JP2002292449A - Casting mold for silicon casting and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that when a molten liquid of silicon is tapped into a casting mold, in solidification thereafter or in melting of a silicon raw material tapped in the casting mold, a mold releasing agent is released, the mold releasing agent is stuck to the casting mold to cause incapability of reusing the casting mold, an oxidation and ablation are caused when the mold releasing material is used. SOLUTION: In the casting mold for silicon coated with the mold releasing material on the inner surface of the casting mold, a primary material comprising a silicon nitride and an admixture, wherein the silicon nitride and silicon dinitride are mixed at the wt.% ratio of 28:72-75:25, are coated overlappingly as the mold releasing material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a casting mold for silicon and a method for producing the same, and more particularly to a casting mold for polycrystalline silicon for forming a solar cell and the like and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, polycrystalline silicon has been used as a kind of semiconductor substrate for forming a solar cell. Such polycrystalline silicon is formed by pouring a silicon melt heated and melted at a high temperature into a mold and solidifying it, or by putting a silicon raw material into a mold and once melting and then solidifying again. Is formed by

As such a mold, a mold in which a release material is applied to the inner surface of a splittable graphite mold is usually used, and the release material is silicon nitride (Si ₃ N ₄ ) which is a nitride of silicon. Is used. Generally, silicon nitride,
It is a known technique to mix powder such as silicon carbide and silicon oxide into a solution composed of an appropriate binder and a solvent and stir to obtain a slurry, which is then coated or sprayed on the inner wall of a mold. known as ^{(e.g., 15 th Photovoltaic Specialists Conf. (} 198
1), P576 ~ P580, "A NEW DIRECTIONAL SOLIDIFICATION
TECHNIQUE FORPOLYCRYSTALLINE SOLAR GRADE SILICON "
See).

[0004] However, when silicon nitride is applied to the inner surface of a graphite mold to cast silicon, the silicon nitride film is fragile. In addition, there has been a problem that the silicon nitride film is damaged, the silicon melt comes into contact with the mold, and the silicon ingot is chipped when the mold adheres to the silicon ingot and is released from the mold. In addition, there is a problem that the silicon nitride film is damaged when dissolving the silicon raw material put in the mold.

[0005] It has also been proposed to cast silicon dioxide by coating silicon dioxide (SiO ₂ ) on the inner surface of a graphite mold. However, when silicon dioxide is used as a mold release material, silicon dioxide adheres to graphite. In addition, since the ingot of silicon dioxide and silicon has good adhesion, silicon dioxide adheres to the mold and the mold cannot be reused, or the mold adheres to the silicon ingot through the release material. However, there is a problem that a part of the silicon ingot is chipped when the mold is removed.

In order to solve such a problem, Japanese Patent Application Laid-Open No. Hei 7-206419 discloses a method in which silicon dioxide is applied to a first layer, a mixture of silicon dioxide and silicon nitride is applied to a second layer, and a third layer is applied. It has been proposed to apply silicon nitride to the surface.

However, when the release material is applied in a three-layer structure as described above, it is necessary to prepare and apply the release material corresponding to each layer, and it takes time to apply and mix the release material. there were.

Accordingly, the present inventors have proposed a mold release material in which the mixing ratio of silicon dioxide and silicon nitride is optimized (Japanese Patent Application Laid-Open No. Hei 9-175809).

However, according to this technique, when the base material on which the release material is applied is a graphite material, the silicon dioxide in the release material and graphite are partially in contact with each other. There is a problem in that graphite is oxidized during the use of, and the damage to the base material is accelerated. This is most remarkable when applying a silicon dioxide single layer proposed for the purpose of increasing the contact force between the release material and the mold substrate.

In order to prepare a mold release material in the form of a slurry and apply it to a mold, a solvent such as water or alcohol, a binder for coating and forming, and an additive for improving fluidity are mixed appropriately. It is common to stir. PVA is the most widely used material in molding binders.
(Polyvinyl alcohol). Since PVA has excellent adhesiveness, it is suitable for adhesion and bonding of powder.

After molding (coating), degrease 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. PVA rapidly undergoes thermal decomposition at around 300 ° C. and is gasified to CO or the like. As a result, up to about 90% can be rapidly removed, but the remaining 10% can be easily heated to a temperature of 500 ° C. or more. Not removed
It often remains as carbon residue.

Further, when the release material is applied to a carbon-based release material, if the high-temperature degreasing is performed in an oxidizing atmosphere, the release material is oxidized. However, there is a problem that the manufacturing cost is increased.

