JP4192070B2 - Silicon casting mold and manufacturing method thereof - Google Patents

Description

  The present invention relates to a silicon casting mold for producing a polycrystalline silicon ingot used for a silicon substrate for solar cells, and more particularly to a silicon casting mold made of silicon dioxide or the like and a method for producing the same.

  In recent years, the demand for polycrystalline silicon solar cells has increased as interest in environmental issues has increased, and high conversion efficiency is required at lower cost. In order to cope with such a demand, it is necessary to reduce the cost and improve the quality of the polycrystalline silicon substrate.

  Silicon ingots for making this polycrystalline silicon substrate are formed by pouring a melt in which silicon is heated and melted into a mold and solidifying it in one direction from the bottom of the mold. Alternatively, silicon raw materials are placed in the mold and dissolved once. After that, a general method is to form by re-solidifying in one direction from the bottom surface of the mold. The ingot obtained in this way usually cuts and removes the ingot head structure where impurities are concentrated due to solidification and segregation phenomenon, and a multi-wire saw. Etc. and processed into a polycrystalline silicon substrate for solar cells.

Further, as a mold for making such a silicon ingot, a mold made of graphite or silicon dioxide (SiO ₂ ) is used, and a mold having a release material layer formed on the inner surface thereof. As the mold release material, silicon nitride powder such as silicon nitride (Si ₃ N ₄ ) powder, silicon carbide (SiC) powder, silica (SiO ₂ ), or the like is used. In general, powders of silicon nitride, silicon carbide, silicon oxide, etc. are mixed in a solution composed of a suitable binder and solvent and stirred to form a slurry, which is coated on the inner wall of the mold by means such as coating or spraying. This is generally known (see, for example, Non-Patent Document 1).
JP 62-108515 A Japanese Patent Laid-Open No. 2002-321037 JP-A-6-166564 JP 2002-292449 A 15th Photovoltaic Specialist Conf. (1981), P576 ~ P580, “A NEW DIRECTIONAL SOLIDIFICATION TECHNIQUE FOR POLYCRYSTALINE SOLAR GRADE SILICON”

  The mold made of silicon dioxide has high heat resistance, is excellent in shape stability even at a high temperature of about 1450 ° C. where silicon is dissolved and solidified, and has a low content of impurities that reduce the power generation efficiency of the solar cell element. In terms of surface, it is suitable as a mold for silicon casting. However, the conventional quartz mold has a cylindrical shape manufactured by a rotational molding method or a processing of a quartz tube, and the obtained silicon ingot has a cylindrical shape. Normally, the shape of the polycrystalline silicon substrate for solar cells allows the solar cell elements to be spread without waste in the limited space of the solar cell module, and improves the efficiency of handling and transport of the substrate in the solar cell elementization process. It is desirable that it is a square or a rectangle that can be most easily processed as a shape suitable for the above. For this reason, when a cylindrical silicon ingot obtained using a cylindrical mold is processed into a cube or a rectangular parallelepiped, it is cut and removed more than when processing a silicon ingot that has the shape of a cube or a rectangular parallelepiped from the beginning. In addition to the increase in the number of portions to be produced, the raw material yield is lowered, and there is a problem that quartz itself is expensive.

  In addition, a mold made by casting a silicon dioxide raw material into a mold shape by casting or pressure press molding, etc., and firing it, the raw material is molded into a mold shape using a gypsum mold or a mold when the mold is formed. In order to obtain a molded body by demolding, a draft angle (taper) for demolding from the bottom to the top is necessarily required on the inner surface of the mold to be molded. For this reason, since the side surface of the silicon ingot cast using such a mold is also tapered from the bottom of the ingot toward the top, when the silicon ingot is processed into a cube or a rectangular parallelepiped, the silicon removed at the bottom at the top of the ingot More excess silicon had to be removed, and there was a problem that the manufacturing cost of the silicon ingot increased because expensive silicon raw material was removed wastefully.

  In order to avoid such problems, a method has also been attempted in which a plate-like side member and a bottom member are produced using high-purity graphite, and then assembled and screwed to produce a mold (see, for example, Patent Document 1). .

  However, such a high-purity graphite mold is very expensive. Therefore, it is necessary to repeatedly use the graphite mold in order to reduce the cost of the polycrystalline silicon ingot. For this reason, when casting a silicon ingot using a graphite mold, a thick release material layer of several mm or more must be formed on the inner surface of the mold from the viewpoint of protection of the mold material, and the amount of expensive release material used increases. There was a problem that the manufacturing cost of a silicon ingot became high. Further, the release material layer is fragile, and breakage and peeling easily occur as the thickness increases. If the release material layer is peeled off from the mold and mixed into the silicon melt during the process of silicon dissolution or silicon melt cooling, it remains as a foreign substance in the silicon ingot, causing a defect in the silicon ingot and reducing the yield. It was.

  In addition, the assembling-type graphite member is consumed due to repeated use, and loosening occurs in the screwed part and the part where each side member and the bottom member are in contact with each other, so that silicon is melted and silicon melt is cooled and solidified. There was a problem that the melt leaked.

  Further, since the density of silicon is larger than that of solid, that is, the silicon melt is solidified and expanded, when the silicon melt is cooled and solidified in the mold, the side surface and bottom of the mold are stressed in the direction of spreading outward. receive. For this reason, the stress applied to the screwing portion is particularly large among the mold members, and the mechanical wear is severe, so that the screw thread is damaged, and the mold member is loosened and the silicon melt leaks.

  In addition, the mold release material layer formed and used on the mold inner surface of the silicon casting mold has an effect of preventing the silicon melt and the mold from being fused while the silicon is melted or cooled and solidified. The release material layer is generally made by mixing and stirring a ceramic powder such as silicon nitride or silicon oxide with a solvent such as water or alcohol and a binder such as PVA (polyvinyl alcohol) to form a slurry, and applying this release material slurry to the inner surface of the mold. Formed by drying. The release material layer formed by this method has the following problems.

