Patent Publication Number: US-2006016292-A1

Title: Process for producing silicon

Description:
TECHNICAL FIELD  
      The present invention relates to a production process of polycrystalline silicon in which molten silicon is received and cooled. More particularly, the present invention relates to a production process of polycrystalline silicon which, in receiving molten silicon, uses a receiving vessel formed of a plurality of divided silicon plate members.  
     BACKGROUND ART  
      Various production processes for producing polycrystalline silicon for use as a raw material for semiconductors or solar light power generation batteries have been known in the art. An example of the production process of polycrystalline silicon is a process called a Siemens process in which the surface of a silicon rod placed within a bell jar is heated and a raw material gas for silicon deposition, containing chlorosilanes, such as trichlorosilane (SiHCl 3 ; hereinafter referred to as TCS) and monosilane (SiH 4 ), and a reducing gas such as hydrogen is brought into contact with the heated silicon rod to deposit polycrystalline silicon.  
      The Siemens process is characterized in that high-purity silicon is produced, and is carried out as the most common process. Since, however, the deposition is carried out in a batch process, the Siemens process suffers from a problem that a very troublesome procedure comprised of installation of a silicon rod as a seed, electrical heating, deposition, cooling, taking-out, and cleaning of a bell jar should be carried out.  
      To overcome the above problem, as a reaction apparatus for the production of polycrystalline silicone in a continuous and stable manner, for example, Japanese Patent Laid Open No. 29726/2002 suggests a new apparatus. In the apparatus, a raw material gas for silicon deposition is fed into a cylindrical vessel, which can be heated to a temperature above the melting point of silicon, the cylindrical vessel is heated to deposit silicon, the deposited silicon is continuously melted and dropped from the lower end of the cylindrical vessel, and the dropped silicon is received.  
      In the above silicon production apparatus, the base material used for the vessel in which the molten silicon is received and cooled is generally a nonmetallic material such as graphite, quartz glass, or silicon nitride, or a metallic material such as stainless steel, copper, or molybdenum. It is known that the inside of the metallic receiving chamber is lined with silicon from the viewpoint of simultaneously realizing that the silicon production apparatus is rendered sturdy as an industrial apparatus and that a high-purity silicon is received (for example, Japanese Patent Laid Open No. 2626/2003).  
      The conventional silicon lining is generally one prepared by integrating a silicon lining material through bonding or welding, or one bonded or flame-coated to a metallic vessel or a nonmetallic vessel. The integrated silicon lining material, however, sometimes poses a problem that, upon contact with molten silicon, the lining material is cracked by thermal stress, or otherwise the broken lining material is adhered to molten silicon and, accordingly, impurities in the vessel are also incorporated, resulting in contamination of a silicon product.  
      As a result, the step of preparing the expensive silicon lining material each time and the step of removing the adhered contaminant and the like from the silicon product is necessary. Thereby, the productivity is lowered, and, thus, the cost effectiveness is disadvantageously lowered.  
      Further, regarding lining other than silicon, for example, as disclosed in Japanese Patent Laid Open No. 2626/2003, in nonmetallic materials such as quartz glass or Teflon (registered trademark), upon contact with molten silicon, the solidified silicon is solidly adhered or fused to the nonmetallic material, and, consequently, the separation of silicon from the lining material becomes sometimes difficult.  
      Accordingly, the development of a production process of silicon, which can reutilize the receiving vessel without breaking of the receiving vessel and further is free from inclusion, in silicon, of impurities from the receiving vessel in contact with molten silicon, has been desired in the art.  
      The present inventor has made extensive and intensive studies with a view to solving the above problems of the prior art and, as a result, has found that, in receiving molten silicon in a receiving vessel to produce polycrystalline silicon,  
      the use of a receiving vessel formed of a plurality of divided silicon plate members can diffuse thermal expansive stress caused by contact with silicon melt and volume expansive force in the solidification of the melt, prevents cracking of the silicon plate member, and can reutilize an expensive silicon lining material.  
