JP2005095924A - Mold for casting silicon - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive mold for casting silicon by saving the labor and time, wherein in the case of pouring molten silicon into the mold and thereafter, in the case of solidifying this molten silicon or in the case of melting the silicon raw material charged into the mold, such troubles that mold releasing agent is released and mixed into the molten silicon or the mold can not be reused because of the sticking of the mold releasing agent to the mold, are restrained, and in the case of using graphite as the mold, such troubles as the oxidized consumption of the graphite and an increase in oxygen concentration in a cast block, are restrained. <P>SOLUTION: In this mold for casting the silicon, the molten silicon in the mold having a mold releasing agent layer on the surface of the mold, is solidified, the above mold releasing agent layer is provided with a first mold releasing agent layer arranged at the contacting side with the mold and a second mold releasing agent layer arranged at the contacting side with the molten silicon, and respective mold releasing agent layers contain silicon nitride powder having a silica layer formed on the surface, and a content ratio of the silicon nitride in the second mold releasing agent layer is less than that in the first mold releasing agent layer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a silicon casting mold that can be suitably used for producing a polycrystalline silicon wafer particularly for photovoltaic power generation.

  Conventionally, polycrystalline silicon has been used as a kind of semiconductor substrate for forming solar cells. Such polycrystalline silicon is formed by pouring and solidifying a silicon melt heated and melted at a high temperature into a mold, or by solidifying a silicon raw material after being melted in the mold. Forming.

As such a mold, usually, a separable graphite mold having an inner surface coated with a release material is used. As the release material, silicon nitride (Si ₃ N ₄ ) powder, silicon carbide (SiC) powder, Silica (SiO ₂ ) powder or the like is used. In general, powders such as silicon nitride, silicon carbide, and silicon oxide are mixed in a solution composed of an appropriate 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).

  However, the release material layer formed only from silicon nitride disclosed above is weak in the strength of the layer itself and weakly adherent to the graphite mold. Therefore, when the silicon melt is poured, the release material layer is weak. The mold material layer breaks, the silicon melt contacts the mold, the silicon adheres to the mold and cannot be removed, and the silicon ingot is chipped during cooling due to the difference in thermal expansion coefficient between silicon and graphite. there were. Furthermore, since the reaction between the silicon melt and silicon nitride is active, there is also a problem that fine silicon nitride powder is mixed in the melt to generate silicon nitride-based precipitates. If such precipitates exist, there is a risk that the yield will be lowered when a silicon ingot is sliced to produce a silicon wafer.

It has also been proposed to cast silicon by applying silica (SiO ₂ ) to the inner surface of a graphite mold (see, for example, Patent Document 1). When silica is used as a mold release material, the mold release layer becomes strong and contact with the silicon melt mold can be prevented. On the other hand, since silica is highly adherent to graphite and silicon ingots, There is a problem that it becomes difficult to reuse the mold due to adhesion to the mold, or the mold adheres to the silicon ingot via a release material, and a part of the silicon ingot is chipped when demolding. there were. Furthermore, at a high temperature near the melting point of silicon, a slight amount of oxygen is supplied from silica, and the oxidation of graphite is promoted at the portion where the silica and the graphite mold are in contact with each other in the mold release material. There is also a problem that the damage of the material is accelerated, or the oxygen concentration in the silicon melt is increased to adversely affect the characteristics of the silicon wafer.
JP 2002-292449 A Japanese Patent Laid-Open No. 7-206419 JP-A-9-175809 JP 2001-198648 A 15th Photovoltaic Specialist Conf. (1981), P576 ~ P580, “A NEW DIRECTIONAL SOLIDIFICATION TECHNIQUE FOR POLYCRYSTALINE SOLAR GRADE SILICON”

  In order to solve the above problem, Patent Document 2 discloses that a multilayer surface is formed by applying silicon dioxide on the first layer of the mold surface, a mixture of silicon dioxide and silicon nitride on the second layer, and silicon nitride on the third layer. It has been proposed to form a mold layer. When the release material layers are applied in multiple layers in this way, it is necessary to prepare and apply the release materials corresponding to the respective layers, and it takes time to apply and prepare the release materials, and the time required for forming the release material layers. Inevitably cause economic and economic losses. In addition, since the silicon nitride provided on the silicon melt side of the release material layer is brittle, it peels off and mixes into the silicon melt, or the silicon nitride and the silicon melt react to melt the fine silicon nitride powder. The risk of contamination in the liquid was inevitable.

  Patent Document 3 also proposes forming a single layer in which silicon nitride and silica are mixed at a specific ratio. Specifically, a release material obtained by mixing silicon nitride and silica in a weight ratio of 28:72 to 75:25 is applied to a graphite mold and dried, and the silica component is vitrified to form silicon nitride inside. The effect of fixing the powder can be brought out. However, since the silica in the mold release material and graphite are in contact with each other, there is a problem in that the oxidation of graphite is accelerated near the melting point of silicon and damage of the mold base material is accelerated. The higher this is, the more prominent this problem becomes. Further, since silica particles are excessively present on the outermost surface portion of the release material, the contact ratio between the silicon melt and the silica in the mixed layer increases, and the oxygen concentration in the silicon melt increases. Oxygen in the melt is manifested as precipitates in various processing steps in the device process after forming the wafer, and adversely affects the wafer characteristics. Therefore, a large amount of oxygen is present in the release material layer that is in contact with the silicon melt for a long time. The presence of silica is undesirable.

