METHOD FOR DEPOSITING AN IN SITU COATING ONTO THERMALLY AND CHEMICALLY LOADED COMPONENTS OF A FLUIDIZED BED REACTOR FOR PRODUCING HIGH-PURITY POLYSILICON

Abstract

In situ coating of the reactor tube of a CVD fluidized bed reactor for producing granular polysilicon allows a wider selection of reactor tube materials to be used and provides granular polysilicon product of higher purity.

Claims

1.-8. (canceled)

9. A process for coating thermally and chemically stressed components of a fluidized-bed reactor containing a reactor tube for producing granular polysilicon, comprising flushing the fluidized-bed reactor which is free of bed material or which contains a reduced amount of bed material as compared to the amount of bed material present during steady state production of granular polysilicon, with a reactive gas mixture at an average reactor tube wall temperature of from 600 to 1400 C. for a period of from 1 hour to 8 days and at a pressure of from 1 to 15 bar abs, and thereby providing surfaces of the reactor which have a temperature of more than 600 C. with an in-situ coating of Si and/or Si.sub.3N.sub.4 by means of a CVD process.

10. The process of claim 9, wherein the reactive gas mixture is a mixture of compounds of the formula SiH.sub.4xCl.sub.x (I) where 0x4 and a nitrogen source and/or a carrier gas selected from the group consisting of Ar or H.sub.2 and/or an organic compound having from 1 to 10 carbon atoms; or is a mixture of one or more compounds of the formula R.sub.xSiHyCl.sub.4-x-y, (II) where 1x4, 1y3 and x+y4 and RC.sub.nH.sub.2n+1 (n=1-5) and a carrier gas selected from the group consisting of Ar, H.sub.2, and mixtures thereof.

11. The process of claim 9, wherein the compound of the formula (I) or (II) is used in an amount of from 0.01 to 50% by volume.

12. The process of claim 9, wherein the compound of the formula (I) or (II) is used in an amount of from 0.01 to 10% by volume.

13. The process of claim 9, wherein coating takes place at a surface temperature which differs by not more than 250 C., from the surface temperature in the steady-state fluidized-bed deposition process.

14. The process of claim 9, wherein coating takes place at a surface temperature which differs by not more than 150 C., from the surface temperature in the steady-state fluidized-bed deposition process.

15. The process of claim 9, wherein coating takes place at a surface temperature which differs by not more than 100 C., from the surface temperature in the steady-state fluidized-bed deposition process.

16. The process of claim 9, which takes place at an absolute pressure of 1.5-8 bar.

17. The process of claim 9, wherein the thermally and chemically stressed components of the fluidized-bed reactor are the surface of the reactor tube facing the reaction space and further parts of the fluidized-bed reactor which are exposed to process gas and granular material.

18. The process of claim 9, which is carried out to provide a coating thickness of from 1 to 200 000 m.

19. A process for producing granular polycrystalline silicon, wherein a process of claim 9 is used during running-in of the reactor.

Description

EXAMPLE 1

[0052] A fluidized-bed reactor which is free of bed material and whose reactor tube consists of iso graphite is heated to the temperature at which the production of granular polycrystalline silicon is subsequently to be carried out (1300 C. at the outside of the reactor tube) and a gas mixture of N.sub.2 and trichlorosilane (300 standard m.sup.3/h of N.sub.2, 5 standard m.sup.3/h of trichlorosilane) is fed in for a period of 12 hours. This results in deposition of an Si.sub.3N.sub.4 layer having an average thickness of 150 m on the inside of the reactor tube. The temperatures on the outside of the reactor tube are measured by means of two pyrometers and in each case kept constant to 50 C. by adapting the heater power of the heaters. The reactor pressure during the coating process is 5 bar abs.

[0053] Subsequently, the flow of nitrogen is decreased. At the same temperature regulation, a layer of silicon having an average thickness of 250 m is additionally applied over a period of 8 hours from a gas mixture of H.sub.2 and trichlorosilane (200 standard m.sup.3/h of H.sub.2, 10 standard m.sup.3/h of trichlorosilane) to the inside of the tube. The reactor pressure during this coating process is 3 bar abs.

[0054] During the further running-in process, during the course of which the reactor is first charged with granular silicon and the steady-state deposition conditions are subsequently set, the measured tube temperature is also kept constant to 50 C. by adapting the heater power.

[0055] In the subsequent production of granular polycrystalline silicon, hydrogen is used as fluidizing gas. 17.5 mol % of the feed gas consists of trichlorosilane and the balance consists of hydrogen. The deposition takes place at a pressure of 3 bar (abs) and a fluidized-bed temperature of 1000 C. in a reactor tube having an internal diameter of 500 mm. Product is continuously taken off and the introduction of seed is regulated so that the Sauter diameter of the product is 100050 m. The intermediate jacket is flushed with nitrogen.

[0056] The residence time of the reaction gas in the fluidized bed is 0.9 s.

EXAMPLE 2

[0057] The steady-state granule deposition process as per Example 1 is carried out in a fluidized-bed reactor in which, in contrast to Example 1, the reactor tube consists of fused silica. In the steady-state deposition process, a temperature at the outside of the tube of 1400 C. is established in the reaction zone. At such temperatures, fused silica becomes soft under constant loading, so that the reactor tube would become deformed and no longer be sealed from the intermediate jacket.

[0058] For this reason, a supporting and at the same time highly pure layer of silicon is applied to the tube during the running-in process. During this, no bed or fluidized particles is/are present in the region which is to be coated. The tube temperature is kept constant at 110050 C. by adapting the heater power. Deformation of the tube during coating can be avoided in this way.

[0059] In the coating process, a gas mixture of H.sub.2 and trichlorosilane (200 standard m.sup.3/h of H.sub.2, 10 standard m.sup.3/h of trichlorosilane) is fed to the reactor over a period of 64 hours and a layer of silicon having an average thickness of 2500 m is in this way applied to the inside of the tube. The coating is applied at an absolute pressure of 4 bar abs.

[0060] During the further running-in process, during the course of which the reactor is first charged with granular silicon and the steady-state deposition conditions are subsequently set, the measured tube temperature is kept constant to 150 C. by adapting the heater power.

[0061] After a deposition time of 18 days, the entire silicon wall deposit in this reactor is corroded away in a corroding process using HCl. During this operation, a gas mixture of 80 standard m.sup.3/h of H.sub.2 and 100 standard m.sup.3/h of HCl is fed to the reactor. Here too, the measured tube temperature is kept constant by adapting the reactor heating power. In this way, the tube is freed of the unwanted thick Si deposit above the fluidized bed. However, this also has the secondary effect that the Si deposit applied deliberately is corroded away.

[0062] After corroding, the coating process according to the invention as described above is carried out again. Subsequently, the steady-state deposition process for producing granular polycrystalline silicon is again carried out as described above.