JP2005211937A - Mold for casting silicon and apparatus for casting silicon using this mold - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mold for casting silicon and an apparatus for casting the silicon having high productivity at a low cost with which in the case of casting the silicon by using the mold, in the inner surface of which releasing material layer is coated, even when the molten silicon heated from the upper part soaks up the releasing material layer, the molten silicon is not brought into contact with the mold, and the crack on a silicon ingot can be prevented. <P>SOLUTION: The mold 1 for casting the silicon having the opening hole part opened toward the upper part and the surface in the mold is provided with the inner surface releasing material layer 2 coated in the inner surface of the mold and an opening hole part releasing material layer 7 continued with the inner surface releasing material layer 2 and coated on the upper edge part 6 of the opening hole part. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a silicon casting mold suitable for casting a polycrystalline silicon ingot suitably used for forming a solar cell and the like, and a silicon casting apparatus using the same.

  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 the 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.

  A general silicon casting apparatus for producing such a polycrystalline silicon ingot is shown in FIG.

  A melting crucible 31 for melting the raw material silicon 30 is disposed on the upper part while being held by a holding crucible 32, and at the bottom of the melting crucible 31 and the holding crucible 32, a hot water outlet 33 for discharging a silicon melt is provided. . Further, an upper heating means 34 and a side heating means 35 are respectively arranged on the upper and side portions of the melting crucible 31 and the holding crucible 32, and a mold in which the silicon melt 24 is poured into the lower portion of the melting crucible 31 and the holding crucible 32. 21 is disposed, and a mold heat insulating material 23 is provided outside thereof. Further, a cooling means 29 is provided below the mold 21, and a mold heating means 8 for controlling solidification of the silicon melt 24 is disposed above the mold 21.

  For example, the silicon raw material put in the melting crucible 31 made of high-purity quartz or the like is formed by the upper and side upper heating means 34 and the side heating means 35 made of a resistance heating type heater, an induction heating type coil or the like. It is heated and melted to form a silicon melt, which is poured from the bottom hot water outlet 33 into the lower mold 21.

As such a mold 21, one in which a mold release material is coated on the inner surface of a mold made of graphite, or one in which a mold release material is coated on an inner surface of a mold made of silica, which is an integral structure, is used. It is done. Generally, as the release material layer 22, powder such as silicon nitride (Si ₃ N ₄ ) which is a silicon nitride, silicon carbide (SiC) which is a silicon carbide, silicon dioxide (SiO ₂ ) which is an oxide of silicon, or the like. It is known as a known technique that these powders are mixed in a solution composed of a suitable 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).

The mold heat insulating material 23 suppresses heat removal from the mold 21 containing a silicon melt, 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 28, a resistance heating type heater, an induction heating type coil or the like is used. The side wall of the mold 21 is covered with a mold heat insulating material 23 made of a graphite-like molded body and the like, and the silicon melt 24 poured into the mold 21 is cooled from the lower part by the cooling means 29, so that silicon is By simply heating the 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"

  However, in the case where silicon is cast by coating the release material layer 22 made of silicon nitride or the like on the inner surface of a graphite mold or silica mold, the mold located above the silicon melt surface 25 is the upper part. The release material layer 22 on the inner surface of the mold is heated by the heating means 28. When the melted portion of the silicon melt 24 that is soaked and wetted by capillary action is heated, the surface tension of the melt is reduced and the wettability is increased, so that the soaked portion becomes larger.

  As a result, the upper edge 26 of the mold and the swelled portion of the silicon melt come into contact with each other, and there is a problem that the silicon ingot is cracked due to the difference in thermal expansion coefficient between silicon and the mold.

  In order to solve this problem, conventionally, for example, the height of the mold is made sufficiently higher than the position of the silicon melt or the silicon melt so that the silicon melt does not come into contact with the upper edge 26 of the mold. Techniques such as reducing the amount of melt input were taken. In addition, there has been devised to reduce or prevent the heating from the mold heating means 28 to the release material layer 22 provided on the inner surface of the mold 21, which increases the size of the apparatus, the cost of the mold, or the productivity. There was a problem of decline.