  Furthermore, the silica used for the conventional release material has an average particle size of about 20 μm (see, for example, Patent Document 3). Such silica can be obtained by pulverizing and classifying quartz glass, but when mixed with silicon nitride, the silica powder is not well filled in the voids between the silicon nitride powder particles, and the strength of the release material is high. There was a problem of weakening.

  The present invention has been made in view of such problems of the prior art. When pouring a silicon melt into a mold, solidifying it thereafter, or dissolving a silicon raw material placed in the mold. This prevents the release material from being peeled off and mixed into the silicon melt, or the release material adheres to the mold and the mold cannot be reused. An object of the present invention is to provide a silicon casting mold that suppresses an increase in oxygen concentration in the lump at a low cost by saving labor.

  In order to achieve the above object, a silicon casting mold according to claim 1 is a silicon casting mold for solidifying a silicon melt in a mold in which a mold releasing material layer is provided on an inner surface of the mold. The mold releasing material layer includes: A first release material layer provided on the side in contact with the mold and a second release material layer provided on the side in contact with the silicon melt, and each of these release material layers has a silica layer on the surface The second release material layer has a content ratio of the silicon nitride powder smaller than that of the first release material layer.

  Since the release material layer provided on the inner surface of the mold in this manner contains silicon nitride powder having a silica layer formed on the surface, the mold is maintained at a high temperature in a casting furnace to obtain a silicon melt. During the pouring, the silicon nitride powders in the release material layer sinter together, the strength of the release material layer improves moderately, and when the silicon melt is poured, the release material layer breaks, There is no contact between the mold and the mold. Since the content ratio of the silicon melt and the highly active silicon nitride powder is reduced on the silicon melt side of the release material layer, silicon nitride is less likely to be mixed into the silicon melt.

  Further, in the silicon casting mold according to claim 2, since the silica layer in the silicon casting mold according to claim 1 includes an amorphous phase, the silicon nitride powder of the release material layer is sintered together. And the strength of the release material layer is easily improved.

  Furthermore, the silicon casting mold according to claim 3 is the silicon casting mold according to claim 1 or 2, wherein the second release material layer contains silica powder having an average particle size smaller than that of the silicon nitride powder. It is characterized by doing. As described above, since the release material layer on the silicon melt side contains silica powder having an average particle size smaller than that of silicon nitride powder and high sinterability, the silica powder is closely packed between the silicon nitride powders. To do. Since the silica layer is also formed on the surface of the silicon nitride powder, it is strongly sintered to each other, and the silicon melt side becomes a release material layer having a high density and a strong bond, and the component of the release material is added to the silicon melt. Is difficult to melt or the release material is peeled off and mixed. And since silica is filled between silicon nitride powder and the area which contacts a silicon melt is restrained small, it can suppress that oxygen is supplied to a silicon melt and raises oxygen concentration.

  The silicon casting mold according to claim 4 is the silicon casting mold according to claim 3, wherein the silica powder contains an amorphous phase. And the strength of the release material layer is easily improved.

  According to a fifth aspect of the present invention, there is provided a silicon casting mold, wherein the silicon nitride powder in the silicon casting mold according to the third aspect is obtained by subjecting a spherical powder of silicon nitride obtained by an imide pyrolysis method to an oxidation reforming treatment. And an amorphous silica layer is formed on the surface. Since it did in this way, the width | variety of the particle size distribution of silicon nitride powder is narrow, and the powder particle with which the magnitude | size was uniform is obtained. As a result, it is possible to prevent the particles from aggregating and fusing due to variations in the particle size of the silicon nitride powder particles when fired, so that the strength of the release material layer is stabilized.

  The silicon casting mold according to claim 6 is the silicon casting mold according to claim 5, wherein the first release material layer contains 90% by weight or more of the silicon nitride powder subjected to the oxidation modification treatment. It is characterized by doing. As described above, the main component of the first release material layer in contact with the mold side is composed of the silicon nitride powder having improved sinterability, so that the release material can be used without adding silicon dioxide powder as in the prior art. The strength of the layer is high and it does not easily peel off from the mold. In addition, when using a graphite mold, silicon nitride hardly reacts with the mold, so it is firmly fixed and does not prevent reuse of the mold, and silicon dioxide is added that causes the mold to oxidize at high temperatures. Therefore, it is possible to prevent the template from being oxidized and consumed.

  Furthermore, the silicon casting mold according to claim 7 is the silicon casting mold according to claim 5 or 6, wherein the second release material layer includes silicon nitride powder and silica powder subjected to the oxidation modification treatment. A mixed layer containing 10 to 70% by weight of the silica powder. In this way, the amount of silica powder in the second release material layer in contact with the silicon melt side is optimally controlled, so that the effect of increasing the density by filling the space between the silicon nitride powders with this silica powder is maximized. Can do.

