JP4340124B2 - Silicon casting equipment - Google Patents

Description

  The present invention relates to a silicon casting apparatus, for example, a polycrystalline silicon casting apparatus used for manufacturing a semiconductor substrate for a solar cell.

  Solar cells are expected to be put to practical use in a wide range of fields, from small households to large-scale power generation systems, as a clean petroleum alternative energy source. These are classified into crystalline, amorphous, and compound types depending on the type of raw 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 is easy to increase 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, polycrystalline silicon solar cells have been distributed in the market for a long time, but in recent years, the demand is increasing as interest in environmental issues is increasing, 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 produce a high-purity silicon ingot with a high yield.

  A polycrystalline silicon substrate used for a polycrystalline silicon solar cell is generally manufactured by a method called a casting method. In this casting method, a silicon melt melted by heating at a high temperature is poured into a mold made of graphite or the like coated with a release material and solidified in one direction from the bottom of the mold, or a silicon raw material is placed in the mold and dissolved once. Then, the silicon ingot is formed by unidirectionally solidifying from the bottom again. The ends of the silicon ingot are removed, cut into a desired size, and the cut silicon ingot is sliced to a desired thickness to obtain a polycrystalline silicon substrate for forming a solar cell.

  FIG. 5 shows a general silicon casting apparatus for producing such a polycrystalline silicon ingot.

  A melting crucible 10 for melting the raw material silicon 12 is disposed on the upper portion and held by a holding crucible 11, and a tapping opening 13 for pouring silicon melt is provided at the bottom of the melting crucible 10 and the holding crucible 11. . Further, heating means 14 and 15 are disposed on the upper and side portions of the melting crucible 10 and the holding crucible 11, respectively, and the mold 1 into which the silicon melt 4 is poured is disposed on the lower portion of the melting crucible 10 and the holding crucible 11, The mold heat insulating material 3 is provided on the outside. Further, a cooling plate 9 is provided below the mold 1, and a mold heating means 8 for controlling the solidification of the silicon melt 4 is disposed above the mold 1.

  For example, a silicon raw material put in a melting crucible 10 made of high-purity quartz or the like is heated and melted by upper and side heating means 14 and 15 made of a resistance heating type heater, an induction heating type coil, etc. It is melted and poured into the mold 1 at the bottom from the bottom outlet 13.

The mold 1 is made of, for example, graphite, and is composed of, for example, a divided mold in which one bottom member 1a and four side members 1b are combined and assembled. The release material layer 2 is made of powder such as silicon nitride (Si ₃ N ₄ ), which is silicon nitride, silicon carbide (SiC), which is silicon carbide, or silicon oxide (SiO ₂ ), which is an oxide of silicon. It is known as a known technique that these powders are mixed in a solution composed of an appropriate binder and a solvent, stirred to form a slurry, and coated on the inner wall of the mold by means such as coating or spraying ( For example, refer nonpatent literature 1). The mold heat insulating material 3 suppresses heat removal and is made of a material containing carbon as a main component in consideration of heat resistance, heat insulating properties, and the like. As the mold heating means 8, a resistance heating type heater, an induction heating type coil or the like is used. The side wall portion of the mold 1 is covered with a mold heat insulating material 3 made of a graphite-like molded body and the like, and the silicon melt 4 poured into the mold 1 by the cooling plate 9 is cooled from below, so that only from above the mold 1. By simply heating the silicon melt, the silicon melt can be solidified in one direction from the lower part to the upper part to obtain a polycrystalline silicon ingot (see, for example, Patent Document 1). These are all arranged in a vacuum vessel (not shown).
Japanese Patent Laid-Open No. 9-263489 15th Photovoltaic Specialists Conf. (1981), P576-P580, "A NEW DIRECTIONAL SOLIDIFICATION TECHNIQUE FOR POLYCRYSTALLINE SOLAR GRADE SILICON"

  In the general silicon casting apparatus shown in FIG. 5, since the radiation of the mold heating means 8 on the upper part of the mold 1 is directly received, the temperature of the release material layer 2 provided on the mold 1 tends to be high. When the release material layer 2 is heated, sublimation decomposition of the release material layer 2 proceeds under conditions of high temperature and low pressure where the mold 1 is placed, so that the thickness of the release material layer 2 is reduced. As a result, there is a problem that the silicon melt 4 and the mold 1 are likely to contact each other.

