JP2012171841A - Method for producing polycrystalline silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce polycrystalline silicon having a fixed quality by using a silicon waste material generating in a processing step of the polycrystalline silicon a recycling raw material without refining it.SOLUTION: In production of polycrystalline silicon, a high-concentration impurity region existing in an outermost surface portion of a bottom material 1b or a casting surface material 2b which is a silicon waste material cut out from a cast polycrystalline silicon ingot 1 is removed by grinding, and then the silicon waste material is used as a recycling raw material. When casting the polycrystalline silicon ingot, a mixed raw material is used, which contains the recycling raw material and a virgin material comprising a natural raw material, and which is blended so that the virgin raw material has a higher ration than the recycling raw material.

Description

  The present invention relates to a method for producing polycrystalline silicon, and more particularly to a method for recycling silicon waste material obtained during processing of cast polycrystalline silicon.

  In recent years, there has been a strong demand for recycling silicon waste materials against the background of a decline in quality resources and the deterioration of the global environment. For example, Patent Document 1 discloses, for example, solidification of silicon melt remaining in the bottom of a quartz crucible after pulling up the silicon single crystal (crucible silicon lump), or a silicon single crystal for semiconductors having a high impurity concentration. It is disclosed that a material that does not satisfy the required purity as a raw material is used as a raw material for manufacturing a solar cell. This recycling method takes into consideration that the quality specifications of the raw materials for solar cells are significantly looser than those of the raw materials for semiconductors, and there is no problem even if the impurity concentration in the raw materials is somewhat high.

  Further, in Patent Document 1, in a method for producing a silicon single crystal by the CZ method, a crucible residual silicon lump or a material that does not satisfy the purity required as a silicon raw material for a semiconductor is melted in a quartz crucible, and a crystal by the CZ method is obtained. There has been proposed a method of obtaining a raw material crystal for producing a silicon single crystal by pulling up. According to this method, the crystal pulling by the CZ method is performed twice, the raw material is purified by the first pulling, the obtained silicon raw material crystal is melted again in the quartz crucible, and the product is recovered by the second pulling. It is possible to adopt an operation method of obtaining a silicon single crystal.

  In Patent Document 2, paying attention to the fact that the resistivity of the evaluation used sample used for the quality evaluation of the straight body part of the silicon single crystal is known, the evaluation used sample that has been discarded in the past is used for growing the silicon single crystal. And a method of using it as a dopant in the case of growing a high-resistance silicon single crystal or the like. The evaluation used sample here is a quality evaluation sample part after being used for the evaluation, and the quality evaluation sample part is a sample for evaluating the quality of the straight body portion of the silicon single crystal. It is a part cut out from the trunk part with a thickness of about 3 mm or less.

  Further, in Patent Document 3, considering that the amount of silicon discarded in the process of manufacturing a semiconductor device from a silicon ingot is 96%, waste liquid or waste material discharged from a silicon ingot or silicon wafer processing apparatus is disclosed. There has been proposed a method of recovering and recycling silicon that is discarded by collecting and purifying waste liquid or waste material.

JP 2009-249233 A JP 2008-156185 A JP 2009-298650 A

  As described above, there is known a method for recycling waste materials generated during the production of silicon wafers for semiconductor devices. However, recycling of waste materials generated during the production of solar cell silicon substrates is also required. A polycrystalline silicon substrate for a solar cell is manufactured by subjecting a polycrystalline silicon ingot to processing such as cutting, grinding, polishing, and etching. Waste materials generated at this time include top materials, bottom materials, cast skin materials, defective wafer materials, etc., but these do not meet the quality specifications of semiconductor device raw materials, so they can be reused as raw materials for semiconductor devices. Can not. Of these, the defective wafer material is a high-purity material at the product level, so it is recycled as it is to the raw material for solar cells, but the other waste materials have low purity. It was recycled as a raw material.

  However, there is a problem that if silicon waste material is refined for recycling, the cost becomes high and the productivity is poor.

  The present invention solves the above-mentioned problems, and the object of the present invention is to obtain a certain quality by using silicon waste material generated in the process of manufacturing a polycrystalline silicon substrate as a recycled material without purification. An object of the present invention is to provide a method for producing a polycrystalline silicon.

