oxidizing atmosphere.

The debinding treatment is not limited to a crucible or a mold made of a carbon-based material, but to a crucible or a mold made of silicon oxide such as silicon nitride, silicon carbide, or quartz. Is also effective.

The silicon is melted and solidified using the template debound by the above method. After the silicon is completely solidified, the silicon ingot is manufactured by gradually cooling so that no residual stress remains in the ingot. The silicon ingot that has completed the casting step is taken out of the mold and turned into a wafer through a cutting / slicing step that is a subsequent step.

[0026]

[Example] 8.7% to silicon nitride powder (purity 99% or more)
A slurry obtained by adding a PVA (polyvinyl alcohol) aqueous solution as a binder was applied to the inner surface of a mold as shown in FIG. 1 at a thickness of 4 mm, dried, and then placed in a furnace kept in an oxidizing atmosphere. The mixture was heated to 450 ° C. in 4 hours, kept at 450 ° C. for 4 hours, cooled to room temperature again, and taken out.

FIG. 2 shows the results of analysis of the thermogravimetry of the release material slurry before the binder removal treatment, and FIG. 3 shows the results of the analysis of the thermogravimetry of the release material slurry after the binder removal treatment (FIG. 2, FIG. In both cases, the TG curve (TG analysis (thermogravimetric analysis) measures the change in the weight of a substance caused by a change in temperature. By monitoring the change in the weight of a sample during heating, weight loss due to thermal decomposition, oxidation / adsorption The TG curve indicates a weight change curve of a sample during heating), which clarifies the weight increase due to the temperature, and what kind of reaction is occurring in which temperature range.))

As shown in FIG. 2, in the slurry of the release material before the binder removal treatment, a decrease in weight due to the thermal decomposition reaction of the binder is observed as the temperature increases. Here, since this weight loss is not observed at about 440 ° C. or higher, it is understood that the binder in the slurry is completely thermally decomposed near this temperature. That is, the absence of weight loss means that the decomposition reaction has been completed, and the temperature required to decompose PVA in the release material slurry is 44 ° C.
It means 0 ° C.

FIG. 3 shows that after performing the binder removal treatment under the above conditions, the release material on the inner surface of the mold is carefully removed from the mold.
It is the result of having analyzed thermogravimetry. As shown in FIG. 3, no weight reduction was observed, indicating that the binder in the release material slurry was completely removed by the debinding treatment. FIG. 3 shows the change in weight when the release material is heated at a constant speed, with the horizontal axis representing temperature and the vertical axis representing weight. FIG. 3 shows the same analysis result for the sample before the debubbling treatment.
No weight loss is seen compared to the weight loss curve of
Conversely, weight increase due to oxidation is observed. Accordingly, it can be seen that the binder was removed by the de-buying treatment.

Further, the release material subjected to the binder removal treatment forms a release material layer on the inner surface of the mold without returning to the powder. When casting silicon using this mold,
A very good mold was obtained as a mold for silicon casting without the silicon melt being fused with the mold.

[0031]

As described above, in the silicon casting method according to the present invention, in a silicon casting method in which a silicon melt is poured into a mold and solidified, silicon nitride, silicon carbide, or silicon oxide is formed on the inner surface of the mold. After applying a release material containing a main constituent substance and a binder composed of any one or more of the above and degreased at 450 to 600 ° C. in an oxidizing atmosphere, the silicon melt is poured into the mold. Since the solidification is performed, the binder in the slurry of the release material can be removed without depleting the carbon-based mold, and the release material layer has a carbon residue remaining in the release material, suppresses the amount of carbon residue remaining, and withstands silicon casting. A silicon casting mold can be used, and a good mold can be obtained to cast silicon.

[0032]

[Brief description of the drawings]

FIG. 1 is a view showing a mold used in a silicon casting method according to the present invention.

FIG. 2 is a diagram showing a thermogravimetric analysis result of a release material before a binder removal treatment in a silicon casting method according to the present invention.

FIG. 3 is a diagram showing a thermogravimetric analysis result of a release material after a binder removal treatment in the silicon casting method according to the present invention.

[Explanation of symbols]

 1: Mold, 2: Release material

Claims (3)

[Claims] 1. A silicon casting method in which a silicon melt is poured into a mold and solidified, wherein a main constituent substance of at least one of silicon nitride, silicon carbide and silicon oxide is formed on an inner surface of the mold. A mold casting method comprising applying a mold release material containing a mold and a binder, degreased the material at 450 to 600 ° C. in an oxidizing atmosphere, and then pouring the silicon melt into the mold to solidify it. 2. The silicon casting method according to claim 1, wherein the mold is made of a carbon-based material such as graphite or a graphite composite material. 3. The silicon casting method according to claim 1, wherein the binder is made of polyvinyl alcohol.