Closed Loop Process for Producing Polycrystalline Silicon and Fumed Silica

Home , Dichlorosilane, Trichlorosilane

^ ^ 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).

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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 is converted to dichlorosilane and sil- tial cost savings in the production of silane and in icon tetrachloride and the dichlorosilane is converted to 45 the production of fumed silica through the unique silane. The silane is then converted to electronic grade recycling arrangement described. polycrystalline silicon and hydrogen through pyrolysis in a fluidized bed or fixed rod reactor. In accordance with [0011] As various changes could be made in the the invention, instead of subjecting the silicon tetrachlo- above process without departing from the scope of the ride by-product to high pressure hydrochlorination to 50 invention, it is intended that all matter contained in the convert it backto trichlorosilane, the silicon tetrachloride above description shall be interpreted as illustrative and is transferred to a co-located fumed silica plant where it not in a limiting sense. is reacted by flame pyrolysis with hydrogen and oxygen to produce fumed silica and hydrogen chloride as a by- product. The hydrogen chloride by-product is then re- Claims turned to the initial step described above and used for hydrochlorination of impure silicon in a fixed bed or flu- 1. A closed loop process for producing electronic idized bed reactor. grade polycrystalline silicon from silane and fumed

3 5 EP 0 921 098 A1 silica from silicon tetrachloride comprising the steps of:

a) subjecting impure silicon to hydrochlorination with hydrogen chloride to produce trichlo- s rosilane and silicon tetrachloride together with minor amounts of dichlorosilane and monochlorosilane; b) converting said trichlorosilane to silicon tetrachloride and silane; 10 c) converting said silane to polycrystalline silicon and hydrogen; d) reacting the silicon tetrachloride from steps a) and b) with hydrogen and oxygen to produce fumed silica and hydrogen chloride; and 15 e) recycling the hydrogen chloride from step d) for use in step a).

A process according to claim 1 , wherein step a) is carried out in a fixed bed or fluidized bed reactor at 20 a temperature of approximately 300-400°C.

A process according to claim 1 or claim 2, wherein in step b) said trichlorosilane is first converted to dichlorosilane and silicon tetrachloride and said 25 dichlorosilane is converted to silane.

A process according to any one of claims 1 to 3, wherein in step c) said silane is converted to electronic grade polycrystalline silicon and hydrogen 30 through pyrolysis in a fluidized bed or fixed rod reactor.

A process according to any one of claims 1 to 4, wherein the reaction of step d) is carried out by 35 flame pyrolysis of said silicon tetrachloride in the presence of hydrogen and oxygen.

55 EP 0 921 098 A1

European Patent EUROPEAN SEARCH REPORT Application Number Office EP 98 30 9027

DOCUMENTS CONSIDERED TO BE RELEVANT Category Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE of relevant passages to claim APPLICATION (lnt.CI.6) US 4 515 762 A (GRIESSHAMMER RUDOLF ET 1,4,5 C01B33/029 AL) 7 May 1985 C01B33/035 * the whole document * C01B33/18 C01B33/04 DE 38 09 784 C (HULS AG) 13 July 1989 1,2 C01B33/107 * the whole document *

EP 0 258 027 A (ETHYL CORP) 2 March 1988 * cl aims 1-6 *

US 4 519 999 A (TAMB0 WILLIAM ET AL) 28 May 1985

TECHNICAL FIELDS SEARCHED (lnt.CI.6) C01B

The present search report has been drawn up for all claims Place of search Date of completion of the search THE HAGUE 17 February 1999 Rigondaud, B CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D 1 document cited in the application document of the same category I : document cited for other reasons A : technological background O : non-written disclosure & ■ member of the same patent family, corresponding P ■ intermediate document document

5 EP 0 921 098 A1

ANNEX TO THE EUROPEAN SEARCH REPORT ON EUROPEAN PATENT APPLICATION NO. EP 98 30 9027

This annex lists the patent family members relating to the patent documents cited in the above-mentioned European search report. The members are as contained in the European Patent Office EDP file on The European Patent Office is in no way liable for these particulars which are merely given for the purpose of information. 17-02-1999

Patent document Publication Patent family Publication cited in search report date member(s) date US 4515762 A 07-05-1985 DE 3203743 A 04-08-1983

0E 3809784 C 13-07-1989 US 5063040 A 05-11-1991

EP 0258027 A 02-03-1988 US 4820587 A 11-04-1989 CA 1294755 A 28-01-1992 DE 3781223 D 24-09-1992 JP 1966837 C 18-09-1995 JP 6102532 B 14- 12-1994 JP 63129012 A 01-06-1988 US 4883687 A 28-11-1989 us 4784840 A 15- 11-1988

US 4519999 28-05-1985 CA 1185071 A 09-04-1985

For more details about this annex : see Official Journal of the European Patent Office, No. 12/82