  The silicon casting mold according to claim 8 is the silicon casting mold according to any one of claims 1 to 7, wherein the main body of the mold contains graphite as a main component. It becomes an optimal combination with the release material layer described in claims 1 to 7, and the above-described effects can be maximized.

  In the present invention, if the release material layer has at least two layers of the first release material layer and the second release material layer as described above, the effects of the invention can be obtained. A release material layer can be formed.

  In the present invention, silicon nitride in which a silica layer containing an amorphous phase is formed on the surface by oxidation modification treatment or silica powder containing an amorphous phase is derived from moisture in the atmosphere on the surface. Since it has a silanol group (Si—OH), each forms a siloxane bond (Si—O—Si) in a state in which a release material layer is formed, and bonds to each other firmly. For this reason, even when an organic binder used by mixing at the time of application of the release material is heated in an oxidizing atmosphere to remove the binder, the release material is unlikely to fall apart. Therefore, it is assumed that the mold release materials are easily sintered when the mold having the mold release material layer is kept at a high temperature in the casting furnace and the silicon melt is poured.

  Regarding the presence of an amorphous phase in the silica layer on the surface of the silicon nitride powder, or the presence of an amorphous phase in the silica powder, means such as electron diffraction or X-ray diffraction using TEM Thus, it can be detected by the presence of a halo pattern peculiar to amorphous.

  The average particle diameter of each of the silicon nitride powder and the silica powder defined in claim 3 is determined by a laser Doppler method suitable for measuring the particle diameter of the fine particles. As the measurement conditions, water was used as a solvent, an ultrasonic homogenizer was used, and after pretreatment by ultrasonic dispersion for 6 minutes at an output of 300 to 400 μA, the particle size distribution of each sample was measured, and 50% of the cumulative frequency The value is the average particle size.

  Furthermore, regarding the content ratio of the material in the release material layer in claim 6 and claim 7, when the release material layer contains an organic substance such as a binder, the weight ratio to the whole excluding these organic substances Is defined to refer to

  According to the silicon casting mold according to claim 1 of the present invention, the mold release material layer provided on the inner surface of the mold contains the silicon nitride powder having the silica layer formed on the surface. When the silicon melt is poured while being kept at a high temperature in the casting furnace, the silicon nitride powders of the release material layer are sintered to moderately improve the strength of the release material layer. As a result, the mold release material peels off during pouring, and the mold and the silicon ingot do not come into contact with each other, and the mold release material layer between the silicon nitride powders separates when the silicon ingot is removed from the mold. Since the mold can be removed while being broken, it is possible to prevent chipping of silicon that occurs when the mold and the silicon ingot adhere. And since the content ratio of silicon melt and highly active silicon nitride powder is reduced on the silicon melt side of the release material layer, silicon nitride is less likely to be mixed into the silicon melt and suppresses the precipitation of silicon nitride-based foreign matter Therefore, when a silicon wafer is produced by slicing a silicon ingot, the yield does not decrease due to such foreign matters.

  According to the silicon casting mold according to claim 2 of the present invention, since the silica layer includes an amorphous phase, the sinterability between the silicon nitride powders of the release material layer is further improved, and the release material layer It becomes easy to improve the strength. As a result, it is possible to more effectively suppress problems such as separation of the release material from the mold and contact between the silicon melt and the mold, and precipitation in the silicon ingot.

  According to the silicon casting mold according to claim 3 of the present invention, the release material layer on the silicon melt side contains high sinterability and contains silica powder having an average particle size smaller than that of silicon nitride powder. The silica powder is filled with high density between the silicon nitride powders. Since the silica layer is also formed on the surface of the silicon nitride powder, it is strongly sintered to each other, and the silicon melt side becomes a release material layer having a high density and a strong bond, and the component of the release material is added to the silicon melt. Is difficult to melt or the release material is peeled off and mixed. Since the silica powder is filled between the silicon nitride powders and the area in contact with the silicon melt is suppressed, it is possible to suppress the oxygen concentration from being increased by supplying oxygen to the silicon melt. The adverse effect on the photoelectric characteristics of the device when it is made into a solar cell element, for example, the decrease in the conversion efficiency of the solar cell can be suppressed.

  According to the silicon casting mold according to claim 4 of the present invention, since the silica powder includes an amorphous phase, the sinterability between the silicon nitride powder and the silica powder of the release material layer is further improved, The strength of the release material layer is easily improved. As a result, the problem of mixing and precipitation of the release material into the silicon ingot can be further effectively suppressed.

  According to the silicon casting mold according to claim 5 of the present invention, the silicon nitride powder has a narrow particle size distribution width obtained by the imide pyrolysis method, and a silicon nitride spherical powder having a uniform size, A material having an amorphous silica layer formed on the surface by oxidation modification treatment was used. As a result, it is possible to prevent the particles from aggregating and fusing due to the variation in the particle size of the powder particles when fired, and a partially fragile portion can be formed in the release material layer. And a stable product as a whole can be obtained.

