JP2005104743A - Silicon casting mold - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a casting mold for avoiding a silicon ingot from cracking by avoiding contact between a silicon melt and a mold when silicon is cast by coating a mold release material layer on an inner surface of the mold.
A mold for silicon casting in which a mold release material in which silicon and silicon dioxide are mixed is coated on the inner surface of the mold, and a mold release material layer at the upper part of the inner surface of the mold is made thicker than a mold release material layer at the lower part. A silicon casting mold characterized by the above.
[Selection] Figure 1

Description

  The present invention relates to a silicon casting mold and a method for manufacturing the same, and more particularly to a polycrystalline silicon casting mold for forming a solar cell or the like.

  Conventionally, polycrystalline silicon has been used as a kind of semiconductor substrate for forming solar cells. Such polycrystalline silicon can be formed by pouring and solidifying a silicon melt that has been heated and melted at a high temperature in a mold, or by solidifying the silicon raw material once dissolved in the mold. Is formed by.

As such a mold, those in which a mold release material is coated on the inner surface of a mold made of graphite that is separable or those in which a mold release material is coated on the inner surface of a mold made of silica that is an integral structure are usually used. . In general, as the release material, silicon nitride (Si ₃ N ₄ ), which is silicon nitride, silicon carbide (SiC), which is silicon carbide, silicon dioxide (SiO ₂ ), which is an oxide of silicon, or the like is used. It is known as a known technique that these powders are mixed in a solution composed of an appropriate binder and solvent and stirred to form a slurry, and the inner wall of the mold is coated by means such as coating or spraying. (For example, refer nonpatent literature 1).
15th Photovoltaic Specialists Conf. (1981), P576-P580, "A NEW DIRECTIONAL SOLIDIFICATION TECHNIQUE FOR POLYCRYSTALLINE SOLAR GRADE SILICON"

  However, in the case where silicon nitride is coated on the inner surface of a graphite mold and silicon is cast, the release material layer has a porous structure, so that the silicon melt penetrates the release material layer and comes into contact with the mold. There was a problem that the silicon ingot was broken when demolding. Therefore, it is necessary to prevent the silicon ingot from cracking by covering the release material layer thickly to avoid contact between the silicon melt and the mold. Further, in order to reuse the graphite mold, it is necessary to avoid contact between the silicon melt and the mold, and a method of thickening the release material layer has been used. However, when the release layer is coated thickly, the fragile silicon nitride film is broken into the silicon melt when the silicon melt is poured and when the silicon melt is subsequently solidified. There was a problem.

  In addition, when silicon nitride is coated on a mold made of quartz and silicon is cast, in order to demold the silicon ingot by breaking the quartz mold, the release layer is thickened in consideration of reuse. There is no need to coat, but as with graphite molds, the silicon ingot breaks due to the contact between the silicon melt and the mold, and depending on the size of the crack, the good silicon ingot becomes smaller than the desired size. Has occurred. For this reason, as in the case of the graphite mold, a countermeasure method for covering the release material layer thickly is used, but there is a problem similar to that of the graphite mold in which the silicon nitride film is broken and mixed into the silicon melt.

  In general, the part where the mold release material layer and the silicon ingot are in contact with each other is used when the quality of the silicon ingot is poor compared to the part that is not in contact and a high-quality solar cell is required. Because it was not done, it was possible to avoid the problem that a good silicon ingot would be small by taking a large excision area, but in the case of an excellent quality silicon ingot, it was said that the area that does not need to be excised should be excised There was waste.

In addition, there is a method of preventing the release material layer from being damaged by coating a release material mixed with silicon dioxide (SiO ₂ ) that increases the adhesive force between the release material and the mold base material. When the weight ratio of (SiO ₂ ) increases, the mold release layer and the mold adhere to each other, so that the mold cannot be reused. When the weight ratio of silicon dioxide decreases, the mold release material layer breaks and the silicon melt The same problem of mixing in occurred.