On the other hand, when degreasing is carried out in an inert atmosphere, the thermal decomposition reaction of the organic polymer proceeds rapidly, so that hydrogen atoms are extracted and CH is arranged in a straight line, which becomes cyclic to form benzene and other benzene. It becomes a cyclic compound. In addition, the dehydrogenation reaction is repeated to greatly condense and grow into carbon-rich soot. Once stabilized as soot, it is difficult to remove it by thermal decomposition, so it comes into contact with the silicon melt while adhering to the mold release material or on the surface of the release material. Soot in contact with the melt or carbon dissolved in the melt not only degrades solar cell characteristics, but also often causes precipitation and causes poor processing when slicing the ingot. There are various organic binders replacing PVA, but none of them has both applicability and adhesiveness. In addition, there is no effective method that has sufficient strength as a release material, does not use an organic binder, and shortens the application time of the release material.

The present invention has been made in view of such problems of the prior art, and is intended to be used when pouring a silicon melt into a mold, during subsequent solidification, or using a silicon raw material placed in the mold. To provide a mold for silicon casting that suppresses oxidative consumption when the mold release material peels off when melted, the mold release material adheres to the mold and the mold cannot be reused, or the mold material is graphite. Aim.

Further, by coating the inner surface of the mold with a mixture of silicon nitride and silicon dioxide using a plasma spraying machine, chipping of silicon caused by the ingot of silicon adhering to the mold is prevented. It is another object of the present invention to provide a silicon casting method in which a debinding step for removing an organic binder, which has been conventionally used, is omitted, and the production cost of a silicon ingot is reduced.

[0016]

According to a first aspect of the present invention, there is provided a silicon casting mold in which a release material is applied to an inner surface of the mold.
A base material made of silicon nitride as the release material, and silicon nitride and silicon dioxide of 28:72 to 75:25
And a mixed material mixed at a weight ratio of 1.

In the above-mentioned mold for silicon casting, it is desirable to apply the mixed material at 0.04 g / cm ² or more.

In the above-mentioned silicon casting mold, it is preferable that the base material is applied in an amount of 0.04 g / cm ² or more.

In the above-mentioned silicon casting mold, it is preferable that the main body of the mold is made of graphite.

[0020] In the silicon casting mold according to claim 5,
In a silicon casting mold in which a mold release material is applied to the inner surface of a mold, a base material made of silicon nitride and silicon nitride and silicon dioxide are used as the mold release material in a ratio of 28:72 to 7: 7.
A mixed material mixed at a weight ratio of 5:25 and a surface material made of silicon nitride are applied in an overlapping manner.

In the above-mentioned silicon casting mold, it is desirable to apply the base material at 0.04 g / cm ² or more.

In the above-mentioned silicon casting mold, it is desirable that the mixed material is applied in an amount of 0.04 g / cm ² or more.

In the above-described silicon casting mold, it is preferable that the surface material is applied in an amount of 0.04 g / cm ² or more.

In the above-described silicon casting mold, it is preferable that the main body of the mold is made of graphite.

According to a tenth aspect of the present invention, in the method for manufacturing a silicon casting mold, a mixture of silicon nitride and silicon dioxide is formed by a plasma spraying method. In the method for producing a casting mold for silicon casting, it is preferable that the mixing ratio of the silicon nitride and the silicon dioxide is 1: 9 to 9: 1.

In the method for manufacturing a casting mold for silicon, the mixed material of silicon nitride and silicon dioxide preferably has a thickness of 20 μm or more.

In the method for manufacturing a silicon casting mold, it is preferable that the main body of the mold is made of graphite.

[0028]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a configuration of a vehicle according to an embodiment of the present invention; FIG. 1 is a view showing an example of a mold used for the silicon casting mold according to claims 1 and 5.

The mold 1 is made of, for example, graphite or the like, and is composed of a split mold that can be divided and assembled by combining one bottom member 1a and four side members 1b. In addition, the bottom member 1a and the side member 1b may be assembled in a dividable manner by fixing with a bolt (not shown) or the like, or may be fixed with a frame member (not shown) where the bottom member 1a and the side member 1b just fit. Assembled divisible by

On the inner surface of the mold 1, a release material 2 is applied so that the bottom member 1a and the side members 1b can be used repeatedly. As such a release material 2, a base material made of silicon nitride (Si ₃ N ₄ ) is applied, and then silicon nitride (Si ₃ N ₄ ) and silicon dioxide (SiO ₂ )
₂ ) A mixed material obtained by mixing 28) at a weight ratio of 28:72 to 75:25 is applied. The powder of silicon nitride and the powder of silicon nitride and silicon dioxide are mixed with an aqueous solution of polyvinyl alcohol and applied to the inner surface of the mold 1. By mixing silicon nitride and silicon dioxide with a polyvinyl alcohol aqueous solution or the like, the powdered silicon nitride and silicon dioxide are slurried, and the graphite mold 1
It becomes easy to apply.