  According to the silicon casting mold according to claim 6 of the present invention, the first release material layer contains 90% by weight or more of the silicon nitride powder subjected to the oxidation modification treatment, so that it comes into contact with the mold side. The main component of the first release material layer is made of silicon nitride. Since this silicon nitride powder has an amorphous silica layer formed on its surface, the silicon nitride powder of the release material layer has good sinterability, and silicon dioxide powder is not added as in the conventional case. However, the strength of the release material layer is high and it does not easily peel off from the mold. As a result, especially when a graphite mold is used, silicon nitride is difficult to react with the mold, so that it is firmly fixed and does not hinder the reuse of the mold, which causes oxidation of the mold at high temperatures. Since it is not necessary to add silicon, it is possible to prevent the mold from being oxidized and consumed.

  According to the silicon casting mold according to claim 7 of the present invention, the second release material layer in contact with the silicon melt side is composed of silicon nitride powder subjected to oxidation modification treatment and amorphous fine silica powder. Since it is a mixed layer and is optimally controlled so that the amount of fine silica powder is contained in an amount of 10 to 70% by weight, the space between the silicon nitride powders is filled with this amorphous fine silica powder to obtain a high density. The effect can be maximized.

  According to the silicon casting mold according to claim 8 of the present invention, since the main body of the mold contains graphite as a main component, the release material layer according to claims 1 to 7 of the present invention. It is possible to enjoy the above-described operation of the present invention as much as possible.

  In the present invention, if the release material layer has at least two layers of the first release material layer and the second release material layer as described above, the effects of the invention can be obtained. A release material layer can be formed. Furthermore, since it has a two-layer structure, it can be made easier to peel than a single-layer structure when the release material is peeled from the mold, and damage to the mold caused by the peeling work can be reduced.

  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, the mold 1 is made of, for example, graphite, and is composed of a split mold that can be divided and assembled by combining one bottom member 101 and four side members 102. It is also possible to form with silica other than graphite. In this case, the bottom and side surfaces are often formed as a single mold. The bottom member 101 and the side member 102 are assembled in a separable manner by being fixed with bolts (not shown) or a frame member (not shown) in which the bottom member 101 and the side member 102 are fitted. In addition, it is preferable that at least one surface of the bottom member 101 and the side member 102 is subjected to uneven processing. This is for improving the adhesiveness and fixing property of the release agent slurry to the mold member described later.

  On the inner surface of the mold 1, a release material layer 2 having a configuration to be described later is provided so that it can be used repeatedly many times. In addition, although the details of the production method of the release material layer 2 will be described later, a predetermined amount of the release material powder (various if necessary) is weighed and mixed in, for example, a 10 wt% PVA (polyvinyl alcohol) aqueous solution, If agitated, the release material powder can be made into a slurry. By applying this to the inner surface of the mold 1 and drying, the release material layer 2 of the silicon casting mold of the present invention can be formed. it can.

  The mold 1 provided with the release material layer 2 in this manner is placed in an argon (Ar) atmosphere whose pressure is reduced to 7.0 to 90 Torr, and the mold 1 is heated at a temperature similar to or slightly lower than the silicon melt. Pour the liquid. Then, after cooling and solidifying, demolding to obtain a silicon ingot. A silicon raw material may be placed in the mold 1 and dissolved directly. Thereafter, the temperature is gradually lowered from the bottom of the mold 1 to gradually solidify the silicon melt from the bottom of the mold 1. Finally, the mold 1 is divided and the silicon ingot is removed. In addition, after removing the mold, after removing the release material layer 2 from the member of the mold 1 with a spatula or the like, it can be coated again with the release material layer 2 and used repeatedly a plurality of times.

  A cross-sectional view of the silicon casting mold of the present invention is shown in FIG. In the present invention, the release material layer 2 provided on the inner surface of the mold 1 is provided so as to face the first release material layer 2a provided facing the mold 1 side and the silicon melt side. At least two layers including the second release material layer 2b are included. The first release material layer 2a and the second release material layer 2b contain silicon nitride powder having a silica layer formed on the surface.

  FIG. 2 shows a conceptual diagram of the structure of silicon nitride powder provided with a silica layer on the surface according to the present invention. The figure shows an example in which an amorphous silica layer 201 a is formed around the silicon nitride powder 201.

  As the silicon nitride powder 201 used in the present invention, it is desirable to use a spherical powder obtained by a thermal decomposition method of silicon diimide. The silicon nitride powder 201 obtained by this method has a narrow particle size distribution and has a uniform size. Therefore, when the release material layer 2 is baked by the method described later, it is possible to prevent the particles from aggregating and fusing due to the variation in the particle size of the silicon nitride powder 201, and the release material layer 2. This is because the strength of the is stabilized. As the silicon nitride powder 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 agglomerated particles or fused particles in the silicon nitride powder 201 subjected to the oxidation modification treatment is undesirably high.

  Then, the spherical silicon nitride powder 201 obtained by the thermal decomposition method of silicon diimide can be subjected to an oxidation reforming treatment in an electric furnace in an oxidizing atmosphere to form an amorphous silica layer 201a on the surface. In this oxidation reforming treatment, the silicon nitride powder 201 may be placed in, for example, an electric furnace (oxidation furnace) and heated in an oxidizing atmosphere at 850 ° C. to 1300 ° C. for about 30 minutes to 600 minutes. If the temperature is too high, there may be a problem in the sinterability of the silicon nitride powder 201 due to an increase in the crystalline phase in the silica layer formed on the surface. First, there is a problem that a long time is required for processing.