  In the case of a graphite mold, it is desirable to reuse it. However, since the silicon melt soaks into the graphite mold, there is a problem that it cannot be reused because the thermal conductivity of the mold 21 changes.

  The present invention has been made in view of such problems of the prior art, and was heated from above when silicon was cast using a silicon casting mold in which a mold release material layer was coated on the inner surface of the mold. Silicon casting mold and silicon that can prevent cracking of the silicon ingot without causing contact between the silicon melt and the mold even when the silicon melt soaks up the release material layer, and has high productivity at low cost. An object is to provide a casting apparatus.

  In order to achieve the above object, a silicon casting mold according to claim 1 of the present invention is a silicon casting mold having an opening opened upward and an inner surface of the mold, and the inner surface of the mold is coated. An inner surface release material layer, and an opening part release material layer that is continuous with the inner surface release material layer and is covered with an upper end edge of the opening.

  A silicon casting mold according to a second aspect of the present invention is the silicon casting mold according to the first aspect, wherein the opening parting material layer includes silicon nitride and silicon dioxide, and the silicon nitride has a weight thereof. The ratio is smaller than the weight ratio of silicon dioxide.

  A silicon casting mold according to a third aspect of the present invention is the silicon casting mold according to the first or second aspect, wherein the mold is made of a material whose main component is silica.

  A silicon casting mold according to a fourth aspect of the present invention is the silicon casting mold according to the first or second aspect, wherein the mold is made of a carbon fiber reinforced carbon material.

  A silicon casting apparatus according to a fifth aspect of the present invention includes a silicon casting mold according to any one of the first to fourth aspects, and a mold heating unit disposed above the silicon casting mold. And a mold heat insulating material disposed so as to surround an outer periphery of the silicon casting mold, and a cooling means formed by arranging the silicon casting mold on the upper part thereof.

  As described above, in the present invention, a mold having an opening that opens upward, an inner surface release material layer coated on the inner surface of the mold, and the inner surface release material layer, Since the opening part release material layer coated on the upper edge of the opening part is provided, the inner surface release material layer covering the inner surface of the mold is heated from the outside, and the silicon melt is caused by capillary action. Even when the soaked and wet state is reached, the opening mold release material layer is coated on the upper edge of the mold in a state continuous with the inner surface mold release material layer, so that the silicon melt and the mold are in contact with each other. Can be suppressed. As a result, the silicon ingot produced by solidifying the silicon melt can be removed without breaking. Further, when a graphite mold is used, the problem that the graphite mold cannot be reused due to contact between the silicon melt and the mold can be suppressed.

  Furthermore, in order to prevent the upper edge of the mold from coming into contact with the silicon melt, the height of the mold is made sufficiently higher than the position of the silicon melt, or the amount of silicon melt input is reduced. There is no need, and good productivity can be obtained at low cost.

  The opening release material layer covering the upper edge includes silicon nitride and silicon dioxide, and the silicon nitride has a weight ratio smaller than the weight ratio of silicon dioxide, so that the opening release material layer And the silicon melt can be further reduced, so that the penetration of the silicon melt can be effectively suppressed, and fusion between the mold release layer and the mold can be prevented.

  Further, when the silicon casting mold of the present invention is made of a material whose main component is silica or a carbon fiber reinforced graphite material, the effect of the present invention can be obtained by coating the release material layer having the structure of the present invention. It will play remarkably.

  And the above-described silicon casting mold of the present invention, a mold heating means disposed above the silicon casting mold, a mold heat insulating material disposed around the outer periphery of the silicon casting mold, The silicon casting apparatus provided with a cooling means in which the silicon casting mold is arranged on the upper part of the silicon casting apparatus, when casting silicon, the silicon melt heated from the upper part by the mold heating means soaks the release material layer. Even if it rises, the silicon ingot can be prevented from cracking without the silicon melt and the mold coming into contact with each other. Therefore, this silicon casting apparatus can widen the margin for the heating conditions, and can optimally solidify the silicon ingot in one direction.

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

  FIG. 1 is a schematic sectional view of an embodiment of a silicon casting apparatus according to the present invention. The mold 1 according to the present invention into which the silicon melt 4 is poured is provided with a mold heat insulating material 3 so as to surround the outer periphery thereof. Further, a cooling means 9 is provided in the lower part of the mold 1, and a mold heating means 8 for controlling the solidification of the silicon melt 4 is arranged in the upper part of the mold 1.