  Further, with respect to the liquid surface position 5 of the silicon melt 4 before the start of solidification, the silicon melt 4 oozes up by a capillary phenomenon with respect to the release material layer located in the vicinity thereof, and is in a wet state. When the swelled portion 6 is heated, the surface tension of the silicon melt is reduced and the wettability is further increased in this portion, so that the silicon melt in the spilled portion 6 easily penetrates into the release material layer 2. . As a result, the silicon melt 4 and the mold 1 are easily brought into contact with each other.

  Thus, when the silicon melt 4 is solidified in a state where the silicon melt 4 and the mold 1 are in contact with each other, the mold 1 and the polycrystalline silicon ingot are fused. Therefore, when the polycrystalline silicon ingot is removed from the mold 1, the silicon ingot is cracked due to the difference in thermal expansion coefficient between the silicon and the mold 1. Further, there arises a problem that the release material in the release material layer is mixed in the silicon melt 4 and causes a foreign matter defect.

  Note that, even if a separable graphite mold is used as the mold 1 or a quartz mold having an integral structure is used, the mold 1 is used as the mold heating means 8 on the upper part thereof. Similar problems occurred when receiving radiation directly. In particular, in the case of a separable graphite mold, it has been prepared with the in mind that it can be used over and over again. Since the reuse becomes difficult, there is a problem that the number of times the mold is used is reduced and the cost is increased.

  The present invention has been made in view of such conventional problems, by preventing the contact between the mold and the silicon melt when casting a polycrystalline silicon ingot, to prevent the silicon ingot from cracking, An object of the present invention is to provide a silicon casting apparatus that prevents the release material layer from being mixed into the melt.

  In order to achieve the above object, a silicon casting apparatus according to claim 1 of the present invention includes a mold having an opening in the upper part and a release material layer on the inner surface thereof, and a mold inside the mold above the mold. A mold heating means for heating the silicon melt, and a heat shielding plate is interposed between the mold heating means and the mold to block radiation heat from the mold heating means to the release material layer. Become.

  As a result, heating to the mold release material layer of the mold is suppressed, and the phenomenon that the mold release material layer undergoes sublimation decomposition due to heat and the silicon melt becomes easy to soak into the mold release material layer can be suppressed. it can. As a result, the silicon melt and the mold can be prevented from contacting each other.

  A silicon casting apparatus according to claim 2 of the present invention is the silicon casting apparatus according to claim 1, wherein the mold is covered with a mold heat insulating material on the outer periphery of the mold, and the heat shielding plate is insulated from the mold. It is connected so as to be connected to the upper end of the material.

  As a result, when the silicon melt is held inside the mold, heat removal from the side of the mold can be suppressed, and at the same time, radiant heat from the mold heating means on the outside of the mold can be blocked. Therefore, heating of the release material layer due to heat transfer from the outside of the mold can be suppressed. As a result, the silicon melt can be easily solidified in one direction from below, and contact between the silicon melt and the mold can be prevented.

  Since the silicon casting apparatus according to claim 3 of the present invention is the silicon casting apparatus according to claim 2, the heat shielding plate and the mold heat insulating material are made of the same material. Since it can be manufactured integrally from the same material, the configuration of the present invention can be obtained very easily.

  A silicon casting apparatus according to a fourth aspect of the present invention is the silicon casting apparatus according to any one of the first to third aspects, wherein a material constituting the heat shielding plate and / or the mold heat insulating material is a main component. Since it is carbon and its surface is coated with carbon powder, even if the silicon melt adheres to this material, it does not easily deteriorate.

Silicon casting apparatus according to claim 5 of the present invention, the silicon casting apparatus according to claim 4, the bulk density of the material, 0.05 g / cm ³ or more 0.50 g / cm ³ so as: Therefore, radiation from the mold heating means can be optimally shielded, and heating of the mold release material layer on the inner surface of the mold can be suppressed while maintaining heating of the silicon melt inside the mold.

  A silicon casting apparatus according to a sixth aspect of the present invention is the silicon casting apparatus according to any one of the first to fifth aspects, wherein the heat shielding plate is perpendicular from the opening edge of the mold to the top. It arrange | positions so that it may protrude inside from the imaginary line extended in this.