  The present invention has been made in view of such problems of the prior art, and avoids contact between the silicon melt and the mold when casting silicon by coating the mold release material layer on the inner surface of the mold. An object of the present invention is to provide a casting mold that prevents cracking of an ingot.

  In addition, when casting the silicon melt into the mold, solidifying afterwards, or when melting the silicon raw material put in the mold, the silicon casting that suppresses the fall of the damaged release material into the silicon melt An object is to provide a casting mold.

  In order to achieve the above object, a silicon casting mold according to the present invention is a silicon casting mold in which an inner surface of a mold is coated with a release material in which silicon and silicon dioxide are mixed, and is formed on the upper surface of the inner surface of the mold. The release material layer is thicker than the release material layer in the lower part.

  The silicon casting mold of the present invention is a silicon casting mold in which the inner surface of the mold is coated with a release material in which silicon and silicon dioxide are mixed, and includes an upper release material layer and a lower release material layer. An intermediate release material layer that is thicker than these release material layers is provided therebetween.

  In the silicon casting mold of the present invention, the release material has a mixing ratio of silicon nitride and silicon dioxide of 10: 0 to 1: 9.

Further, in the silicon casting mold of the present invention, the density of the release material after drying is 1 to 2 g / cm ³ , and the thickness of the thickened part of the release material layer is 0.5 mm or more. And

Further, in the silicon casting mold of the present invention, the density of the release material after drying is 2 to ³ g / cm ³ , and the thickness of the thickened part of the release material layer is 0.15 mm or more. And

  In the silicon casting mold of the present invention, it is preferable that the main body of the mold is made of graphite or silica.

  Further, the graphite is better if it is a carbon fiber reinforced material.

  The silicon casting mold according to claim 1 is a silicon casting mold in which an inner surface of the mold is coated with a release material in which silicon and silicon dioxide are mixed, and a release material layer on an upper portion of the inner surface of the mold Is made thicker than the release material layer in the lower portion, so that it is possible to prevent the silicon from being cracked by the mold adhering to the silicon ingot.

  The silicon casting mold according to claim 2 is a silicon casting mold in which a mold release material in which silicon and silicon dioxide are mixed is coated on the inner surface of the mold. Since the part in which the release material layer is thicker than the release material layer is provided, it is possible to prevent silicon cracking caused by the mold adhering to the silicon ingot. Moreover, it is possible to prevent the release material from dropping into the silicon ingot, and to reduce the number of defective foreign matters.

  Hereinafter, the invention according to each claim will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram showing a mold used for a silicon casting mold according to the present invention, FIG. 2 is a diagram showing an example of a silicon casting mold according to the present invention, and FIG. 3 is another silicon casting mold according to the present invention. It is a figure which shows an example of a casting_mold | template.

  In FIG. 1, a mold 1 is made of, for example, graphite, and is composed of a divided mold that can be assembled and assembled by combining one bottom member 1a and four side members 1b.

  The bottom member 1a and the side member 1b are assembled so as to be separable by fixing with bolts (not shown) or the like, or fixed with a frame member (not shown) in which the bottom member 1a and the side member 1b just fit. Is assembled in a separable manner.

A mold release material 2 is coated on the inner surface of the mold 1 so that the bottom member 1a and the side member 1b can be used repeatedly. As such a mold release material 2, silicon nitride (Si ₃ N ₄ ) powder and a polyvinyl alcohol aqueous solution are mixed to coat the inner surface of the mold 1. By mixing silicon nitride with an aqueous polyvinyl alcohol solution or the like, the powdered silicon nitride becomes a slurry and can be easily coated on the graphite mold 1.

  As the silicon nitride powder, one having an average particle diameter of about 0.4 to 0.6 μm is used, and mixed with such silicon nitride and a polyvinyl alcohol aqueous solution having a concentration of about 5 to 15% by weight. The inner surface of the mold 1 is covered with a spatula or a brush. In this state, the silicon melt 3 is poured into the mold 1 after natural drying or drying on a hot plate and drying.