  The embodiment of FIG. 1 is a casting apparatus of a type in which a silicon melt melted from the outside is poured into a mold 1. A melting crucible 11 for melting the raw material silicon 10 is disposed on the upper part of the mold 1 while being held by a holding crucible 12. At the bottom of the melting crucible 11 and the holding crucible 12, a hot water outlet 13 for discharging a silicon melt. Is provided. Further, an upper heating means 14 and a side heating means 15 are arranged on the upper and side portions of the melting crucible 11 and the holding crucible 12, respectively.

  Next, a method for manufacturing a silicon ingot using this silicon casting apparatus will be described. First, the mold 1 having the configuration of the present invention is placed in an argon (Ar) atmosphere whose pressure is reduced to 70 to 90 Torr, and the mold 1 is heated at a temperature similar to or slightly lower than that of the silicon melt 4. Here, for example, the silicon raw material placed in the melting crucible 11 made of high-purity quartz or the like is composed of a resistance heating type heater, an induction heating type coil or the like, and upper and side upper heating means 14 and side heating means. 15 is heated and melted to form a silicon melt, which is poured into the mold 1 at the bottom from the outlet 13 at the bottom. Thereafter, the cooling means 9 gradually lowers the temperature from the bottom of the mold 1 to gradually solidify the silicon melt 4 from the bottom of the mold. Finally, the mold 1 is divided to complete the silicon ingot.

  2A and 2B show examples of a silicon casting mold according to the present invention, in which FIG. 2A is a perspective view and FIG. 2B is a cross-sectional structure diagram.

  The mold 1 has an opening that opens upward, and the silicon melt 4 can be held and solidified therein. The mold 1 is made of graphite, a silica member, a carbon fiber reinforced carbon material (C / C material), etc., and includes one bottom member 1a constituting the bottom of the mold and four side members 1b constituting the side of the mold. It is composed of divided molds that can be assembled and divided molds that can be assembled.

  The bottom member 1a and the side member 1b are assembled so as to be separable by being fixed 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 are just fitted. And assembled in a splittable manner.

An inner surface release material layer 2 is coated on the inner surface of the mold 1. By such an inner surface release material layer 2, the inner wall of the mold 1 and the silicon ingot are not fused after the silicon melt 4 inside the mold 1 is solidified, and the bottom member 1 a and the side member 1 b are fixed several times. Can also be used repeatedly. As such an inner surface release material layer 2, for example, a powder of silicon nitride (Si ₃ N ₄ ) is mixed with a polyvinyl alcohol aqueous solution and coated on the inner surface of the mold 1 by coating and drying. By mixing silicon nitride with an aqueous polyvinyl alcohol solution, 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 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. Usually, when coating the inner surface release material layer 2 made by mixing powder and an organic binder aqueous solution such as polyvinyl alcohol to form a slurry, the thermally decomposable product of the organic binder is contained in the silicon melt by subsequent heating. A degreasing process is performed to prevent the mixture from being mixed into the mold 1, and the silicon melt 4 is poured into the mold 1 after natural drying or degreasing process by drying on a hot plate.

  The inner surface 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. In addition to applying slurry, the inner surface release material layer 2 may be provided directly using a plasma spraying machine.

  In addition, as a coating method, a brush, a spatula, or the like can be used, or a spray method or the like can be used. From the viewpoint of productivity, a brush that can secure a coating thickness by a single coating is used. desirable. Further, as a drying method, a conventionally known method such as a hot plate or an oven can be used.

  As described in the conventional problem, in order to solidify the silicon melt 4 unidirectionally, in the portion corresponding to the upper part of the silicon melt surface 5, the inner surface mold release material layer 2 on the inner surface of the mold is formed by the mold heating means 8 located at the upper part. Becomes heated. When the melted portion of the silicon melt 4 that has been soaked and wetted by capillarity is heated, the surface tension of the melt is reduced and the wettability is increased, so that the soaked portion becomes larger. As a result, conventionally, when the upper end edge 6 of the mold 1 and the swelled portion of the silicon melt come into contact with each other and are removed from the mold, the silicon ingot is cracked due to the difference in thermal expansion coefficient between silicon and the mold material. It was. Therefore, in the silicon casting mold of the present invention, as shown in FIGS. 1 and 2B, the upper end edge 6 is opened continuously with the inner surface release material layer 2 provided on the inner surface of the mold 1. This problem was solved by constituting the part release material layer 7 so as to be covered.