As the powder of silicon nitride, 0.4 to
Those having an average particle size of about 0.6 μm are used.
As the silicon dioxide powder, a powder having an average particle size of about 20 μm is used. Such silicon nitride and silicon dioxide are mixed with an aqueous solution of polyvinyl alcohol having a concentration of about 5 to 15% by weight to form a slurry, and the slurry is applied to the inner surface of the mold 1 with a spatula or a brush. In this state, the silicon melt is poured into the mold 1 by air drying or drying on a hot plate.

The mixing ratio of the mixed material of silicon nitride and silicon dioxide in the release material 2 is 28:72 by weight.
Use the one adjusted between 75:25. However, as the weight ratio of silicon nitride approaches 28%, the reaction between the mold release material 2 and the mold 1 becomes remarkable, so that a base material made of silicon nitride is applied on the mold material 1 by 0.04 g / cm ² or more. Then, a mixed material of silicon nitride and silicon dioxide is applied thereon at 0.04 g / cm ² or more. Or not be maintained, the strength of the releasing agent layer 2 when the coating weight per unit area of the base material and the mixed material is less than 0.04 g / cm ^2, silicon dioxide in the mixed material as (SiO ₂₎ The graphite in the mold 1 is partially fused.

The pouring and solidification of the silicon melt are performed, for example, by coating the mold release material 2 on the inner surface of the mold 1 and drying it.
Was reduced to 7.0 to 9.0 Torr by argon (Ar).
The mold 1 is placed in an atmosphere, and the mold 1 is heated at a temperature approximately equal to or slightly lower than the silicon melt to pour the silicon melt. Alternatively, a silicon raw material may be placed in the mold 1 and directly dissolved. Thereafter, the temperature is gradually lowered from the bottom of the mold 1 to gradually solidify the silicon melt from the bottom of the mold. Finally, the mold 1 is divided and a silicon ingot is taken out to complete the casting.

Next, one embodiment of the silicon casting mold according to claim 5 will be described. In the present invention, a base material made of silicon nitride, a mixed material in which silicon nitride and silicon dioxide are mixed at a weight ratio of 28:72 to 75:25, and a surface material made of silicon nitride are applied in layers as the release material 2. A surface material made of silicon nitride is further applied on the base material and the mixed material at 0.04 g / cm ₂ or more to form an oxygen-resistant elution layer. If the amount of the surface material applied is small, the contact between the silicon melt and silicon dioxide in the mixed material becomes active, and the oxygen concentration in the melt increases. Oxygen in the melt is manifested as a precipitate in various processing steps in a device process after forming a wafer and affects the quality, so that the oxygen concentration is appropriately optimized.

Next, a method for manufacturing a silicon casting mold according to claim 10 will be described. A mold release material 2 is coated on the inner surface of the mold 1 by using a plasma spraying method. 2. Description of the Related Art A plasma spraying machine is an apparatus for forming a coating by feeding various powder materials into a plasma stream and spraying them. Thermal spray temperature is 3
The spray velocity of the sprayed particles reaches Mach 1 using a temperature range of about 10000 ° C. in a plasma stream extending to 2000 ° K. As a result, a very high-quality and dense coating layer is formed.

However, silicon nitride is about 1900
Sublimation decomposition at a temperature of 1 ° C. (1 atm, in N ₂ ) makes it impossible to form a coating layer by thermal spraying without forming a liquid phase by itself. Therefore, by mixing silicon dioxide, which forms a glass layer at a relatively low temperature, as a sintering aid, the silicon nitride powder is melted into the molten silicon dioxide and fused to the base material of the mold 1 to rapidly melt the powder. It is fixed by cooling.

For pouring and solidifying the silicon melt, for example, after the mold release material 2 is applied to the inner surface of the mold and dried, the mold 1 is placed in an argon (Ar) atmosphere reduced to 7.0 to 9.0 Torr. Then, the mold 1 is heated at a temperature approximately equal to or slightly lower than the silicon melt, and the silicon melt is poured. Alternatively, a silicon raw material may be placed in the mold 1 and directly dissolved. Thereafter, the temperature is gradually lowered from the bottom of the mold 1 so that the silicon melt is gradually solidified from the bottom of the mold 1. Finally, the mold 1 is divided and a silicon ingot is taken out to complete the casting.

