JP2006326670A - Method for forming mold-released layer, and method for producing silicone ingot - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for forming a mold-released layer on a mold for forming a silicon ingot by which the mold-released layer can be formed on the inner surface of the mold at uniform thickness and strength by restraining the settling of ceramic particles contained in slurry with time. <P>SOLUTION: In the method for forming the mold-released layer 2 by coating and drying the slurry on the inner surface of the mold 1 for forming the silicon ingot, the slurry contains ceramic particles and a cross-linking assistant. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for forming a release layer on a silicon ingot forming mold and a method for manufacturing a silicon ingot.

  Solar cells convert incident light energy into electrical energy, and are expected to be put to practical use from small households to large-scale power generation systems as clean oil alternative energy sources. These are classified into crystalline, amorphous, and compound types depending on the type of materials used, and most of them currently on the market are crystalline silicon solar cells. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. Single crystal silicon solar cells have the advantage that conversion efficiency can be easily increased because the quality of the substrate is good, but the disadvantage is that the manufacturing cost of the substrate is high. On the other hand, the polycrystalline silicon solar cell has the advantage that the quality of the substrate is inferior to that of the single crystal silicon substrate, but it can be manufactured at a low cost. For this reason, polycrystalline silicon solar cells have been distributed in the market. However, in recent years, the demand has increased as interest in environmental problems has increased, and higher conversion efficiency is required at lower cost. In order to cope with such a demand, it is necessary to reduce the cost and quality of the polycrystalline silicon substrate, and it is required to manufacture a high-purity silicon ingot with a high yield.

  A polycrystalline silicon ingot is formed by pouring a silicon melt heated and melted at a high temperature into a mold and solidifying it unidirectionally from the bottom of the mold. It is formed by unidirectionally solidifying from the bottom of the mold.

  As such a mold, a mold in which a release layer is formed on the inner surface of a quartz or fused silica mold or a graphite mold is usually used. This is to prevent the silicon and the mold from being fused during the melting or solidification of the silicon. When silicon and the mold are fused, when the silicon ingot solidified in the mold is cooled, stress acts on the silicon ingot due to the difference in thermal shrinkage between the mold member and silicon, and the silicon ingot is cracked or demolded. There is a problem that the silicon ingot is chipped or the yield is lowered.

  In general, from the viewpoint of economy, the release layer is made by mixing ceramic particles such as silicon nitride and silicon dioxide into a solution composed of a solvent such as water or alcohol and an appropriate organic binder, and stirring the slurry. Is known to be applied to the inner surface of a mold.

  However, for example, when casting silicon by applying a slurry made of silicon nitride to the inner surface of the mold, the release layer made of silicon nitride is easy to peel off from the mold because of its weak adhesion to the mold, and the coating itself Since the strength is fragile and easily damaged, the silicon melt penetrates into the release layer, and the silicon nitride in the release layer dissolves in the silicon melt because it is in contact with the high temperature silicon melt for a long time. There was a problem. For this reason, when the silicon melt is poured into the mold, or when the silicon raw material is melted in the mold or the silicon melt is solidified, the release layer is damaged or the silicon melt enters the release layer. Since the silicon melt comes into contact with the mold and melts, when the silicon ingot solidified in the mold is cooled, stress is applied to the silicon ingot due to the difference in thermal shrinkage between the mold member and silicon. In some cases, the ingot is broken, the silicon ingot cannot be removed from the mold, and the silicon ingot is lost during the removal, resulting in a decrease in yield. Further, when the damaged release layer is peeled off from the mold and mixed into the silicon melt, it remains in the silicon ingot as a foreign substance, resulting in a problem that the yield of the silicon ingot is lowered.