  Since it comprised in this way, it becomes possible to concentrate the radiation from the mold heating means with respect to the mold release material layer from the opening edge of the mold to the lower part thereof.

  The silicon casting apparatus according to claim 7 of the present invention is the silicon casting apparatus according to claim 6, wherein the heat shielding plate has a length of a portion protruding inward from the imaginary line of 15 mm to 50 mm. Thus, the balance between the effect of keeping the radiation from the mold heating means to the silicon melt inside the mold and the effect of suppressing the radiation to the release material layer on the inner surface of the mold can be kept optimal.

  As described above, according to the silicon casting apparatus of the first aspect of the present invention, the heating of the mold release material layer is suppressed, and the mold release material layer is subjected to sublimation decomposition due to heat, and the silicon melt is thinned. Can be prevented from being easily soaked into the release material layer, and contact between the silicon melt and the mold can be prevented. As a result, there is a problem that the silicon ingot is prevented from cracking when the mold is fused to the silicon ingot, or that the release material in the release material layer is mixed into the silicon melt, causing foreign matter defects. Can be suppressed.

  Moreover, according to the silicon casting apparatus according to claim 2 of the present invention, when the silicon melt is held inside the mold, heat removal from the lateral portion of the mold can be suppressed, and at the same time, Since the radiant heat from the mold heating means to the outside can be blocked, heating of the release material layer due to heat transfer from the outside of the mold can be suppressed. As a result, the silicon melt can be easily unidirectionally solidified from below, and the silicon melt and the mold can be prevented from coming into contact with each other, so that the silicon ingot generated when the mold is fused to the silicon ingot can be prevented. Polycrystals that can be cracked and that the release material in the release material layer is mixed into the silicon melt, resulting in foreign matter defects, and at the same time efficiently solidified in one direction. A silicon ingot can be obtained.

  Furthermore, according to the silicon casting apparatus according to claim 3 of the present invention, since the same material can be integrally manufactured, the configuration of the present invention can be obtained very easily using, for example, an integral molding method. Thus, it is possible to reduce the cost by eliminating the labor for manufacturing.

  According to the silicon casting apparatus according to claim 4 of the present invention, since the silicon melt is hardly deteriorated even if it adheres to this material, for example, when the silicon melt is discharged from a crucible, Even if the liquid scatters and adheres to the heat shielding plate or the mold heat insulating material, it can be prevented that they contain the silicon melt and become brittle and mixed into the silicon melt.

  Further, according to the silicon casting apparatus according to claim 5 of the present invention, radiation from the mold heating means can be optimally shielded, and the mold release material on the inner surface of the mold can be maintained while heating the silicon melt inside the mold. Since the heating to the layer can be suppressed, the silicon ingot is prevented from cracking without adversely affecting the purpose of obtaining a unidirectionally solidified polycrystalline silicon ingot, and the release material is contained in the silicon melt. It is possible to prevent a problem that the foreign matter is mixed and the foreign matter is defective.

  Furthermore, according to the apparatus for silicon casting according to claim 6 of the present invention, radiation from the mold heating means to the mold release material layer from the edge of the mold opening to the lower part thereof can be intensively shielded. It is possible to prevent the portion that is most susceptible to radiation from the mold heating means from being heated. As a result, the effect of the present invention can be optimally achieved by preventing cracking of the silicon ingot and preventing the problem that the release material is mixed into the silicon melt and causes foreign matter defects.

  According to the silicon casting apparatus according to claim 7 of the present invention, the balance between the effect of maintaining the radiation from the mold heating means to the silicon melt inside the mold and the effect of suppressing the radiation to the mold release material layer on the inner surface of the mold. Can be kept optimal. As a result, the silicon ingot is prevented from cracking without adversely affecting the purpose of obtaining a unidirectionally solidified polycrystalline silicon ingot, and the release material is mixed into the silicon melt, thereby preventing foreign matter defects. The effect of the present invention for preventing the problem of waking up is optimally achieved.