  As a result of intensive research on a highly productive recycling method, the inventors of the present application have found that the bottom material and the cast skin material of the polycrystalline silicon ingot can be removed by grinding only the outermost surface portion, so that many It was found that it can be recycled as a raw material for crystalline silicon. Even in the case of silicon for solar cells, raw materials with an extremely large amount of impurities are undesirable because they adversely affect product characteristics. Unlike semiconductor devices, however, some impurities are allowed, so the impurity concentration is somewhat low. If it is a raw material, it can be used by mixing with a virgin raw material without going through a purification step.

  The present invention is based on such technical knowledge, and the method for producing polycrystalline silicon according to the present invention is the bottom surface or the outermost surface of the cast skin material, which is silicon waste material cut out from a cast polycrystalline silicon ingot. The high-concentration impurity region existing in the part is removed by grinding, and then the silicon waste material is used as a recycled material. Further, the method for producing polycrystalline silicon according to the present invention includes a mixed raw material containing a recycled raw material made of silicon waste and a virgin raw material made of a natural raw material and blended at a higher ratio of the virgin raw material than the recycled raw material. It is characterized by casting a crystalline silicon ingot.

  According to the present invention, it is possible to provide a method for producing polycrystalline silicon that can be used as a recycled raw material to obtain a certain quality without refining silicon waste generated in the processing step of polycrystalline silicon. it can.

FIG. 1 is a flowchart showing a manufacturing process of polycrystalline silicon for solar cells according to a preferred embodiment of the present invention. FIG. 2 is a schematic sectional side view showing the structure of the polycrystalline silicon casting apparatus according to the preferred embodiment of the present invention. FIG. 3 is a schematic cross-sectional view showing the configuration of the bottomless crucible shown in FIG. FIG. 4 is a schematic diagram for explaining a cutting step of cutting a silicon block from a polycrystalline silicon ingot.

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

  FIG. 1 is a flowchart showing a manufacturing process of polycrystalline silicon for solar cells according to a preferred embodiment of the present invention.

  As shown in FIG. 1, in the manufacture of a polycrystalline silicon substrate for a solar cell, first, a silicon raw material is prepared (step S101). In the present embodiment, it is preferable to use a mixture of a virgin raw material and a recycled raw material in a predetermined ratio, but only a virgin raw material may be used. Here, the “virgin raw material” is a raw material made of only natural silicon that does not include a recycled raw material, and the “recycled raw material” is a raw material made only of silicon waste material generated in the processing step of the silicon product. The silicon product here is preferably polycrystalline silicon, but may be single crystal silicon. Furthermore, the recycled raw material may be a mixture of silicon waste material generated in the polycrystalline silicon processing step and waste material generated in the single crystal silicon processing step.

  In order to ensure a certain quality of polycrystalline silicon, it is preferable that a larger amount of virgin material is blended than recycled material. Specifically, the virgin raw material is preferably 70% or more by weight, the recycled raw material is preferably 30% or less, particularly preferably 80% or more and the recycled raw material 20% or less. Usually, the amount of recyclable silicon waste obtained from a single polycrystalline silicon ingot processing process does not exceed 30%. Therefore, even if it is used as it is for casting the next polycrystalline silicon ingot, the ratio of the recycled materials does not exceed 30% of the total raw materials, and efficient recycling is possible while ensuring a certain product quality.

  Next, a polycrystalline silicon ingot is cast using the silicon raw material (step S102). As a casting method of the polycrystalline silicon ingot, an electromagnetic casting method (EMC method) is preferable. In the electromagnetic casting method, the silicon raw material charged in the bottomless crucible is melted by electromagnetic induction heating, and the molten silicon is drawn out from the bottom of the bottomless crucible and cooled to grow crystals. A silicon ingot is obtained. Hereinafter, a method for producing a polycrystalline silicon ingot by electromagnetic casting will be described.

  FIG. 2 is a schematic sectional side view showing the structure of the polycrystalline silicon casting apparatus according to the preferred embodiment of the present invention.

