JP4931432B2 - Molds for the production of polycrystalline silicon slabs - Google Patents

Description

  The present invention relates to an internal structure of a silicon casting furnace used for solidifying a silicon melt, and more particularly to a mold used for obtaining a polycrystalline silicon slab for manufacturing a substrate for a solar cell.

  As a substrate for building a photovoltaic power generation cell, a silicon crystal substrate that has shared the function of the power generation cell itself is widely used. Among them, the crystallinity of the substrate can be divided into a single crystal type and a polycrystalline type, and the polycrystalline type is most frequently used at present. A polycrystalline type substrate is made by melting a high-purity silicon raw material and solidifying it in a mold in one direction to produce a polycrystalline slab and then slicing it into a thin piece. The polycrystalline silicon slab manufacturing process by the unidirectional solidification method for making the polycrystalline slab is important.

  On the other hand, silicon used for solar cells generally requires a purity of about 99.9999% by mass, various metal impurities are 0.1 mass ppm or less, and B is at least 0.3 mass ppm or less, preferably 0. .1 mass ppm or less is required. As silicon satisfying this purity, there is silicon for semiconductors, that is, high-purity silicon obtained by pyrolyzing silicon chloride after distillation. However, this Siemens method is expensive and unsuitable for solar cells that require a large amount of silicon.

  Therefore, various techniques for producing inexpensive silicon that can be used for solar cells have been studied. Various metal impurities other than B and P, such as Fe, Al, and Ca, can be removed by a unidirectional solidification method. It is common. That is, when the silicon melt is solidified, it is a purification method using the phenomenon that a large amount of metal impurities are distributed to the coexisting melt silicon and only a small amount is taken into the solidified silicon. That is, in the technology for producing inexpensive silicon that can be used for solar cells, a polycrystalline silicon slab manufacturing process by a unidirectional solidification method is important.

  In a conventional polycrystalline silicon slab manufacturing process using a unidirectional solidification method, for example, Patent Document 1 discloses that a material having a high porosity and a low thermal conductivity is used for a side wall of a mold, thereby removing heat from the side wall surface. A method of manufacturing polycrystalline silicon with good unidirectional solidification by suppressing crystal growth and reducing crystal growth from the side wall surface is disclosed. Further, Patent Document 2 not only gives a temperature gradient in the vertical direction upward from the bottom as in the conventional manufacturing method, but also uses an auxiliary heater provided on the side surface of the mold at the start of cooling, A method of manufacturing crystalline silicon having crystal grains with a large diameter in the horizontal direction by applying a temperature gradient in the horizontal direction from a part of the bottom is disclosed.

JP 2000-135545 A JP 2000-327487 A

  The polycrystalline silicon slab manufacturing furnace as disclosed in the above-mentioned Patent Documents 1 and 2 generally has a main structure in a sealed metal container in which the internal air can be expelled by a vacuum pump or the like. A heater, a heat insulating material, a mold, and a mold base are accommodated as members. The heat insulating material is disposed for heat insulation between the heater, the mold, the mold base, and the container metal, and a molded heat insulating material made of carbon fiber is often used. The heater is disposed on the upper surface, the side surface, the upper surface, and the side surface of the mold.

  The mold pedestal is often used to cool the lower part of the mold and itself or directly to provide a temperature gradient suitable for unidirectional solidification of the slab to the slab and melt during casting. In general, it has a cooled structure and can move up and down so that the positional relationship between the heater and the mold can be controlled, or it can be rotated in order to keep the heat in the in-plane direction inside the mold. Is. The inside of the silicon polycrystalline slab manufacturing apparatus using the unidirectional solidification method requires a complicated structure as described above.

  On the other hand, the materials constituting these members must be materials that can withstand high temperatures of 1400 ° C or higher and high-purity materials that do not allow impurities to diffuse into silicon slabs. Made of quartz or quartz. Moreover, it is usually processed in a complicated manner to suit the above application.