  The thickness of the amorphous silica layer 201a formed on the surface of the silicon nitride powder 201 by such oxidation modification treatment can be controlled by the treatment temperature and time, but it should be in the range of 1 to 100 nm thickness. desirable. Below this range, the effect of improving the sinterability of the silicon nitride powder 201 is poor. On the other hand, exceeding this range, the silicon nitride powders 201 subjected to the oxidation modification treatment are bonded or fused together at the portion of the amorphous silica layer 201a, and as a result, hard coarse particles are formed. When there are many hard and coarse particle groups in the powder, when used as a release agent slurry, workability in the production or application is poor, which is not preferable. Specifically, since the coarse particles settle in the slurry, a uniform slurry cannot be prepared and cannot be uniformly applied, resulting in variations in the thickness and strength of the formed release material layer 2 and the release material layer. There is a problem that the silicon melt and the mold 1 are easily fused at a thin portion 2 or a damaged portion. Further, since the coarse particles in the release material layer 2 have a weak cohesive force in the layer and are poor in adhesion to the mold 1 and easily peel off, the release material layer 2 containing the coarse particles peels off and melts silicon. This is not suitable due to the problem that it enters into the liquid and becomes a foreign substance. The thickness of the amorphous silica layer 201a can be measured by a TEM (Transmission Electron Microscope) image and elemental analysis thereby.

  In the amorphous silica layer 201a on the surface of the silicon nitride powder 201 oxidized and modified by such a method, Si—OH (silanol group) derived from moisture in the atmosphere is formed. And since Si-O-Si (siloxane bond) arises between Si-OH (silanol group) of mutual silicon nitride powder 201, the adhesiveness of silicon nitride powder 201 is improved significantly, and mold release material layer 2 becomes. It will be solid.

  Thus, since the release material layer 2 contains the silicon nitride powder 201 having the amorphous silica layer 201a formed on the surface thereof, the mold 1 is kept at a high temperature in the casting furnace and the silicon melt is melted. When pouring, the silicon nitride powders 201 of the release material layer 2 are sintered to moderately improve the strength of the release material layer 2. Therefore, the mold release material layer 2 does not peel off when the silicon melt is poured, and the mold 1 does not come into contact with the silicon ingot, and the mold release material layer 2 is removed when the silicon ingot is removed from the mold 1. Since it becomes disjoint, it is easy to remove the mold, and it is possible to prevent the chipping of silicon that occurs when the mold 1 adheres to the silicon ingot.

  Further, the first release material layer 2a desirably contains, as a main component, silicon nitride powder 201 that has been oxidized and modified to form an amorphous silica layer 201a on the surface, and the content thereof is 90% by weight or more. Is desirable. Conventionally, when such a silicon nitride powder is used as a main component and formed as a release material, it is very fragile. When a silicon melt is poured, the release material layer breaks, and the mold and the silicon ingot come into contact with each other. In the present invention, as described above, the silicon nitride powders 201 having the amorphous silica layer 201a formed on the surface are sintered and bonded together, as described above. Such a problem does not occur because it has an appropriate strength. The effect of the present invention is obtained even if the content ratio of the silicon nitride powder 201 is less than 90% by weight. However, in order to keep the effect of preventing the contact between the mold 1 and the silicon melt, this range is used. Is desirable.

  The second release material layer 2b provided facing the silicon melt side contains the silicon nitride powder 201 and the silica powder having the amorphous silica layer 201a formed on the surface.

  FIG. 3 shows a conceptual diagram of the structure when silicon nitride powder and silica powder having a silica layer on the surface according to the present invention are mixed. An amorphous silica layer 201 a is formed around the silicon nitride powder 201, and the silica powder 202 exists so as to surround the periphery.

  As such a silica powder 202, amorphous spherical silica fine powder obtained by injecting silicon tetrachloride into a high-temperature flame of combustible gas and auxiliary gas, for example, hydrogen gas and oxygen gas and heat-treating ( It is preferable to use a so-called fumed silica fine powder or a fused silica powder), and it is desirable to use a fine powder having a particle size of about 0.01 to 0.1 μm.

  As shown in FIG. 3, since the second release material layer 2b is a mixed layer in which the silicon nitride powder 201 and the silica powder 202 are mixed, the silica powder 202 surrounds the silicon nitride powder 201, This induces an effect of strongly bonding the silicon nitride powders 201 to each other.

In addition, as the silica powder 202 used here, it is desirable to use an amorphous fine silica powder, and among the amorphous fine silica powders, those 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 ) aqueous solution contains a large amount of alkali metal impurities and causes contamination of silicon ingots. It must be used after being removed.

