JPH06115922A - Method for purifying silicon - Google Patents

Abstract

PURPOSE:To provide a method for economically purifying silicon to a high-purity one capable of using as a raw material for solar cells in high productivity by removing boron and carbon content from a silicon containing large amounts of boron and carbon, e.g. metal silicon to obtain a purified silicon having lowered contents of them useful as a raw material silicon for solar cells. CONSTITUTION:A melted silicon 1 is put in a vessel made of silica or a material consisting essentially of silica and plasma gas jet 6 is injected to the surface of the melted silicon. Boron and carbon in silicon can be removed in a short time without increasing plasma torch number by keeping an angle theta between the jetting direction 6 of the plasma and the surface at an angle satisfying the equation, 60 deg.>=theta>=10 deg.C.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for purifying high-purity silicon that can be used as a raw material for solar cells.

[0002]

2. Description of the Related Art High-purity silicon used in solar cells has, for example, a specific resistance of 0.1 Ωcm or more. In such silicon, the content of impurities contained in silicon is removed to the order of ppm. Need to be. On the other hand, various technologies have been studied,
Boron and carbon are the most difficult elements to remove.

Japanese Unexamined Patent Publication No. 3-10 proposed by the present inventors.
Japanese Patent No. 4342 discloses a method and an apparatus for holding a molten silicon in a silica or a container containing silica as a main component and injecting a plasma gas jet into the container, so that boron and carbon in the silicon can be efficiently discharged. It was explained that it will be removed. However, the technique disclosed in Japanese Patent Laid-Open No. 3-104342 has a limit in shortening the processing time because the removal reaction of boron and carbon proceeds in the portion of the silicon bath where the plasma gas jet is projected. For this reason, it has been desired to develop a processing technique capable of processing in a shorter time, that is, having a high reaction rate.

[0004]

SUMMARY OF THE INVENTION The present invention is intended to provide a method for purifying silicon which solves the above problems. That is, to provide a silicon refining method that increases the area of a portion of a silicon bath onto which a plasma gas jet is projected, and significantly shortens the boron-carbon removal reaction time without increasing the number of plasma torches. Is an issue.

[0005]

Means for Solving the Problems The present invention is to solve the above-mentioned problems, in which molten silicon is held in a container containing silica or silica as a main component, and a plasma gas jet is provided on the molten metal surface of the molten silicon. Was applied to the silicon refining method of removing impurities by adopting the following method.
That is, when the jet angle of the plasma gas jet with respect to the molten metal surface of the molten silicon is θ, it is 10 ° ≦ θ ≦ 60 °.

[0006]

[Function] By making the entire silicon in the container in a molten state, it is possible to easily stir the silicon bath by the plasma jet, thereby eliminating a region where impurities such as boron and carbon in the silicon bath are stagnated, and boron or carbon is removed. The removal reaction of is progressing every moment. In the method disclosed in Japanese Patent Laid-Open No. 3-104342, a portion where the plasma gas jet is projected onto the molten silicon surface is considered as a place where the reaction for removing boron and carbon proceeds. The present inventors have earnestly studied to improve the reaction rate for removing boron, and as a result, obtained the result that the portion where the above-mentioned plasma gas jet is projected is a promising reaction site.

Increasing the area of the portion where the plasma gas jet is projected increases the boron removal reaction rate.
There is a method of increasing the number of plasma torches as a method of increasing the area of the portion on which the plasma gas jet is projected, but a method of inclining the torch to reduce the angle θ formed by the molten silicon surface and the plasma gas jets. Is effective. The method of reducing the angle θ between the molten silicon surface and the plasma gas jet is a cheaper processing method because it is not necessary to increase the number of plasma torches.

As a result of investigating the boron removal reaction rate by variously changing the angle θ formed between the molten silicon surface and the plasma gas jet, no significant difference was observed in the boron removal reaction rate above 60 °. Therefore, θ is 60 °
The following are valid: Furthermore, when θ is set to 40 ° or less, the increase in the boron removal reaction rate is more remarkable and more effective.