  The inner surface of the mold 1 can be coated with the release material 2 by mixing a mixture of silicon nitride and silicon dioxide powder using a plasma spraying machine. Normally, when coating a release material that is made by mixing powder and an organic binder aqueous solution such as polyvinyl alcohol into a slurry, the heat decomposable product of the organic binder is mixed into the silicon melt by subsequent heating. A degreasing treatment is performed to prevent this.

  In plasma spraying, since the spraying temperature is about 10000 ° C. in a plasma stream extending to 32000 ° K, the spray particles are sprayed and coated, so that the step of removing the organic binder that has been conventionally used is omitted. be able to.

  For the pouring and solidification of the silicon melt 3, for example, the inner surface of the mold 1 is coated with the release material 2 and dried, and then the mold 1 is placed in an argon (Ar) atmosphere reduced to 70 to 90 Torr. The silicon melt is poured by heating at a temperature that is the same as or slightly lower than that of the silicon melt 3. Alternatively, a silicon raw material may be placed in the mold 1 and dissolved directly. Thereafter, the temperature is gradually lowered from the bottom of the mold 1 to gradually solidify the silicon melt 3 from the bottom of the mold. Finally, the mold 1 is divided to complete the silicon ingot.

  However, when the amount of the slurry coating consisting of silicon nitride and polyvinyl alcohol is small, the release material layer 2 has a porous structure, so that the silicon melt 3 penetrates into the release material layer 2 and comes into contact with the mold. There is a problem that the silicon ingot breaks when demolding. When the temperature of the release material layer is particularly high, the surface tension of the silicon melt 3 is reduced and wettability is increased, so that the penetration of the silicon melt 3 into the release material layer is likely to occur.

  In order to solve this problem, in the silicon mold according to claim 1 of the present invention, as shown in FIG. 2, the release material layer at the upper part of the inner surface of the mold is made thicker than the release material layer at the lower part. At this time, the silicon melt is injected so as to be at the same level as the lowest position 7 of the release material layer that has been thickened. In the position below the silicon melt position 4, the contact between the release material layer 2 and the silicon melt 3 is maintained at around 1412 ° C., which is the melting point of silicon, and the release material layer 2 is not extremely thin. Can be released without any problem.

  When the silicon melt is left in the solidified state of the silicon melt surface due to large heat removal from the silicon melt surface, the silicon ingot erupted due to expansion when the remaining silicon melt solidified It will be in a state and a crack will arise in a silicon ingot. In order to prevent this problem, the silicon melt 3 is solidified by removing heat from the lower part of the mold while being heated by a heater located above the silicon melt 3 so as not to solidify the surface of the silicon melt. The method is taken.

  Therefore, in the part which hits the upper part from the silicon melt position 4, the release material layer 2 is heated by the heater located at the upper part. When the release material layer is heated, sublimation decomposition proceeds under high temperature and low pressure conditions, so that the release material layer thickness is reduced and the silicon melt and the mold are easily brought into contact with each other. In addition, when the melted silicon melt is heated by capillary action and the wet melt is heated, the surface tension of the melt decreases and the wettability increases. As a result of easy penetration, the silicon melt and the mold come into contact with each other, so that the melt is infiltrated into the release material layer. This results in cracking. Therefore, in the silicon casting mold according to claim 1 of the present invention, as shown in FIG. 2, the release material layer on the upper part of the inner surface of the mold is made thicker than the release material layer on the lower part. At this time, the silicon melt is injected so as to be at the same level as the lowest position 7 of the release material layer that has been thickened. Compared with the case where the whole is thickly coated in order to thicken the portion corresponding to the upper portion, not only the cost of the release material can be reduced, but also the coating time can be shortened.

  Further, in the silicon casting mold according to claim 1, when the silicon melt is injected to a position higher than the lowest position 7 of the release material layer having been thickened, the silicon nitride film is broken and enters the silicon melt. The effect of reducing the cost of the release material and shortening the coating time can be obtained while avoiding the problem of mixing.