  First, a brush, a spatula, a spray, or the like is generally used for applying the release material slurry. However, the thickness of the formed release material layer varies and a non-uniform layer is formed. Since the binder component and solvent that connect ceramic powders are removed from the release material slurry after coating by a heat treatment process such as a drying process or debinding process, the release material layer when casting silicon has only the cohesive force of the powder. It is thought that it adheres to the inner surface of the mold. For this reason, the release material slurry cannot be uniformly applied, and when the thickness of the release material layer varies or a non-uniform layer is formed, the release material layer is peeled off from the mold at an excessively thick layer. There is a problem that it becomes fragile, for example. If the mold release material layer is fragile, the mold release material will be damaged when the silicon raw material is dissolved in the mold or when the silicon melt is poured into the mold, or when the silicon melt is cooled and solidified. A part may be peeled off from the inner surface of the mold and mixed into the silicon melt. Further, if the mold release material is peeled off and the mold material is exposed, it does not play its original role as a mold release material.

  In addition, the release material layer forming method in which the release material slurry is spray-coated on the inner surface of the mold can be applied relatively uniformly, but the thickness of the release material layer that can be formed by one application is thin, so it takes time to apply and the productivity There is a problem that it is bad and expensive. And, in order to increase the thickness of the release material layer in a short time, if the applied release material slurry is overcoated without sufficiently drying, there is a problem that the application time cannot be shortened because cracks and peeling occur in the applied release material layer. is there. The release material slurry used for this spray coating needs to increase the solvent component to the powder in order to prepare a viscosity lower than that of the release material slurry when applied using a brush or spatula. The powder density is small, and when the solvent or binder component is removed by drying or heat treatment after coating, the release material layer shrinks, and cracking and peeling are likely to occur. Further, since the powder density is small, the cohesive force of the powder is weak, and the adhesive force to the mold is reduced, so that the release material layer is easily peeled from the mold. For this reason, in the spray application of the release agent slurry, it is necessary to dry the thinly applied slurry sufficiently and repeatedly apply it repeatedly.

  In addition, the organic binder added to the release agent slurry is usually turned into a tar-like soot where the carbon residue that has been pyrolyzed when heated to a high temperature in an inert gas such as vacuum or Ar where silicon is cast. And adheres to the inner surface of the mold. Therefore, when silicon is cast in such a situation, carbon residues are mixed in the ingot, resulting in precipitates such as hard SiC in the silicon, which deteriorates the workability of the subsequent ingot or becomes a current leakage source. There is a possibility of deteriorating the characteristics of the solar cell element. Such a binder can be removed by a debinding step in which the binder is heated at a high temperature in an oxygen-containing atmosphere. However, it takes time to provide the debinding step, and there is a problem in terms of cost and productivity. In addition, when a graphite mold is heated at a high temperature in an oxidizing atmosphere, the mold is subject to oxidative wear. Therefore, it is difficult to provide such a debinding step.

  Patent Document 2 discloses a technique that avoids this problem by using PVA as a binder and performing the binder removal step in a temperature range of 450 to 600 ° C. at which the graphite mold is not oxidized. However, the problem that it was difficult to apply to all organic binders remained.

  The present invention has been made in view of the above-described conventional problems, and an object thereof is to provide a casting mold for a polycrystalline silicon ingot for manufacturing a high-quality silicon ingot at a low cost and a method for manufacturing the same. And

The silicon casting mold of the present invention is a silicon casting mold made of silicon dioxide in which a release material layer containing at least silicon nitride is formed on the inner surface of the mold, and the inner surface of the mold has one or more inner side surface portions. And one inner bottom surface portion, and the side member having the inner side surface portion and the bottom surface member having the inner bottom surface portion are heat-bonded, and 1 nm is formed on the surface of silicon nitride contained in the release material layer. An oxide layer having the above thickness is provided.

  In the above-mentioned silicon casting mold, the release material layer contains silicon dioxide.

  In the silicon casting mold described above, the silicon dioxide forming the mold is fused silica.

  In the above-described silicon casting mold, the inner bottom surface portion and the inner side surface portion are both substantially flat.

  The above-mentioned silicon casting mold is formed such that the inner surface of the mold is substantially perpendicular to the inner bottom surface portion.

  The method for manufacturing a silicon casting mold includes a step of forming a release material layer containing at least silicon nitride on the inner surface of the mold, and a step of heating the release material layer in an atmosphere containing oxygen. .

  Manufacture of a silicon casting mold comprising one or a plurality of inner side surface portions and one inner bottom surface portion, wherein the side surface member having the inner side surface portion and the bottom surface member having the inner bottom surface portion are heat bonded. A method comprising: forming a release material layer containing at least silicon nitride on the inner surface of the mold; and heating the release material layer in an atmosphere containing oxygen.

  In the above-described silicon casting mold manufacturing method, the silicon casting mold includes a side member having the inner side surface portion and a bottom surface member having the inner bottom surface portion, and the inner side surface portion and the bottom surface member of the side surface member. A first step of forming a release material dry layer by applying a release material slurry containing at least silicon nitride to each of the inner bottom surface portions of the inner bottom surface portion, the side member on which the release material dry layer is formed, and the A second step of heating the bottom surface member on which the release material dry layer is formed in an oxygen-containing atmosphere to form the release material layer; and heating and joining the side surface member and the bottom surface member. 3 steps.

Manufacture of a silicon casting mold comprising one or a plurality of inner side surface portions and one inner bottom surface portion, wherein the side surface member having the inner side surface portion and the bottom surface member having the inner bottom surface portion are heat bonded. The silicon casting mold includes a side surface member having the inner side surface portion and a bottom surface member having the inner bottom surface portion, and an inner side surface portion of the side surface member and an inner bottom surface portion of the bottom surface member. A first step of forming a release material drying layer by applying a release material slurry containing at least silicon nitride to each of them and drying, and forming the side member on which the release material drying layer is formed and the release material drying layer A second step of heating the formed bottom member in an oxygen-containing atmosphere to form the release material layer, and a third step of heating and joining the side member and the bottom member. It becomes. Further, in the above-described method for producing a silicon casting mold, after the first step, the side member and the bottom member are combined and heated in an atmosphere containing oxygen, and the second step and the The third step is performed simultaneously.

The silicon casting mold of the present invention is a silicon casting mold made of silicon dioxide in which a release material layer containing at least silicon nitride is formed on the inner surface of the mold, and the inner surface of the mold has one or more inner side surface portions. And one inner bottom surface portion, and the side surface member having the inner side surface portion and the bottom surface member having the inner bottom surface portion are heat-bonded. Since no mechanical parts such as screws are used, the silicon melt does not leak.

