REACTOR FOR THE DEPOSITION OF POLYCRYSTALLINE SILICON

Abstract

Reflective silver coatings on the inside surfaces of a Siemens reactor for polycrystalline silicon production are improved by a cold forming after-treatment of the silver coating.

Claims

1.-8. (canceled)

9. A reactor for deposition of polycrystalline silicon, comprising: a metallic base plate, a coolable bell jar placed thereover and forming a gas-tight seal therewith, nozzles for supplying gas and openings for removing reaction gas, and holders for filament rods and input and output leads for electric current, wherein the inner walls of the bell jar are coated with silver, and the silver coating has been mechanically after-treated by cold forming such that the coating has undergone plastic deformation during the mechanical after-treatment, wherein the cold forming takes place by peening, hammering or cold rolling.

10. The reactor of claim 9, wherein cold forming takes place by hammering.

11. The reactor of claim 9, wherein the bell jar inner walls and the base plate are coated with silver.

12. The reactor of claim 9, wherein the coating has a thickness of 0.5-5 mm.

13. The reactor of claim 12, wherein the coating is a coating comprising fine silver.

14. The reactor of claim 13, wherein the coating is a coating comprising fine grain silver.

15. The reactor of claim 9, wherein the coating comprises indentations after cold forming.

16. In a process for producing polycrystalline silicon, which comprises introducing a reaction gas comprising a silicon-containing component and hydrogen, by means of one or more nozzles, into a reactor which comprises at least one heated filament rod upon which silicon is deposited, the improvement comprising employing a reactor of claim 9.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 shows a schematic representation of a reactor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The invention provides for working the coating by mechanical forming such that the coating has either a smooth, even structure or an irregular, non-smooth structure comprising indentations, dents or other depressions.

[0034] The coating preferably has a minimum thickness of 0.5 mm.

[0035] The mechanical forming may be a hot forming procedure and/or a cold forming procedure, preferably a cold forming procedure. Hot forming comprises plastic working of the surface above the recrystallization temperature, for example forging or welding. Cold forming comprises plastic working of the surface below the recrystallization temperature, for example peening and hammering.

[0036] Materials preferred for employment as the coating material are those which improve the reflection properties of the reactor inner wall with reference to the carrier material. These are in particular metals and metal alloys having an emissivity coefficient of less than 0.3, preferably less than 0.15. Preference is given to stainless steel, nickel, nickel alloys such as Hastelloy or Inconel, silver or gold. The use of silver is particularly preferred.

[0037] The invention provides for targeted aftertreatment of the coating by mechanical forming.

[0038] In one embodiment the base plate also has such a coating on its reactor-side surface, i.e. on the surface facing into the reactor space.

[0039] In contrast to the typical processing steps in coating processes, which are known from the prior art and referred to at the outset, the forming of the coating seeks to mechanically expel the coating-dissolved oxygen and also oxygen inclusions by plastic deformation of the coating.

[0040] This reduces the susceptibility to detachment of the coating from the carrier wall. The mechanically aftertreated coating exhibits improved adhesion of the coating to the metal carrier sheet. The formation of undesired metal oxide compounds which could negatively affect the reflection properties of the inner wall is reduced.

[0041] The surface may have a smooth appearance, such as after cold rolling and warm rolling for example, or may exhibit dents, indentations or other depressions, referred to in general terms as indentations hereinbelow, such as after hammering for example, wherein the surface treatment of the coating does not have a negative effect on the reflection properties of the coating.

[0042] Possible indentations preferably have a diameter of 1-100 mm, more preferably 5-30 mm, and a depth of preferably 0.1-2 mm, more preferably 0.1-1 mm.

[0043] The indentations may be discrete. In one embodiment at least some of the indentations are contiguous.

[0044] The forming of the coating may be hot forming or cold forming, i.e. mechanical working of the coating with plastic deformation of the coating. Hot forming is performed above the recrystallization and cold forming is performed below the recrystallization temperature. Preference is given to cold forming since the solubility of the oxygen is lower here.

[0045] Cold forming results in a microstructure change toward smaller crystallites and a higher dislocation density. This leads to an increase in the hardness of the coating.

[0046] On account of the higher hardness the coated surface is hardly, or at least less severely, damaged by toppling silicon rods. Cold forming accordingly expels oxygen dissolved in the coating and/or trapped oxygen bubbles and increases the hardness of the coating.

[0047] The production of the coating or plating, in particular of the silver coating/silver plating is carried out, for example, according to the processes described in DE 956 369 C and DE 1 033 378 B.

[0048] Plating is to be understood as meaning the application and secure joining of a layer, which has a film thickness of not less than of 0.5 mm and is composed of a metal or a metal alloy, to a carrier metal. The coating material may be coated onto the carrier metal by explosion plating, build-up welding, roll application, cold gas dynamic spraying or other known processes. These processes are usually performed at high temperature and/or pressure.