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

  The block diagram of the casting apparatus concerning this invention is shown in FIG. A melting crucible 10 for melting the raw material silicon 12 is disposed on the upper portion and held by a holding crucible 11, and a tapping opening 13 for pouring silicon melt is provided at the bottom of the melting crucible 10 and the holding crucible 11. . Further, heating devices 14 and 15 are arranged on the side and upper part of the melting crucible 10 and the holding crucible 11, respectively, and the mold 1 into which the silicon melt 4 is poured is arranged at the lower part of the melting crucible 10 and the holding crucible 11, The mold heat insulating material 3 is provided on the outside. Further, a cooling plate 9 is provided below the mold 1, and a mold heating means 8 for controlling the solidification of the silicon melt 4 is disposed above the mold 1.

  For example, a silicon raw material put in a melting crucible 10 made of high-purity quartz or the like is heated and melted by an upper heating means 14 and a side heating means 15 made of a resistance heating type heater or an induction heating type coil, A silicon melt is poured into the mold 1 at the bottom from the outlet 13 at the bottom. The mold containing the silicon melt 4 is made of, for example, carbon or a carbon fiber reinforced carbon material, 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 splittable manner

A mold release material layer 2 is applied to 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 release material layer 2, silicon nitride (Si ₃ N ₄ ) powder and a polyvinyl alcohol aqueous solution are mixed and applied to the inner surface of the mold 1. By mixing with silicon nitride and an aqueous polyvinyl alcohol solution, the powdered silicon nitride becomes a slurry and can be easily applied to the graphite mold 1.

  As the silicon nitride powder, one having an average particle size 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. And is applied to the inner surface of the mold 1 with a spatula or a brush. In this state, the silicon melt 4 is poured into the mold 1 after natural drying or drying on a hot plate and drying.

  The mold release material layer 2 can be applied to the inner surface of the mold 1 by applying a mixture of silicon nitride and silicon dioxide powder using a plasma spraying machine. Usually, when applying a release material that is made into a slurry by mixing powder and an organic binder aqueous solution such as polyvinyl alcohol, the heat decomposable product of the organic binder is mixed into the silicon melt by subsequent heating. In order to prevent this, a binder removal (degreasing) treatment is performed. In plasma spraying, spraying is performed by spraying sprayed particles using a temperature range of about 10,000 ° C. in a plasma stream covering 32000 ° K. Therefore, the binder removal step that has been conventionally used can be omitted. .

  The side surface of the mold 1 is insulated by the mold heat insulating material 3 and heated from the upper part by the mold heating means 8, and the cooling plate 9 is brought into contact with or brought close to the lower part to remove heat from the lower part to realize unidirectional solidification. Since the silicon melt 4 has a large heat removal from the surface, the surface of the silicon melt 4 is first solidified, and if the silicon melt is left inside, the silicon melt left behind is It solidifies and expands, and the surface of the silicon ingot becomes like an eruption and cracks occur in the silicon ingot. In order to prevent this problem, it is necessary to heat the surface of the silicon melt 4 by the mold heating means 8 located above the silicon melt 4 so that the surface of the silicon melt 4 is not solidified. In this state, the silicon melt 4 can be solidified in one direction by removing heat from the lower part by bringing the cooling plate 9 under the mold 1 into contact with or approaching it.

  As the mold heating means 8, a resistance heating type heater, an induction heating type coil, or the like can be used. Further, as the cooling plate 9, for example, a metal plate such as stainless steel (SUS) can be used, and a coolant such as water is circulated in the inside, so that the cooling plate 9 is effectively removed from the silicon melt 4 inside the mold 1. It is configured to remove heat. As the mold heat insulating material 3, a material having carbon as a main component, such as graphite felt, is desirable. Particularly, when the surface of the mold heat insulating material 3 is coated with carbon powder, it deteriorates when silicon melt adheres. This is desirable because it can reduce the problem of doing so.

  After solidification, the silicon ingot is cooled, the mold heat insulating material 3 is removed, and finally the silicon ingot is taken out from the mold 1 to complete the polycrystalline silicon ingot.

  Here, in the silicon casting apparatus according to the present invention, as shown in FIG. 1, a heat shielding plate 7 is provided on the upper end of the mold heat insulating material 3 that covers the outer periphery of the side surface of the mold 1. The heat shielding plate 7 is disposed so as to be interposed between the release material layer 2 provided on the inner surface of the mold 1 and the mold heating means 8. Radiant heat is shielded. As a result, the ratio of heating by the direct radiation from the mold heating means 8 to the release material layer 2 is reduced, so that it is possible to prevent the temperature of the release material layer 2 from being excessively increased. Therefore, in the environment in the silicon casting apparatus, that is, under high temperature and low pressure, the release material layer 2 is not easily thinned due to sublimation decomposition due to heat.