  That is, even if the inner surface mold release material layer 2 covering the inner surface of the mold 1 is heated by the mold heating means 8 and the silicon melt 4 is soaked and wetted, the inner surface mold release material Since the upper edge 6 of the mold 1 is covered with the opening parting material layer 7 continuously with the layer 2, the silicon melt 4 and the mold 1 are prevented from coming into contact and the silicon ingot is cracked. It can be demolded. Further, when a graphite mold is used, there is no problem that the graphite mold cannot be reused due to the contact between the silicon melt and the mold.

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, for about 9% volume expansion occurs when the transition from liquid to solid, the mold high The minimum required height is that the silicon melt surface 5 before the start of solidification has a volume expansion of about 9% and a soaking area. Usually, the inner surface release material layer 2 has a difference in wettability due to temperature variations, so that it is necessary to make the mold higher in view of safety. However, since the upper edge 6 is covered with the opening parting material layer 7, the contact between the silicon melt 4 and the mold 1 can be avoided, so that the height of the mold is increased more than necessary or the silicon melt is used. There is no need to reduce the amount of input. As a result, it is possible to reduce the cost of the mold or improve the productivity.

  Further, it is desirable that the opening parting material layer 7 covering the upper edge 6 of the mold contains silicon nitride and silicon dioxide so that the weight ratio of silicon nitride is smaller than the weight ratio of silicon dioxide. Since silicon dioxide has a lower wettability with the silicon melt 4 than silicon nitride, the inner surface release material layer 2 is made to have the upper edge 6 by applying the release material having such a structure to the opening release material layer 7. The silicon melt that has soaked up can be effectively suppressed, and fusion between the opening parting material layer 7 and the mold 1 can be prevented.

  When the weight ratio of silicon nitride is larger than that of silicon dioxide, the wettability of the opening release material layer 7 covering the upper edge 6 of the mold 1 increases, so that the inner surface release material layer 2 penetrates to the upper edge 6. The risen silicon melt may also permeate into the opening parting material layer 7, and may come into contact with the outer surface of the mold, making it impossible to crack the silicon ingot or reuse the mold.

  Note that the upper limit of the weight ratio of silicon dioxide is desirably 90%. When the weight ratio is higher than this, particularly when a graphite mold is used, silicon dioxide has good adhesion to graphite, and both silicon dioxide and silicon ingot have good adhesion. This is because there is a possibility that a part of the silicon ingot may be chipped when adhering to the silicon ingot via the layer 7 and demolding.

  The basic method for forming the opening parting material layer 7 is the same as that for the inner surface parting material layer 2. Here, a case where a release material layer containing silicon nitride and silicon dioxide is formed will be described.

  As the raw material silicon nitride powder, one having an average particle size of about 0.4 to 0.6 μm is used, and as silicon dioxide powder, one having an average particle size of about 20 μm is used. . Such a powder of silicon nitride and silicon dioxide is weighed so as to have a predetermined weight ratio, and is mixed and stirred in a PVA aqueous solution of about 5 to 15% by weight to obtain a release material mixture slurry. Then, the mold release material slurry is continuously applied from the inner surface mold release material layer 2 to the upper edge 6 of the mold 1 and dried so as not to be interrupted, thereby separating the opening of the silicon casting mold of the present invention. The mold material layer 7 can be formed.

The density of the opening parting material layer 7 is desirably 1 g / cm ³ or more and 2 g / cm ³ or less. When the density decreases, the opening parting material layer 7 has a more porous structure, so that the soaked silicon melt easily penetrates into the opening parting material layer 7. In order to optimally adjust the density, the concentration of the organic binder PVA may be adjusted, the average particle size of the silicon nitride and silicon dioxide powder used may be adjusted, or the shape may be adjusted. For example, in order to decrease the density, operations such as increasing the organic binder concentration, increasing the average particle diameter of the powder, and increasing the linearity are effective. The linearity is a ratio of the maximum length and area (maximum cross-sectional area) of one particle ((absolute maximum length) ² / area) * π / 4. In the case of a perfect circle, the linearity is 1. In the case of an ellipse, the linearity increases as the flatness ratio increases. Linearity can be measured with a scanning laser microscope.