  Further, preferably, like the silicon casting mold according to claim 2, a part where the release material layer is thicker than the upper and lower release material layers is provided on the inner surface of the mold (FIG. 3).

The density of the silicon melt is about 2.55 g / cm ^3, density of the silicon ingot is about 2.33 g / cm ^3, from about 9% volume expansion occurs when the transition from liquid to solid. Therefore, the silicon melt surface rises with the solidification volume. The silicon melt and the mold are most likely to come into contact with each other when the release material layer thickness is reduced under high temperature and low pressure conditions. This is when the surface tension of the liquid is reduced and the wettability is increased, so that the silicon melt at the spotted and wetted portion easily penetrates into the release material layer. Therefore, it is most preferable to thicken the parting material layer in this part, and it is sufficient to thicken the parting material layer where the silicon melt rises by 9% due to solidification expansion, and the part that gets wet and gets wet. Since it differs depending on the atmosphere, the type of release material, etc., it is considered desirable to make the release material layer thicker in the region of the silicon melt height of about 10% to 13% than the silicon melt position before solidification.

  Since only the part that really needs to be thickened is coated by thickening the parting material layer, the cost of the parting material and the coating time can be shortened as compared with the silicon casting mold according to claim 1. Moreover, the fall of the foreign material in the silicon melt due to breakage of the release material can be reduced.

  In consideration of demolding, melting into the silicon melt, and mixing, it is preferable that the mixing ratio of silicon nitride and silicon dioxide is 10: 0 to 1: 9. When the mixing ratio is 10: 0, it means that the release material is made of silicon nitride.

When the density of the release material is reduced, the release material has a more porous structure, so that the silicon melt easily penetrates the release material layer, and the silicon melt and the mold come into contact with each other. In order to prevent contact between the silicon melt and the mold, when the density of the release material is 1 to 2 g / cm ³ , 0.3 mm or more, and when the density of the release material is 2 to ³ g / cm ³ , 0. A thickness of 05 mm or more is required at a minimum even in a thin portion of the lower release material layer.

In contrast, in the portion where it is necessary to increase the upper part of the releasing agent layer, the density of the releasing member is 0.5mm or more when the 1 to 2 g / cm ^3, the density of the releasing agent is 2 to 3 g / cm ³ In some cases, it is preferable to cover a release material of 0.15 mm or more. When the coating amount is small, contact between the silicon melt and the mold tends to occur.

  If the release material layer is sufficiently thick below the position of the silicon melt before the start of solidification, it is not necessary to increase the thickness of the release material layer at the upper part of the inner surface of the mold, but this leads to an increase in the cost of the release material. Moreover, when the release material layer becomes thick, it results in melting or mixing into the silicon melt.

  Although the method for increasing the thickness of the release material layer at the required part on the inner surface of the mold has been described, multiple types of release materials can be used as long as the release material conditions (mixing ratio, density, required thickness) are satisfied. It can also be used.

  A silicon nitride powder having an average particle size of 0.5 μm and a silicon dioxide powder having an average particle size of 20 μm are weighed and mixed in a weight ratio of 10: 0 to 1: 9, and then the mixed powder and 8.7% polyvinyl alcohol. Three types of release materials that were stirred and mixed with an aqueous solution to form a slurry were obtained. The release material was coated on the inner surface of the graphite mold with a brush, placed on a hot plate and dried. As shown in Table 1, with reference to the position of the silicon melt before solidification, silicon that accompanies the solidification and expansion of silicon in a portion that hits the lower part of the melt position, a part that hits the upper part, and a part that hits the upper part of the silicon melt position before the start of solidification A sample in which the thickness of the release material layer is changed at the part where the melted surface rises and the part that rises (from the silicon melt height before solidification starts to 10-13% of the silicon melt height above) Produced.