     DISCLOSURE OF THE INVENTION  
      The present inventor has surprisingly found that, when the receiving vessel is formed of a plurality of silicon plate members, even though the individual members are not bonded to each other, the silicon melt is not leaked from the gap between the members and silicon can be efficiently recovered while preventing contamination of the silicon product, which has led to the completion of the present invention.  
      According to the present invention, there is provided a process for producing silicon, comprising the steps of:  
      depositing silicon in a solid state or molten state by contacting gas mixture of hydrogen and silanes to the surface having the temperature range of 600 to 1700° C.;  
      melting a part or the whole of the precipitated silicon, dropping the melt from a precipitation surface, and receiving the dropped molten silicon in a receiving vessel, wherein  
      said receiving vessel  
      comprises a silicon bottom plate member(s) and a plurality of silicon side plate members that are installed upright direction from the peripheral part of the bottom plate member.  
      A frame body may be provided around the receiving vessel.  
      A silicon block may be mounted on the silicon bottom plate member(s).  
      In the production process of silicon according to the present invention, even when a large amount of molten silicon is dropped, the breakage of the receiving vessel can be significantly reduced and, in addition, silicon as a product can be efficiently received. Further, the level of contamination of the received silicon block with impurities is very small. Furthermore, the silicon member used as the receiving vessel can be reutilized. Since a silicon member is used, the resultant silicon block as such can also be fed without being separated from the silicon member to the step of producing an ingot. Furthermore, off-specification products of polycrystalline silicon can be used as the silicon member, and, thus, the material can be effectively utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram of one embodiment of a receiving vessel used in the present invention;  
       FIG. 2  is a schematic diagram of another embodiment of a receiving vessel used in the present invention;  
       FIG. 3  is a schematic diagram of another embodiment of a contact surface between silicon side plate members themselves;  
       FIG. 4  is a schematic diagram of still another embodiment of a contact surface between a silicon side plate member and a bottom plate; and  
       FIG. 5  is a schematic diagram of another embodiment of a receiving vessel used in the present invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      The production process of silicon according to the present invention will be described in more detail.  
      In the present invention, a receiving vessel formed of a plurality of silicon members is used as a receiving vessel for receiving molten silicon.  
      Receiving Vessel  
      Silicon Plate Member  
      In the receiving vessel used in the present invention, a silicon bottom plate member(s) is mounted on the bottom, and silicon side plate members are stood upright direction from the peripheral part of the bottom plate member. An opening for receiving molten silicon is provided on the upper side.  
      The silicon bottom plate member is not particularly limited so far as it is a plate formed of silicon. The thickness, however, is generally not less than 5 mm, preferably not less than 10 mm, most preferably not less than 15 mm. The size of the silicon bottom plate member is properly selected according to the size of the bottom plate. A plurality of silicon bottom plate members may be arranged, and the disposition of the divided silicon bottom plate members can diffuse the stress created by thermal impact, and the vessel is less likely to become broken.  
      The silicon side plate member is not particularly limited so far as it is a plate formed of silicon. The thickness, however, is generally not less than 5 mm, preferably not less than 10 mm, most preferably not less than 15 mm. The size of the silicon side plate member is properly selected according to the size of the receiving vessel. A rectangular plate member is preferred from the viewpoints of easiness on arrangement, easiness on leaning and the like. The length of the narrow side of the rectangular plate is preferably not more than 30 cm, more preferably not more than 25 cm. The length of the long side is not particularly limited.  
      A part, which is quarried from the resultant polycrystalline silicon ingot or the like, may be used as the silicon member. Further, for example, off-specification products at the time of switching of the production lot may also be used. Further, used materials may be reutilized.  