  The weight ratio of the silica powder 202 in the second release material layer 2b is preferably in the range of 10 to 70% by weight, and more preferably 10 to 20% by weight of the silica powder 202 is added. . When the weight ratio of the silica powder 202 is greater than 70%, the release material layer 2 does not adhere to the mold 1 and peels off, making it difficult to reuse the mold 1. Further, when the mold 1 adheres to the silicon ingot through the release material and the mold is peeled off from the silicon ingot, a part of the silicon is chipped. Further, if the weight ratio of the silica powder 202 is smaller than 10%, the fixing effect between the silicon nitride powder 201 and the silica powder 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 powder obtained by injecting silicon tetrachloride into a high-temperature flame formed by hydrogen gas and oxygen gas as the fine silica powder 202 in the release material layer 2 and heat-treating it. If the fine silica is used, since the sinterability is high, the second release material layer 2b can be easily formed on the mold 1 even if the addition ratio is in the range of 5 to 20% by weight. In addition, the range of weight ratios that can be used can be widened compared to when the above silica powder is used (at least 10% by weight).

  In the present invention, the second release material layer 2b provided on the side in contact with the silicon melt contains the silicon nitride powder 201 more than the first release material layer 2a provided on the side in contact with the mold 1. The ratio is decreasing. This is because, as described above, the first release material layer 2a is made of silicon nitride powder 201 as a main component, and the second release material layer 2b is made of a mixed layer of silicon nitride powder 201 and silica powder 202. Can be easily achieved. Thereby, in the second release material layer 2b on the silicon melt side, the content ratio of the silicon melt and the highly active silicon nitride powder 201 is reduced, so that silicon nitride is less likely to be mixed into the silicon melt, Precipitation of silicon nitride-based foreign matters can be suppressed.

  By the way, in Patent Document 4, a release material layer comprising a mixed base obtained by mixing fine silica (so-called colloidal silica) obtained by an ion exchange method of sodium silicate and silicon nitride powder and a stucco layer of fine fused silica sand is provided. Although an example of forming is disclosed, this invention is an invention devised to use fine spherical silica obtained by a wet method in order to produce a silicon ingot having a low internal residual stress. For the purpose of strengthening the mold material layer 2, it is greatly different from using fine spherical silica powder obtained by a vapor phase method.

  In particular, when the oxidized silicon nitride powder 201 and the amorphous fine silica powder 202 according to the present invention are used, it is possible to effectively avoid the insufficient adhesion force between the silicon nitride powders, which has been a problem in the past. The reason is that the silicon nitride powder 201 oxidized and modified by the above-mentioned method and the amorphous fine silica powder 202 have Si—OH (silanol group) derived from moisture in the atmosphere on the surface. Is formed. Then, water is eliminated and Si—O—Si (siloxane bond) is generated between the silicon nitride powders 201 or between the silanol groups of the silicon nitride powder 201 and the amorphous fine silica powder 202, so that the powder particles This is because the adhesion between each other is greatly improved, and the release material layer 2 becomes strong. As a result, when the silicon melt is poured into the mold 1 or solidified thereafter, the release material layer 2 is peeled off or missing and mixed into the silicon melt, or the fine silicon nitride powder 201 Can be prevented from being caught in the silicon melt.

  In addition, the use of the amorphous fine silica powder 202 obtained by the above-described vapor phase method causes a problem of contamination (contamination) unlike silica obtained by pulverizing conventional quartz glass. Absent.

  Next, the formation method of the mold release material layer 2 of this invention is demonstrated.

  First, in order to form the first release material layer 2a, the silicon nitride powder 201 oxidized and modified by the above-mentioned method is mixed with 5 to 15% by weight of PVA (polyvinyl alcohol) aqueous solution and stirred. Since the silicon nitride powder 201 as a body can be made into a slurry, it can be easily applied to the mold 1. By applying this mold release material slurry to the inner surface of the mold 1 and drying, the first mold release material layer 2a of the mold release material layer 2 of the silicon casting mold of the present invention can be formed.

  Next, in order to form the second release material layer 2b, the silicon nitride powder 201 and the silica powder 202 produced by the above-described method are set to a predetermined mixing ratio, which is exactly the same as in the case of the first release material layer 2a. Thus, a release material slurry is prepared. Then, the 1st mold release material layer 2b can be formed by apply | coating on the 1st mold release material layer 2a formed in the casting_mold | template 1, and drying.

  In addition, for the formation of the release material layer 2 according to the present invention, it is preferable to use a method of applying to the member of the mold 1 with a brush or a spatula as described above, and drying on a hot plate. A method of applying to the inner surface of the mold 1 using a spray or the like and drying it, or a method of thermocompression bonding using a laminating apparatus provided with a heating plate / silicon rubber diaphragm is also possible. Furthermore, when the inner surface of the target mold 1 is a plane, screen printing can be used.

  The first release material layer 2a is preferably 0.1 to 2 mm thick, and the second release material layer 2b is preferably formed within a thickness range of 0.1 to 2 mm. If this range is exceeded, there is a problem that the drying time of the release material layer 2 becomes long. Below this range, the inner surface of the mold 1 cannot be completely covered with the release material layer 2, and the silicon melt and the mold 1 are not covered. This is because contact may occur.

  In order to make these release material layers have a required thickness, the amount to be applied may be changed, or after repeated application and drying, the same release material slurry may be applied and dried.

