Closed Loop Process for Producing Polycrystalline Silicon and Fumed Silica

^ ^ H ^ I H ^ H ^ H ^ II ^ ^ ^ II ^ II ^ H ^ ^ ^ ^ ^ ^ ^ I ^ European Patent Office Office europeen des brevets EP 0 921 098 A1 EUROPEAN PATENT APPLICATION (43) Date of publication: (51) |nt CI * C01 B 33/029, C01 B 33/035, 09.06.1999 Bulletin 1999/23 C0-| B 33/1 8 CQ1 B 33/04, P01 R 33/1 07 (21) Application number: 98309027.5 (22) Date of filing: 04.11.1998 (84) Designated Contracting States: (72) Inventor: The designation of the inventor has not AT BE CH CY DE DK ES Fl FR GB GR IE IT LI LU yet been filed MC NL PT SE Designated Extension States: (74) Representative: Eyles, Christopher Thomas ALLTLVMKROSI W.P. THOMPSON & CO. Celcon House (30) Priority: 10.11.1997 US 966798 289-293 High Holborn London WC1V7HU (GB) (71) Applicant: MEMC Electronic Materials, Inc. St. Peters, Missouri 63376 (US) (54) Closed loop process for producing polycrystalline silicon and fumed silica (57) A closed loop process for producing electronic monochlorosilane; b) converting the trichlorosilane to grade polycrystalline silicon from silane and fumed silica silicon tetrachloride and silane; c) converting the silane from silicon tetrachloride involves the steps of a) sub- to polycrystalline silicon and hydrogen; d) reacting the jecting impure silicon to hydrochlorination with hydrogen silicon tetrachloride from steps a) and b) with hydrogen chloride to produce trichlorosilane and silicon tetrachlo- and oxygen to produce fumed silica and hydrogen chloride together with minor amounts of dichlorosilane and ride; and e) recycling the hydrogen chloride from step d) for use in step a). 00 O) o CM O) o a. LU Printed by Jouve, 75001 PARIS (FR) 1 EP 0 921 098 A1 2 Description proximately 88%) of trichlorosilane, approximately 10-12% of silicon tetrachloride and minor amounts of Background of the Invention dichlorosilane and monochlorosilane. The trichlorosilane is used to make dichlorosilane and silicon tetrachlo- [0001] This invention relates to a process for produc- 5 ride and the dichlorosilane produced is further redistrib- ing polycrystalline silicon and, more particularly, to an uted to make silane as in the previous reaction scheme improved, cost-effective process for producing poly- shown above. The silane is then used to make polycrys- crystalline silicon from silane and fumed silica from sili- talline silicon by pyrolysis. This sequence of reactions con tetrachloride. may be represented by the following simplified equa- [0002] In known commercial processes for producing 10 tions: polycrystalline silicon (also called polysilicon) from silane (SiH4), a major cost element is the cost of produc- Trichlorosilane Synthesis: ing silane by either of two commercial production processes, i.e. reduction of silicon tetrafluoride by sodium 4 HCI + Si -> HSiCI3 (88%) + SiCI4(1 0%) + H2 aluminum hydride (NaAIH4) or redistribution of trichlorosilane produced by high pressure hydrochlorination of metallic silicon and silicon tetrachloride. In the latter Redistribution Reactions: process, as practised commercially, the first step involves the hydrochlorination of silicon tetrachloride in a 4 HSiCI, -> 3SiCI, +SiH, fluidized bed of silicon to produce the trichlorosilane as 20 3 4 4 the starting material employed in the redistribution reactions. This hydrochlorination reaction carried out at a Pyrolysis: temperture of 500-550°C and a pressure of 500 psig (3447.38 kPa gauge) is the most costly of the com- step SiH, -> 2H„ +Si mercial process for making silane. The trichlorosilane is 25 4 2 converted by redistribution to silicon tetrachloride and redistribution to silicon tetrachloride and silane. The si- This process of producing silane has heretofore not lane is then subjected to pyrolysis in either a fixed rod been regarded as a practical commercial process type or fluidized bed reactor to produce polycrystalline because of the large amount of silicon tetrachloride silicon and hydrogen. The overall process may be sum- 30 produced as a by-product even though the hydro- marized as follows: chlorination of silicon with HCI to make trichlorosilane can produce the latter at a lower cost than that of the hydrogenation previously outlined 1 Hydrochlorination: + + Si -> 4 process . 3SiCI4 2H2 HSiCI3 above. 35 [0004] It is also known to produce fumed silica by 2. Redistribution: 2HSiCI3 -> SiCI4 + H2SiCI2 burning silicon tetrachloride and hydrogen with oxygen in a burner. This combustion process produces fumed silica and hydrogen chloride (HCI). 