JPH06227808A - Method for purifying metallic silicon - Google Patents

Abstract

PURPOSE:To enable the mass production of high-purity silicon in a short time at a low cost by charging metallic silicon into a crucible nonoxidizing the metallic silicon, melting the metallic silicon in a nonoxidizing atmosphere and then keeping the silicon in this molten state in the atmosphere under a reduced pressure. CONSTITUTION:Metallic silicon is purified. In the process, the silicon is charged into a crucible nonoxidizing the silicon and then melted in a nonoxidizing atmosphere. The resultant silicon in this molten state is kept in the atmosphere under a reduced pressure of <=10Pa to carry out the purification of the metallic silicon. P, As, Ca, O, N, etc., can be removed to <=1ppm content according to this method for purification and the yield of the silicon after purification thereof is high. The high-purity silicon can be mass-produced at a low cost in a short time by using inexpensive metallurgical silicon as a starting raw material. Thereby, the cost of a solar cell heretofore using conventional expensive silicon for semiconductors can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high-purity metallic silicon used for producing solar cells and the like.

[0002]

2. Description of the Related Art Impurity elements such as B, P, C, O, Fe, Al, and Ti in silicon used for manufacturing solar cells must have a low concentration of 1 ppmw or less in order to secure required semiconductor characteristics. is there. Therefore, as the current silicon as a raw material for solar cell production, excessively refined silicon produced as a semiconductor raw material is used. However, since silicon for semiconductors is expensive, it has hindered the spread of solar cells.

Therefore, in order to inexpensively produce silicon for solar cells, various proposals have been made on a method of refining inexpensive metallic silicon for metallurgy as a raw material, such as acid extraction method, flux melting method, The vacuum melting method and the like have been reported. However, none of the methods for removing P in silicon has been proposed for its effective and concrete measures.

[0004]

In the removal of P in silicon, the acid extraction method is used when the silicon is dissolved and solidified.
Is precipitated on the crystal grain boundaries of silicon together with heavy metal elements, and this is subjected to acid extraction.Since it is necessary to grind the silicon mass as finely as possible before acid extraction, and it is also necessary to wash it after acid extraction. There is a problem that the number of processes increases and the cost increases.

In the case of the flux processing method, P is 1 pp
In order to remove up to mw or less, the amount of flux is the same as or more than that of silicon, and the process of removing flux and the process of removing flux components dissolved in silicon are required, and there is little merit in process and cost. . On the other hand, Suzuki et al.
As reported in Vol. 54, No. 2 (1990) 161-167 ", it is a method of evaporating and removing P with high vapor pressure in a vacuum atmosphere. This method removes P in a high vacuum of 10 ^-3 Pa. In this method, a crucible made of quartz is used in JP-A-62-292613, which treats silicon in a reduced pressure atmosphere, although the crucible and the atmosphere are not described in this method. In such a method, there is a problem that quartz and silicon react with each other and the yield of silicon decreases.

An object of the present invention is to solve the above problems and to provide a method for refining metallic silicon by removing impurity elements such as As and O in silicon for which no refining method has been proposed so far. It is a thing.

[0007]

In order to solve the above problems, the present invention solves the above problems by dissolving silicon in a non-oxidizing crucible such as graphite or copper to obtain a non-oxidizing gas such as Ar, N ₂ or H _2. In the atmosphere
It has been found that P can be effectively removed from metallic silicon by maintaining a reduced pressure atmosphere of 10 Pa or less.

That is, according to the present invention, when refining metallic silicon, silicon is charged into a crucible that is non-oxidizing with respect to silicon, melted in a non-oxidizing atmosphere, and 10 Pa or less in a molten state. The method for refining metallic silicon is characterized in that it is maintained in a reduced pressure atmosphere.

[0009]

In order to remove components having a high vapor pressure such as P, As and Ca and gas components such as O and N contained in silicon, it is effective to hold the melted silicon in a vacuum state.
By maintaining a high vacuum of 0.027Pa, Suzuki et al.
The concentration of is reduced from 32 ppmw to 6 to 7 ppmw. However, although the graphite crucible is used here, the influence of other crucibles and gas atmosphere is not described.

When silicon is dissolved in an oxidizing atmosphere,
A silica film is formed on the silicon bath surface. When the pressure is reduced in such a state, the reaction of Si + SiO ₂ → 2SiO ₂ proceeds on the silicon bath surface, and silicon is evaporated in the form of SiO 2 gas. on the other hand,
P is removed by evaporation from the silicon bath surface to the gas phase, but if there is a silica coating on the silicon bath surface, there is transfer of P from silicon to silica and from silica to the gas phase. Therefore, when the pressure of silicon is reduced in an oxidizing atmosphere, the yield of silicon is reduced and the removal rate of P is reduced.

Therefore, from the above, when removing P, it is necessary to maintain a non-oxidizing atmosphere. Furthermore, FIG. 1 shows the relationship between the degree of vacuum and the reduction amount of P when the pressure is reduced under such conditions. This figure shows a graphite crucible with P of 30ppmw in Ar atmosphere.
This is a graph showing the amount of reduction of P after 100 g of silicon contained was melted at 1500 ° C. under a predetermined pressure and kept for 1 hour. It can be seen from the figure that P can hardly be removed at a pressure exceeding 10 Pa, but P can be removed by reducing the pressure to 10 Pa or less. The same tendency applies to elements with high vapor pressure such as As and Ca and O,
It was observed for gas components such as N.

From the above, in order to remove elements such as P, As, Ca, O and N from silicon with a good yield, non-oxidizing crucibles such as graphite and copper are used to remove Ar, N ₂ , H _{2 and the} like. It is understood that it is preferable to reduce the pressure to 10 Pa or less in a non-oxidizing gas atmosphere.

[0013]

[Embodiment] Each of the raw material metal silicon having the composition shown in Table 1
00 g The crucible shown in Table 1 was melted in a vacuum melting furnace (50 kw) in the atmosphere shown in Table 1, and then held at 1500 ° C. at the predetermined pressure shown in Table 1 for 1 hour. The metallic silicon composition at that time is also shown in Table 2.

[0014]

[Table 1]

[0015]

[Table 2]

According to the present invention, it has been shown that P, As, Ca, O, N and the like can be removed up to 1 ppmw or less. Further, it is understood that the yield after refining silicon is high.

[0017]

INDUSTRIAL APPLICABILITY According to the present invention, high-purity silicon having a low content of P, As, Ca, O, N, etc. can be mass-produced at low cost in a short time using inexpensive metallurgical silicon as a starting material. It is possible to reduce the cost of a solar cell that uses silicon for semiconductors. As a result, the use of solar cells can be greatly advanced, and a great social contribution will be brought about.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the degree of vacuum and the amount of P removed.

Claims (1)

[Claims] 1. When refining metallic silicon, silicon is charged into a crucible that is non-oxidizing with respect to silicon, melted in a non-oxidizing atmosphere, and a reduced pressure atmosphere of 10 Pa or less is maintained in the molten state. A method for refining metallic silicon, characterized in that it is kept.