  And it is desirable for the thickness of the opening part release material layer 7 to be in the range of 0.1 mm or more and 1 mm or less. If the thickness is smaller than 0.1 mm, the soaked silicon melt permeates the opening parting material layer 7 and the silicon melt and the mold may come into contact with each other. If it is larger than 1 mm, the temperature between the upper part of the mold 1 where the opening parting material layer 7 covering the upper edge 6 of the mold is heated by the mold heating means 8 and the lower part of the mold 1 cooled by the cooling means 9 is used. The distribution may be changed, which may adversely affect the quality of the formed polycrystalline silicon substrate, and the coating weight of the opening parting material layer 7 increases, leading to an increase in cost.

  Further, when the mold 1 is made of a material whose main component is silica, it is desirable to apply the configuration of the present invention. Since silica materials usually contain more impurities such as iron and aluminum than a mold made of graphite, for example, when the mold 1 and the silicon melt 4 come into contact, impurities such as iron and aluminum are present in the silicon melt. It may melt and adversely affect solar cell characteristics. However, according to the structure in which the upper edge 6 of the mold 1 according to the present invention is covered with the opening parting material layer 7, the contact between the soaked silicon melt 4 and the mold 1 is prevented, and impurities such as iron and aluminum are prevented. This is because it is possible to suppress the dissolution of silicon into the silicon melt 4.

  When graphite is used as the mold 1, it is desirable to use a carbon fiber reinforced carbon material that is high in strength and is not easily damaged, and the carbon fiber reinforced carbon material may be laminated in the thickness direction of the mold 1. More desirable. In this way, the thermal conductivity of the mold 1 is low in the thickness direction of the mold 1 and high in the surface direction, so that the unidirectional solidification of the ingot can be improved. However, in this case, the carbon fiber reinforced carbon material constituting the mold 1 has a greater penetration of the silicon melt 4 on the side surface, that is, on the portion where the upper edge 6 of the mold 1 exists than on the inner surface. Therefore, when the upper end edge 6 of the mold and the swelled portion of the silicon melt come into contact with each other, cracking of the silicon ingot is likely to occur when demolding. Thus, by covering the upper edge 6 of the mold 1 with the opening release material layer 7 according to the present invention, contact between the silicon melt 4 and the mold 1 can be prevented.

  Next, the configuration and operation of the silicon casting apparatus using the silicon casting mold according to the present invention as described above will be described in more detail with reference to FIG.

  A mold heating means 8 is disposed above the mold 1 of the present invention. As the mold heating means 8, for example, a resistance heating type heater or an induction heating type coil can be used. The purpose of providing the mold heating means 8 is to control the solidification of the silicon melt 4 held inside the mold 1 by supplying heat from the upper part, and the mold heat insulating material 3 and the cooling means 9 described later. The purpose of this is to perform unidirectional solidification.

  A mold heat insulating material 3 is disposed on the outer periphery of the mold 1 of the present invention so as to surround the mold 1. 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. The purpose of providing the mold heat insulating material 3 is to suppress the heat removal from the side of the mold 1 when the silicon melt 4 is held in the mold 1, and the mold heating means 8 described above, the cooling means 9 described later, It is intended to facilitate the unidirectional solidification by developing a cooperative action. Normally, if the silicon melt 4 is left in the state where the surface of the silicon melt is solidified due to large heat removal from the surface of the silicon melt 4, the silicon melt is expanded due to the expansion of the remaining silicon melt. Although there is a problem that the ingot erupts and the silicon ingot is cracked, this mold heat insulating material 3 can prevent this problem.

  And the cooling means 9 is comprised so that the casting_mold | template 1 may be arrange | positioned in the upper part. As the cooling means 9, for example, a metal plate such as stainless steel (SUS) can be used, and heat can be effectively removed from the silicon melt 4 inside the mold 1 by circulating a coolant such as water inside. It is configured as follows. This cooling means 9 has the effect of removing heat from below the silicon melt 4 held inside the mold 1.