Further, since the oxide layer having a 1nm or more in thickness on the surface of the silicon nitride contained in the prior KiHanare type material layer was set to be provided, and the contact between the silicon melt and the silicon nitride, silicon by it It is possible to effectively suppress the dissolution of silicon nitride into the melt and the formation of silicon nitride precipitates in the silicon ingot.
Furthermore, the oxide layer formed on the silicon nitride surface of the release material layer is softened at a high temperature during silicon casting to promote the bonding between the silicon nitride powders in the release material layer, thereby improving the strength of the release material layer. It is possible to effectively reduce breakage and peeling of the release material layer and mixing into the silicon ingot, which were problems in the past.
Furthermore, as a result, a high-quality polycrystalline silicon ingot can be manufactured.

In addition, the release agent layer to bind the fact that as silicon dioxide is formed by containing silicon dioxide of the release material layer is softened at a high temperature in the silicon cast silicon nitride powder particles of the release material layer The action is promoted.
As a result, the strength of the release material layer is improved, breakage and peeling of the release material layer, mixing into the silicon ingot and the like can be extremely reduced, and a high-quality polycrystalline silicon ingot can be manufactured. .

And, the silicon dioxide to form a pre-Symbol template, since the so as fused silica, on inexpensive, can be molded freely in a shape required, the mold silicone cast by firing the shaped body The required mechanical strength can be obtained.

Also, the pre-Symbol in the bottom portion and the inner surface portion, since both have a substantially flat surface, a wider range of methods of applying these inner bottom portion and the release material slurry to the inside surface, the optimum depending on the application You will be able to choose a method.
As a result, a uniform and strong release material layer can be formed inside the mold with high productivity.

And, the front Symbol mold inner surface, since the inner side surface portion with respect to the inner bottom portion is set to be formed so as to be substantially perpendicular, conventional mold is tapered such draft molded inside the mold As compared with the above, the amount of silicon to be removed at the time of cutting the polycrystalline silicon ingot can be reduced, and the polycrystalline silicon ingot can be manufactured at a low cost.

In the following, the manufacturing method of divorced casting mold, comprising the steps of forming a release agent layer containing at least silicon nitride on the inner surface of the mold, and a step of heating the releasing agent layer in an atmosphere containing oxygen Thus, the silicon casting mold of the present invention can be manufactured very easily.

Each Also, before Symbol silicon casting mold is configured to and a bottom member having the inner bottom portion and the side members having the inner surface portion, an inner bottom portion of the bottom member and the inner surface of the side member First, a release material slurry containing at least silicon nitride is applied and dried to form a release material dry layer, and the side member on which the release material dry layer is formed and the release material dry layer are formed. A second step of heating the bottom member in an oxygen-containing atmosphere to form the release material layer, and a third step of heating and joining the side member and the bottom member. Become.
Thus, the second step of forming the mold release material layer by heating the mold release material dry layer in an oxygen-containing atmosphere at the same time that the silicon casting mold of the present invention can be produced very easily. By this, organic components such as the organic binder contained in the release material slurry can be almost completely removed, so that a silicon casting mold capable of obtaining a high quality silicon ingot can be obtained.

And, the after the previous SL first step, by heating in an atmosphere containing oxygen in combination with the bottom member and the side member, at the same time allowed to proceed with the second step and the third step Therefore, the mold release material layer and the mold according to the present invention can be joined at the same time, and the silicon casting mold of the present invention can be produced efficiently with extremely high productivity. It becomes possible.

  Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

  FIG. 1 is a diagram for explaining a silicon casting mold according to the present invention. FIG. 1A is a perspective view showing an example of a silicon casting mold, and FIG. 1B is a cross-sectional view in the Aa direction of FIG.

  As shown in FIG. 1A, in this example, four side members 11 and one bottom member 12 are assembled and heat-bonded to form an integral mold 1. The inner surface of the mold of the bottom member 12 and the side member 11 is preferably subjected to uneven processing. This is to improve the adhesiveness and fixability of the release material slurry described later.

  Since the side member 11 and the bottom member 12 are made of silicon dioxide, even when the temperature is higher than the melting point of silicon (1412 ° C.) or higher, the silicon melt is stably held and cooled to form silicon. Ingots can be made.

  The silicon dioxide is preferably fused silica. Such fused silica is known, for example, in Patent Document 3, and is formed into a free shape by pressure-molding fused silica powder, slurry casting as slurry, or extrusion molding. can do. And it sinters at the temperature of 1200-1500 degreeC, and turns into a fused silica sintered compact. Further, such fused silica is amorphous and highly pure, and gradually softens at a temperature of about 1100 ° C. or higher. Therefore, by combining the members of this sintered body, for example, 1200 ° C. to 1500 ° C. It becomes possible to sinter in a temperature range of about.

  By using fused silica in this way, it is possible to form a mold 1 having a complicated shape with high purity, high moldability, high dimensional accuracy, and good reproducibility. Furthermore, since fused silica is less expensive than crystalline materials such as quartz, it can be a low-cost process.

  In addition, although the mold member formed from these fused silicas is softened in such a temperature range, the bottom and side surfaces of the mold are supported by a plate-like refractory member such as a graphite plate or a ceramic plate from the outside of the mold. Thus, the stability of the shape sufficient to hold the silicon melt can be easily obtained, so that the mechanical strength required for the silicon casting mold can be obtained, which is not a problem.

  Further, since the members are joined by heating, the members are joined without a gap, and no mechanical parts such as screws are used, so that the silicon melt held in the mold 1 does not leak. Furthermore, each member of the mold 1 can be easily disassembled apart as compared with the case where all are formed simultaneously by integral molding, so that the silicon ingot can be easily taken out from the mold 1.

  Furthermore, in the silicon casting mold according to the present invention, a release material layer 2 containing at least silicon nitride is provided on the inner surface of the mold 1 as shown in the sectional view of FIG. Further, it is more desirable that the release material layer 2 contains silicon dioxide in addition to silicon nitride. The release material layer 2 is prepared by weighing a predetermined amount of the release material powder (various if necessary), mixing it in, for example, a 10% by weight PVA (polyvinyl alcohol) aqueous solution, and stirring it. Is applied to the inner surface of the mold 1 and dried to form a release material drying layer, which is further heated at a predetermined temperature in an atmosphere containing oxygen. The mold release material layer 2 of the silicon casting mold can be formed.

