Removal of metal impurities in molten silicon by directional solidification with electron beam heating

Noriyoshi Yuge, Kazuhiro Hanazawa, Yoshiei Kato

Research output: Contribution to journalArticle

47 Citations (Scopus)

Abstract

An industrial-scale pyrometallurgical method of removing metallic impurities from metallurgical grade silicon (MG-Si) was developed as an element technology in a sequential purification process for manufacturing high-purity silicon for solar grade silicon (SOG-Si) by segregation of metallic impurities during solidification. Metallic impurities were removed from MG-Si using an electron beam heating equipment. Molten silicon was supplied continuously at a constant mass to a water-cooled copper mold and was allowed to solidify gradually in an unidirectional manner from the bottom upward. This process is termed directional solidification. The iron concentration after solidification can be expressed by Pfann's and Burton's equations, and was reduced from an initial 1500 mass ppm to below 10 mass ppm. Aluminum removal was excessive, presumably due to vaporization to the gas phase. Above a certain height in the ingot, it became impossible to remove metallic impurities by partition during directional solidification. This phenomenon showed a correlation with the concentration of enriched iron in the silicon pool. The mechanism of metallic impurity removal was estimated based on visual examination of the solidified structure and EPMA. The iron concentration profile of ingots and critical purification height were estimated experimentally using a 20 kg scale device and verified as being applicable on an industrial scale in experiments with 150kg scale equipment. Solar grade silicon was test-produced by this process and showed satisfactory quality for solar cell use.

Original languageEnglish
Pages (from-to)850-857
Number of pages8
JournalMaterials Transactions
Volume45
Issue number3
DOIs
Publication statusPublished - Mar 2004
Externally publishedYes

Keywords

  • Aluminum
  • Directional solidification
  • Electron beam
  • Iron
  • Metallurgical grade silicon
  • Silicon
  • Solar grade silicon
  • Titanium

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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