  Furthermore, with respect to the liquid surface position 5 of the silicon melt 4 before the start of solidification, the silicon melt 4 oozes up by a capillary phenomenon with respect to the release material layer located in the vicinity thereof, and is in a wet state. Since the temperature of the part of the release material layer 2 corresponding to the soaking-up part 6 does not rise too much, the phenomenon that the surface tension of the silicon melt is reduced and the wettability is further increased in this part can be suppressed. As a result, it is possible to prevent the silicon melt in the spread-up portion 6 from easily penetrating into the release material layer 2.

  Thus, by providing the heat shielding plate 7, it is possible to make the silicon melt 4 and the mold 1 difficult to contact with each other, so that the mold 1 and the polycrystalline silicon ingot are prevented from being fused together. Can be reduced. Further, the release material in the release material layer 2 can be hardly mixed into the silicon melt 4, and foreign matter defects can be reduced.

The density of the silicon melt is about 2.55 g / cm ^3, the 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 liquid surface position 5 of the silicon melt 4 rises with the solidification volume. The silicon melt 4 and the mold 1 are most likely to come into contact with each other when the release material layer 2 is thin under high temperature and low pressure conditions, and the silicon melt soaks up due to capillary action. 6 is heated, the surface tension of the silicon melt is decreased, the wettability is increased, and the silicon melt of the spilled portion 6 is likely to penetrate into the release material layer 2. Therefore, it is most preferable to prevent the temperature rise of the swelled portion 6 by blocking the radiation of the mold heating means 8 with the heat shielding plate 7.

  The material of the heat shielding plate 7 is preferably a material having carbon as a main component, such as graphite felt. Particularly, when the surface is coated with carbon powder, the silicon melt is adhered. In addition, the problem of deterioration can be reduced, which is desirable. Thus, if the surface of the heat shielding plate 7 is coated with carbon powder, when the silicon melt is poured from the hot water outlet 13 of the melting crucible 10, the silicon melt jumps around and heats up. Even if it adheres to the shielding plate 7, it does not deteriorate. Moreover, it can suppress that the material of the heat-shielding board 7 which the silicon melt adhered and became brittle mixes into the silicon melt.

Furthermore, when using such a material, the bulk density is preferably set to 0.05 g / cm ³ or more 0.50 g / cm ³ or less. When the bulk density is less than 0.05 g / cm ³ , it is difficult to sufficiently block the radiant heat of the mold heating means 8, and the release material layer 2 becomes high temperature. On the other hand, if it is larger than 0.50 g / cm ³ , the density is too high, and radiation from the mold heating means 8 is blocked more than necessary, and a sufficient amount of heat cannot be supplied to the silicon melt 4. There is a risk that the surface solidifies and cannot be solidified in one direction. The bulk density can be adjusted by changing the weaving method for each carbon fiber felt.

  Moreover, when using this material, it is desirable that the thickness of the heat shielding plate 7 be in the range of 3 mm to 40 mm. When it is smaller than this range, the effect of shielding radiant heat is poor, and when it is larger than this range, radiation from the mold heating means 8 is blocked more than necessary, and a sufficient amount of heat is supplied to the silicon melt 4. This is because the surface cannot be solidified and the surface cannot be solidified in one direction.

  Further, in the present invention, as shown in FIG. 1, it is desirable that the upper end of the mold heat insulating material 3 that covers the outer periphery of the side surface of the mold 1 and the heat shielding plate 7 be continuous. If there is a gap in this portion, there is a possibility that when the silicon melt 4 is held inside the mold 1, there is a slight heat removal from the lateral part of the mold 1, and the temperature of the silicon melt 4 may be lowered. Because.

  Furthermore, in order to make it the structure which the upper end of the mold heat insulating material 3 which covers the outer periphery of the side surface of the casting_mold | template 1 in this way, and the heat shielding board 7 are connected, the heat shielding board 7 and the mold heat insulating material 3 are comprised with the same material. It is desirable to do. Rather than being manufactured by separate members and then assembled, if they are manufactured integrally from the same material, for example, they can be manufactured by integral molding, and the configuration of the present invention can be obtained very easily.