[0049] Cold gas dynamic spraying comprises accelerating very small particles of the coating material onto the surface to be coated at very high speed using a gas stream. Impact gives rise to plastic deformation of the sprayed material and the near-surface layers of the metal carrier sheet. This builds up a firmly adherent layer.

[0050] However, all coated metal carrier sheets are in principle formable, irrespective of the coating technology and irrespective of the material of the metal carrier sheets or of the coating.

[0051] The cold forming of the coating may be affected by cold rolling, deep drawing, bending, peening, hammering, shot peening or other processes for cold forming which cause dislocations in the microstructure and improve the hardness of the coating.

[0052] In these cold forming operations a metal carrier sheet (steel or stainless steel with the coating or plating disposed thereupon) is worked using a suitable tool on the coating side. The cold forming operation may be performed either as a last processing step after putting together the coated metal carrier sheets to form the deposition reactor or else beforehand on individual coated metal carrier sheets in an intermediate fabrication step.

[0053] Peening, hammering and cold rolling have proven to be particularly suitable cold forming operations. Hammering is particularly preferred.

[0054] Hammering cold forms the surface in indented regions.

[0055] After cold forming or hot forming the coating is preferably 0.5 to 5 mm thick, more preferably 0.5 to 3.5 mm.

[0056] It is preferable when silver is chosen as the coating material.

[0057] It is possible to employ as the silver not only silver of the highest possible purity (so-called ‘fine silver’) but also silver comprising alloying constituents (for example comprising nickel or the like).

[0058] Fine silver (Ag 4N) comprises at least 99.99 wt % of silver.

[0059] Silver comprising small proportions of alloying constituents, in particular fine grain silver (AgNi 0.15 having a nickel proportion of 0.15 wt %), is particularly preferable since fine grain silver has a higher hardness than silver and fine silver.

[0060] It is preferable when the inner walls of the reactor bell jar are silverplated sheet steel, wherein the silver plating is hammered.

[0061] The base plate/the reactor-side surface of the base plate is preferably also made of silverplated steel or stainless steel. In this case all surfaces of the reactor interior, delimited by the base plate and the bell jar, are silverplated.

[0062] The invention further relates to a process for producing polycrystalline silicon in such a reactor, which comprises introducing a reaction gas comprising a silicon-containing component and hydrogen into a CVD reactor containing at least one filament rod supplied with current by means of electrodes and thus heated by direct passage of current to a temperature at which polycrystalline silicon is deposited upon the filament rod.

[0063] Preferably, pairs of filament rods are connected at one end via a bridge to form a support body having an inverse U-shape. At the other end the filament rods are each connected to a respective electrode disposed on the reactor base plate. The two electrodes have opposite polarities.

[0064] The inverse U-shaped support body—when composed of silicon—requires initial preheating to approximately at least 250° C. to become electrically conductive and to be able to be heated by direct passage of current.

[0065] Finally, reaction gas comprising a silicon-containing component is supplied. The silicon-containing component of the reaction gas is preferably monosilane or halosilane of general formula SiH.sub.nX.sub.4-n (n=0, 1, 2, 3, 4; X=Cl, Br, I). The silicon-containing component is particularly preferably a chlorosilane or a chlorosilane mixture. The use of trichlorosilane is very particularly preferred. Monosilane and trichlorosilane are preferably employed in a mixture with hydrogen.

[0066] High-purity polysilicon is deposited upon the heated filament rods and the horizontal bridges to increase the diameter thereof with time. The process is terminated once the desired diameter has been achieved.

[0067] The polycrystalline silicon rods obtained by deposition are preferably comminuted into chunks, optionally cleaned, and packed in subsequent processing steps.

[0068] The features cited in connection with the abovedescribed embodiments of the process according to the invention may be correspondingly applied to the product according to the invention. Conversely, the features cited in connection with the abovedescribed embodiments of the product according to the invention may be correspondingly applied to the process according to the invention. These and other features of the embodiments according to the invention are elucidated in the description of the figures and in the claims. The individual features may be realized either separately or in combination as embodiments of the invention. These features may further describe advantageous implementations eligible for protection in their own right.

LIST OF REFERENCE NUMERALS EMPLOYED

[0069] 1 base plate [0070] 2 bell jar [0071] 3 reactor wall

[0072] The reactor, as shown in FIG. 1 comprises a bell jar 2 disposed upon a base plate 1.

[0073] The reactor-interior-facing surface of the reactor wall 3 of the bell jar is silverplated and hammered.

[0074] In one embodiment the surface of the base plate 1 facing the reactor interior is also silverplated and hammered.

[0075] The description of illustrative embodiments hereinabove is to be understood as being exemplary. The disclosure made thereby enables a person skilled in the art to understand the present invention and the advantages associated therewith and also encompasses alterations and modifications to the described structures and processes obvious to a person skilled in the art. All such alterations and modifications and also equivalents shall therefore be covered by the scope of protection of the claims.