  As shown in FIG. 2, the polycrystalline silicon casting apparatus 10 according to the present embodiment electromagnetically induces a chamber 11, a bottomless crucible 12 provided at the upper center in the chamber 11, and a silicon raw material in the bottomless crucible 12. An induction coil 13 for heating, an auxiliary heater 14 for heating the silicon raw material in the bottomless crucible 12 together with the induction coil 13, a support shaft 15 for supporting the silicon ingot 20 drawn from the bottomless crucible 12, and a bottomless A heat retaining heater 16 that retains the temperature of the silicon ingot 20 obtained by cooling the silicon melt 21 in the crucible 12 is provided.

  An inert gas introduction port 11 a is provided in the upper part of the chamber 11, and a vacuum suction port 11 c is provided in the lower part of the chamber 11. Further, a raw material inlet 11b partitioned by a blocking means is provided at the top of the chamber 11, and an ingot outlet 11d is provided at the bottom.

  FIG. 3 is a schematic cross-sectional view showing the configuration of the bottomless crucible 12.

  As shown in FIG. 3, the bottomless crucible 12 is a square tube body made of copper, and is divided in the circumferential direction by a partial vertical slit 12 a. An induction coil 13 is provided around the bottomless crucible 12. The bottomless crucible 12 is a water-cooled type, and is forcibly cooled by cooling water flowing through the inside. Although not particularly limited, the opening size of the bottomless crucible 12 can be 350 × 530 mm.

  As shown in FIG. 2, a duct 18 is provided in the chamber 11, and a granular or lump shape supplied from a raw material hopper (not shown) provided outside the chamber 11 via a raw material input port 11 b. The silicon raw material 22 is introduced into the bottomless crucible 12 through the duct 18.

  The induction coil 13 is disposed around the bottomless crucible 12 and is connected to a power source via a coaxial cable. In addition, in order to heat and melt the silicon raw material together with the induction coil 13, an auxiliary heater 14 by means of a plasma arc torch is provided above the bottomless crucible 12 so that it can be raised and lowered. It has become so.

  The support shaft 15 is provided so as to be able to move up and down through the ingot outlet 11d. The tip of the support shaft 15 can reach the lower end of the bottomless crucible 12, thereby forming a movable bottom surface with respect to the bottomless crucible 12, and a silicon ingot drawn downward from the bottomless crucible 12 Can be supported.

  The heat retaining heater 16 is, for example, a resistance heater, and is provided below the bottomless crucible 12 and heats the ingot pulled down from the bottomless crucible 12 to keep the temperature at a constant temperature of 1100 ° C., for example. The heat retaining heater 16 according to the present embodiment uniformly heats the ingot without giving a temperature gradient in the axial direction (vertical direction). In order to increase the heat insulation efficiency of the heat insulation heater 16, a heat insulating material 17 is provided on the outer periphery of the heat insulation heater 16.

  Next, a method for casting polycrystalline silicon will be described. In the production of polycrystalline silicon by electromagnetic casting, first, the inside of the chamber 11 is evacuated from the vacuum suction port 11c, and then an inert gas such as argon is introduced into the chamber 11 from the inert gas introduction port 11a. Is an inert gas atmosphere. Next, the support shaft 15 with the initial silicon raw material attached to the tip is raised and inserted from below the bottomless crucible 12, and the opening at the bottom of the bottomless crucible 12 is closed with the initial silicon raw material.

  Next, electromagnetic force is applied to the initial silicon raw material by the induction coil 13, and the initial silicon raw material in the bottomless crucible 12 is melted using the induction coil 13 and the auxiliary heater 14 to generate a silicon melt 21. At this time, the silicon melt 21 in the bottomless crucible 12 is held in a non-contact state with respect to the inner surface of the bottomless crucible 12. Thereafter, the silicon melt 21 is gradually lowered together with the support shaft 15 to be solidified. At the same time, the silicon raw material 22 is added to the silicon melt 21 and the additional raw material is dissolved by the combined use of electromagnetic induction heating by the induction coil 13 and plasma heating by the auxiliary heater 14.

  By continuing this operation, the silicon ingot 20 is continuously manufactured in the bottomless crucible 12 by electromagnetic induction heating by the induction coil 13 and is continuously drawn out from the bottomless crucible 12. The silicon ingot 20 is pulled out below the chamber 11 while being kept warm by the warming heater 16.