[0038]

Example 1 A silicon nitride powder having an average particle diameter of 0.5 μm was weighed and stirred with an 8.7% aqueous solution of polyvinyl alcohol to obtain a release material in the form of a slurry. The release material was applied to the inner surface of a graphite mold with a brush, placed on a hot plate, and dried (base material). The coating weight per unit area was changed to 0, 0.02, 0.04, 0.2 g / cm ² . Thereafter, a release material obtained by weighing a silicon nitride powder having an average particle diameter of 0.5 μm and a silicon dioxide powder having an average particle diameter of 20 μm on an underlayer material and stirring and mixing with an 8.7% aqueous polyvinyl alcohol solution to obtain a slurry. Was applied with a brush and placed on a hot plate and dried (mixed material). After drying is completed, the mold is placed in an argon atmosphere depressurized to 8.0 Torr, and while being heated to 1000 ° C. using a graphite heater, 68 kg of a silicon melt is poured into the mold and gradually solidified over 7 hours. Was. The silicon ingot solidified after cooling was taken out of the mold, and the presence or absence of adhesion between the release material and the mold and the presence or absence of adhesion between the silicon ingot and the mold were examined. Table 1 shows the results.

[0039]

[Table 1]

As shown in Table 1, when there is no applied weight per unit area of the base material (condition Nos. 1, 5, 9, 13, 1)
7, 21, 25 and 29), oxidation of the mold material surface was observed under all conditions. When the application weight per unit area of both the base material and the mixed material is small (conditions No. 1, 5,
9 and 13), fusion of the melt to the mold material was observed, and it was found that the melt did not have a function as a release material. Further, when the application weight per unit area of the mixed material is small (condition No. 1).
In Nos. To 16), it was found that the release material peeled off from the mold, and there was a problem with the release material strength itself. In conclusion, the conditions described in claims 1 to 3 (No. 19, 20, 23, 2
4, 27, 28, 31, 32).

[0041]

Example 2 A silicon nitride powder having an average particle size of 0.5 μm was weighed and stirred and mixed with an 8.7% aqueous polyvinyl alcohol solution to obtain a slurry-like release material. The release material was applied to the inner surface of a graphite mold with a brush, placed on a hot plate, and dried (base material). The coating weight per unit area was 0.04 g / cm ² . Thereafter, a silicon nitride powder having an average particle size of 0.5 μm and an average particle size of 20
A μm silicon dioxide powder was weighed and stirred and mixed with an 8.7% aqueous polyvinyl alcohol solution to form a slurry. The release material obtained was applied by a brush, placed on a hot plate and dried (mixed material). Further, a release material obtained by weighing silicon nitride powder having an average particle size of 0.5 μm on one or two layers, stirring and mixing with an 8.7% aqueous solution of polyvinyl alcohol to obtain a slurry, and applying the release material with a brush, Placed on plate and dried (surface material). The coating weight per unit area of the surface material was changed to 0, 0.04, 0.1, 0.2 g / cm ² . After drying, the mold was placed in an argon atmosphere depressurized to 8.0 Torr, and the mold was heated to 100 Torr using a graphite heater.
While heating to 0 ° C., 68 kg of the silicon melt was poured into the mold and gradually solidified over 7 hours. After cooling, the solidified silicon ingot was taken out of the mold, and the presence or absence of adhesion between the mold release material and the mold and the presence or absence of adhesion of the silicon ingot and the mold were examined. Then, a solar cell was fabricated and the conversion efficiency was examined. The results are shown in FIG.

[0042]

[Table 2]

As is clear from FIG. 2 and Table 2, when the coating weight per unit area of the surface material is 0 g / cm ² , the conversion efficiency is obviously reduced, but the coating weight is 0.04 g / cm ² or more. Under the conditions, the conversion efficiency hardly decreases.
As a result, it has been found that it is desirable to use under the conditions described in claims 5 to 8.

[0044]

Example 3 Silicon nitride powder having an average particle size of 0.5 μm and silicon dioxide powder having an average particle size of 20 μm were weighed and mixed at a weight ratio of 1: 9 to 9: 1. It was coated on a graphite mold material. The film thickness was adjusted to be 10, 25, 40, and 75 μm. The mold was placed in an argon atmosphere depressurized to 8.0 Torr, and heated to 1000 ° C. using a graphite heater, 70 kg of a silicon melt was poured into the mold and gradually solidified over 7 hours. The silicon ingot solidified after cooling was removed from the mold, and the presence or absence of adhesion between the release material and the mold and the presence or absence of adhesion of the silicon ingot and the mold were examined. Table 3 shows the results.

