JP4365669B2 - Silicon casting mold - Google Patents

Description

  The present invention relates to a silicon casting mold and a method for manufacturing the same, and more particularly to a polycrystalline silicon casting mold for forming a solar cell or the like.

  Conventionally, polycrystalline silicon has been used as a kind of semiconductor substrate for forming solar cells. Such polycrystalline silicon is formed by pouring a silicon melt heated and melted at a high temperature into a mold and heating it from the upper part of the mold and cooling it from the lower part to solidify in one direction. It is formed by being melted once and then heated from the upper part of the mold, cooled from the lower part and solidified in one direction.

As such a mold, those in which a mold release material is coated on the inner surface of a mold made of graphite that is separable or those in which a mold release material is coated on the inner surface of a mold made of silica that is an integral structure are usually used. . Generally, as a release material, silicon nitride (Si ₃ N ₄ ) which is a silicon nitride, silicon carbide (SiC) which is a silicon carbide, silicon oxide (SiO ₂ ) which is an oxide of silicon, or the like is used. These powders 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 of coating or spraying, or sprayed with silicon nitride or silicon oxide. It is known as a known technique to coat a film (for example, see Non-Patent Document 1).
15th Photovoltaic Specialists Conf. (1981), P576-P580, "A NEW DIRECTIONAL SOLIDIFICATION TECHNIQUE FOR POLYCRYSTALLINE SOLAR GRADE SILICON"

  However, when the silicon melt is unidirectionally solidified using the mold as described above, the solid-liquid interface shape is flattened and the solidification expansion stress is allowed to escape upward. In the process of cutting and slicing the ingot due to the stress remaining inside the ingot during the solidification and cooling process, cracks are generated on the cut surface due to the lateral expansion, and the work efficiency is improved. There have been problems of worsening and lowering the manufacturing yield.

  In order to solve the above problems, it is necessary to cool and solidify the crystal at a uniform solidification rate and solidification direction to obtain a healthy ingot with a uniform thermal stress distribution. When discussing the directional solidification phenomenon, it is important to control the characteristic values of the solidification rate X (m / s), the cooling rate θ (K / s), and the temperature gradient G (K / s). It is necessary to increase the / X ratio. However, since the necessary G / X ratio cannot be achieved simply by solidifying the mold within the mold, it is usually necessary to control G and X independently to increase G and decrease X. Various unidirectional solidification furnaces such as a drawer-type unidirectional solidification furnace and a power-down unidirectional solidification furnace are used.

  However, if such a unidirectional solidification furnace is used, a sound ingot having an ideal unidirectional solidification structure can be obtained. However, the initial investment for introducing the apparatus is expensive, and various ingot sizes are available. There was a problem that it was not possible to respond flexibly.

  The present invention has been made in view of such problems, and an object of the present invention is to obtain a silicon ingot that does not cause surface defects such as vertical cracks and transverse cracks generated on the surface of the silicon ingot in the silicon ingot cutting step. It is said.

  The inventors of the present invention have studied to achieve the above object by a simple and economical method, and have realized the result as the present invention. That is, one rectangular or square bottom plate and four rectangular or square side plates are fitted together and assembled into a box shape, and the inner surface is composed of at least one powder of silicon nitride and silicon oxide and an organic binder aqueous solution. In the upper open mold in which the release material slurry is applied, the shape of the release material in the engaging portion composed of the four corners and the eight ridges formed by the bottom plate and the side plate is applied in a substantially R shape. When the release material thickness of the surface portions of the bottom plate and the side plate is t1, and the release material thickness at the locking portion is t2, √2 × t1 <t2 is satisfied.

(Function)
The silicon casting mold of the present invention includes a rectangular bottom plate and four rectangular side plates fitted together to form a box shape, and at least one powder of silicon nitride and silicon oxide and an organic binder on its inner surface In the upper open mold to which the release material slurry composed of an aqueous solution is applied, the shape of the release material in the engaging portion composed of the four corners and the eight ridges composed of the bottom plate and the side plate is made substantially R-shaped. In addition to applying, when the release material thickness of the surface portion of the bottom plate and the side plate is t1, and the release material thickness of the locking portion is t2, √2 × t1 <t2 is satisfied. Since the strain due to thermal stress can be reduced at the engaging portion composed of the four corners and the eight ridges formed by the bottom plate and the side plate, the silicon ingot is vertically cut when the silicon ingot is cut. Such as cracks and side cracks Prevent the possible to generate a click.

  Further, the above reason will be specifically described with reference to the drawings.

  FIG. 7 shows an isotherm diagram of the longitudinal section of the mold after a certain time when the inside of the mold is kept at a high temperature and the outside of the mold is kept at a low temperature, which is numerically calculated by the orthogonal difference method. Here, (a) is the mold for silicon casting of the present invention, and (b) is the shape of the mold release material at the locking portion composed of four corners and eight ridges formed by the bottom plate and the side plate. The case where the shape is not used is shown. Moreover, the arrow line in a figure shows the direction of solidification.