  That is, while the silicon melt 4 held inside the mold 1 of the present invention is heated from above by the mold heating means 8, the cooling means 9 removes heat from below, and the mold heat insulating material 3 further removes the side of the mold 1. By preventing heat removal from the parts, the cooperative action by these parts is exhibited well. As a result, an appropriate temperature gradient can be provided from the lower part to the upper part of the silicon melt 4, and the silicon melt 4 is solidified in one direction from the lower part to the upper part to obtain a silicon ingot having good characteristics. be able to.

  As described above, in the conventional casting apparatus shown in FIG. 3, when the release material layer 22 provided on the inner surface of the mold 21 is heated by the mold heating means 28 and the temperature rises too much, the silicon melt 24 becomes the release material layer. 22 oozes up and comes into contact with the upper edge of the mold 21, causing problems. For this reason, the heating condition of the mold heating means 28 has a restriction such that it cannot be heated above a certain strength in order to prevent this problem.

  On the other hand, in the silicon casting apparatus of the present invention using the mold 1 of the present invention, the inner surface mold release material layer 2 is heated by the mold heating means 8 and the silicon melt 4 soaks the inner surface mold release material layer 2. Even if it rises, the mold release material layer 7 provided at the upper edge 6 of the mold 1 continuously with the inner surface mold release material layer 2 may cause a problem when the mold 1 and the silicon melt 4 come into contact with each other. Absent.

  Thus, in the silicon casting apparatus of the present invention, the heating condition of the mold heating means 8 is not restricted by this problem and can be set in a more free range. As a result, the margin of conditions for unidirectionally solidifying the silicon melt 4 is widened, and an optimum solidification condition can be selected from a wider range of conditions. The silicon ingot having better characteristics can be obtained by selecting the optimum conditions according to this.

  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 the above description, the method in which the silicon raw material is melted in the crucible externally in FIG. 1 and then poured into the mold of the present invention is described, but the present invention is not limited to this, and the silicon raw material is placed in the mold. After melting once, it may be solidified again from the bottom to form a silicon ingot.

  In the above description, in the silicon casting apparatus of the present invention, the mold heat insulating material 3 arranged so as to surround the outer periphery of the mold 1 has been described as an example of being integrally formed. However, the present invention is not limited to this. For example, you may comprise so that the mold heat insulating material 3 can be divided | segmented into the vertical direction, horizontal direction, etc. of a casting_mold | template. Moreover, the mold heat insulating material 3 may not completely cover the outer periphery of the mold 1, and a part thereof may be exposed.

  In the above description, the method of coating the upper edge 6 of the mold 1 with one kind of opening parting material layer 7 has been described. However, it is also possible to form a plurality of kinds of parting material layers. At this time, it is desirable to satisfy the above-mentioned conditions (mixing ratio, density, required thickness).

  Also, the inner surface release material layer 2 and the opening part release material layer 7 may be provided together as the same material structure, or they may be applied simultaneously as different material structures, and then dried together. May be formed at the same time.

  Furthermore, in the above description, the method of providing the inner surface release material layer 2 and the opening part release material layer 7 has been described with reference to an example in which the release material slurry is formed by a method of applying and drying with a brush, but is not limited thereto. In the coating method, in addition to the brush, it may be formed using a dispenser or screen printing, or as a method other than the coating method, a plasma spraying method may 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 at a weight ratio of 60:40, followed by stirring and mixing with the mixed powder and an 8.7% aqueous polyvinyl alcohol solution. Then, the inner surface of the graphite mold 1 having the shape shown in FIG. 2 was covered with a brush, placed on a hot plate and dried to obtain an inner surface release material layer 2.

Thereafter, 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 5:95 to 40:60, and then mixed with the mixed powder to 8.7%. The release material made into a slurry by stirring and mixing with an aqueous solution of polyvinyl alcohol was continuous with the inner surface release material layer 2 on the upper edge 6 of the mold 1 under the conditions of a density of 1.69 g / cm ³ and a thickness of 0.2 mm. It was coated and similarly placed on a hot plate and dried to obtain an opening parting material layer 7.