  The silicon ingot 20 is continuously cast until it reaches a predetermined length (for example, 7000 mm), and during that period, the silicon ingot 20 is kept at a constant temperature (for example, 1100 ° C.) by heating from the heat retaining heater 16. The silicon ingot 20 that has reached a predetermined length is gradually cooled and then removed from the chamber 11.

  Thereafter, the prismatic polycrystalline silicon ingot cast by the electromagnetic casting method is divided into silicon blocks of a predetermined size that are easy to process, and by slicing each silicon block, for example, an approximately square of about 15 × 15 cm. A polycrystalline silicon substrate for solar cells is obtained.

  Specifically, as shown in FIG. 1, first, an ingot is divided to produce a middle ingot (step S103), the middle ingot is divided to produce a small ingot (step S104), and a plurality of silicon blocks are formed from the small ingot. Cut out (step S105). Furthermore, the silicon block of a product size is completed by grinding the outer peripheral surface of the silicon block (step S106). Thereafter, the silicon block is sliced to a predetermined thickness to produce a silicon substrate (step S107), and the front and back surfaces of the silicon substrate are polished to adjust the thickness and flatness (step S108). A crystalline silicon substrate is completed.

  FIG. 4 is a schematic diagram for explaining a cutting step of cutting a silicon block from a polycrystalline silicon ingot.

As shown in FIG. 4 (a), first, the top material 1a and the bottom material 1b at both ends in the longitudinal direction are cut from the prismatic polycrystalline silicon ingot (original ingot) 1, and the product is an effective part as a product. The body part is divided to obtain a plurality of middle ingots 2. Line C ₁₁ indicates a cutting position of the equally divided. The number of ingot divisions is not particularly limited, and may be determined as appropriate according to the final target silicon block length.

  Although the top material 1a and the bottom material 1b are cut out as non-product objects and are made into silicon waste materials, as shown in FIG. 4 (f), the bottom material 1b is made a recycling object. The bottom material 1b is coated with silicon nitride powder in order to prevent the carbon dummy and silicon from being fused, and it is necessary to remove the silicon nitride powder. However, since the silicon nitride powder is formed only on the surface of the bottom material 1b, it is sufficient to grind the surface. Therefore, the bottom material 1b is used as a recycled material after removing the high-concentration impurity region existing in the outermost surface portion by grinding. The grinding depth of the bottom material 1b is preferably 1 to 5 mm from the outermost surface.

  The top material 1a is a site where concentration due to segregation of impurities and precipitation of foreign matters are occurring, and the purity of the top material is very poor in the ingot. Therefore, the top material 1a may be discarded without being recycled by the same processing as the bottom material 1b, or may be purified by being dissolved once. As a purification method, a casting method, EB dissolution, CZ method, or the like can be employed. In particular, in the case of crystal pulling by the CZ method, a very high purification effect can be obtained without pulling up as a single crystal.

Next, as shown in FIG. 4B, the middle ingot 2 is divided into a predetermined number to obtain a small ingot 2a. That is, the middle ingot 2 is cut at the positions of the lines C ₂₁ and C ₂₂ to remove the casting surface, and further cut at the position of the line C ₂₃ and divided into two in the X-axis direction, as shown in FIG. The small ingot 2a shown is obtained.

Further, as shown in FIG. 4C, each of the small ingots 2a is cut at the positions of the lines C ₃₁ and C ₃₂ to remove the casting surface, and further cut at the positions of the lines C ₃₃ and C _34. Divide into three in the Y-axis direction. As described above, as shown in FIG. 4D, the silicon block 3 having a substantially square cross section obtained by finally dividing the middle ingot 2 into 6 can be obtained. If the cross-sectional size of the polycrystalline silicon ingot 1 is, for example, a rectangle of 505 × 345 mm, the middle ingot 2 can be divided into six to obtain a silicon block 3 of about 158 × 158 mm. Thereafter, as shown in FIG. 4E, the silicon block 3 is sliced to obtain a base material of the polycrystalline silicon substrate 4.