  Since the heat flow goes from the inside of the mold to the outside of the mold, the direction of solidification is the opposite direction and the normal direction of the solidification surface (melting point isotherm). That is, in the case of the silicon casting mold of the present invention of (a), the crystal grains from one side and the crystal grains from the other side do not collide in the vicinity of the locking part, but the locking of (b) When the shape of the release material in the part is not R-shaped, the crystal grains from one side face and the crystal grains from the other side collide in the vicinity of the engaging part, so that stress concentration in the vicinity of the engaging part may occur. Increases nature. That is, by using the silicon casting mold of the present invention, it is possible to obtain a sound silicon ingot having a uniform thermal stress distribution and less distortion in the vicinity of the engaging portion.

  According to the silicon casting mold of the present invention, one rectangular or square bottom plate and four rectangular or square side plates are fitted together and assembled into a box shape, and at least one of silicon nitride and silicon oxide is formed on the inner surface thereof. In an upper open mold in which a release material slurry composed of the above powder and an organic binder aqueous solution is applied, the release material in the locking portion composed of the four corners and the eight ridges formed by the bottom plate and the side plate The shape was applied in a substantially R shape, and when the release material thickness of the surface portion of the bottom plate and the side plate was t1, and the release material thickness of the locking portion was t2, √2 × t1 <t2 was satisfied. Therefore, the work of taking out the silicon ingot from the mold is reduced, and surface defects such as vertical cracks and transverse cracks generated on the cut surface of the taken silicon ingot are eliminated in the cutting process. Manufacturing yield of silicon is improved, it is possible to reduce the manufacturing cost.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a longitudinal sectional view showing an embodiment of a silicon casting mold according to the present invention, FIG. 2 is a 1/4 schematic view showing corners and ridges of the inner surface of the silicon casting mold according to the present invention, FIG. 3 is a cross-sectional view showing an embodiment of a silicon casting mold according to the present invention.

  1, 2, and 3, 1a is a side plate, 1b is a bottom plate, 1c is a ring-shaped member, 2 is a mold holder, 3 is a mold insulation material such as carbon fiber felt, 4a is a surface portion in the mold, and 4b is a mold. The inner corners 4c indicate ridge surface portions in the mold.

  The side plate 1a and the bottom plate 1b are most preferably a carbon fiber reinforced carbon material in view of impact resistance, heat insulating properties, light weight, and ease of handling, but may be formed of, for example, graphite or silicon oxide.

  Moreover, it is preferable that the thickness of the said side plate 1a and the baseplate 1b is 1-5 mm. When the plate thickness is less than 1 mm, the durability in repeated use is inferior, which is not preferable. When the plate thickness is more than 5 mm, the cost increases and the heat absorption amount per unit area reaches the saturation region, and the heat insulating property. In addition, the heat balance of heat conductivity is not preferable because it causes waste.

  A release material slurry composed of at least one powder of silicon nitride and silicon oxide and an organic binder aqueous solution is applied to the inner surfaces of the side plate 1a and the bottom plate 1b by means such as brush or spray coating. Then, it heat-drys and makes a mold release material adhere to the side plate 1a and the bottom plate 1b.

In order to assemble the mold from the side plate 1a and the bottom plate 1b, the side plate 1a and the bottom plate 1b may be fixed to each other by screwing or the like.

  The method for assembling the mold from the side plate and the bottom plate is not limited to the above method, and various modifications are possible as long as the external plate can be restrained by the side plate due to the solidification and expansion of the ingot. It is.

  Next, as shown in FIG. 2, a mold release material having the same specifications as the mold release material to be applied to the surface portion 4a is poured into the corner portion 4c and the ridge line portion 4b serving as the locking portions of the mold bottom plate and the side plate by a dispenser. Or using a dedicated jig to form a substantially R shape. In this case, as shown in the cross-sectional view of FIG. 3, the thickness t1 of the surface portion and the thickness t2 of the corner portion are the thickness t1 of the release material of the surface portion, and the release material thickness of the locking portion of the mold bottom plate and the side plate. When t2 is set, t2 is applied so that the thickness t2 of the engaging portion is √2 × t1 <t2.

  The mold release material injected into the locking portion is formed in a substantially R shape while being pressed using a brush or a dedicated jig. However, due to artificial variation in the coating operation, for example, as shown in FIG. The same effect can be obtained as long as it is applied within the shape range. That is, the shape of the release material in the locking portion composed of the four corners and the eight ridge lines formed by the bottom plate and the side plate is t1, the release material thickness of the surface portion of the bottom plate and the side plate is t1, When the release material thickness is t2, √2 × t1 <t2, and there are 9 or more sides on the silicon melt side of the release material, and the release material on the silicon melt side centered on that side If all the angles are larger than 90 °, the same effect can be obtained.