After completion of drying, the mold was placed in an argon atmosphere reduced to 80 Torr, and 70 kg of silicon melt was poured into the mold while being heated to 1000 ° C. using a graphite heater, and was gradually solidified over 8 hours. After cooling, the solidified silicon ingot 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. The removed silicon ingot was cut and sliced, and the finished wafer was visually inspected to confirm the presence or absence of foreign matter. The results are shown in Table 1.

  As shown in Table 1, in the samples (samples 1 and 2) having the same thickness of the mold release material at the upper and lower portions of the silicon melt position before the start of solidification from the mold release material type A, the sample with the thick release material layer (sample 2) ) Does not adhere to the silicon melt and the mold, but adheres to the sample (sample 1) with a thin release material layer. From this, it can be seen that a sample having a thin release material layer does not have a function as a release material that suppresses adhesion between the silicon melt and the mold. In the sample with the thick release material layer (Sample 2), there is no adhesion between the silicon melt and the mold, but when the release material layer becomes thick, the number of defective foreign matters increases, and the release material is damaged and is in the silicon melt. You can see that it is falling. These can also be understood from the result of the mold release material type C.

  In the sample (samples 3 and 4) where the release material layer in the upper part from the position of the silicon melt before the start of solidification is thick (sample 4), the sample in which the release material layer in the upper part is thin (sample 4) adheres to the silicon melt and the mold. However, it can be seen that the sample with the thick release material layer (sample 3) has no adhesion between the silicon melt and the mold. It can also be seen that Sample 3 has a smaller number of foreign matter defects than the sample (Sample 2) coated with the same thickness at both the upper and lower positions of the silicon melt position. This can also be understood from the result of the mold release material type C.

  In addition, a sample (sample 3) where the release material layer is thicker than the sample (sample 3) where the release material layer is thicker than the silicon melt position before the start of solidification. It can be seen that in 5), the number of defective foreign matters is reduced, and the breakage and dropping of the release material into the silicon melt is reduced. This can be seen from the release material types B and C.

From the above results, it was found that it is desirable to use the conditions described in claims 1 to 5 as a conclusion.

It is a schematic diagram which shows the casting mold for silicon which concerns on this invention. It is a figure which shows an example of the casting mold for silicon which concerns on this invention. It is a figure which shows an example of the casting_mold | template for another silicon casting which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Mold 2 ... Release material 3 ... Silicon melt 4 ... Silicon melt position 5 before the start of solidification ... Rising part of silicon melt surface accompanying silicon solidification expansion 6 ... -The part above the silicon melt surface where the silicon melt soaks and gets wet by capillary action
7 ... Position of the lowest part of the thickened release material layer 8 ... Position of the uppermost part of the thickened release material layer

Claims (7)

A mold for silicon casting in which a mold release material mixed with silicon and silicon dioxide is coated on the inner surface of the mold, wherein the mold release material layer at the upper part of the inner surface of the mold is made thicker than the mold release material layer at the lower part. Silicon mold for casting. A mold for silicon casting in which the inner surface of a mold is coated with a release material in which silicon and silicon dioxide are mixed, and between the upper release material layer and the lower release material layer, compared to each of these release material layers A silicon casting mold characterized in that a thick intermediate release material layer is provided. The silicon casting mold according to claim 1 or 2, wherein a mixing ratio of silicon nitride and silicon dioxide in the release material is 10: 0 to 1: 9. The density after drying of the mold release material is 1 to 2 g / cm ³ , and the thickness of the thickened part of the mold release material layer is set to 0.5 mm or more. The mold for silicon casting as described. The density after drying of the mold release material is 2 to ³ g / cm ³ , and the thickness of the thickened part of the mold release material layer is 0.15 mm or more. The mold for silicon casting as described. 6. The silicon casting mold according to claim 1, wherein the main body of the mold is made of graphite or silica. The silicon casting mold according to claim 6, wherein the graphite is made of a carbon fiber reinforced material.

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