  Next, a method for arranging the heat shielding plate 7 according to the present invention will be described with reference to FIG. FIG. 2 is a main part schematic diagram showing the configuration of the mold part. The reference numerals of the members are the same as those in FIG. In the present invention, it is desirable that the heat shielding plate 7 is disposed so as to protrude toward the inside of the mold 1 from an imaginary line L extending vertically from the opening edge of the mold 1 toward the upper part. . As a result, radiation from the mold heating means 8 to the mold release material layer 2 from the edge of the opening of the mold 1 to the lower part thereof can be intensively shielded, so that the radiation from the mold heating means 8 is received most. It is possible to prevent easy heating of the part. As a result, the effect of the present invention can be optimally achieved, which prevents cracking of the silicon ingot and prevents the problem that the release material layer 2 is mixed into the silicon melt 4 to cause foreign matter defects.

  And it is desirable for the length d of the part protruded toward the inner side of the casting_mold | template 1 rather than the virtual line L to be 15 mm or more and 50 mm or less. The reason is that if the length of this portion is shorter than 15 mm, radiation cannot be sufficiently blocked, and the release material layer 2 becomes high temperature. On the other hand, if it is longer than 50 mm, the radiation of the mold heating means 8 is blocked more than necessary, so that the surface of the silicon melt 4 is solidified and unidirectional solidification is impossible. In the case where 7 is fixed in a cantilever state or formed integrally, the portion of the heat shielding plate 7 cannot withstand its own weight and falls into the silicon melt 4 to cause a foreign matter defect. Because there is a risk of becoming. By setting the optimum range as described above, the balance between the effect of maintaining the radiation from the mold heating means 8 to the silicon melt 4 inside the mold 1 and the effect of suppressing the radiation to the mold release material layer 2 on the inner surface of the mold 1 is optimized. Can keep.

  In FIG. 3, the principal part perspective view which shows an example of member arrangement | positioning of the apparatus for silicon casting of this invention is shown. The reference numerals of the members are the same as those in FIG. 1, but the mold heat insulating material 3 and the cooling plate 9 are omitted for easy viewing. In the example shown in FIG. 3, the mold 1 has a rectangular parallelepiped shape in which the bottom member 1 a and the side member 1 b can be assembled / divided. Further, a donut ring-shaped heater is used as the mold heating means 8, and the heat shielding plate 7 has a rectangular frame shape corresponding to a rectangular opening formed by assembling the side member 1 b of the mold 1. ing. Thus, if the shape of the heat shielding plate 7 is made to correspond to the shape of the opening of the mold 1, the release material layer 2 provided on the inner surface of the mold 1 can be efficiently shielded from the radiant heat of the mold heating means 8. .

  FIG. 4 shows another embodiment of the heat shielding plate and the mold heat insulating material according to the present invention.

  FIG. 4A shows an example in which the heat shielding plate 7a and the mold heat insulating material 3 are formed of different members and connected so that the heat shielding plate 7a is continuous with the upper end of the mold heat insulating material 3. FIG. It differs from the case in that a different member is used for the heat shielding plate and the mold heat insulating material. Thus, when the heat shielding plate 7a and the mold heat insulating material 3 are constituted by different members, for example, the heat shielding plate 7a is not easily deteriorated with respect to the silicon melt as described in claim 4 of the present invention. Even if the mold heat insulating material 3 is a material lacking in chemical stability with respect to the silicon melt, it can be protected by the heat shielding plate 7a. Therefore, since the range of selection of the material used as the mold heat insulating material 3 is widened, an optimal combination of materials for unidirectionally solidifying the polycrystalline silicon ingot is selected, or an inexpensive material is selected for cost reduction. The material can be freely selected according to the purpose.

  FIG. 4B is an example in which the upper end of the mold heat insulating material 3 and the heat shielding plate 7b are not fixed in FIG. 4A. These members may be arranged apart from each other or in contact with each other. If comprised in this way, since the heat-shielding board 7b can be positioned freely, for example, the amount which protrudes inside with respect to the virtual line L of FIG. 2 can be adjusted freely, and the claim of this invention 7 can be easily obtained. Due to this, for example, when the silicon casting apparatus is started up, it is affected by various conditions such as the surrounding environment, so there may be a deviation between the optimum value and the design value. The apparatus can be adjusted to be optimal.