  The release material thickness t1 and the inner radius r of the surface portion can be arbitrarily set according to the size of the ingot, etc. In the embodiment of the present invention, the release material thickness t1 of the surface portion is 1 to 3 mm, the inner radius r Is preferably 1 to 7 mm. When the thickness t1 of the surface portion is smaller than 1 mm, it is not preferable because silicon is likely to leak from the corner of the mold when the molten silicon is poured into the mold or cooled and solidified. Further, if the thickness t1 of the surface portion is set to be larger than 3 mm, the ingot is unnecessarily small or a prescribed casting size cannot be obtained at the corners, which is not preferable. Further, the inner radius r is preferably within the allowable range for the same reason as the description of the surface thickness t1 described above.

  A rectangular bottom plate made of a carbon fiber reinforced carbon material and a rectangular bottom plate having a thickness of 2 mm and a rectangular side plate having a thickness of 5 mm on one side are formed of powder of silicon nitride: silicon oxide = 50: 50 (wt%) and an organic binder aqueous solution. The mold material slurry was applied with a scissors, and the release material thickness after heat drying was changed to 1 mm, 2 mm, and 3 mm. Next, the rectangular bottom plate 1a is placed on the mold holder 2, the rectangular side plates 1b are fitted to each other and installed perpendicularly to the rectangular bottom plate 1b, the ring-shaped members 1c are arranged on the outer periphery of the side plate at substantially equal intervals, and a wedge ( The side plates are firmly fixed to each other with a not-shown carbon fiber felt heat insulating material 3 disposed on the outer periphery of the side plates.

  A mold release material having the same specifications as the surface portion is injected into the locking portion (corner corner portion) between the mold bottom plate and the side plate with a dispenser, and pressed using an R-shaped dedicated jig, and the inner radius R is approximately 1 mm, 3 mm. The thickness was changed to 5 mm and 7 mm. Further, as shown in FIG. 5 as Comparative Example 1, the mold has an inner radius of 0 mm, that is, a right angle, and as Comparative Example 2 as shown in FIG. 6, the thickness of the corner is smaller than the thickness of the surface part. A mold coated with a release material was prepared.

Next, these molds are placed in a silicon casting apparatus, 70 kg of silicon melt is poured, the mold 1 and the silicon melt are heated by a heating element placed above the mold 1, and the bottom 1b of the mold 1 is cooled. The silicon melt was then unidirectionally solidified and cooled. After cooling, the silicon ingot taken out from the mold was cut into a specified size with a band saw, and the occurrence rate of defects such as cracks and cracks generated on the cut surface was examined. The defect defect rate is represented by the ratio of defect defect length to the entire length of the ingot. The casting is repeated 30 times and the average defect rate is 0.0% or more and less than 0.1%, and the defect rate is 0.1%. The case of less than 5% is indicated by Δ and the case of the defect rate of 5% or more is indicated by x, and the results are shown in Table 1.

  As is apparent from Table 1, the ingots under conditions 1, 2, 3, 4, 6, 7, 8, 10, 11, and 12 were free from cracks during cutting. On the other hand, cracks are observed when √2 × t1> t2 as in conditions 5 and 9 and Comparative Example 1, and the release material at the corners becomes concave as in Comparative Example 2. In the case, the occurrence of cracks was particularly noticeable.

It is a longitudinal cross-sectional view which shows one Example of the casting_mold | template for silicon casting which concerns on this invention. It is the 1/4 schematic which shows the corner | angular corner part and ridgeline part of the inner surface of the casting_mold | template for silicon casting which concerns on this invention. It is a cross-sectional view showing an embodiment of a silicon casting mold according to the present invention. It is a cross-sectional view showing another embodiment of the silicon casting mold according to the present invention. It is a cross-sectional view showing an example of a conventional mold for silicon casting. It is a cross-sectional view showing another example of a conventional silicon casting mold. It is a figure which shows the isotherm and solidification progress direction in a corner corner of a casting_mold | template.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a ... Side wall part 1b ... Bottom part 1c ... Ring-shaped member 2 ... Mold holder 3 ... Mold heat insulating material 4a ... Surface part 4b in a mold ... Corner part 4c in a mold ... Ridge surface in mold

Claims (1)

One rectangular or square bottom plate and four rectangular or square side plates are fitted together and assembled into a box shape, and the inner surface is composed of at least one powder of silicon nitride and silicon oxide and an organic binder aqueous solution. In the upper open mold in which the release material slurry is applied, the shape of the release material in the engaging portion composed of the four corners and the eight ridges formed by the bottom plate and the side plate is applied in a substantially R shape, A silicon casting mold characterized in that √2 × t1 <t2 is satisfied, where t1 is the thickness of the release material at the surface portions of the bottom plate and the side plate, and t2 is the thickness of the release material at the locking portion.

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