  FIG. 2 shows a conceptual diagram of the structure of silicon nitride contained in the release material layer of the silicon casting mold of the present invention. In the present invention, as shown in FIG. 2A, the silicon nitride 201 contained in the release material layer 2 provided on the inner surface of the mold 1 is provided with an oxide layer 201a having a thickness of 1 nm or more on the surface. It is desirable that The silicon nitride 201 contained in the release material layer 2 has the function and effect as a release material even if such an oxide layer 201a is not provided. In this way, the oxide layer 201a is formed around the silicon nitride 201. When provided, the effect of effectively suppressing the contact between the silicon melt and the silicon nitride 201, the dissolution of the silicon nitride 201 into the silicon melt, and the formation of silicon nitride precipitates in the silicon ingot. There is an advantage that increases further.

  Furthermore, the oxide layer 201a formed on the surface of the silicon nitride 201 of the release material layer 2 is softened at a high temperature of about 1412 ° C. during silicon casting, and the bonding between the powders of the silicon nitride 201 is strengthened, and the strength of the release material layer 2 is increased. There is also an effect that it can be improved.

  When the release material layer 2 contains silicon dioxide, as shown in FIG. 2B, the silicon dioxide 202 surrounds the silicon nitride 201 around which the oxide layer 201a is formed as a finer powder. It is preferable to have such a configuration. Since this silicon dioxide 202 has the effect of softening at a high temperature during silicon casting and bonding the powders of silicon nitride 201 in the release material layer 2, the strength of the release material layer 2 can be improved. It is possible to effectively reduce breakage and peeling of the release material layer 2 and mixing into the silicon ingot, which are problems, and as a result, a high-quality polycrystalline silicon ingot can be manufactured.

  As the silicon nitride 201 contained in the release material layer 2 of the present invention, it is desirable to use a spherical powder obtained by a thermal decomposition method of silicon diimide. The silicon nitride 201 obtained by this method has a narrow particle size distribution and a uniform size. Therefore, when the release material layer 2 is fired by the method described later, the particle size of the silicon nitride 201 This is because it is possible to prevent the particles from aggregating and fusing due to the variation of the above, and the strength of the release material layer 2 is stabilized. As the silicon nitride 201, it is preferable to use a spherical powder having a particle size of 0.1 to 1.5 μm. Outside this range, the content of coarse particles such as aggregated particles or fused particles of the silicon nitride 201 in the release material layer 2 is undesirably high.

  Then, if the spherical silicon nitride 201 obtained by the thermal decomposition method of silicon diimide is subjected to heat treatment in a furnace under an oxidizing atmosphere, the oxide layer 201a can be formed on the surface. In this heat treatment, silicon nitride 201 is placed in, for example, an electric furnace (oxidizing furnace) and heated in an oxidizing atmosphere at 850 ° C. to 1300 ° C. for about 30 minutes to 600 minutes, so that a large amount of amorphous silica is contained. An oxide layer 201a can be formed. If the temperature is too high, the silicon nitride powders are bonded or fused together at the oxide layer 201a and aggregate to form hard coarse particles, which is not preferable. When there are many hard and coarse particle groups in the powder, workability in preparation or application of the release agent slurry is poor, which is not preferable. Specifically, since coarse particles settle in the slurry, a uniform slurry cannot be produced and cannot be uniformly applied, resulting in variations in the thickness and strength of the formed release material film, resulting in a thin part of the release material film. In addition, there is a problem that the silicon melt and the mold are likely to be fused at a damaged part. Furthermore, the coarse particles in the release material film have a weak cohesive force in the film and have poor adhesion to the mold and are easy to peel off. Therefore, the release material film containing these coarse particles peels and mixes into the silicon melt. The problem of becoming is unsuitable. On the other hand, if the temperature of the heat treatment is too low, the formation of the oxide layer 201a does not proceed, and there is a problem that a long time is required for the treatment.

  Note that the heat treatment may be performed on the silicon nitride 201 in advance, and the release material layer 2 may be formed using the silicon nitride 201 having the oxide layer 201a formed on the surface. Since an excellent silicon dioxide mold is used, a mold release material slurry containing silicon nitride 201 without forming oxide layer 201a is applied to the inner surface of mold 1 and dried, and then mold 1 is 850 ° C. in an oxidizing atmosphere. By heating to the above temperature, the oxide layer 201a can be formed on the surface of the silicon nitride 201 contained in the release material dry layer, and the silicon casting mold of the present invention can be produced very easily. This is desirable.

  By such heat treatment, the thickness of the oxide layer 201a formed on the surface of the silicon nitride 201 can be controlled by treatment temperature and time, but is preferably 1 nm or more. When the thickness is 1 nm or less, the effect of improving the sinterability of the silicon nitride 201 is poor. Note that the thickness of the oxide layer 201a can be measured by a TEM (transmission electron microscope) image and elemental analysis using the image. Note that the upper limit of the thickness of the oxide layer 201a formed on the surface of the silicon nitride 201 is not particularly limited. However, if the thickness exceeds 200 nm, the silicon nitride 201 may be slightly brittle. It is desirable to make it below this value.

  In the oxide layer 201a on the surface of the silicon nitride 201 manufactured by such a method, Si—OH (silanol group) derived from moisture in the air is formed. And since Si-O-Si (siloxane bond) arises between Si-OH (silanol group) of mutual silicon nitride 201, the adhesiveness of silicon nitride 201 is improved significantly, and the mold release material layer 2 is strong. Become a thing.

  As described above, if the mold release material layer 2 contains the silicon nitride 201 having the oxide layer 201a formed on the surface, the mold 1 is kept at a high temperature in the casting furnace and the silicon melt is poured. At this time, the silicon nitrides 201 of the release material layer 2 are sintered together via the oxide layer 201a on the surface, so that the strength of the release material layer 2 is appropriately improved. Therefore, the mold release material layer 2 is not peeled off when the silicon melt is poured, and the mold 1 and the silicon ingot do not come into contact with each other, and the mold release material layer 2 is separated when the silicon ingot is removed from the mold 1. Therefore, demolding is facilitated, and chipping of the silicon ingot that occurs when the mold 1 and the silicon ingot adhere can be prevented.