  In this way, the mold 1 provided with the release material layer 2 according to the present invention is placed in an argon (Ar) atmosphere inside a casting furnace or the like depressurized to 7.0 to 90 Torr, and the mold 1 is about the same as the silicon melt. The silicon melt is poured by heating at a slightly lower temperature. Then, after cooling and solidifying, demolding to obtain a silicon ingot. A silicon raw material may be placed in the mold 1 and dissolved directly.

  At this time, since the release material layer 2 according to the present invention contains the silicon nitride powder 201 having a silica layer, preferably an amorphous silica layer 201a formed on the surface, the mold 1 is heated at a high temperature in a casting furnace. When held at the temperature, the silicon nitride powders 201 are sintered to obtain an effect that the strength of the release material layer 2 is appropriately improved.

  Thereafter, the temperature is gradually lowered from the bottom of the mold 1 to gradually solidify the silicon melt from the bottom of the mold 1. Finally, the mold 1 is divided and the silicon ingot is demolded. However, the release material layer 2 according to the present invention is separated when demolding, and the silicon ingot can be demolded smoothly. Usually, after demolding, the mold release material layer 2 is removed from the member of the mold 1 with a spatula, etc., and again coated with the mold release material layer 2 and used repeatedly a plurality of times, but when the mold release material layer 2 is removed Also, the member of the mold 1 is not damaged and can be peeled off very easily.

  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, as the oxidation modification treatment, the silicon nitride powder is heated in advance in the air or in an atmosphere containing oxygen, and the surface modification is performed to form an oxide layer. However, the present invention is not limited to this, and it is also possible to provide a silica layer on the surface of the silicon nitride powder by subjecting the release material layer containing the silicon nitride powder previously applied to the mold to oxidation modification treatment. . This is particularly effective when the release material layer is porous or when a mold having high oxidation resistance, for example, a mold made of silica is used.

  Further, the oxidation reforming treatment is not limited to heat treatment. For example, when a raw material of a release material containing silicon nitride powder is used as a slurry, for example, by appropriately adding an acidic chemical such as acetic acid. A silica layer may be provided on the surface of the silicon nitride powder. Furthermore, a silica layer may be formed on the surface of the silicon nitride powder by forming a release material layer by plasma spraying a raw material powder of a release material containing the silicon nitride powder in the air or the like. Either method can achieve the effects of the present invention.

  In the above description, the release material layer 2 has been described with an example composed of two layers of the first release material layer 2a and the second release material layer 2b. However, the present invention is not limited to this. Another layer may be provided between the first release material layer 2a and the second release material layer 2b. The layer structure of the release material layer 2 of the present invention is only required to include the first release material layer 2a on the mold 1 side and the second release material layer 2b on the silicon melt side.

  Further, in the above description, the example in which the amorphous silica layer 201a is formed on the surface of the silicon nitride powder 201 has been described. However, the present invention is not limited to this, and the silica layer partially includes an amorphous phase. Even if it is a thing, even if it is a crystalline silica layer, there can exist the effect of this invention. However, the amorphous silica layer 201a as described above is preferable because the effects of the present invention can be optimally achieved.

  In the above description, the first release material layer 2a is mainly composed of the silicon nitride powder 201, and the second release material layer 2b is a mixed layer of the silicon nitride powder 201 and the silica powder 202. However, the present invention is not limited to this, and if the second release material layer 2b is formed so that the content ratio of the silicon nitride powder 201 is smaller than that of the first release material layer 2a, another material may be used. However, it is necessary that the material does not affect the mold 1 or the silicon melt. It goes without saying that the combination of materials presented in the above description is the best embodiment of the present invention.

  In the above description, the material of the mold 1 has been described as a material containing graphite as a main component. In this case, the material is not limited to a single material of high-purity graphite, and a graphite material reinforced with carbon fibers is used. May be. Furthermore, a mold formed of fused silica or the like other than graphite may be used, or an integral mold may be used instead of a split mold.

  Further, in the above description, the silicon nitride powder 201 has been described as being manufactured by the imide pyrolysis method, but it is also possible to use a silicon nitride powder obtained by a direct nitridation method of metal silicon. However, silicon nitride powder obtained by the direct nitriding method contains a large amount of coarse powdered unmilled particles produced during the direct nitriding reaction and has a wide range of particle size distribution, so the particle size of aggregated particles and / or fused particles after heating and firing Control may be difficult. It is desirable to keep the width of the particle size distribution narrow by means such as classification as appropriate.

  Examples of the present invention will be described below.

  A silicon nitride powder 201 having an average particle size of 0.5 μm obtained by a thermal decomposition method of silicon diimide is heated at 1000 ° C. for 3 hours in a batch-type electric furnace (oxidation furnace) in an air atmosphere to obtain about 20 nm An amorphous silica layer 201 a was formed on the surface of the silicon nitride powder 201. The thickness is confirmed by TEM.

  A release material slurry obtained by stirring and mixing the silicon nitride powder 201 with an aqueous 8% polyvinyl alcohol solution was applied to the inner surface of a graphite mold plate as a first release material layer 2a with a scissors. Then, it was dried on a hot plate to obtain a first release material layer 2a having a thickness of 0.8 mm.

