Methods for purifying metallurgical silicon

• US 8,257,492 B2
• Filed: 11/16/2010
• Issued: 09/04/2012
• Est. Priority Date: 11/16/2009
• Status: Active Grant

• Current Assignee: Cheng C. Kao, Masahiro Hoshino
• Original Assignee: Cheng C. Kao, Masahiro Hoshino
• Inventors: Hoshino, Masahiro, Kao, Cheng C.
• Primary Examiner(s): SONG, MATTHEW J
• Application Number:
  US12/947,777
• Publication Number:
  US 20110114012A1
• Time in Patent Office: 658 Days
• Field of Search: 117/2, 117/4, 117/11, 117/37, 117/41, 117/42, 117/933
• US Class Current: 117/37
• CPC Class Codes: C01B 33/037 Purification by zone-melting C30B13/00
• CPC Class Codes: H01L 31/182 Special manufacturing methods for polycrystalline Si, e.g. Si ribbon, poly Si ingots, thin films of polycrystalline Si
• CPC Class Codes: Y02E 10/546 Polycrystalline silicon PV cells
• CPC Class Codes: Y02P 70/50 Manufacturing or production processes characterised by the final manufactured product

Abstract

A method for purifying silicon bearing materials for photovoltaic applications includes providing metallurgical silicon into a crucible apparatus. The metallurgical silicon is subjected to at least a thermal process to cause the metallurgical silicon to change in state from a first state to a second state, the second stage being a molten state not exceeding 1500 Degrees Celsius. At least a first portion of impurities is caused to be removed from the metallurgical silicon in the molten state. The molten metallurgical silicon is cooled from a lower region to an upper region to cause the lower region to solidify while a second portion of impurities segregate and accumulate in a liquid state region. The liquid state region is solidified to form a resulting silicon structure having a purified region and an impurity region. The purified region is characterized by a purity of greater than 99.9999%.

Claims

1. A method for purifying silicon bearing materials for photovoltaic applications, the method comprising:

providing metallurgical silicon into a crucible apparatus;

subjecting the metallurgical silicon to at least a thermal process to cause the metallurgical silicon to change in state from a first state to a second state, the second stage being a molten state not exceeding 1500 Degrees Celsius, the power to the thermal process being provided intermittently;

providing at least a purification gas towards the surface of the silicon melt through an independent supply tube, the supply of the purification gas being provided intermittently;

causing at least a first portion of impurities to be removed from the metallurgical silicon in the molten state;

cooling the molten metallurgical silicon from a lower region to an upper region to cause the lower region to solidify while a second portion of impurities segregate and accumulate in a liquid state region; and

solidifying the liquid state region to form a resulting silicon structure having a purified region and an impurity region,

whereupon the purified region is characterized by a purity of greater than 99.9999%.

2. The method of claim 1 wherein the thermal process uses a plasma gun or arc heater.

3. The method of claim 2 wherein the plasma gun or arc heater provides H₂ gas towards the surface of the silicon melt;

and wherein the independent supply tube provides O₂ towards the surface of the silicon melt, the H₂ from the plasma gun or arc heater and the O₂ from the independent supply tube forming water vapor (H₂O) that is injected into the silicon melt.

4. The method of claim 2 wherein the duty cycle of intermittent plasma irradiation is below 50%.

5. The method of claim 1 wherein the thermal process has a source of about 3000 Degrees Celsius.

6. The method of claim 1 wherein the molten metallurgical silicon has a temperature profile characterized by a higher temperature region within a center region and a lower temperature region within an edge region of the crucible apparatus.

7. The method of claim 1 further comprising rotating and shifting the crucible through a manipulating mechanism to move the solidus-liquidus interface of the silicon melt to achieve one-directional cooling purification.

8. The method of claim 1 wherein the thermal process comprises a convection process of the molten state.

9. The method of claim 1 wherein the molten state is substantially free from any foreign objects for circulating the molten metallurgical silicon.

10. The method of claim 1 further comprising subjecting an upper molten region of the metallurgical silicon with an inert blanket of gas;

and subjecting a portion of the upper molten region to a hydrogen gas to cause a reaction and remove a boron impurity with the hydrogen gas to remove the oxygen impurity,

wherein the metallurgical silicon has a purity of about 99%.

11. The method of claim 1 further comprising subjecting a region of the molten metallurgical silicon to a hydrogen species to remove a boron impurity using a gas selected from at least one of B₂O₃, B₂O₃H₂O, BH₄, B₂H₆, BH₃, H₃BO₃, HBO₂, HBO₃, H₄B₂O₄, H₃BO₂, H₃BO, H₂B₄O₇, B₂O₂, B₄O₃ or B₄O₅.

12. The method of claim 11 wherein the hydrogen species is injected using one or more channel regions.

13. The method of claim 1 wherein the crucible is maintained in a vacuum to keep the molten metallurgical silicon free from external contaminants.

14. The method of claim 13 wherein the vacuum is selected to achieve a desired impurity removal characteristic from the molten metallurgical silicon.