  However, the furnace structure described above is designed for the purpose of producing polycrystalline silicon with good crystallinity in one direction. The mold is damaged due to the occurrence of troubles during solidification, and the associated melt. However, when the melt flows out, the melt infiltrates into the casting furnace member, making it impossible to reuse the above expensive member. . That is, it has been an important problem to find a mold material and / or structure that can fundamentally prevent the melt from spreading to the casting furnace inner member.

  An object of the present invention is to provide a mold capable of preventing silicon from impregnating and damaging members such as a heater and a heat insulating material in a casting furnace even when a melt flows out due to breakage of the mold.

To achieve the above objective,
(1) A mold used for producing a polycrystalline silicon slab by a unidirectionally solidified casting method, comprising at least an outer mold, a double mold composed of an inner mold, and a silica-based powder between both molds. A mold for producing polycrystalline silicon slabs, characterized in that a filler is disposed;
(2) The mold for producing a polycrystalline silicon slab according to (1), wherein the silica-based powder is one or both of silica sand and silica glass,
(3) The mold for producing a polycrystalline silicon slab according to (1) or (2), wherein the maximum particle size of the silica-based powder is less than 3 mm,
Is used.

  According to the present invention, the casting mold has a double structure, and the gap between the outer casting mold and the inner casting mold is filled with silica-based powder. Is received by the silica-based powder filled between the inner mold and the outer mold, so that it does not reach the outer mold and flows out of the mold, and the heater, heat insulating material and mold in the casting furnace are removed. It does not reach the supporting structural members, and prevents damage to the outer molds, heaters and heat insulating materials in the casting furnace, and structural members that support the molds. Therefore, the polycrystalline silicon manufacturing apparatus can be stably operated.

  A silicon casting mold for a polycrystalline silicon slab manufacturing apparatus according to the present invention first has a double structure.

  The outer mold is not limited to two, such as high-strength carbon or carbon composite, but it can be used repeatedly and is strong enough to support the mass of the mold and silicon slab. A material that does not diffuse impurities into the silicon slab is selected.

  The inner mold is not particularly limited, such as carbon, carbon composite, quartz, silica powder or silicon nitride powder hardened with phenolic resin, but it has a large shape at the temperature and atmosphere at which it casts silicon. It is not changed, and it is repeatedly used by spraying silica or silicon nitride powder on the surface as it is in contact with the silicon slab, or by dissolving and applying with a solvent such as water in which ethanol or polyvinyl alcohol is dispersed. Alternatively, it may be broken every time it is cast, but one that does not diffuse impurities into the silicon slab is selected.

  Further, a silica-based filler is filled in the gap between the outer mold and the inner mold. The filler is made of silica-based powder, and even if the size of the gap between the outer mold and the inner mold changes, the gap between the outer mold and the inner mold is indefinite, or it is made of the above-mentioned various materials. Therefore, even if the size and shape of the gap greatly change due to thermal expansion accompanying an increase in temperature, it is possible to always fill the gap due to the fluidity due to the powder. Furthermore, since silica has a very poor wettability with the silicon melt and has a property of repelling the silicon melt, the leakage of the silicon melt can be stopped. Therefore, leakage of the silicon melt can be stopped by combining the fluidity and the repelling property.

  Silicon melt has a very low viscosity even near the freezing point, and a large amount of melt often leaks from a small gap. This melt leakage damages members in the furnace. However, in the case of the above mold, it is assumed that the inner mold is cracked, melted and missing, has nests due to problems in the manufacturing process, or the mold has some defects, and the inner mold is Even if it leaks out, the silica-based powder having the property of repelling silicon is filled without any gap, and the leakage of silicon can be stopped just before reaching the outer mold.

  For the purpose of reliably preventing leakage, it is desirable to place the filler made of silica-based powder between the bottom of the inner mold and the bottom of the outer mold, and both on the side of the inner mold and the side of the outer mold. . The thickness of the packed bed of the silica-based powder is preferably 3 times or more because, based on experimental knowledge, if the maximum particle size of the silica-based powder is at least 3 times or more, the hot water leakage preventing function is exhibited.

