Abstract:
A process for reducing waste and increasing yield of chlorosilane monomers is performed by cracking polychlorosiloxane and polychlorosilane by-products generated during production of trichlorosilane useful for the manufacture of polycrystalline silicon.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/119,391 filed on 3 Dec. 2008. U.S. Provisional Patent Application No. 61/119,391 is hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY FUNDED RESEARCH 
       [0002]    None. 
       BACKGROUND 
       [0003]    1. Technical Field 
         [0004]    This invention relates to a method for cracking high boiling polymers to improve yield and minimize waste in a process for making trichlorosilane (HSiCl 3 ). The polymers include tetrachlorodisiloxane (H 2 Si 2 OCl 4 ), pentachlorodisiloxane (HSi 2 OCl 5 ), hexachlorodisiloxane (Si 2 OCl 6 ), and hexachlorodisilane (Si 2 Cl 6 ). The cracking process produces additional HSiCl 3  and/or tetrachlorosilane (SiCl 4 ) useful in process for producing polycrystalline silicon. 
         [0005]    2. Problem to be Solved 
         [0006]    SiCl 4  is a by-product produced when silicon is deposited on a substrate in a chemical deposition (CVD) reactor that uses a feed gas stream comprising HSiCl 3  and hydrogen (H 2 ). It is desirable to convert the SiCl 4  back to HSiCl 3  to be used in the feed gas stream. One process for converting SiCl 4  back to HSiCl 3  comprises feeding H 2  and SiCl 4  to a fluidized bed reactor (FBR) having silicon particles therein. The FBR operates at high pressure and temperature where the following reaction occurs. 
         [0000]      3SiCl 4 +2H 2 +Si         4HSiCl 3    
         [0007]    Partial conversion of the H 2  and SiCl 4  to HSiCl 3  is achieved due to equilibrium limitations. H 2  is separated from the chlorosilanes and is recycled back to the feed. Likewise, unconverted SiCl 4  is distilled from the product HSiCl 3  and is recycled. Product HSiCl 3  may be further distilled to remove impurities. 
         [0008]    Residue is generated in the FBR along with the intended product HSiCl 3 . Residue, which is heavier than SiCl 4 , accumulates in the bottoms from the distillation apparatus. Residue typically comprises polychlorosilanes and/or polychlorosiloxanes exemplified by partially hydrogenated species, including tetrachlorodisiloxane (H 2 Si 2 OCl 4 ) and pentachlorodisiloxane (HSi 2 OCl 5 ); and other high boiling species, including hexachlorodisiloxane (Si 2 OCl 6 ) and hexachlorodisilane (Si 2 Cl 6 ). Residue further comprises silicon particulates, which must periodically be removed. The residue is periodically pumped out and disposed of. 
         [0009]    One approach for converting polychlorosilanes and polychlorosiloxanes has been proposed in which these species are fed back to the FBR for making HSiCl 3 . However, it is thought that this process may not be industrially desirable because of limitations presented by reaction kinetics at typical reactor temperatures unless considerable recycle passes are undertaken. This process is also complicated by the interference of the recycle stream with hydrodynamics in the reactor and the intended HSiCl 3  generating reaction itself. 
       SUMMARY 
       [0010]    A process for cracking polychlorosilanes and/or polychlorosiloxanes comprises: recycling a clean mixture comprising polychlorosilanes and/or polychlorosiloxanes to a distillation apparatus; thereby producing trichlorosilane, tetrachlorosilane, or a combination thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a process flow diagram showing a process of this invention. 
           [0000]    
         
           
                 
               
                 
                 
                 
                 
               
             
                 
                     
                 
                 
                   Reference Numerals 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                   101  
                   SiCl 4  feed line 
                   111 
                   sump 
                 
                 
                   102  
                   H 2  feed line 
                   113 
                   distillation feed line 
                 
                 
                   103  
                   fluidized bed reactor 
                   115 
                   overheads mixture removal line 
                 
                 
                   105  
                   silicon particle feed line 
                   117 
                   residue removal line 
                 