In addition, silicon nitride is refined to an average particle size of 0.1 to 0.5 μm, and various dispersants and surfactants are added to the slurry to apply a slurry in which silicon nitride particles are uniformly dispersed. It has been proposed to improve the strength of the release layer and the adhesion to the mold (see, for example, Patent Document 1).
JP 2002-239682 A

  However, in the above method, although the silicon nitride is uniformly dispersed in the slurry at the beginning of the slurry formation, the precipitation of the silicon nitride with the passage of time cannot be sufficiently suppressed. The mold release layer formed by coating on the inner surface of the mold has a problem in that the thickness and strength vary, and the silicon melt and the mold are easily fused at a thin part or a damaged part of the mold release layer. Therefore, it is necessary to stir the precipitated silicon nitride by frequently stirring the slurry, and the silicon nitride once settled is difficult to disperse due to the strong connection between the particles, and it takes time to stir. There was a problem that workability deteriorated.

  The present invention has been made in view of such conventional problems, and it is possible to suppress the ceramic particles contained in the slurry from settling with the passage of time, and to provide a uniform release layer on the inner surface of the mold. An object of the present invention is to provide a method for forming a release layer on a silicon ingot-forming mold that can be formed with an appropriate thickness and strength, and a method for producing a silicon ingot.

  The mold release layer forming method of the present invention is a mold release layer forming method in which a slurry is applied to the inner surface of a silicon ingot forming mold and dried. The slurry contains ceramic particles and a crosslinking aid. It is characterized by.

  Here, the ceramic particles preferably contain silicon dioxide or silicon nitride, and the crosslinking aid is contained in a proportion of 0.05% by weight to 3% by weight with respect to the ceramic particles. Is preferred.

  The slurry is preferably applied by spraying.

  The method for producing a silicon ingot according to the present invention is a method for forming a silicon ingot by solidifying a silicon melt in a mold for forming a silicon ingot having a release layer formed on the inner surface by using the method for forming a release layer. It is characterized by manufacturing.

  The release layer forming method of the present invention is obtained by applying and drying a slurry on the inner surface of a silicon ingot forming mold, and the slurry contains ceramic particles and a crosslinking aid. Since the ceramic particles contained in the slurry can be prevented from settling with time, the release layer can be formed on the inner surface of the mold with a uniform thickness and strength. As a result, the fusion between the silicon melt and the mold can be suppressed.

  Further, since the ceramic particles are crosslinked in the slurry, even when the ceramic particles settle, the ceramic particles can be easily dispersed by stirring for a relatively short time.

  Here, it is preferable that the ceramic particles are made of silicon dioxide or silicon nitride, which can suppress the mixing of impurities from the release layer into the silicon ingot, improve the releasability, and improve the silicon dioxide. In addition, it is possible to suppress the formation of hard coarse particles due to bonding between particles in silicon nitride.

  Further, the crosslinking aid is preferably contained in a proportion of 0.05% by weight or more and 3% by weight or less with respect to the ceramic particles, and thereby, the sedimentation of the ceramic particles can be more effectively suppressed. It becomes.

  Furthermore, the slurry is preferably applied by spraying from the viewpoint of film uniformity of the release layer.

  Further, the method for producing a silicon ingot according to the present invention comprises the step of forming a silicon ingot by solidifying a silicon melt in a mold for forming a silicon ingot having a release layer formed on the inner surface by using the method for forming a release layer. Therefore, since the fusion between the silicon melt and the mold can be effectively suppressed by the release layer having high uniformity, it is possible to manufacture a highly reliable silicon ingot with high productivity.

  Hereinafter, embodiments of the present invention will be described in detail. FIG. 1 is a view showing an example of a mold in which a release layer is formed by the release layer forming method according to the present invention. The mold 1 is made of, for example, graphite, quartz, silica, ceramic, or the like, and is configured by an integrally molded mold or a divided mold that can be assembled separately. A release layer 2 is formed on the inner surface of the mold 1 in order to prevent the silicon melt from fusing with the mold 1.

  The release layer 2 according to the present invention is prepared by mixing and stirring ceramic particles made of silicon nitride, silicon dioxide, silicon carbide or the like in a solution composed of an organic binder and a solvent, and forming a slurry therein. Containing. Then, the produced slurry is applied and dried on the inner surface of the mold 1 using a spatula, a brush, a spray method or the like to form the release layer 2.