  As shown in FIG. 4G, the cast skin material 2b cut out from the middle ingot 2 and the small ingot 2a is silicon waste material, but is subject to recycling. Here, the high concentration impurity region existing in the outermost surface portion of the casting surface material 2b is removed by grinding, and this silicon waste material is used as a recycled raw material. In particular, the top surface portion of the ingot is heavily impregnated and oxidized, and cannot be easily treated by acid cleaning. Therefore, the cast skin material 2b can be recycled by mechanically grinding and removing the outermost surface portion. As described above, it is possible to dissolve and purify the casting surface material 2b. However, since the processing cost is very high compared to grinding, grinding is preferable.

  The concentrations of the impurities contained in the high concentration impurity regions of the bottom material 1b and the casting skin material 2b are Fe> 1E15 atoms / cc, Ni> 1E14 atoms / cc, Cr> 1E14 atoms / cc, Cu> 1E15 atoms / cc. Although the concentration of each metal impurity present in the outermost surface portion is very high as described above, a high-quality recycled raw material can be generated by removing the portion.

  The polycrystalline silicon substrate 4 shown in FIG. 4E is inspected for cracks, chips, dirt, and foreign matter through a predetermined inspection process, and those that do not satisfy the predetermined quality are recognized as defective wafer materials, Turned into recycled material. At this time, a wafer material having a defective shape such as a crack or a chip can be recycled as an initial raw material by simply crushing it. Moreover, what has a dirt and a foreign material can be used as a recycled raw material by carrying out melt | dissolution refinement | purification and washing | cleaning.

  As described above, according to the present embodiment, for the bottom material 1b and the casting skin material 2b cut from the polycrystalline silicon ingot, only the outermost surface portion is removed by grinding and then used as a recycled material. It can be used as a part of the silicon raw material without requiring a high-cost process such as refining or reducing the quality of the final product. Further, according to the present embodiment, the virgin raw material and the recycled raw material are blended at a predetermined ratio, and in particular, the ratio of the virgin raw material is larger than the recycled raw material, which affects the product characteristics. Therefore, a polycrystalline silicon substrate having a desired quality can be manufactured.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Needless to say, the invention is included in the invention.

  For example, in the above-described embodiment, the case where polycrystalline silicon is manufactured by an electromagnetic casting method has been described as an example, but the present invention is not limited to the electromagnetic casting method, and is manufactured by a normal casting method. There may be.

DESCRIPTION OF SYMBOLS 1 Polycrystalline silicon ingot 1a Top material 1b Bottom material 2 Medium ingot 2a Small ingot 2b Cast skin material 3 Silicon block 4 Polycrystalline silicon substrate 10 Polycrystalline silicon casting apparatus 11 Chamber 11a Inert gas inlet 11b Raw material inlet 11c Vacuum suction Mouth 11d Ingot outlet 12 Bottomless crucible 12a Slit 13 Inductive coil 14 Auxiliary heater 15 Support shaft 16 Heat retaining heater 17 Heat insulating material 18 Duct 20 Silicon ingot 21 Silicon melt 22 Silicon raw material

Claims (7)

  After removing the high-concentration impurity region existing in the outermost surface portion of the bottom material or cast skin material, which is silicon waste material cut out from the cast polycrystalline silicon ingot, the silicon waste material is used as a recycled raw material. A method for producing polycrystalline silicon.   The method for producing polycrystalline silicon according to claim 1, wherein the high concentration impurity region is within a range of 1 to 5 mm from the outermost surface.   3. The method for producing polycrystalline silicon according to claim 1, wherein a concentration of Fe among impurities contained in the high concentration impurity region is Fe> 1E15 atoms / cc. 4.   4. The method for producing polycrystalline silicon according to claim 1, wherein the concentration of Ni in the impurities contained in the high concentration impurity region is Ni> 1E14 atoms / cc. 5.   5. The method for producing polycrystalline silicon according to claim 1, wherein a concentration of Cr among impurities contained in the high concentration impurity region is Cr> 1E14 atoms / cc. 6.   6. The method for producing polycrystalline silicon according to claim 1, wherein a concentration of Cu among impurities contained in the high concentration impurity region is Cu> 1E15 atoms / cc. 7.   The polycrystalline silicon ingot is cast using a mixed raw material containing the recycled raw material and a virgin raw material made of a natural raw material, and the virgin raw material is blended at a higher ratio than the recycled raw material. The method for producing polycrystalline silicon according to any one of 1 to 6.

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