3. Redistribution: 2H2SiCI2 -> SiCI4 + SiH4 40 [0005] There is a continuing need to develop more cost efficient processes for producing polycrystalline silicon from silane and fumed silica from silicon tetrachlo- 4. Pyrolysis: -> + Si SiH4 2H2 ride. Equations 1 , 2 and 3 represent partial reactions. If one 45 Summary of the Invention recycles all the unreacted starting materials and by- products, a closed loop process is achieved with the [0006] Among the several objects of the present in- overall, net result being the conversion of metallurgical vention may be noted the provision of an improved proc- silicon from equation 1 to polycrystalline silicon in equa- ess for producing electronic grade polycrystalline silicon tion 4. Thus, the hydrogen and chlorine are merely car- 50 from silane and fumed silica from silicon tetrachloride; riers and, after the initial charge represented by equa- the provision of such a process which advantageously tion 1, only makeup hydrogen and silicon tetrachloride utilizes the silicon tetrachloride by-product formed from are needed to replenish the processing loss of these two the production of polycrystalline silicon from silane to reactants. produce fumed silica and recycles the hydrogen chlo- [0003] It is also known to produce silane by the reac- ride formed from the production of fumed silica for use tion of metallurgical or impure silicon and hydrogen chlo- in the hydrochlorination of impure silicon; and the pro- ride in a fluidized bed reactor at a temperature of about vision of such a closed loop process which is practical 300 to 400°C. The reaction produces a high yield (ap- and cost efficient. Other objects and features will be in 2 3 EP 0 921 098 A1 4 part apparent and in part pointed out hereinafter. [0010] The steps of the process of the present inven- [0007] Briefly, the present invention is directed to a tion as described above may be represented by the fol- closed loop process for producing electronic grade poly- lowing simplified chemical equations: crystalline silicon from silane and fumed silica from silicon tetrachloride which comprises the following steps: 5 Fumed Silica Production: a) subjecting impure silicon to hydrochlorination SiCI4 + 2H2 + X02 -> SiOx + 4 HCI with hydrogen chloride to produce trichlorosilane and silicon tetrachloride together with minor amounts of dichlorosilane and monochlorosilane; 10 Trichlorosilane Synthesis: b) converting the trichlorosilane to silicon tetrachloride and silane; 4 HCI + Si -> HSiCI3 (88%) + SiCI4 (10%) + H2 c) converting the silane to polycrystalline silicon and hydrogen; d) reacting the silicon tetrachloride from steps a) 15 Redistribution Reactions: and b) with hydrogen and oxygen to produce fumed silica and hydrogen chloride; and 4 HSiCI3 -> 3SiCI4 +SiH4 e) recycling the hydrogen chloride from step d) for use in step a). 20 Pyrolysis: Description of the Preferred Embodiments SiH4 -> Si + 2H2 [0008] In accordance with the present invention, it has now been found that electronic grade polycrystalline silicon may be produced from silane and fumed silica pro- 25 As can be seen, the HCI from the fumed silica produced from silicon tetrachloride in a more cost efficient duction is converted by reaction with impure silicon and practical manner through the use of the silicon tet- to trichlorosilane in high yield with a smaller amount rachloride by-product from the production of polycrys- of silicon tetrachloride. The trichlorosilane is then talline silicon in forming fumed silica and recycling the redistributed to form silicon tetrachloride and silane by-product hydrogen chloride from the production of 30 which is in turn converted to electronic grade poly- fumed silica to the process for producing polycrystalline crystalline silicon. The silicon tetrachloride and hy- silicon with the plants for producing the respective end drogen by-products thus generated are returned for products being co-located to facilitate the transfer of the the step of fumed silica production to form a closed respective by-products in the overall closed loop proc- loop process. In this closed loop process, the hy- ess. 35 drogen and chlorine are merely carriers. The net [0009] In carrying out the novel and unique process overall result of the improved cost-efficient process of the present invention, impure or metallurgical silicon of the invention is that fumed silica is produced from is first subjected to hydrochlorination with hydrogen impure silicon and oxygen and polycrystalline sili- chloride in a fixed bed or fluidized bed reactor at a tem- con is produced from impure silicon, the silicon tet- perature in the range of 300 to 400°C to produce mostly 40 rachloride, hydrogen and hydrogen chloride by- trichlorosilane and silicon tetrachloride together with mi- products being fully utilized through recycling in the nor amounts of dichlorosilane and monochlorosilane. closed loop process as described. As is evident, the Through conventional redistribution reactions or steps, process of the present invention results in substan- the trichlorosilane

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