  The packed bed of silica-based powder arranged on the side surface contributes as a heat insulating layer when the heater position is only on the upper surface of the mold. It is possible to create an organization with the same degree of parallelism. Incidentally, the upper limit value of the thickness of the silica-based powder packed layer is not particularly defined, and can be appropriately set within a range that contributes as a desired heat insulating layer.

  In general, heat is often removed from the bottom surface of the mold, but in that case, it is advantageous to create a unidirectionally solidified structure if the thickness of the silica-based powder packed layer on the bottom surface is as thin as possible. There is no need to fill the gap between the bottom and the bottom of the outer mold with silica powder, and it is sufficient to fill the gap between the side of the inner mold and the outer mold with silica-based powder to prevent melt leakage from the mold side. It is.

  The silica-based powder described above may be silica sand which is crystalline silica, silica glass which is amorphous silica, a mixture thereof, or an intermediate substance between crystalline and amorphous. Silica sand is generally readily available and inexpensive. Preferably, it is obtained by pulverizing high-purity silica and removing impurities mixed with the pulverization using means such as magnetic separation. Silica glass is generally readily available and inexpensive. Preferably, it is silica glass obtained by melting and spray-solidifying high-purity silica stone. The high-purity silica-based powder does not diffuse impurities other than Si and O, and if it contains at least 80% by mass or more of high-purity silica-based powder, the solar cell characteristics of the silicon slab will be affected. Absent.

  The particle size of the silica-based powder described above is preferably less than 3 mm. There is always a gap between silica-based powders, but if the particle size of the silica-based powder is large, the voids also become large. If the maximum particle size of the powder is 3 mm or more, silicon leaks through the voids and melts. . Therefore, the maximum particle size of the silica-based powder is preferably less than 3 mm, and the maximum particle size of the silica-based powder is preferably less than 1 mm in order to ensure the melt leakage preventing effect.

  From the viewpoint of powder fluidity, the maximum particle size is preferably less than 3 mm. That is, if the maximum particle size is 3 mm or more, the fluidity is deteriorated. In addition, the lower limit of the maximum particle size of the silica-based powder is not particularly specified, but if the maximum particle size is less than 10 μm, the silica-based powders sinter at a high temperature near the silicon melting point to inhibit fluidity. The maximum particle size is preferably 10 μm or more.

  Next, an embodiment of a mold for preventing molten silicon leakage of a polycrystalline silicon manufacturing apparatus according to the present invention will be described with reference to the drawings. FIG. 1 is a schematic partial longitudinal sectional view in a polycrystalline silicon production apparatus solidification furnace in which a mold for preventing molten silicon leakage according to the present invention is installed.

  In the embodiment of FIG. 1, the heater 1 made of high-purity high-density graphite is arranged above the mold and in the furnace ceiling. The inner mold 2 is made of quartz, and silicon nitride is coated on the inner side. The outer mold 3 is a high-strength carbon fiber reinforced carbon composite material (hereinafter referred to as C / C material). Silica sand is arranged in the filling layer 4 between the inner mold 2 in contact with the silicon slab and the outer mold 3 in contact with the external heat insulating material. Silica sand is a specification in which high-purity silica stone is crushed and sieved to select only 0.5 to 1 mm grains, and then magnetically separated to remove iron-based impurities.

  Similar to the outer mold, a wall made of C / C material is arranged on the wall above the outer mold, and a ceiling wall made of C / C material is also arranged above the heater. That is, the inside of the furnace of FIG. 1 has a structure surrounded by six surfaces with C / C material. On the surface of the six surfaces except the bottom of the outer mold, a molded heat insulating material 5 made of graphite is arranged outside the outer mold so that the heat inside the furnace is not easily transmitted to the outside of the furnace. A water-cooled metal member 6 is disposed only at the bottom of the outer mold, and heat is extracted from the lower part of the mold.

  The C / C material and the graphite heat insulating material are very expensive and need to be used repeatedly. Also, since the water-cooled metal member is placed at the bottom of the mold, if the molten silicon leaks out and reaches the water-cooled metal member, it may cause more serious equipment damage such as water leakage. become.