      This receiving vessel is shown in  FIG. 1 .  FIG. 1  is a schematic typical diagram showing one embodiment of the receiving vessel used in the present invention. As shown in  FIG. 1 , a plurality of silicon side plate members may be arranged in a direction vertical to the silicon bottom plate member.  
      As shown in  FIG. 2 , a plurality of silicon side plate members may be stacked in a direction horizontal to the silicon bottom plate member (that is, vertically).  
      The silicon side plate member can stand by the own weight. Alternatively, the silicon side plate members may be mutually sustained by each other for self-sustaining. For example, the face at which the silicon side plate members are brought into contact with each other is generally vertical, or alternatively may be obliquely cut. This can realize mutual supporting for self-sustaining (see  FIG. 3 ).  
      The silicon side plate members may be provided upright direction from the peripheral part of bottom plate member. The silicon side plate members may be surrounded the outside of silicon bottom plate member, or alternatively may be superimposed on the peripheral part.  
      Further, when inclination is provided in the peripheral part of the silicon bottom plate member, the silicon side plates can easily be rested against each other. The provision of inclination on the lowermost end face of the silicon side plate member (that is, the end face in contact with-the-bottom plate) can also facilitate resting of the silicon side plates against each other (see  FIG. 4 ).  
      Crushed pieces of a silicon block may be mounted on the bottom plate member.  
      For example, crushed pieces and mechanical cut pieces of silicon may be mentioned as the silicon block.  
      Off-specification products of polycrystalline silicon and cut and crushed pieces of a used silicon plate member having a desired size may be used as the silicon block.  
      The mounting of the silicon block can fully prevent fusing between dropped silicon and the silicon bottom plate member when the molten silicon has a high temperature or when a large amount of melt is dropped on one point in the receiving vessel. Further, in this case, scattering of a large amount of dropped melt upward can be prevented.  
      Further, as shown in  FIG. 5 , a frame body may be provided around the receiving vessel. When the frame body is provided, the silicon member can easily retain the shape as the receiving vessel. As a result, falling or breaking of the silicon member during transport can be suppressed.  
       FIG. 5A  is a diagram showing an embodiment in which a plurality of silicon side plate members are arranged perpendicularly to the silicon bottom plate member, and  FIG. 5B  is a diagram showing an embodiment in which silicon side plate members are stacked in a direction horizontal to the silicon bottom plate member (that is, vertically). In both the embodiments, a frame body is provided around the receiving vessel.  
      The frame body may be provided as an auxiliary member around the receiving vessel for the silicon member to retain the shape as the receiving vessel. That is, the frame body may be in contact with the receiving vessel without any space, or alternatively there may be a space between the frame body and the receiving vessel. The presence of a space between the frame body and the receiving vessel can cope with expansion in the solidification of the melt. Further, in order to prevent such an unfavorable phenomenon that the silicon plate member is disadvantageously cracked, broken, or chipped by impact during handling or transport, and, at the same time, to regulate the cooling rate of the received molten silicon, a felt-like cushioning material (for example, carbon felt) may be inserted into between the frame body and the silicon plate member.  
      The silicon side plate members may be lean against the frame body, or alternatively the silicon side plates may be fastened to the frame through a holding jig such as a clip or a fastening.  
      The material for the frame body is not particularly limited. In general, however, a frame body of a metal such as tungsten, molybdenum, or SUS is used. The frame body may be comprised of a frame only as shown in  FIG. 5 . Alternatively, the frame body may be a boxy-type frame body having on its bottom a bottom plate, or a bottom-free cylindrical frame body.  
      The holding jig is not particularly limited so far as it does not contaminate, as an impurity, silicon. In general, however, a holding jig formed of tungsten or molybdenum is used.  