                 
                   107  
                   crude product line 
                   119 
                   solids removing apparatus 
                 
                 
                   108  
                   dust removing apparatus 
                   121 
                   solids removal line 
                 
                 
                   109  
                   silicon particle recycle line 
                   123 
                   clean mixture line 
                 
                 
                   110  
                   distillation column 
                 
                 
                     
                 
               
            
           
         
       
       
    
    
     DETAILED DESCRIPTION 
       [0012]    A process for cracking polychlorosilanes and/or polychlorosiloxanes is described herein. The process may comprise:
       a. producing a mixture comprising a polychlorosilane and/or a polychlorosiloxane;   optionally b. removing solids from the mixture to form a clean mixture;   c. recycling the clean mixture to a distillation apparatus; thereby producing trichlorosilane, tetrachlorosilane, or a combination thereof.       
 
         [0016]      FIG. 1  shows a process flow diagram of an exemplary process for preparing HSiCl 3 . SiCl 4  is fed through line  101 , and H 2  is fed through line  102 , into a FBR  103 . Silicon particles are fed into the FBR through line  105  and form a fluidized bed in the FBR  103 . A crude product stream comprising HSiCl 3 , SiCl 4 , silicon solids, and H 2  is drawn off the top of the FBR  103  through line  107 . The silicon solids may be removed with a dust removing apparatus  108  such as a cyclone, and returned to the FBR  103  through line  109 . The resulting effluent mixture is fed to the sump  111  of a distillation column  110  through line  113 . 
         [0017]    The sump  111  of the distillation column  110  may contain a catalyst that facilitates cracking of the polychlorosiloxane and polychlorosilane species. Some catalysts may inherently form in the sump  111  of the distillation column  110  resulting from impurities such as tin, titanium, or aluminum. Examples such catalysts include, but are not limited to, titanium dichloride, titanium trichloride, titanium tetrachloride, tin tetrachloride, tin dichloride, iron chloride, AlCl 3 , and a combination thereof. The amount of such catalyst depends on various factors including how frequently the residue is removed from the distillation apparatus  110  and the level of the catalyst present in the effluent mixture from the FBR 103. Alternatively, a catalyst can be added to the sump  111 . Platinum group metal catalysts such as platinum, palladium, osmium, iridium, or heterogeneous compounds thereof can be used. The platinum group metal catalysts may optionally be supported on substrates such as carbon or alumina. The amount of catalyst may vary depending on the type of catalyst and the factors described above, however, the amount may range from 0 to 20%, alternatively 0 to 10% of the residue. One skilled in the art would recognize that different catalysts have different catalytic activities and would be able to select an appropriate catalyst and amount thereof based on the process conditions in the distillation apparatus  110  and the sump  111 . 
         [0018]    A mixture including SiCl 4 , HSiCl 3 , and H 2  is removed from the top of the distillation column  110  through line  115 . The SiCl 4  and H 2  may be recovered and fed back to the FBR  103 , as described above. The HSiCl 3  may optionally be used as a feed gas for a CVD reactor (not shown) for the production of polycrystalline silicon. 
         [0019]    Residue is generated in the FBR  103  along with the intended product HSiCl 3 . Residue, which is heavier than SiCl 4 , accumulates in the sump  111 . The residue is periodically removed through line  117 . Residue typically comprises a polychlorosilane and/or a polychlorosiloxane. Such polychlorosilanes and polychlorosiloxanes are exemplified by partially hydrogenated species, including tetrachlorodisiloxane (H 2 Si 2 OCl 4 ) and pentachlorodisiloxane (HSi 2 OCl 5 ); and other high boiling species, including hexachlorodisiloxane (Si 2 OCl 6 ) and hexachlorodisilane (Si 2 Cl 6 ). The exact amount of each species of polychlorosilane and polychlorosiloxane in the residue may vary depending on the process chemistry and conditions that produce the residue. However, residue may contain 0 to 15% H 2 Si 2 OCl 4 , 5% to 35% HSi 2 OCl 5 , 15% to 25% Si 2 OCl 6 , and 35% to 75% Si 2 Cl 6 , based on the combined weights of the polychlorosilanes and polychlorosiloxanes in the residue. Residue may further comprise solids, which are insoluble in the species described above. For example, the solids may be polychlorosiloxanes having 4 or more silicon atoms and higher order polychlorosilanes. The solids may further comprise silicon particulates, which may optionally be recovered as described below and optionally recycled to the FBR  103 . 