  The silicon dioxide 202 contained in the release material layer 2 of the present invention is obtained by injecting silicon tetrachloride into a high-temperature flame of a combustible gas and an auxiliary combustion gas, for example, hydrogen gas and oxygen gas, and performing heat treatment. It is preferable to use amorphous spherical silica fine powder (so-called fumed silica fine powder or fused silica powder), and it is desirable to use fine powder having a particle size of about 0.01 to 0.1 μm.

  As shown in FIG. 2B, the release material layer 2 of the present invention is a mixed layer in which silicon nitride 201 and silicon dioxide 202 are mixed. Therefore, silicon dioxide 202 surrounds silicon nitride 201. The surrounding effect induces the effect of strongly bonding the silicon nitrides 201 to each other.

Note that as the silicon dioxide 202 used here, it is desirable to use amorphous fine silica powder, and among the amorphous fine silica powder, the one obtained by the above-described vapor phase method may be used. desirable. Amorphous spherical silica fine powder obtained by hydrolysis and ion exchange of sodium silicate (Na ₂ O · nSiO ₂ ) solution contains many alkali metal impurities and causes contamination of silicon ingots. It is necessary to use it after.

  The weight ratio of the silicon dioxide 202 in the release material layer 2 is preferably in the range of 10 to 70% by weight, and more preferably 10 to 20% by weight of the silicon dioxide 202. When the weight ratio of silicon dioxide 202 is greater than 70%, the mold 1 adheres to the silicon ingot via the release material, and chipping occurs in the silicon ingot when the mold is peeled off from the silicon ingot. Further, if the weight ratio of the silicon dioxide 202 is smaller than 10%, the fixing effect between the silicon nitride 201 and the silicon dioxide 202 is reduced, and the strength of the release material layer 2 is lowered, which is not preferable.

  In the present invention, a spherical amorphous obtained by spraying silicon tetrachloride into a high-temperature flame formed by hydrogen gas and oxygen gas as the fine silicon dioxide 202 in the release material layer 2. If fine silica is used, since the sinterability is high, the release material layer 2 can be easily formed on the mold 1 even if the addition ratio is in the range of 5 to 20% by weight. The range of weight ratios that can be used can be broadened compared with the case of using silica powder (at least 10% by weight).

  As described above, the mold 1 provided with the release material layer 2 containing at least silicon nitride 201 is placed in an argon (Ar) atmosphere whose pressure is reduced to 7 to 100 Torr, and the mold 1 is at a temperature similar to or slightly lower than the silicon melt. Heat and pour silicon melt. Alternatively, the silicon raw material may be placed in the mold 1 and directly heated and melted. Thereafter, the temperature is gradually lowered from the bottom of the mold 1, and the silicon melt is gradually solidified in one direction from the bottom of the mold. Finally, the silicon ingot is taken out from the mold 1 and cut, and sliced using a multi-wire saw or the like to obtain a silicon substrate for a solar cell.

  In the silicon casting mold of the present invention, the side member 11 and the bottom member 12 are assembled and heated to form an integral mold as follows.

  The side member 11 as shown in FIG. 1 can produce a molded body by, for example, press-molding a raw material made of silicon dioxide as a main component, preferably a powder raw material of fused silica. At this time, it is desirable to form the inner side surface portion 111 corresponding to the inner surface of the mold 1 so as not to warp when the side surface members 11 are assembled. Moreover, it is desirable to form a groove part and a convex part (not shown) or the like so that the side surface members 11 come into contact with each other and are joined to the portion where the side surface members 11 are combined and joined. The members can be securely fixed to each other.

  Similarly, the bottom member 12 is also formed by press-molding a raw material made of silicon dioxide as a main component, preferably a powdered raw material of fused silica, for example, under pressure. At this time, it is desirable that the inner bottom surface portion 121 is formed so as not to be warped. Moreover, a groove part and a convex part (not shown) are formed in a part where the bottom surface member 12 and the side surface member 11 are combined and joined so that the bottom surface member 12 and the plurality of side surface members 11 come into contact with each other. It is desirable that the members can be securely fixed to each other.

  Further, on the inner surface of the mold, the shape of the portion where the side members 11 and the side member 11 and the bottom member 12 are joined to each other is a right angle, an R shape with a radius less than 5 mm, or a C chamfered shape with a radius less than 5 mm. desirable. This is because, when a square or rectangular silicon substrate is cut out from the completed silicon ingot, it is possible to reduce the portion to be cut and removed excessively.

  Moreover, it is desirable that the thickness of the side member 11 and the bottom member 12 be in the range of 5 mm to 25 mm. The reason is that when the thickness is 5 mm or less, the mold becomes thin, the upper part of the side member 11 exposed to a high temperature of about 1450 ° C. that melts or solidifies silicon is softened and deformed, and a normal-shaped silicon block cannot be cast. . Further, if the thickness is 25 mm or more, the mold becomes thicker and heavier than necessary even when considering the mechanical strength required for the mold, and handling becomes difficult.

  In the method for producing a casting mold for silicon casting according to the present invention, a release material slurry containing at least silicon nitride is applied to the side that becomes the inner surface of the mold when the side member 11 and the bottom member 12 are assembled into a mold shape, and dried. And a second step of forming the release material dry layer into the release material layer 2 by heating the release material dry layer in an oxygen-containing atmosphere. And a third step in which the side member 11 and the bottom member 12 are assembled into a mold shape and heat-bonded.

  First, as a first step according to the method for producing a silicon casting mold of the present invention, at least silicon nitride 201, preferably silicon dioxide 202, is added to the mold inner surface, and a predetermined amount is weighed. When mixed in a 15% by weight PVA (polyvinyl alcohol) aqueous solution and stirred, these raw materials that are powders can be made into a slurry and can be easily applied to the mold 1. The release material dry layer is formed by applying and drying the release material slurry thus prepared on the inner surface of the mold 1.

  In addition, for forming the release material drying layer according to the present invention, a method of applying to a member of the mold 1 with a brush or a spatula and drying on a hot plate may be used. For example, the mold 1 may be used by using a spray or the like. A method of coating the inner surface of the substrate and drying it, and a method of thermocompression bonding using a laminating apparatus provided with a heating plate / silicon rubber diaphragm are also possible.