      In the present invention, the silicon bottom plate members are disposed close to each other, the silicon side plate members are disposed close to each other, and the silicon side plate member is disposed close to the silicon bottom plate member. In this case, however, there is no particular need to bond them with the aid of an adhesive or the like. Specifically, as described above, even though there is some space between the silicon members, upon contact of the silicon melt with a low-temperature structure, the viscosity of the silicon melt is rapidly increased and, consequently, the contacted melt per se functions as a sealant, whereby leakage of the melt to the outside of the silicon plate members can be prevented. In order to minimize the amount of leakage of the melt, however, the maximum space between the silicon plate members is not more than 10 mm, preferably not more than 5 mm.  
      In assembling a receiving vessel formed of silicon lining plate member, partial welding or coating of an adhesive for temporary joining is possible. In this connection, however, as described above, it should be noted that the essential point of the present invention is that the stress of thermal expansion caused by the contact of the silicon melt with the silicon plate member and the volume expansive force caused in the solidification of the melt are diffused by taking advantage of the use of a plurality of divided silicon plate members to prevent the breaking of the silicon plate members. Accordingly, adhesive strength should be fully taken into consideration so that, upon action of these stresses, the welded part or bonded part is preferentially separated to protect the plate members. Examples of adhesives usable herein include oxide adhesives such as silica, alumina, and magnesia or carbon adhesives. They may be used alone or as a mixture of two or more of them.  
      Bottom Plate  
      A bottom plate may be provided under the silicon bottom plate member of the receiving vessel. The material for constituting the bottom plate is not particularly limited. Preferably, however, the material is one that does not contaminate silicon, and such materials include carbon materials such as graphite and metal materials such as tungsten, molybdenum, and stainless steel.  
      The form of the bottom plate is not particularly limited. In general, however, a bottom plate in a form similar to the silicon bottom plate member is used. The size of the bottom plate is not particularly limited. Preferably, however, the size of the bottom plate is larger than that of the silicon bottom plate member. The bottom plate may be doubled or triplied, or alternatively may be in a finely split form. The bottom plate may be provided only at the corner of the silicon bottom plate material. Further, the bottom plate may be provided with a water-cooled jacket or the like for cooling.  
      Production Process of Silicon  
      In the present invention, molten silicon is received in the above receiving vessel and is cooled to produce polycrystalline silicon.  
      Specifically, silicon is deposited in a solid state or molten state by contacting gas mixture of hydrogen and silanes to the surface having the temperature range of 600 to 1700° C., a part or the whole of the deposited silicon is melted and is dropped from a deposition surface, and the dropped molten silicon is received in the receiving vessel.  
      More specifically, a raw material gas for silicon deposition, containing chlorosilanes such as TCS and hydrogen is contacted with a heated silicon deposition substrate (for example, a substrate formed of a carbon material such as graphite) to deposit silicon. The deposited silicon is melted, and the molten silicon is dropped and is received in the receiving vessel.  
      The molten silicon received in the receiving vessel is preferably 1600° C. or below. When the temperature of the molten silicon is high, in some cases, the molten silicon is adhered to and cannot be separated from the bottom plate or the silicon side plate members.  
      The molten silicon is dropped on one point of the silicon bottom plate member and further is intermittently or continuously dropped thereon. Silicon is spread on the whole vessel by the own weight of the molten silicon. In this case, however, the temperature is lowered to some extent. Therefore, positions other than the dropping point, for example, the bottom plate member and the silicon side plate member in their parts distant from the dropping point, are in contact with cold silicon, and, thus, fusing of silicon does not occur. When the temperature of silicon melt to be dropped is high, in some cases, a part of dropped silicon is fused to a part of the bottom plate around the silicon dropped point in the silicon bottom plate member. This fusing, however, can be significantly reduced by dropping molten silicon having a temperature of 1600° C. or below.  
      The molten silicon used in the present invention is silicon at least a part of which is molten silicon. For example, the whole molten silicon may consist of molten silicon alone, or alternatively the molten silicon may be a mixture of molten silicon with solid silicon. Embodiments in which molten silicon and solid silicon are present in a mixed state include: (1) a state that, before dropping, solid silicon is contained as a part in molten silicon; (2) a state that, before dropping, molten silicon is contained as a part in solid silicon; (3) a state that solid silicon cooled during dropping is contained as a part in molten silicon; and (4) a state that molten silicon is contained as a part in solid silicon cooled during dropping.  