  Thereafter, the mold 1 was placed in an argon atmosphere reduced to 80 Torr using a silicon casting apparatus having the structure shown in FIG. A graphite felt mold heat insulating material 3 whose surface is coated with carbon powder is disposed around the mold 1, and the mold 1 is made of a stainless steel metal plate in which water is circulated as a coolant. It mounted on the cooling means 9 which becomes.

  Then, 70 kg of silicon melt was poured into the mold 1 while being heated to 1000 ° C. by the mold heating means 8 comprising a graphite heater, and gradually solidified over 8 hours.

  After cooling, the mold 1 was divided and solidified silicon ingot was taken out, and the presence or absence of silicon adhesion to the mold, the cracking and chipping of the silicon ingot were examined.

As a comparative example, a mold having a conventional structure in which the opening parting material layer 7 is not provided was also produced and evaluated in the same manner as in the example of the present invention. These results are shown in Table 1.

  As shown in Table 1, a sample No. using a mold having a conventional structure in which the upper edge of the mold is not covered with a release material layer. In No. 1, silicon adhered to the mold, and when the silicon ingot was removed from the mold, the silicon ingot was cracked.

  In contrast, sample No. using the mold according to the present invention. 2-No. 6 regarding the presence or absence of silicon adhering to the mold and the cracking and chipping of the silicon ingot. An improved result compared to 1 was obtained.

  In particular, sample no. In 3-5, the adhesion of the silicon | silicone to a casting_mold | template was not seen, but the crack and notch | chip were not confirmed by the silicon ingot. On the other hand, Sample No. produced so that the weight ratio of silicon nitride is larger than that of silicon dioxide. In No. 6, the adhesion of silicon was observed on the outer surface of the mold, and the silicon ingot was chipped to such an extent that it could be removed by the slicing process, and the effect was somewhat poor. In addition, the sample No. 1 in which the weight ratio of silicon nitride is smaller than 10%. In No. 2, silicon was not attached to the mold, but the mold was attached to the silicon ingot via the release material layer, and the silicon ingot was chipped to such an extent that it could be removed by a slicing process, and the effect was slightly poor. .

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

It is a cross-sectional schematic diagram which shows the silicon | silicone casting apparatus concerning this invention. (A) is a perspective view which shows an example of the casting_mold | template for silicon casting concerning this invention, (b) is a schematic sectional drawing. It is a cross-sectional schematic diagram which shows the conventional silicon | silicone casting apparatus.

Explanation of symbols

1: Mold 1a: Bottom member 1b: Side member 2: Inner surface mold release material layer 3: Mold heat insulating material 4: Silicon melt 5: Silicon melt surface 6: Upper edge 7: Opening mold release material layer 8: Mold heating Means 9: Cooling means 10: Raw material silicon 11: Melting crucible 12: Holding crucible 13: Outlet 14: Upper heating means 15: Side heating means 21: Mold 22: Release material layer 23: Mold heat insulating material 24: Silicon melt 25: Silicon melt surface 26: Upper edge 28: Mold heating means 29: Cooling means 30: Raw material silicon 31: Melting crucible 32: Holding crucible 33: Hot water outlet 34: Upper heating means 35: Side heating means

Claims (5)

A mold for silicon casting having an opening opened upward and an inner surface of the mold,
An inner surface release material layer coated on the inner surface of the mold;
A silicon casting mold comprising: an opening part release material layer which is continuous with the inner surface release material layer and is coated on an upper end edge of the opening part. The opening release material layer includes silicon nitride and silicon dioxide,
The silicon casting mold according to claim 1, wherein the silicon nitride has a weight ratio smaller than that of silicon dioxide. The silicon casting mold according to claim 1, wherein the mold is made of a material whose main component is silica. The silicon casting mold according to claim 1, wherein the mold is made of a carbon fiber reinforced carbon material. A silicon casting mold according to any one of claims 1 to 4,
A mold heating means disposed above the silicon casting mold,
A mold heat insulating material disposed around the outer periphery of the silicon casting mold, and
A silicon casting apparatus comprising: cooling means having the silicon casting mold disposed thereon.

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