  In particular, when the inner side surface portion and the inner bottom surface portion constituting the inner surface of the mold 1 are substantially flat in the side surface member and the bottom surface member, respectively, it is preferable because many coating methods can be applied. It is desirable to apply and form the release agent slurry using screen printing. If the release material slurry is applied to the inner side surface portion and the inner bottom surface portion using this screen printing, the uniformity of the release material dry layer can be remarkably improved. As a result, the release material layer 2 of extremely high quality is formed into the mold 1. This is because the yield of silicon ingots can be increased.

  It is desirable to form the release material dry layer so that the release material layer 2 has a thickness in the range of 0.3 mm to 1.2 mm. The reason is that if the release material layer 2 is thinner than 0.3 mm, the silicon melt penetrates the release material layer 2 and is fused with the mold 1, and the cast silicon ingot is cracked, chipped, and the like. If the mold material layer 2 is thicker than 1.2 mm, it is sufficient as a mold release material that does not fuse the silicon melt to the mold 1, but it takes time to form the mold release material layer 2, and the thickness of the mold release material layer 2 is necessary. If it is too thick, the release material layer 2 is likely to be damaged or peeled off, and the release material layer 2 peeled off from the mold 1 is mixed as foreign matter in the silicon during the process of silicon dissolution or silicon melt cooling and solidification. This is because a defect is generated and the yield is lowered. Note that when the release material dry layer is heated to form the release material layer 2, the thickness thereof is slightly reduced, but may be considered to be substantially the same.

  Next, the second step according to the method for producing a silicon casting mold of the present invention is as follows. First, if the side member 11 and the bottom member 12 on which the release material dry layer is formed by the above-described method are fired in an oxidizing atmosphere of 850 to 1500 ° C., the silicon nitride 201 contained in the release material dry layer is formed. The release layer 2 can be obtained by forming the oxide layer 201a on the surface.

  The mold release material layer 2 obtained in this way, as already described above, when the silicon nitride 201 is poured into the mold release material layer 2 when the mold 1 is held at a high temperature in the casting furnace and the silicon melt is poured. Is sintered through the oxide layer 201a on the surface, and the strength of the release material layer 2 is appropriately improved. Therefore, the mold release material layer 2 is not peeled off when the silicon melt is poured, and the mold 1 and the silicon ingot do not come into contact with each other, and the mold release material layer 2 is separated when the silicon ingot is removed from the mold 1. Therefore, demolding is facilitated, and chipping of silicon that occurs when the mold 1 and the silicon ingot adhere can be prevented.

  Furthermore, in the second step, the release material dry layer is baked in a high-temperature oxidizing atmosphere, so that the organic component in the release material dry layer, for example, a wide variety of binders added to the release material slurry, 2 can be almost completely removed. Therefore, it is possible to extremely effectively prevent the carbon residue from being mixed into the silicon ingot from the release material layer 2, so that a high quality solar cell element substrate can be obtained.

  Then, as a third step according to the method for producing a silicon casting mold of the present invention, the side member 11 and the bottom member 12 so that the inner side surface and the inner bottom surface on which the release material layer 2 is formed become the inner surface of the mold 1. Are assembled by heat joining at 1200 to 1500 ° C. to obtain a mold 1. As this heat bonding method, a glass bonding method in which only the bonding portion is locally heated may be used, or the whole may be heated and bonded in a state where the members are in contact with each other.

  Further, when assembling the side surface member 11 and the bottom surface member 12 to form the mold 1, it is desirable that the inner surface of the mold is formed so that the inner side surface portion is substantially perpendicular to the inner bottom surface portion. If comprised in this way, compared with the conventional mold | type with the taper of the shaping | molding draft etc. inside the casting_mold | template 1, the quantity of the silicon | silicone removed at the time of the cutting process of a polycrystalline silicon ingot can be reduced, and a polycrystalline silicon ingot is made. This is because it can be manufactured at low cost.

  In addition, in the above-mentioned 3rd process, if it heats to predetermined temperature in the atmosphere containing oxygen, a 2nd process and a 3rd process can be advanced simultaneously. That is, the side surface member 11 and the bottom surface member 12 on which the release material dry layer is formed are fired in an oxidizing atmosphere of 1200 ° C. to 1500 ° C. with the inner side surface portion and the inner bottom surface portion being the inner surface of the mold 1. For example, the oxide layer 201a is formed on the surface of the silicon nitride 201 contained in the release material dry layer to obtain the release material layer 2, and at the same time, the side member 11 and the bottom member 12 are joined together by heating. Thus, the template 1 can be obtained.

  Thus, if the 2nd process and the 3rd process are advanced simultaneously, the time and labor which a process requires can be saved, and it is very advantageous at the point of cost or productivity.

  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, after joining the joint portions of the side member 11 and the bottom member 12 provided with the release material drying layer or the release material layer 2 with a slurry made of silicon dioxide or the like to form a box shape, it is baked and integrated. The mold 1 may be produced by improving the adhesion between members.

  Further, before forming the release material layer 2, the members may be heat-bonded to each other, and the release material layer 2 containing at least silicon nitride may be provided later. At this time, before the release material layer 2 is provided, the side member 11 and the bottom member 12 are previously heated and integrated at a predetermined temperature to obtain a mold 1, and at least silicon nitride is formed on the inner surface of the mold 1. The release material layer 2 having an oxide layer 201a having a thickness of 1 nm or more on the surface of the silicon nitride 201 can be obtained by applying and drying a release material slurry preferably containing silicon dioxide and then heating at a predetermined temperature in an oxidizing atmosphere. It is more desirable because it can be formed.

  Further, the method of forming the release material layer 2 is not limited to the method of applying a slurry of the release material, and can be formed by plasma spraying or the like. For the release material layer 2 by plasma spraying, for example, the method disclosed in Patent Document 4 by the applicant can be used. Patent Document 4 is a method of coating a mixture of silicon nitride and silicon dioxide powder in a weight ratio of 28:72 to 75:25 using a plasma spraying machine, whereby the mold adheres to the silicon ingot. As a result, the chipping of silicon generated can be prevented, and the binder removal step for removing the organic binder, which has been conventionally used, can be omitted, and the manufacturing cost of the silicon ingot can be reduced. Further, since the release material layer 2 by thermal spraying is formed while being heated to a high temperature in the atmosphere, an oxide layer 201a is preferably formed on the surface of the silicon nitride 201 contained in the release material layer 2.

  Further, the side member 11 and the bottom member 12 are not limited to the above-described shapes, and various shapes can be used within the scope of the gist of the present invention. Another example of a silicon casting mold according to the present invention is shown in FIGS.

