PROCESS FOR PREPARING POLYCRYSTALLINE SILICON

• US 20200131043A1
• Filed: 12/14/2016
• Published: 04/30/2020
• Est. Priority Date: 12/14/2016
• Status: Active Grant

• Current Assignee: Wacker Chemie AG (Dr. Alexander Wacker Familiengesellschaft Mit Beschrã¤Nkter Haftung)
• Original Assignee: Wacker Chemie AG (Dr. Alexander Wacker Familiengesellschaft Mit Beschrã¤Nkter Haftung)
• Inventors: Hertlein, Harald, Weckesser, Dirk
• Granted Patent: US 11,440,803 B2
• Time in Patent Office: Days
• CPC Class Codes: C01B 33/02 Silicon forming single crystals or homogeneous polycrystalline material with defined structure C30B
• CPC Class Codes: C01B 33/03 by decomposition of silicon halides or halosilanes or reduction thereof with hydrogen as the only reducing agent

Abstract

The invention relates to a method for producing polycrystalline silicon granulate in a fluidized bed reactor. The method comprises a fluidization of silicon seed particles by means of a fluidizing gas in a fluidized bed, which is heated by a heating device, wherein elemental silicon is deposited by pyrolysis on the silicon seed particles by the addition of a reaction gas containing hydrogen and silane and/or halosilane to form the polycrystalline silicon granulate. In a continuous process, waste gas is discharged from the fluidized bed reactor and hydrogen recovered from said waste to gas is again supplied to the fluidized bed reactor as a circulating gas. The circulating gas has a nitrogen content of less than 1000 ppmv. The invention further relates to polycrystalline silicon granulate having a nitrogen content of less than 2 ppba.

Claims

1. A method for producing a polycrystalline silicon granulate in a fluidized bed reactor comprising:

fluidizing silicon seed particles using a fluidizing gas in a fluidized bed reactor which is heated a heating apparatus;

adding a reaction gas comprising a hydrogen and a silane and/or a halosilane; and

depositing elemental silicon, by pyrolysis, onto the silicon seed particles to form the polycrystalline silicon granulate;

wherein, in a recycle gas system for workup of offgases, an offgas is continuously discharged from the fluidized bed reactor and hydrogen recovered from the offgas is sent back to the fluidized bed reactor as a recycle gas, wherein the recycle gas system has a nitrogen content of less than 1000 ppmv.

2. The method of claim 1, wherein the nitrogen content is less than 500 ppmv, preferably less than 100 ppmv, particularly preferably less than 10 ppmv, in particular less than 0.5 ppmv.

3. The method of claim 1, wherein the deposition of the elemental silicon is carried out at a deposition temperature between 1000°

C. and 1300°

C., preferably between 1080°

C. and 1250°

C., particularly preferably between 1100°

C. and 1200°

C.

4. The method of claim 1, wherein the nitrogen content of the recycle gas is determined by a measuring instrument, in particular a gas chromatograph.

5. The method of claim 1, wherein when a nitrogen threshold value between 0.01 and 1000 ppmv is exceeded, either a shutdown of the fluidized bed reactor is effected or a supply of the recycle gas is interrupted and the method is operated with external hydrogen until the nitrogen threshold value is achieved.

6. The method of claim 1, wherein upon exceeding a nitrogen threshold value of between 0.01 and 1000 ppmv, the temperature of the fluidized bed reactor and/or of a reactor wall is increased until the nitrogen threshold value has been achieved.

7. The method of claim 1, wherein the recycle gas is admixed with less than 90%, preferably 40%, particularly preferably 10%, of external hydrogen.

8. The method of claim 1, wherein the halosilane is a chlorosilane, in particular trichlorosilane.

9. The method of claim 1, wherein before commencement of the method a pressurization with hydrogen followed by a depressurization is carried out inside the fluidized bed reactor, wherein a maximum pressure during pressurization is in the range between 3.1 and 15 bar, in particular about 6.5 bar, and a minimum pressure during depressurization is in the range between 1.1 and 3 bar, in particular about 1.4 bar.

10. The method of claim 9, wherein the pressurization and the depressurization each have a duration between 1 and 60 min.

11. The method of claim 9, wherein the pressurization is carried out with a hydrogen volume flow per reactor volume V_R between 10 and 7000 m³/hVR, preferably between 300 and 1500 m³/hVR, particularly preferably between 500 and 1000 m³/hVR, in particular of about 520 m³/hVR.

12. A polycrystalline silicon granulate comprising an Si₃N₄ content of less than 1 ppba.

13. The polycrystalline silicon granulate of claim 12, comprising a surface roughness Ra≥

400 nm, preferably Ra≥

700 nm.

14. (canceled)

15. (canceled)