Energy efficient method for growing polycrystalline silicon

• US 20030150378A1
• Filed: 02/13/2003
• Published: 08/14/2003
• Est. Priority Date: 02/14/2002
• Status: Active Grant

• Current Assignee: Advanced Silicon Materials LLC (REC Silicon ASA)
• Original Assignee: Advanced Silicon Materials LLC (REC Silicon ASA)
• Inventors: Hill, John P., Winterton, Lyle C.
• Granted Patent: US 7,105,053 B2
• Time in Patent Office: Days
• US Class Current: 117/87
• CPC Class Codes: C01B 33/035 by decomposition or reduction of gaseous or vaporised silicon compounds in the presence of heated filaments of silicon, carbon or a refractory metal, e.g. tantalum or tungsten, or in the presence of heated silicon rods on which the formed silicon is deposited, a silicon rod being obtained, e.g. Siemens process
• CPC Class Codes: C30B 25/005 Growth of whiskers or needles
• CPC Class Codes: C30B 29/06 Silicon

Abstract

Polysilicon dendrites are grown by depositing silicon on a polysilicon rod or other substrate. Surface temperature is increased to a temperature at which needle-like dendrites develop due to the deposition of silicon from silicon-containing molecules in the surrounding reactor atmosphere. Thereafter, the surface temperature is gradually reduced so that silicon that deposits on the needle-like dendrites causes the dendrites to grow and assume a generally flared shape.

Claims

1. A method for producing polycrystalline silicon comprising:

heating a surface of a substrate;

exposing the heated surface to an atmosphere that contains at least one silicon-containing compound under conditions that cause a silicon-containing compound in the atmosphere to decompose and deposit needle-like dendrites of silicon on the surface; and

gradually reducing the surface temperature of the needle-like dendrites while exposing the needle-like dendrites to an atmosphere that contains at least one silicon-containing compound such that silicon deposits on the surfaces of the needle-like dendrites and forms multiple larger dendrites.

2. The method of claim 1 wherein the heating the surface of the substrate and the exposing the heated surface to the atmosphere are conducted such that larger dendrites are distributed over substantially the entire heated surface of the substrate.

3. The method of claim 1 wherein the gradually reducing of the surface temperature is at a rate such that the resulting larger dendrites have surfaces that flare outwardly from the substrate.

4. The method of claim 3 wherein the gradually reducing the surface temperature while exposing the needle-like dendrites to the atmosphere is conducted such that at least some of the larger dendrites are generally teardrop-shaped.

5. The method of claim 1 wherein the gradually reducing the surface temperature while exposing the needle-like dendrites to the atmosphere is conducted such that at least some of the larger dendrites have a stem portion that is no greater than about 8 mm in diameter at its narrowest point.

6. The method of claim 1 wherein the atmosphere comprises a silicon-containing compound selected from the group consisting of silane gases, halosilane gases, and mixtures thereof.

7. The method of claim 6 wherein the atmosphere comprises a silicon-containing compound selected from the group consisting of silane (SiH₄), disilane (Si₂H₆), trisilane (Si₃H₈), dichlorosilane (SiH₂Cl₂), trichlorosilane (SiHCl₃), silicon tetrachloride (SiCl₄), dibromosilane (SiH₂Br₂), tribromosilane (SiHBr₃), silicon tetrabromide (SiBr₄), diiodosilane (SiH₂I₂), triiodosilane (SiHI₃), silicon tetraiodide (SiI₄), and mixtures thereof.

8. The method of claim 7 wherein the atmosphere comprises silane.

9. The method of claim 7 wherein the atmosphere comprises trichlorosilane.

10. The method of claim 7 wherein the atmosphere comprises trisilane.

11. The method of claim 1 wherein:

the silicon-containing compound is silane (SiH₄); and

the exposing of the heated surface to an atmosphere that contains at least one silicon-containing compound under conditions that cause a silicon-containing compound in the atmosphere to decompose and deposit needle-like dendrites comprises maintaining the surface at a temperature in a range such that the temperature is at least 875°

C. and is below the melting point of silicon during at least a portion of the time when the needle-like dendrites are being deposited.

12. The method of claim 11 wherein the decomposing of the silane to form needle-like dendrites is at a temperature of at least 975°

C. during at least a portion of the time when the needle-like dendrites are being deposited.

13. The method of claim 1 further comprising breaking the larger dendrites off of the substrate.

14. The method of claim 1 wherein:

silane (SiH₄) is the silicon-containing compound;

the decomposing of the silane to form needle-like dendrites is conducted in a surface temperature range such that the temperature is at least 875°

C. and is below the melting point of silicon during at least a portion of the time when the needle-like dendrites are being formed; and

the decomposing of the silane to cause the needle-like dendrites to grow into larger dendrites is at a temperature below the melting point of silicon.

15. The method of claim 14 wherein the decomposing of the silane to form needle-like dendrites is at a temperature of at least 975°

C. during at least a portion of the time when the needle-like dendrites are being formed.

16. A method for producing polycrystalline silicon comprising:

providing an elongated polycrystalline silicon core rod;

exposing the surface of the core rod to an atmosphere that contains at least one silicon-containing compound while maintaining the surface temperature of the core rod below the melting point of silicon and sufficiently high that a silicon-containing compound in the atmosphere pyrolytically decomposes and deposits silicon on the surface to provide a rod encrusted with needle-like dendritic formations; and

gradually reducing the surface temperature of the rod encrusted with needle-like dendritic formations while continuing to expose the needle-like dendritic formations to an atmosphere that contains at least one silicon-containing compound such that silicon deposits on the needle-like dendritic formations and forms a rod encrusted with multiple larger silicon dendrites.