  Thus, it becomes possible to suppress sedimentation of the sedimentation of ceramic particles by including a crosslinking aid in the slurry. As this mechanism, the cross-linking aid cross-links between the ceramic particles in the slurry to form weak aggregates (floc) due to weak bonding between the ceramic particles, and the flocs are likely to be trapped in the slurry. Is considered to be suppressed. Therefore, the release layer 2 formed by applying such a slurry to the inner surface of the mold 1 suppresses variations in thickness and strength, and the silicon melt and the mold 1 are formed in a thin portion or a damaged portion of the release layer 2. Can be prevented from being easily fused. Furthermore, even when this floc settles, it can be easily recovered to a separated state by stirring for a short time compared to the conventional case. The problem that time is shortened and workability | operativity deteriorates can be suppressed. As the crosslinking aid, polyethylene glycol, N-vinyl acrylamide, divinyl benzene, glycidyl acrylate, or the like is used.

  In addition, the organic binder is used for moldability that strongly bonds particles after coating and holds them together. For example, polyvinyl alcohol, polyvinyl butyral, methyl cellulose, carboxymethyl cellulose, ethyl cellulose, hydroxypropyl cellulose, wax Are generally used.

When polyvinyl alcohol is used as the organic binder and polyethylene glycol is used as the crosslinking aid, the effects of the present invention could be remarkably confirmed. From the viewpoint of suppressing contamination of the silicon ingot with silicon dioxide, Silicon nitride is preferable, and it is more preferable to use mixed particles of silicon dioxide and silicon nitride in consideration of releasability. In addition, silicon dioxide and silicon nitride tend to aggregate and bind strongly, and as a result, hard coarse particles are easily formed. Further, such coarse particles have a weak cohesive force in the release layer 2 and have poor adhesion to the mold 1 and are easy to peel off. Therefore, the release layer 2 is peeled off and mixed into the silicon melt. There was a problem of becoming a foreign object. However, the problem could be effectively suppressed by adding a crosslinking aid to the slurry. Further, particles having an average particle diameter of silicon dioxide of about 0.1 to 40 μm and particles having an average particle diameter of silicon nitride of about 0.1 to 5 μm are used.

  In addition, when the slurry mixed with the aqueous organic binder solution is coated on the inner surface of the mold 1, a degreasing treatment may be performed in order to prevent the thermally decomposable product of the organic binder from being mixed into the silicon melt by subsequent heating. preferable. Moreover, as a drying method, degreasing is performed by natural drying or drying using a conventionally known method such as a hot plate or an oven.

  The crosslinking aid is preferably contained in an amount of 0.05 to 3% by mass with respect to the weight of the ceramic particles in the slurry. By containing the crosslinking aid in the above range, flocs of the ceramic particles are easily formed, and sedimentation of the ceramic particles can be suppressed. When the content of the crosslinking aid is less than 0.05% by weight, the effect of suppressing the sedimentation of the ceramic particles is small. When the content is more than 3% by weight, the viscosity of the slurry increases, so that the workability of the coating work is poor. In addition, it is not preferable because it may cause impurity contamination of the silicon ingot.

  Moreover, it is preferable to perform application | coating of the slurry of this invention by the spray method. By applying the slurry to the inner surface of the mold 1 using a spray device, more uniform application is possible. In addition, since the slurry of the present invention suppresses the settling of ceramic particles and further suppresses the formation of hard coarse particles, it is possible to always apply a uniform slurry without any problem of clogging at the tip of the spray nozzle.

  A mold release layer 2 is formed on the inner surface of the mold 1, and then the mold 1 is heated at a temperature similar to or slightly lower than that of the silicon melt in an argon (Ar) atmosphere reduced to, for example, 7 to 100 Torr. Pour the melt. Alternatively, a silicon raw material may be put into the mold 1 and heated and melted. 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 1 in one direction. Finally, the silicon ingot is formed by removing the silicon ingot from the mold 1.