[0045]

[Table 3]

Regardless of the mixing ratio of silicon dioxide, a coating layer having a thickness of 10 μm cannot be used because the mold material and silicon are fused. However, 25μ
With a thickness of m or more, it was found that a mixture of silicon nitride and silicon dioxide at a ratio of 1: 9 to 9: 1 was spray-coated to have a function as a charge-separation mold material. From the viewpoint of the adhesion between the coat layer and the release material, the best adhesion was obtained when the ratio of silicon nitride: silicon dioxide was 7: 3, but sufficient adhesion was obtained when the silicon dioxide ratio was 10% or more. It was found to have

[0047]

As described above, according to the silicon casting mold of the first aspect, the base material made of silicon nitride as the mold release material, and the silicon nitride and silicon dioxide are 28:
Since the mixed material mixed at a weight ratio of 72 to 75:25 is applied in a superposed manner, it is possible to prevent the release material from adhering to the mold and preventing the chipping of silicon caused by the mold adhering to the silicon ingot. And consumption of the mold material can be suppressed.

According to the silicon casting mold of the fifth aspect, the base material made of silicon nitride as the mold release material, and the silicon nitride and silicon dioxide are mixed in a ratio of 28:72 to 7: 7.
Since the mixed material mixed at a weight ratio of 5:25 and the surface material made of silicon nitride were applied in layers, the conversion efficiency of the solar cell caused by the contact between the silicon dioxide and the silicon melt in the mixed material was reduced. It is possible to prevent the lowering, and to prevent chipping of silicon caused by the release material adhering to the mold or the mold adhering to the ingot of silicon.

According to the method of manufacturing a silicon casting mold according to the tenth aspect, since a mixture of silicon nitride and silicon dioxide is applied by a plasma spraying method as a release material, the mold is made of a silicon ingot. In addition to preventing the chipping of silicon caused by adhering to the silicon, the debinding step of removing the organic binder used conventionally can be omitted, and the production cost of the silicon ingot can be reduced. .

[Brief description of the drawings]

FIG. 1 is a view showing an example of a silicon casting mold according to claim 1;

FIG. 2 is a view showing an experimental result of a silicon casting mold according to claim 5;

[Explanation of symbols]

 1 ... mold, 2 ... release material

Continued on the front page (72) Inventor Satoshi Kawamura 1166, Haseno, Snake-cho, Yokaichi City, Shiga Prefecture F-term in Kyocera Corporation's Shiga Yokaichi Plant (reference) 4E092 AA01 AA02 AA05 DA03 FA01 GA01 4E093 NB08

Claims (13)

[Claims] 1. A silicon casting mold in which a mold release material is applied to the inner surface of a mold, wherein a silicon nitride and silicon dioxide are mixed in a weight ratio of 28:72 to 75:25 as the mold release material. A mold for silicon casting, wherein the mixed material is applied in an overlapping manner. 2. The silicon casting mold according to claim 1, wherein the mixed material is applied in an amount of 0.04 g / cm ² or more. 3. The mold for silicon casting according to claim 1, wherein the base material is applied in an amount of 0.04 g / cm ² or more. 4. The mold for silicon casting according to claim 1, wherein the main body of the mold is made of graphite. 5. A silicon casting mold in which a mold release material is applied to an inner surface of a mold, wherein a silicon nitride and silicon dioxide are mixed in a weight ratio of 28:72 to 75:25 as the mold release material. A silicon casting mold, wherein a mixed material and a surface material made of silicon nitride are applied in a superposed manner. 6. The silicon casting mold according to claim 5, wherein the base material is applied in an amount of 0.04 g / cm ² or more. 7. The silicon casting mold according to claim 5, wherein the mixed material is applied in an amount of 0.04 g / cm ² or more. 8. The mold for silicon casting according to claim 5, wherein the surface material is applied in an amount of 0.04 g / cm ² or more. 9. The mold according to claim 5, wherein the main body of the mold is made of graphite.
The mold for silicon casting according to item 1. 10. A method for producing a mold for silicon casting, in which a mold release material is applied to the inner surface of a mold, wherein a mixture of silicon nitride and silicon dioxide is applied by plasma spraying as the mold release material. Manufacturing method of silicon casting mold. 11. The method of claim 10, wherein a mixing ratio of the silicon nitride and the silicon dioxide is 1: 9 to 9: 1. 12. The method according to claim 10, wherein a thickness of the mixed material of silicon nitride and silicon dioxide is 20 μm or more. 13. The method for manufacturing a silicon casting mold according to claim 10, wherein the main body of the mold is made of graphite.