  Next, FIG. 4C shows an example in which the heat shielding plate 7c is placed on the edge of the mold 1 and the mold heat insulating material 3 is not used. For example, when a material having low thermal conductivity is used as the material of the mold 1, the thickness of the side surface of the mold 1 is increased, or the release material layer 2 provided on the mold 1 is further increased in thickness, Thus, the silicon ingot can be cast without using the mold heat insulating material 3. In this case, the effect of the present invention can be obtained if the heat shield plate 7c is placed on the edge of the mold 1 as shown in FIG.

  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, in FIG. 3, the heat shielding plate 7 has a quadrangular frame shape corresponding to the quadrangular opening formed by assembling the side member 1 b of the mold 1, but is not limited thereto. When a mold having a cylindrical opening integrally manufactured with quartz or the like is used as 1, a donut-ring shaped heat shield plate 7 may be used in correspondence with the opening.

  Further, in FIG. 3, a donut ring-shaped heater is used as the mold heating means 8, but the present invention is not limited to this, and it may be spherical or square. The gist of the present invention is that a heat shielding plate 7 is interposed between the mold heating means 8 and the mold 1 to shield the radiant heat from the mold heating means 8 to the release material layer 2 provided on the inner surface of the mold 1. As long as it is configured in this way, it goes without saying that the effects of the present invention can be obtained even if the shapes of the mold heating means 8 and the heat shielding plate 7 are changed.

  Examples of the present invention will be described below.

  Four side members 1b having a thickness of 2 mm and one bottom member 1a having a thickness of 5 mm were prepared. To these members, 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 are weighed and mixed with an aqueous solution of 8.7% polyvinyl alcohol to form a slurry. It was applied, placed on a hot plate and dried to obtain a release material layer 2. Thereafter, these members were combined so that the release material layer 2 was inside, and the mold 1 shown in FIG. 1 having an inner size of 330 mm × 330 mm and a depth of 320 mm was produced.

  A member in which the heat shielding plate 7 made of a material obtained by coating the graphite felt having the shape shown in FIG. 1 with carbon powder and the mold heat insulating material 3 are integrally formed is disposed so as to cover the outer periphery of the mold 1. The thickness of the heat shielding plate 7 was 10 mm.

  The mold 1, the mold heat insulating material 3, and the heat shielding plate 7 were set at predetermined positions on the cooling plate 9 inside the silicon casting apparatus. Further, as the mold heating means 8, a donut ring-shaped graphite heater having an outer diameter of 360 mm and an inner diameter of 220 mm shown in FIG. 3 is disposed at a predetermined position, and between the heater and the release material layer 2 provided on the inner surface of the mold 1. The heat shielding plate 7 is interposed between the heat shielding plate 7 and the radiant heat.

  Thereafter, the inside of the apparatus is set to an argon atmosphere whose pressure is reduced to 80 Torr, and 75 kg of silicon melt is poured into the mold 1 while being heated to 1000 ° C. using the mold heating means 8 of the graphite heater, and gradually over 8 hours. To solidify. After cooling, the solidified polycrystalline silicon ingot was taken out from the mold 1 and examined for the presence of fusion between the silicon ingot and the mold. Further, the removed silicon ingot was cut and sliced, and about 2000 finished wafers were visually inspected to confirm the presence or absence of foreign matter. Moreover, the presence or absence of surface solidification of the silicon melt 4 during solidification was also confirmed.

  Note that the length d of the portion of the heat shielding plate 7 described in FIG. 2 that protrudes toward the inside of the mold 1 from the imaginary line L that extends vertically from the opening edge of the mold 1 upward. Modified samples were made. Furthermore, what changed the bulk density of the heat shielding board 7 (and the mold heat insulating material 3) by changing the weaving method for each carbon fiber felt was produced, and the above-described evaluation was performed. Furthermore, a sample outside the scope of the present invention in which the heat shielding plate 7 is not provided was also prepared and evaluated as described above.

  These results are shown in Table 1. In the evaluation results of Table 1, the contents are as follows although indicated by symbols.