  3 (a) and 3 (b), the end portions of the four side members 11a are inclined, and the side members 11a are orthogonal to each other with the end portions in contact with each other, so that the side portions of the mold 1 are formed. Is formed. And the bottom face member 12 is provided in the lower part of this side member 11a, and the casting_mold | template 1 with which the mold release material layer 2 was provided in the casting_mold | template inner surface is formed. The plate-like side member 11 shown in FIG. 3 is obtained by, for example, forming a molded body by press-molding a raw material of fused silica and sintering it at a predetermined temperature to obtain a sintered body member of fused silica. be able to. At this time, it is desirable to form the inner side surface portion so as not to warp. Moreover, you may form the fitting structure which consists of a groove part and a convex part (not shown) in the position which contact | abuts in the junction part of the four corners of the casting_mold | template 1 where each side part joins.

  4 (a) and 4 (b) are formed so as to be side portions of the mold 1 in a state where one or a plurality of frame-like side members 11b are stacked. And the bottom face member 12 is provided in the lower part of this side surface member 11a, and the casting_mold | template 1 with which the mold release material layer 2 was provided in the casting_mold | template inner surface is formed. The plate-like side member 11 shown in FIG. 3 can obtain a sintered body member of fused silica by, for example, producing a molded body by extruding a raw material of fused silica and sintering at a predetermined temperature. it can. At this time, it is desirable to form the inner side surface portion so as not to warp. In addition, the joint portion where each member overlaps is formed with a fitting structure composed of a groove portion and a convex portion (not shown) at a contact position, or a step shape (not shown) on the inner side and the outer side of the upper and lower members. ) May be formed.

  Further, as shown in FIGS. 5 (a) and 5 (b), a cylindrical side member 11c and a columnar bottom member 12a are assembled so that the mold 101 is provided with the release material layer 2 on the inner surface of the mold. It is also possible to form a fitting structure consisting of a groove and a convex (not shown) at the abutting position at the joining portion where each member overlaps.

(Example 1)
Using a raw material made of fused silica, four molded bodies of a plate-like side member 11 having a width of 360 mm, a height of 450 mm, and a thickness of 10 mm shown in FIG. 1 and a plate-like bottom member 12 having a thickness of 370 mm and a thickness of 10 mm are shown. One molded body was produced by pressure molding and fired at 1300 ° C. to obtain a mold member.

  The release material slurry obtained by mixing silicon nitride and silicon dioxide at a weight ratio of 8: 2 on the surfaces of these mold members and stirring and mixing with an 8% PVA aqueous solution is 0.6 mm thick by screen printing. After the coating material is applied to form a release material dry layer (first step), the release material dry layer is assembled so that the surface on which the release material dry layer is formed is inside the container, and is fired at 1250 ° C. in the atmosphere. 2 and each member was heated and joined to produce a silicon casting mold 1 (the second step and the third step were simultaneously performed).

  About 70 kg of silicon raw material is charged into the obtained mold, and after heating and dissolving the silicon raw material in an argon (Ar) atmosphere reduced to 7 to 100 Torr, the temperature is gradually lowered from the bottom surface of the mold to allow the silicon melt to flow in one direction. Solidification was performed to obtain silicon ingots each having a height of about 250 mm.

  With respect to the obtained silicon ingot, the presence or absence of fusion with the mold, the presence or absence of foreign matter, and the presence or absence of leakage of the silicon melt were observed. We also evaluated total processing costs including mold costs. As for the film thickness of the oxide on the surface of silicon nitride in the release material layer 2, a necessary film is obtained after preliminarily determining the relationship between the condition and the film thickness using TEM observation. A thick sample was prepared.

  Regarding the formation of the release material layer 2, samples were similarly prepared for the application method using a brush, the spray application method, and the plasma spraying method in addition to the method using screen printing. The thickness of the release material layer 2 was basically set to 0.6 mm. However, in the case of plasma spraying, the film formation rate per pass was small, and thus the evaluation was performed at 0.1 mm.

On the other hand, as a conventional example, when the release material layer 2 does not contain silicon nitride (No. 9), when the mold type is not a split type but an integral type (No. 14), when the mold material is graphite (No. 16). )), A sample was prepared in exactly the same manner and subjected to the same evaluation. These results are shown in Table 1. In the item of the release material layer in Table 1, “◯” indicates that it was added, and “X” indicates that it was not added. In the result items, “◎” is very good, “○” is good, “Δ” is the limit of the allowable range, and “×” is not possible. In Table 1, the fact that the silicon nitride oxide film thickness cannot be measured means that the thickness of the oxide layer is smaller than 1 nm, and the thickness cannot be measured by TEM measurement.

  As shown in Table 1, No. 1 is within the scope of the present invention. For the samples 1 to 8, 10 to 13, and 15, all acceptable results were obtained. However, other samples outside the scope of the present invention did not satisfy any of the evaluation items, confirming the effects of the present invention.

  No. The sample 11 is the release material layer 2 formed by a coating method using a brush, but there is a problem that it is somewhat difficult to form a uniform release material layer 2 due to coating unevenness and the like. However, the release material peeled off from the mold 1 was mixed into the ingot. No. The sample 12 is a release material layer 2 formed by spray coating. A uniform release material layer 2 is formed, and no foreign matter is mixed into the ingot. However, the number of times of coating is different from that of other coating methods. There are many problems that the number of work man-hours increases, which is disadvantageous in terms of cost. Furthermore, no. The sample 13 is a release material layer 2 formed by plasma spraying, and a release material layer 2 not containing an organic binder is formed, and no quality problem was observed, but it takes time to form a film. There was a disadvantage in terms of cost.

  As described above, the release material layer forming method using screen printing can form the uniform release material layer 2 with high work efficiency, and no fusion or contamination is observed.

(Example 2)
Using a raw material made of fused silica, as shown in FIG. 4, three shaped bodies of a frame-shaped side member 11b having an inner dimension of 350 mm square, a thickness of 10 mm, and a height of 150 mm, and a plate-like bottom surface member having a thickness of 370 mm and a thickness of 10 mm One 12 molded bodies were produced. The corner portion of the inner side surface portion of each side member was processed into an R shape having a radius of 0 mm (that is, a right angle), 5 mm, 10 mm, and 15 mm. These side member 11b and bottom member 12 are assembled into a box shape, and a slurry made of fused silica is applied and bonded to the joint portion, and then fired at 1300 ° C. to obtain an integral mold 1 (from Example 1) Example 5).