      After molten silicon is received and cooled in the receiving vessel, solidified polycrystalline silicon is taken out of the vessel. When the polycrystalline silicon is taken out of the vessel, the silicon bottom plate member and the silicon side plate members are separated from the polycrystalline silicon. The assembly may be supplied to the step of producing an ingod, without separating the silicon bottom plate member and the silicon side plate members from the polycrystalline silicon. The silicon melt may be received either continuously or intermittently a plurality of time.  
      The separated silicon bottom plate member and the silicon side plate members may be reutilized to assemble the receiving vessel. When a large amount of silicon is fused to the bottom plate member, only the bottom plate may be replaced with a fresh one.  
      The molten silicon received in the receiving vessel may be cooled by standing for self-cooling. When the cooling time is long, however, other methods may be adopted including a method in which a cooling gas is introduced into a position near the receiving vessel for accelerating cooling, and a method in which a cooling device is provided near the receiving vessel for cooling.  
      Gas substantially not reactive with silicon, for example, hydrogen gas or nitrogen, may be mentioned as the cooling gas.  
      An example of a method for cooling with the cooling device is a liquid jacket system in which a passage through which water, a heating medium oil, alcohol or other cooling medium liquid is passed for cooling.  
      A metallic chamber for shielding a silicon deposition reaction gas atmosphere from the atmospheric air may function also as the cooling device.  
      The present invention will be described with reference to the following Example. However, the present invention is not limited thereby.  
     EXAMPLE  
      A receiving vessel as shown in  FIG. 5B  was prepared. Fifteen silicon plate members each having a size of 10 cm in width, 30 cm in length, and 1 cm in thickness were prepared. They were assembled within a stainless steel frame body without use of any adhesive. Three silicon plate members as described above were arranged as bottom plate members. Three silicon plate members were vertically stacked on top of each other as side plate members. In this case, the size of the frame body was somewhat larger than a receiving vessel assembled from the silicon plate members. A carbon felt having a thickness of 5 to 10 mm was inserted into between the frame body and the silicon plate members. 5 kg of silicon crushed pieces (5 to 20 g per piece) were mounted on the top of the silicon bottom plates.  
      50 kg of silicon melt of 1570° C. was poured into the receiving vessel in a mixed gas atmosphere composed of hydrogen and nitrogen over a period of about 2 min.  
      When the temperature of silicon melt was satisfactorily lowered, the receiving vessel was observed. As a result, it was found that, although there were cracks in a solidified product of the dropped molten silicon, and separability between the silicon plate members and the solidified product of the dropped molten silicon was good and, in addition, there was no crack in the silicon plate members.  
     Comparative Example 1  
      In the same manner as in the Example, molten silicon was received, except that the silicon plate members were not used and, instead, a receiving vessel of quartz glass having the same dimension as in the Example was used.  
      As a result, the receiving vessel of quartz was cracked, and, further, silicon was fused to quartz.  
     Comparative Example 2  
      Instead of the silicon plate members as used in the Example, five silicon plate members having a width of 30 cm, a length of 30 cm, and a thickness of 1 cm were used. One silicon plate member as described above was used as bottom plate member. Four silicon plate members as the side plate member were provided upright direction from the peripheral part of the bottom plate member, and individual joints between the plates were bonded with a silica alumina adhesive to prepare a receiving vessel. This receiving vessel was received in a stainless steel frame body into which the receiving vessel was just housed. Thereafter, in the same manner as in the Example, molten silicon was received.  
      As a result, the stainless steel frame body was deformed, and, regarding the silicon plate members, both the bottom plate and side plates were cracked.