Chemistry of inorganic compounds – Silicon or compound thereof – Elemental silicon
Reexamination Certificate
1999-09-08
2001-05-15
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Elemental silicon
C422S186220, C422S255000, C023S30800S
Reexamination Certificate
active
06231826
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process and an apparatus for refining silicon, particularly for enhancing removal of impurities such as P, Al and Ca by electron beam melting. The process and apparatus produce silicon having low concentrations of these impurities.
2. Description of the Related Art
In recent years, requirement for diversification in energy sources has caused solar power generation to be spotlighted. Generating apparatus of low cost has actively been researched and developed. Under such circumstances, silicon is likely to be most generally used as a raw material for solar batteries. Crystalline silicon has attracted the most serious attention as the material used for such a power supply.
It is recognized that high purity silicon having a purity of 99.9999% (6N) or more (hereinafter abbreviated as SOG-Si) is required for use as a raw material for solar batteries. Concentrations of impurities in the silicon have to be reduced to an order of parts per million or lower. A process has been proposed for producing high purity silicon from commercially available silicon metal (purity: 99.5%, hereinafter abbreviated as MG-Si), in which metallic impurities such as Al, Fe, Ti and the like are removed by directional solidification making use of small segregation coefficient. In this process C is deposited on the surface of the silicon by solidification, in the case of SiC, and is removed in the form of CO in the case of solid solution C; and B is removed by Ar plasma melting carried out while adding H
2
O, CO
2
or O
2
.
However, in the production process described above, the methods for removing the respective impurity elements are different from each other. The steps and facilities used are complicated. Moreover, a loss is caused in transfer from one step to a following step, and this lowers the silicon yield.
On the other hand, electron beam melting is usually used for melting high-melting metals such as titanium and molybdenum, and it is being researched for production of silicon used for a solar battery.
In Japanese Unexamined Patent Publication No. 61-232295, a copper-made vessel, cooled with water, is used in order to prevent contamination caused by vessel materials in electron beam melting of silicon metal. Japanese Unexamined Patent Publication No. 63-64909 discusses making a silicon sheet for a solar battery by combining a water-cooled copper hearth (having a small depth) with electron beam melting. Further, proposed in Japanese Unexamined Patent Publication No. 5-124809, is a solidification refining process in which a temperature gradient is provided in a vertical direction by melting only an upper part of a silicon metal held in a cast vessel, making use of local heating by an electron beam, and cooling the lower part of the silicon metal.
Also proposed is a method for removing P under reduced pressure by making use of its high vapor pressure. However, it takes a long time to remove P under reduced pressure. Recently, however, it is reported that P contained in silicon can be removed in a short time by electron beam melting [ISIJ International, vol. 32 (1982), No. 5, p. 635 to 642].
Further, an indicated advantage of electron beam melting is that Al and Ca as well as P can be removed together. However, in electron beam melting using conventional techniques, the removal limits for P, Al and Ca contained in silicon are about 3 ppmw, about 470 ppmw and about 150 ppmw, respectively. Since the concentrations of P, Al and Ca contained in silicon become almost fixed values in a melting time of 15 minutes or more, the contents of P, Al and Ca cannot be expected to be reduced any further. It is not reasonable to suspect that the electron beam melting method has been sufficiently investigated to achieve a purity required for SOG-Si, which is about 99.9999% Si or more.
We have independently proposed a so-called skull melting procedure, using water-cooled copper for a vessel (crucible, mold, hearth or the like) for holding the silicon, and that has made it possible to achieve a degree of purity as required for a silicon solar battery (Japanese Unexamined Patent Publication Nos. 7-309614 and 7-325827). High purity silicon is solidified at a location in contact with a water-cooled copper,vessel, and the silicon is melted in the inside of a solidified shell referred to as a skull. According to this method, contamination originating in impurities contained in the silicon adhered to the vessel can be prevented. Regrettably, however, this method has encountered the problem that about two-thirds of the energy held by the electron beam is taken up by the cooling water. Thus, the remaining energy contributing to melting is small, and very much reduces the heat efficiency of the treatment.
SUMMARY OF THE INVENTION
It is accordingly an important object of the present invention to provide a silicon refining method having high heat efficiency while impurities are evaporated and removed from the silicon by electron beam melting.
Another object of the present invention is to provide an efficient and practical technique for evaporating and removing P, Al and Ca from silicon by limiting reduction of rate of impurity removal caused by contamination.
We have discovered that these and other objects can be achieved by specially enhancing the efficiency of heat transfer from an electron beam to silicon and elevating the temperature of the molten silicon. The rate of removing P, Al and Ca is increased as the temperature of the molten silicon is elevated. It has also been confirmed that when electron beam melting is carried out in a water-cooled copper vessel, the amount of heat taken away by the water-cooled copper vessel is as much as about 60 to 70% of the beam input.
We have discovered that a large amount of this heat can be retained by using a vessel having low heat conductivity, or even a vessel having no water-cooling system. In this way the temperature of the molten silicon at a fixed electron beam output can be significantly elevated.
The present invention relates to a process for refining silicon by melting silicon in a graphite vessel by irradiation with an electron beam to evaporate volatile impurity elements contained in the silicon in a graphite vessel. Further, the present invention relates to reducing the concentrations of impurities contained in the graphite vessel by an amount that is greater than the target amounts of selected impurities contained in the refined silicon, particularly by controlling the density of the graphite in the vessel to. about 1.5 g/cm
3
or more.
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Jacobson, C. A. “Encyclopedia of Chemical Reactions”, vol. II, Reinhold Publishing Corp 1948, pp. 208-209, No Month.
“Purification of Metallurgical Silicon for Solar-grade silicon by Electron Beam Button Melting”, vol. 32, No. 5, ISIJ 1992, pp. 635-642, No Month.
Translation of JP 7-315,827, Dec. 1995.
Abe Masamichi
Aratani Fukuo
Baba Hiroyuki
Hanazawa Kazuhiro
Kato Yoshiei
Kawasaki Steel Corporation
Miller Austin R.
Nguyen Ngoc-Yen
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