  In the mold of the present invention, even if molten silicon leaks from the inner mold 2, the silicon is received by the silica sand filled in the filling layer 4, so that the outer mold 3, the graphite insulating material 5 outside thereof, and the water-cooled metal The member 6 is not reached, and it is possible to have a lifetime of the member and to operate stably.

  The embodiment of FIG. 2 is an example in which the mold of the present invention is arranged in a so-called Bridgman furnace. The heater 7 made of high purity high density graphite is disposed above the mold, and the heater 9 is disposed on the side of the outer mold 13. The inner mold 8 is made of quartz, and silicon nitride is coated on the inner side. The outer mold 13 is a C / C material. Silica sand is arranged in the filling layer 10 between the inner mold 8 in contact with the silicon slab and the outer mold 13 in contact with the external heat insulating material.

  Silica sand is a specification in which high-purity silica stone is crushed and sieved to select only 0.5 to 1 mm grains, and then magnetically separated to remove iron-based impurities. A graphite molded heat insulating material 11 is arranged on the ceiling surface and the side surface except the bottom so as not to transmit heat in the furnace. A water-cooled metal member 12 is disposed only at the bottom of the outer mold, and heat is extracted from the lower part of the mold. The C / C material and the graphite heat insulating material are very expensive and need to be used repeatedly.

  In addition, since the water-cooled metal member 12 is arranged at the bottom of the mold, if the molten silicon leaks and reaches the water-cooled metal member 12, more serious equipment damage such as water leakage may occur. Will give. In the mold of the present invention, even if molten silicon leaks from the inner mold 8, the silicon is received by the silica sand filled in the filling layer 10, so that the outer mold 13, the graphite heat insulating material 11, and the water-cooled metal member 12 It does not reach the end of its life, and can have a long life and can be stably operated.

  The melt leakage and impurity contamination tests were conducted using the apparatus of FIG.

  The outer mold 3 is an assembly-type C / C mold with an opening of 600 mm x 600 mm and a depth of 320 mm (both internal dimensions). The inner mold 2 has an opening of 500 mm x 500 mm and a depth of 300 mm. A quartz mold with a wall thickness of 10 mm (both inner dimensions) was used. The gaps between the outer mold 3 and the inner mold 2 which are the filling layer 4 in FIG. 1 were filled with powders of various materials as described below, but the thickness was 40 mm on the side and 10 mm on the bottom.

  In Example 1, the packing layer 4 is silica glass having a particle size of 0.5 to 1 mm, in Example 2, the packing layer 4 is silica sand having a particle size of 0.5 to 1 mm, and in Comparative Example 1, the packing layer 4 is 0.5 to 1 mm in particle size. Alumina powder, in Comparative Example 2, the packed layer 4 is zirconia powder having a particle size of 0.5 to 1 mm, in Comparative Example 3, the packed layer 4 is magnesia powder having a particle size of 0.5 to 1 mm, in Comparative Example 4, the packed layer 4 is carbon fiber, In Comparative Example 5, the filler layer 4 was made of silica / alumina fiber. It shows in Table 1 about the combination of an Example and a comparative example.

  A hole with a diameter of 10 mm was made in advance at a height of 20 mm from the bottom of the side surface of the quartz inner mold. A structure in which molten silicon leaks from the mold was intentionally formed by the holes. After replacing the chamber with argon gas at atmospheric pressure, 100 kg of high-purity silicon (99.999999999% by mass) was melted at 1550 ° C in the melting furnace, and the mold was heated at a heating rate of 5 ° C per minute with a resistance heater. Heated to 1550 ° C.

  Next, 100 kg of molten silicon (silicon melt) in the melting furnace was poured into the mold, and then the temperature of the resistance heating heater was gradually lowered to gradually solidify the silicon from below. After the entire silicon solidified, the resistance heater was turned off and cooled to room temperature. After cooling to room temperature, the solidified silicon was removed from the mold.