  The silicon ingot produced by using the mold 1 coated with the slurry of the present invention on the inner surface has a uniform release layer 2 and thus has a higher strength than the conventional one, so that the problem that the release layer 2 is peeled off is suppressed. Thus, it is possible to suppress problems such as the silicon ingot being fused to the mold 1 and the member of the release layer 2 being mixed into the silicon ingot. Furthermore, since it is a uniform film, it is possible to manufacture a silicon ingot having excellent unidirectionality by suppressing variations in heat entering and leaving the mold 1.

  It should be noted that the embodiment of the present invention is not limited to the above-described example, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

  For example, a ball mill or a vibration mill may be used for the slurry. The agglomeration of the ceramic particles can be crushed, and by subsequently containing a crosslinking aid, the sedimentation of the ceramic particles can be more effectively suppressed, which is suitable for spray application of the slurry to the mold 1.

  The silicon nitride may be silicon nitride that has been subjected to oxidation modification treatment. When the thickness of the oxide film on the silicon nitride surface is increased, silicon nitrides are bonded or fused together at the oxide film layer portion to aggregate, resulting in the formation of hard coarse particles. Therefore, the formation of hard and coarse particles can be suppressed.

  In order to confirm the effect of the method for forming a release layer of the present invention, the following experiment was conducted.

  A mixture of silicon nitride having an average particle diameter of 1.0 μm, silicon dioxide having an average particle diameter of 20 μm, and 4.0% polyvinyl alcohol in a weight ratio of 55:35:80 was crushed by a ball mill for about 3 hours, A slurry was prepared by adding and stirring 1% by mass of polyethylene glycol with respect to the slurry weight.

This slurry was applied to the mold 1 on a hot plate using a spray device and dried. At this time, the clogging at the tip of the spray nozzle was investigated, and the discharge amount from the spray device was measured every 10 minutes. The results are shown in Table 1.

  As shown in Table 1, it can be seen that the use of the slurry containing the crosslinking aid is better than the case where there is no crosslinking aid. This is presumably because the slurry containing no cross-linking aid clogged the spray nozzle because hard coarse particles were formed due to particle aggregation. However, since the slurry containing the crosslinking aid forms an aggregate (floc) in which the particles are fragile, the formation of hard coarse particles can be suppressed and smooth application can be performed.

Next, 30 molds 1 each having a release layer 2 formed by the above method were prepared, placed in an argon atmosphere reduced to 8.0 Torr, and heated in a mold 1 in a state heated with a graphite heater. Was poured and gradually solidified. After cooling, the solidified silicon ingot was taken out from the mold 1 and examined for the presence or absence of fusion (adhesion) between the mold 1 and the silicon ingot. The results are shown in Table 2.

  As shown in Table 2, the slurry containing the crosslinking aid did not show any fusion between the mold 1 and the silicon ingot, whereas the slurry containing no crosslinking aid fused the mold 1 and the silicon ingot. I found out that wearing would happen.

It is a figure which shows an example of the casting mold for silicon casting, (a) is a perspective view, (b) is sectional drawing which shows the state cut | disconnected by the AA surface in (a).

Explanation of symbols

1 ... mold 2 ... release layer

Claims (5)

A method for forming a release layer comprising applying and drying a slurry on the inner surface of a mold for forming a silicon ingot,
The slurry comprises ceramic particles and a crosslinking aid, and is a method for forming a release layer. The method for forming a release layer according to claim 1, wherein the ceramic particles are made of silicon dioxide or silicon nitride. The method for forming a release layer according to claim 1 or 2, wherein the crosslinking aid is contained in a proportion of 0.05% by weight or more and 3% by weight or less with respect to the ceramic particles. 4. The method for forming a release layer according to claim 1, wherein the slurry is applied by spraying. A silicon ingot is manufactured by solidifying a silicon melt in a mold for forming a silicon ingot having a release layer formed on the inner surface thereof, using the method for forming a release layer according to any one of claims 1 to 4. A method for producing a silicon ingot, comprising:

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