(Fusion of silicon and mold)
◎: Not observed at all, ○: Slightly recognized, △: Recognized but within the allowable range, ×: Remarkably not recognized (number of defective foreign matter)
◎: Extremely good (0 to 39 sheets), ○: Good (40 to 79 sheets), △: Tolerable range (80 to 99 sheets), X: Impossible (100 sheets or more)
(Surface solidification in melt)
A: Not observed at all, O: Slightly recognized, Δ: Recognized but within the allowable range, X: Remarkably not recognized, and outside the range of the present invention in which the heat shielding plate 7 was not provided About samples 1-5, the bulk density of the mold heat insulating material 3 was described.

熱遮蔽板７を設けなかった本発明の範囲外の試料１〜５は、シリコンと鋳型との融着が顕著に観察されるとともに、離型材が破損してシリコン融液中に落下したため、異物不良数も多かった。

Samples 1 to 5 outside the scope of the present invention, in which the heat shielding plate 7 was not provided, had a remarkable adhesion between the silicon and the mold, and the release material was damaged and dropped into the silicon melt. There were many defects.

  On the other hand, Samples 6 to 30 that fall within the scope of the present invention have more improved results than Samples 1 to 5, and it has been found that there is an effect of the configuration of the present invention.

In particular, in the sample in which the length d of the portion protruding from the imaginary line L is in the range of 15 to 50 mm and the bulk density of the heat shielding plate 7 is in the range of 0.05 to 0.50 g / cm ³ , The evaluation item was also a good result of ○ or more.

  For samples (samples 6 to 10) in which the length d of the portion protruding from the imaginary line L is 1 mm, the radiation heat shielding to the release material layer 2 is insufficient, but due to the presence of the heat shielding plate 7, the mold The effect of preventing heat radiation to the vicinity of the opening 1 is obtained, and the result is improved because the temperature of the release material layer 2 is lower than in the case without the heat shielding plate 7 (Samples 1 to 5). Seem.

  From the above results, the effect of the present invention could be confirmed.

It is a figure which shows the structure of the apparatus for silicon casting concerning this invention. It is a principal part schematic diagram which shows the structure of the casting_mold | template part concerning this invention. It is a principal part perspective view which shows an example of member arrangement | positioning of the apparatus for silicon casting of this invention. It is a figure which shows other embodiment of the heat shielding board concerning this invention, and a mold heat insulating material. It is a figure which shows the structure of the general apparatus for silicon casting.

Explanation of symbols

1: Mold 1a: Bottom member 1b: Side member 2: Release material layer 3: Mold heat insulating material 4: Silicon melt 5: Liquid surface position 6: Swelling portion 7, 7a to 7c: Heat shielding plate 8: Mold heating means 9: Cooling plate 10: Melting crucible 11: Holding crucible 12: Raw material silicon 13: Outlet 14: Upper heating means 15: Side heating means L: Virtual line d: Length of the protruding portion

Claims (7)

A mold having an opening in the upper part and having a release material layer on its inner surface; and a mold heating means for heating the silicon melt inside the mold above the mold; and the mold heating means A silicon casting apparatus in which a heat shielding plate is interposed between the mold and the mold so as to block radiation heat from the mold heating means to the release material layer. 2. The silicon casting apparatus according to claim 1, wherein an outer periphery of the mold is covered with a mold heat insulating material, and the heat shielding plate is connected to be connected to an upper end of the mold heat insulating material. The silicon casting apparatus according to claim 2, wherein the heat shielding plate and the mold heat insulating material are made of the same material. 4. The silicon casting apparatus according to claim 1, wherein a material constituting the heat shielding plate and / or the mold heat insulating material is carbon as a main component, and a surface thereof is coated with carbon powder. 5. . The bulk density of the material, a silicon casting device according to claim 4 comprising as below 0.05 g / cm ³ or more 0.50 g / cm ^3. The said heat shielding board is arrange | positioned so that it may protrude inside from the virtual line extended perpendicularly | vertically toward the upper part from the opening edge of the said casting_mold | template in any one of Claim 1 to 5 apparatus. The said heat shielding board is a silicon casting apparatus of Claim 6 which becomes 15 mm or more and 50 mm or less in length of the part protruded inside from the said virtual line.

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