[Comparison of molten silicon leaks]
For Example 1, 99.5 kg of silicon was recovered. That is, although 0.5 kg of melt flowed out, the melt that flowed out was stopped by the silica glass powder and did not reach the inner wall surface of the outer mold. In Example 2 as well, 0.5 kg of melt flowed out, but the flowed out melt was stopped by the silica sand powder and did not reach the inner wall surface of the outer mold.

  For Comparative Example 1, 99.5 kg of silicon was recovered. That is, 0.5 kg of the melt flowed out, but the flowed out melt was stopped by the alumina powder and did not reach the inner wall surface of the outer mold. In Comparative Example 2, 0.5 kg of melt flowed out, but the melt that flowed out was stopped by the zirconia powder, and did not reach the inner wall surface of the outer mold. In Comparative Example 3, 0.5 kg of the melt flowed out, but the flowed out melt was stopped by the magnesia powder and did not reach the inner wall surface of the outer mold.

  In Comparative Example 4, only 10 kg of silicon could be recovered. That is, 90 kg of silicon leaked and impregnated into the C / C member, making it impossible to reuse. Comparative Example 5 recovered 99.8 kg of silicon. That is, 0.2 kg of the melt flowed out, but the flowed out melt was stopped by the silica / alumina fiber and did not reach the inner wall surface of the outer mold.

[Mixed silicon contamination comparison]
Furthermore, an analysis sample was taken from the center of the silicon lump and qualitatively analyzed by ICP-AES (ICP emission) analysis.

  As a result, no element other than Si was detected from the silicon mass of Example 1, and it was confirmed that there was no contamination with impurities. The detection limit values in the ICP-AES analysis method are B: 0.05 mass ppm, P: 0.05 mass ppm, and other heavy metals are 0.02 mass ppm. A similar test was performed on Example 2. As a result, elements other than Si were not detected, and it was confirmed that there was no contamination with impurities.

  As a result of conducting the same test for Comparative Example 1, 27 mass ppm of Al was detected, and it was confirmed that there was contamination by Al. As a result of conducting the same test for Comparative Example 2, 17 mass ppm of Zr was detected, and it was confirmed that there was contamination with Zr. As a result of conducting the same test for Comparative Example 3, 7 ppm of Mg was detected, and it was confirmed that there was contamination with Mg. As a result of conducting the same test for Comparative Example 4, it was confirmed that no elements other than Si were detected and there was no contamination by impurities. When a similar test was performed for Comparative Example 5, 22 ppm by mass of Al was detected, and it was confirmed that there was contamination with Al. The evaluation results are summarized in Table 2.

  In Examples 1 and 2 of the present invention, there was no impurity contamination in silicon, and leakage of molten silicon could be stopped. On the other hand, in Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 5, leakage of the silicon melt could be stopped, but there was impurity contamination in silicon, and in Comparative Example 4, impurity contamination in silicon. No, but the leakage of molten silicon could not be stopped.

It is a fragmentary longitudinal cross-sectional view which shows typically the inside of the polycrystal silicon manufacturing apparatus solidification furnace which installed this invention casting_mold | template. It is a figure which shows the aspect which has distribute | arranged this invention casting_mold | template in the furnace of a Bridgeman type | mold furnace.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Resistance heater 2 Inner mold 3 Outer mold 4 Filling layer 5 Graphite heat insulating material 6 Water-cooled metal member 7 Upper resistance heater 8 Inner mold 9 Side resistance heater 10 Filling layer 11 Graphite heat insulating material 12 Water-cooled metal member Cum lift table 13 outer mold

Claims (3)

  A mold for producing polycrystalline silicon slabs by a unidirectional solidification casting method, comprising at least an outer mold, a double mold composed of an inner mold, and a filler made of silica-based powder between both molds. A mold for producing a polycrystalline silicon slab characterized by being arranged.   2. The mold for producing a polycrystalline silicon slab according to claim 1, wherein the silica-based powder is one or both of silica sand and silica glass.   3. The mold for producing a polycrystalline silicon slab according to claim 1, wherein the maximum particle size of the silica-based powder is less than 3 mm.

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