Electromagnetic casting method and apparatus for polycrystalline silicon

Patent metadata

Field	Value
Publication number	US-9553221-B2
Application number	US-201313845507-A
Country	US
Kind code	B2
Filing date	Mar 18, 2013
Priority date	Feb 14, 2011
Publication date	Jan 24, 2017
Grant date	Jan 24, 2017

Abstract

Disclosed is an electromagnetic casting method of polycrystalline silicon which is characterized in that polycrystalline silicon is continuously cast by charging silicon raw materials into a bottomless cold mold, melting the silicon raw materials using electromagnetic induction heating, and pulling down the molten silicon to solidify it, wherein the depth of solid-liquid interface before the start of the final solidification process is decreased by reducing a pull down rate of ingot in a final phase of steady-state casting. By adopting the method, the region of precipitation of foreign substances in the finally solidified portion of ingot can be reduced and cracking generation can be prevented upon production of a polycrystalline silicon as a substrate material for a solar cell.

First claim

What is claimed is:

1. An electromagnetic casting method of polycrystalline silicon for continuously casting polycrystalline silicon, the method comprising:
charging silicon raw materials into a bottomless cold mold;
melting the silicon raw materials using electromagnetic induction heating; and
pulling down the molten silicon to solidify it,
wherein the depth of solid-liquid interface from an upper surface of the molten silicon before the startup of a final solidification process is reduced by reducing a pull down rate of ingot from a prescribed rate in a final phase of a steady-state casting,
wherein the prescribed rate is such that molten silicon on the solidified silicon is confined in the solidified silicon when the silicon ingot is pulled at the prescribed rate in the final solidification process, and
wherein the reduced pull down rate from the prescribed rate is such that molten silicon on the solidified silicon is not confined in the solidified silicon when the silicon ingot is pulled at the reduced rate in the final solidification process.

2. The electromagnetic casting method of polycrystalline silicon according to claim 1 , wherein the pull down rate ingot is reduced by 0.05 to 0.2 mm/min per hour.

3. An electromagnetic casting apparatus for polycrystalline silicon including: a conductive bottomless cold mold in which a part of the mold along an axial direction is divided into a plurality of elements in a circumferential direction; an induction coil surrounding t he mold; and a heat retention heater arranged below the mold for slowly cooling a solidified silicon ingot, in which silicon melted by electromagnetic induction heating using the induction coil is pulled down and solidified, the apparatus comprising:
a pull down rate controller for changing a pull down rate of ingot between a prescribed rate and a reduced rate from the prescribed rate in response to a casting stroke,
wherein the pull down rate controller comprises:
a casting stroke counter for measuring the casting stroke;
a motor drive amount computing unit for computing a drive amount of a shaft-pull-down motor according to a predetermined deceleration rate by inputting the measured casting stroke and then outputting the computation result to the shaft-pull-down motor; and
a shaft-pull-down motor that is driven based on a signal input from the motor drive amount computing unit,
wherein the prescribed rate is such that molten silicon is confined in the solidified silicon when the silicon ingot is culled at the prescribed rate in the final solidification process, and
wherein the reduced rate from the prescribed rate is such that molten silicon is not confined in the solidified silicon when the silicon ingot is pulled at the reduced rate in the final solidification process.

Assignees

• Sumco Corp

Inventors

• Maegawa Koichi
• Onizuka Tomohiro
• Yoshihara Mitsuo

Classifications

• C30B15/08

  Downward pulling · CPC title

• C30B28/06

  by normal freezing or freezing under temperature gradient · CPC title

• B22D11/20

  for removing cast stock · CPC title

• C30B13/20

  by induction, e.g. hot wire technique (C30B13/18 takes precedence) · CPC title

• C30B28/04

  from liquids · CPC title