         [0020]    The residue may be fed to a solids removing apparatus  119 . The solids may be removed through line  121 . The clean mixture (i.e., the mixture comprising tetrachlorodisiloxane, pentachlorodisiloxane, hexachlorodisiloxane, and hexachlorodisilane with the solids removed) may be sent through line  123  back to the sump  111 . 
         [0021]      FIG. 1  is intended to illustrate the invention to one of ordinary skill in the art and should not be interpreted to limit the scope of the invention set forth in the claims. Modifications may be made to  FIG. 1  by one of ordinary skill in the art and still embody the invention. For example, one skilled in the art would recognize that cyclone  108  is optional and that one or more of the feeds in lines  101 ,  102 , and  105  may optionally be combined before being fed into the FBR  103 . One skilled in the art would recognize that the distillation column  110  can have a different configuration than that shown in  FIG. 1 , e.g., a separate reboiler into which gas from line  113  is fed may be used instead of the sump  111 . The residue would then accumulate in the reboiler. Furthermore, an alternative process for producing HSiCl 3  may be used, for example, an alternative FBR  103  that produces HSiCl 3  from HCl and particulate silicon. 
         [0022]    Cracking reactions of the polychlorosilane and/or polychlorosiloxane species in the clean mixture can form monomeric chlorosilane species (HSiCl 3  and SiCl 4 ) and higher order silane and siloxane polymers with each successive reaction of the species in the clean mixture. The siloxane polymers become large enough to form solids at approximately 4 units long. Under the conditions in the distillation apparatus, polychlorosilanes undergo cracking reactions, similarly. The partially hydrogenated species described above exhibit equilibria with HSiCl 3 , and the other (not hydrogenated) species described above, exhibit equilibria with SiCl 4  according to the following reactions: 
         [0000]      H n Si 2 OCl 6-n           H n-1 Si 3 O 2 Cl 8-n +HSiCl 3 , 
         [0000]    where subscript n represents the number of hydrogen atoms, e.g., 1 or 2, 
         [0000]      Si 2 OCl 6           Si 3 O 2 Cl 8 +SiCl 4 . 
         [0000]    When the polychlorosiloxanes reach a degree of polymerization of 4 or greater, a solid may form and the reaction may become irreversible, as illustrated below: 
         [0000]      H n Si 3 O 2 Cl 8-n →H n-1 Si 4 O 3 Cl 10-n (solid)+HSiCl 3 ,
 
         [0000]    and 
         [0000]      Si 3 O 2 Cl 8 →Si 4 O 3 Cl 10 (solid)+SiCl 4 .
 
         [0000]    Based on the kinetic data, the reactions above all occur at different rates in the sump  111  to permit the above equilibria to be reached within the residence time for the species in the sump  111  when the clean mixture is recycled. The sump  111  may operate at 130° C. to 280° C., alternatively to 180° C. to 240° C., and alternatively 200° C. to 220° C., for a residence time ranging from 10 days to 1 hour at a pressure ranging from 25 bar to 40 bar. One skilled in the art would recognize that the residence time selected depends on various factors including the temperature and the presence or absence of a catalyst. The pressure may be selected based on practical limitations. Increasing pressure will increase the boiling temperatures in the distillation apparatus. The range of pressures enable the reaction to occur at the appropriate temperatures, and therefore at sufficient rate. 
       INDUSTRIAL APPLICABILITY  
       [0023]    The process described herein reduces waste and improves yield of chlorosilane monomers (HSiCl 3  and SiCl 4 ) useful for the production of polycrystalline silicon. Polychlorosilanes and polychlorosiloxanes that otherwise would have been disposed of as waste are cracked to form useful HSiCl 3  and SiCl 4 .