  Next, a predetermined amount of amorphous silica powder 202 having an average particle diameter of 0.05 μm obtained by the above-described silicon nitride powder 201 and silicon tetrachloride hydrogen / oxygen combustion method is weighed to 8% A release agent slurry obtained by stirring and mixing with an aqueous polyvinyl alcohol solution is overcoated on the first release material layer 2a and dried on a hot plate, whereby a second release material having a thickness of 1.2 mm is obtained. Layer 2b was obtained. As a result, the release material layer 2 according to the silicon casting mold of the present invention was obtained.

  The presence of an amorphous phase in the amorphous silica layer 201a of the silicon nitride powder 201 and the amorphous silica powder 202 was confirmed by the presence of a halo pattern by electron beam diffraction using TEM.

  The mold 1 according to the present invention produced by the method described above was placed in an argon atmosphere reduced to 8.0 Torr, and 68 kg of silicon melt was poured into the mold 1 while being heated to 1000 ° C. using a graphite heater. The solution was gradually solidified over 7 hours. The silicon ingot solidified after cooling was taken out from the mold 1 and sliced to obtain a silicon wafer. Here, the evaluation items include the presence or absence of peeling of the release material layer 2 from the mold 1 (visual confirmation), the presence or absence of precipitates in the silicon ingot (confirmation by microscopic observation of the sliced silicon wafer), the element of the silicon wafer Evaluation was performed with characteristic evaluation (a bulk type solar cell element was formed by a well-known method and confirmed by its power generation efficiency).

In addition, about the 1st mold release material layer 2a and the 2nd mold release material layer 2b, the content ratio of the silicon nitride powder 201 and the silica powder 202 and the numerical value of an average particle diameter were changed within the range of this invention, and it is to Table 1 Condition No. shown Samples 1 to 16 were prepared. Further, for comparison, as a sample outside the scope of the present invention, a sample in which the content ratio of the silicon nitride powder in the first release material layer 2a is smaller than that in the second release material layer 2b (Condition No. 17). 18), a sample with only the first release material layer 2a (condition No. 19), and a sample with only the second release material layer 2b (condition No. 20) were also prepared and evaluated by the above-described methods. It was. The results are shown in Table 1. In each evaluation result, “◎” indicates no problem at all, “◯” indicates a slightly worse but not problematic level, “Δ” indicates the limit of the allowable range, and “×” indicates “not acceptable”.

  Condition No. in Table 1 Samples 1 to 16 were within the scope of the present invention, but all of the results exceeded the acceptable range. On the other hand, Condition No. which is a sample outside the scope of the present invention. For the samples 17 to 20, one of the evaluation items was not possible (x), and the effect of the present invention was confirmed.

  It is needless to say that the silicon casting mold according to the present invention is not limited to the above embodiments, and can be variously modified within the scope of the gist thereof.

It is a figure explaining the casting_mold | template for silicon casting of this invention, (a) is a perspective view which shows an example of the casting mold for silicon casting, (b) is sectional drawing of the Aa direction of (a). It is a structure conceptual diagram of the silicon nitride powder which provided the silica layer on the surface concerning the mold release material layer provided in the casting mold for silicon casting of the present invention. It is a structure conceptual diagram when the silicon nitride powder and silica powder which provided the silica layer on the surface concerning the mold release material layer provided in the casting mold for silicon of the present invention are mixed.

Explanation of symbols

1: Mold 2: Release material layer 2a: First release material layer 2b: Second release material layer 101: Bottom member 102: Side member 201: Silicon nitride powder 201a: Amorphous silica layer 202: Silica powder

Claims (8)

A silicon casting mold for solidifying a silicon melt in a mold having a mold release material layer provided on an inner surface of the mold, wherein the mold release material layer includes a first mold release material layer provided on a side in contact with the mold and a silicon melt. A second release material layer provided on the side in contact with the first release material layer, and each of the release material layers contains a silicon nitride powder having a silica layer formed on the surface thereof, and the second release material layer The silicon casting mold, wherein the layer has a smaller content ratio of the silicon nitride powder than the first release material layer. The silicon casting mold according to claim 1, wherein the silica layer includes an amorphous phase. 3. The silicon casting mold according to claim 1, wherein the second release material layer includes silica powder having an average particle diameter smaller than that of the silicon nitride powder. The silicon casting mold according to claim 3, wherein the silica powder includes an amorphous phase. 4. The silicon nitride powder according to claim 3, wherein a spherical powder of silicon nitride obtained by imide pyrolysis is subjected to an oxidation modification treatment to form an amorphous silica layer on the surface. Silicon casting mold. 6. The silicon casting mold according to claim 5, wherein the first release material layer contains 90% by weight or more of the silicon nitride powder subjected to the oxidation modification treatment. The second release material layer is a mixed layer containing silicon nitride powder subjected to the oxidation modification treatment and silica powder, and contains 10 to 70% by weight of the silica powder. Item 7. A silicon casting mold according to Item 5 or 6. 8. The silicon casting mold according to claim 1, wherein the main body of the mold contains graphite as a main component.

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