17. The method of claim 16 wherein the gradually reducing of the surface temperature of the rod encrusted with needle-like dendritic formations while continuing to expose the needle-like dendritic formations to an atmosphere that contains at least one silicon-containing compound is conducted such that larger dendrites are distributed over substantially the entire heated surface of the core rod.

18. The method of claim 16 wherein:

the silicon-containing compound is silane (SiH₄); and

the decomposing of the silane to form needle-like dendrites is at a surface temperature of at least 875°

C. during at least a portion of the time when the needle-like dendrites are being formed.

19. The method of claim 18 wherein the decomposing of the silane to form needle-like dendrites is at a surface temperature of at least 975°

C. during at least a portion of the time when the needle-like dendrites are being formed.

20. The method of claim 16 wherein the gradually reducing of the surface temperature of the rod encrusted with needle-like dendritic formations while continuing to expose the needle-like dendritic formations to an atmosphere that contains at least one silicon-containing compound is conducted at conditions such that at least some of the larger dendrites are generally teardrop-shaped.

21. The method of claim 16 further comprising forming the core rod by depositing a growth layer of polycrystalline silicon on a starter filament.

22. The method of claim 16 wherein the core rod consists of a filament and the needle-like dendritic formations are formed directly on the filament.

23. The method of claim 16 wherein the atmosphere comprises a silicon-containing compound selected from the group consisting of silane compounds, halosilane compounds, and mixtures thereof.

24. The method of claim 23 wherein the atmosphere comprises a silicon-containing compound selected from the group consisting of silane (SiH₄), disilane (Si₂H₆), trisilane (Si₃H₈), dichlorosilane (SiH₂Cl₂), trichlorosilane (SiHCl₃), silicon tetrachloride (SiCl₄), dibromosilane (SiH₂Br₂), tribromosilane (SiHBr₃), silicon tetrabromide (SiBr₄), diiodosilane (SiH₂I₂), triiodosilane (SiHI₃), silicon tetraiodide (SiI₄), and mixtures thereof.

25. A method for producing polycrystalline silicon comprising:

providing an elongated polycrystalline silicon filament;

depositing a generally cylindrical layer of polycrystalline silicon on the filament to form a core rod;

exposing the surface of the core rod to an atmosphere that comprises silane (SiH₄) while maintaining the rod surface a temperature in a range such that the temperature is at least 875°

C. and is below the melting point of silicon, during which silane in the gas pyrolytically decomposes and deposits silicon on the surface in needle-like dendritic formations and produces a rod encrusted with needle-like dendrites; and

gradually reducing the surface temperature of the rod encrusted with needle-like dendrites while exposing the needle-like dendrites to an atmosphere that contains silane, the surface temperature being reduced at a rate such that silicon deposits on the needle-like dendritic formations and forms multiple larger silicon dendrites distributed over substantially the entire heated surface of the rod encrusted with needle-like dendrites, at least some of which larger dendrites have a surface that flares outwardly from the core rod; and

separating the larger dendrites from the core rod.

26. The method of claim 25 wherein the exposing of the surface of the core rod to an atmosphere that comprises silane is at a rod surface temperature of at least 975°

C. during at least a portion of the time when the needle-like dendrites are being deposited.

27. A method for forming single crystal silicon ingots comprising:

growing multiple polycrystalline silicon dendrites on a substrate;

separating the dendrites from the substrate;

charging the crucible of a crystal-growing furnace with dendrites separated from the substrate;

melting the dendrites in the crucible; and

pulling a single crystal ingot from molten silicon in the crucible.

28. The method of claim 27 wherein:

the substrate is a core rod of polycrystalline silicon; and

the method further comprises breaking the core rod into chunks and charging the crucible with chunks of the core rod along with the separated dendrites.

29. A body of polysilicon comprising:

a substrate; and

multiple silicon dendrites extending from a surface of the substrate, at least some of the dendrites having a surface that flares outwardly from the substrate.

30. The body of claim 29 wherein at least some of the flared dendrites comprise:

at least one a needle-like dendrite extending from the substrate; and

a flared growth layer of silicon on the at least one needle-like dendrite.

31. The body of claim 29 wherein the dendrites are distributed over substantially the entire surface of the substrate.

32. A rod of polysilicon comprising:

an elongated filament;

a growth layer of silicon surrounding the filament; and

multiple silicon dendrites extending radially from the growth layer, at least some of the dendrites having a surface that flares outwardly from the growth layer.

33. The rod of claim 32 wherein the dendrites are distributed over substantially the entire surface of the growth layer.

34. The rod of claim 32 wherein at least some of the dendrites are generally teardrop-shaped.

35. The rod of claim 32 wherein at least some of the dendrites have a stem portion that is no greater than about 8 mm in diameter at its narrowest point.

36. The rod of claim 32 wherein the growth layer surrounding the filament is generally cylindrical.

37. The rod of claim 32 wherein:

the growth layer surrounding the filament is generally cylindrical;

the dendrites are distributed over substantially the entire surface of the growth layer;

a majority of the dendrites are generally teardrop-shaped; and

a majority of the dendrites have a stem portion that is no greater than about 8 mm in diameter at its narrowest point.

38. A polycrystalline silicon dendrite comprising:

at least one needle-shaped body of polycrystalline silicon having a surface that tapers to a point; and

a growth layer of polycrystalline silicon on the surface of the at least one needle-shaped body, the growth layer having a surface that flares in the same general direction as the at least one needle-shaped body tapers.

39. A batch of polycrystalline silicon material suitable for use as a raw material in the crucible of a crystal-growing furnace, the batch comprising:

a plurality of chunks of a broken cylindrical polycrystalline core rod that comprises an elongated filament and a growth layer of silicon surrounding the filament; and

multiple polycrystalline silicon dendrites that were grown on and broken away from the surface of a core rod.