  On the other hand, the inner dimensions of the mold are 350 mm square on the bottom and 450 mm in height, and each side has a 15 mm thick integrated fused silica mold with a taper of 6 mm toward the opening above the mold. It was. Each side or four corners inside the mold has an R shape with a radius of 15 mm (Example 6).

  A mold release material slurry (PVA 8) containing, in a weight ratio of 7: 3, silicon nitride having an oxide layer having a thickness of 20 nm on the surface, which has been previously oxidized on the inner surfaces of these molds. (Weight% aqueous solution) was applied to a thickness of 0.9 mm with a brush and dried to prepare a silicon casting mold.

  The obtained silicon melt heated and melted in the mold 1 was poured into a non-tapered mold at 85.6 kg and into a tapered mold at 87.6 kg, and the mold was placed in an argon (Ar) atmosphere reduced to 7 to 100 Torr. The silicon melt was unidirectionally solidified by gradually lowering the temperature from the bottom surface, and silicon ingots each having a height of 300 mm were obtained.

  Each side part of the obtained ingot was cut using a cutting device having a blade thickness of 2 mm. In order to perform cutting while avoiding the R shape at the end of the ingot, the thickness of the cut silicon end material was secured at 3 mm, 5 mm, 8 mm, 13 mm, and 18 mm at the bottom of each ingot.

The raw material yield of each ingot and the presence or absence of foreign matter on the side surface were observed from the weight of the silicon scrap material cut and removed with respect to the obtained ingot weight. The results are shown in Table 2. In the determination column, “◎” is very good, “○” is good, and “Δ” indicates the limit of the allowable range.

  As shown in Table 2, the larger the R shape of each side or four corners inside the mold and the greater the taper of the inner surface of the mold, the thicker the end material during cutting, and the lower the silicon raw material yield. To do. On the other hand, when the side face material thickness was 3 mm, the release material was mixed in a very small part of the side surface of the silicon ingot. From the above results, it was found that it is desirable to remove the end material of the silicon ingot with a thickness of 5 mm or more.

It is a figure which shows the casting_mold | template for silicon casting concerning this invention, (a) is a perspective view, (b) is sectional drawing of the Aa direction of (a). (A), (b) is a schematic diagram which shows the structure of the mold release material layer concerning this invention. It is a figure which shows other embodiment of the casting_mold | template for silicon casting concerning this invention, (a) is a top view, (b) is sectional drawing. It is a figure which shows other embodiment of the casting_mold | template for silicon casting concerning this invention, (a) is a perspective view, (b) is sectional drawing. It is a figure which shows other embodiment of the casting_mold | template for silicon casting concerning this invention, (a) is a perspective view, (b) is sectional drawing.

Explanation of symbols

1: Mold 2: Release material layers 11, 11a to 11c: Side members 12, 12a: Bottom member 101: Mold 111: Inner side 121: Inner bottom 201: Silicon nitride 201a: Oxide layer 202: Silicon dioxide

Claims (10)

A silicon casting mold made of silicon dioxide, in which a release material layer containing at least silicon nitride is formed on the inner surface of the mold,
The inner surface of the mold has one or a plurality of inner side surface portions and one inner bottom surface portion, and the side surface member having the inner side surface portion and the bottom surface member having the inner bottom surface portion are heat-bonded, and the separation is performed. A silicon casting mold in which an oxide layer having a thickness of 1 nm or more is provided on the surface of silicon nitride contained in a mold material layer . The silicon casting mold according to claim 1, wherein the release material layer contains silicon dioxide. Silicon dioxide forming the mold is silicon casting mold according to claim 1 or 2 comprising as fused silica. The silicon casting mold according to any one of claims 1 to 3 , wherein both the inner bottom surface portion and the inner side surface portion are substantially flat. 5. The silicon casting mold according to claim 1, wherein the inner surface of the mold is formed such that the inner side surface portion is substantially perpendicular to the inner bottom surface portion. A method for producing a silicon casting mold according to any one of claims 1 to 5 ,
A method for producing a silicon casting mold, comprising: forming a release material layer containing at least silicon nitride on an inner surface of the mold; and heating the release material layer in an atmosphere containing oxygen. Manufacture of a silicon casting mold comprising one or a plurality of inner side surface portions and one inner bottom surface portion, wherein the side surface member having the inner side surface portion and the bottom surface member having the inner bottom surface portion are heat bonded. A method,
A method for producing a silicon casting mold, comprising: forming a release material layer containing at least silicon nitride on an inner surface of the mold; and heating the release material layer in an atmosphere containing oxygen. A method for manufacturing a mold for a silicon casting according to any one of claims 1-5, wherein the silicon casting mold is configured to and a bottom member having the inner bottom portion and the side members having the inner surface portion A first step of forming a release material drying layer by applying and drying a release material slurry containing at least silicon nitride on each of an inner side surface portion of the side surface member and an inner bottom surface portion of the bottom surface member; and the release material A second step of forming the release material layer by heating the side member on which the dry layer is formed and the bottom member on which the release material dry layer is formed in an atmosphere containing oxygen; and the side member; A method for producing a silicon casting mold, comprising a third step of heating and joining the bottom surface member. Manufacture of a silicon casting mold comprising one or a plurality of inner side surface portions and one inner bottom surface portion, wherein the side surface member having the inner side surface portion and the bottom surface member having the inner bottom surface portion are heat bonded. A method,
The silicon casting mold includes a side member having the inner side surface portion and a bottom surface member having the inner bottom surface portion, and at least silicon nitride is provided on each of the inner side surface portion of the side surface member and the inner bottom surface portion of the bottom surface member. A first step of forming a release material dry layer by applying and drying a release material slurry containing, the side member on which the release material dry layer is formed, and the bottom member on which the release material dry layer is formed A second step of forming the release material layer by heating in an atmosphere containing oxygen, and a third step of heating and joining the side member and the bottom member Mold manufacturing method. The second step and the third step are simultaneously performed after the first step by heating the side member and the bottom member in an oxygen-containing atmosphere in combination. 10. A method for producing a silicon casting mold according to 8 or 9 .

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