As θ decreases, the molten silicon surface moves violently. In particular, when θ is smaller than 10 °, the splash increases remarkably, and it becomes difficult to continue the plasma gas jet projection. Therefore, in practice, θ is preferably 10 ° or more. As disclosed in JP-A-3-104342, an inert gas such as Ar or Ar-He may be used as the gas for generating plasma. Water vapor is added to this to accelerate the reaction of removing boron and carbon.

[0010]

EXAMPLES Examples of the present invention will be described below. Figure 1
FIG. 3 is an explanatory view of a vertical cross section of a silicon refining apparatus that preferably implements the present invention. Fused silicon 1 is silica stamp 3
It is held inside a container constituted by the heat insulating lining 4, and the induction heating coil 2 is attached to the outside thereof.
A plasma torch 5 is installed above the molten silicon, and a plasma jet 6 jetted from the plasma torch 5 is projected onto the molten silicon surface at an angle θ. Water vapor is added to the plasma jet 6 from an auxiliary pipe (not shown).

Using this silicon refining apparatus, an inner diameter of 120
2 kg of metallic silicon was melted by induction heating in a container having a depth of 90 mm and a depth of 90 mm, and a plasma jet 6 generated by a 30 KW non-transfer type plasma torch 5 was projected onto the surface of the silicon bath at an angle of θ = 90 °. . At this time, a change in the concentration of boron in the molten silicon 1 was observed as shown in FIG. Ar1 is used as the plasma generating gas at this time.
5 Nl / min was used and 5% steam was added.

When the silicon refining apparatus and the plasma generation conditions are the same as the above and only the angle θ is changed to 60 ° and 40 °, the boron concentration in the molten silicon is observed as shown in FIG. 2, and the angle θ = 90. The deboron reaction rate increased more than at 0 °. As shown in FIG. 2, the deboron reaction was observed as a primary reaction of boron as shown in the following formula (1).

-D [B] / dt = k [B] + C (1) where [B]: boron concentration in silicon (ppmw) t: time (min) k: coefficient, amount of water vapor added Becomes a function of.

C: constant k in the formula (1) becomes a deboron reaction rate coefficient, and the larger this value, the more efficiently boron is removed. Furthermore, when the angle θ was changed and examined, it was found that the coefficient k was changed depending on the angle θ, and the deboron reaction proceeded favorably when θ was 60 ° or less. Further, θ is a more preferable condition of 40 ° or less. That is, as shown in FIG. 3, it was found that the deboron reaction rate was increased by setting the angle θ to 60 ° or less. If θ> 60 °, θ = 9
0 °, that is, no significant increase in the deboron reaction rate was observed compared to the case of projecting perpendicularly to the molten metal surface, and θ>
60 ° (-90 °) is less inevitable to limit.

Comparative Example Using the apparatus shown in FIG. 1, 2 kg of metallic silicon was melted by induction heating in a silica stamp container lined with an inner diameter of 120 mm and a depth of 90 mm. A plasma jet generated by a 30 KW plasma torch was projected onto this at θ = 90 °. The concentrations of boron and carbon in silicon changed as shown in Table 1.

<Example 1> Using the same apparatus as in the comparative example, 2 kg of metallic silicon was melted under the same conditions as in the comparative example.
Onto this, a plasma jet was projected at θ = 60 ° similarly to the comparative example. The concentrations of boron and carbon in silicon changed as shown in Table 1. The deboron reaction rate was improved to about twice that of the comparative example.

Example 2 Using the same apparatus as in Comparative Example and Example 1, 2 kg of metallic silicon was melted under the same conditions, and a plasma jet was similarly projected at θ = 40 °.
The concentrations of boron and carbon in silicon changed as shown in Table 1.

[0018]

[Table 1]

[0019]

INDUSTRIAL APPLICABILITY The present invention provides a method for economically producing high-purity silicon used as a solar cell raw material. By carrying out the present invention, inexpensive metallic silicon is used as a starting raw material in a short time. It becomes possible to manufacture high-purity silicon by removing boron and carbon, and it is possible to reduce the cost of a solar cell that has conventionally used expensive silicon for semiconductors. The present invention is a technology that makes a great contribution to society, such that it can greatly improve the use of solar cells.

[Brief description of drawings]

FIG. 1 is an explanatory view of a vertical cross section of a silicon refining apparatus that preferably implements the present invention.

FIG. 2 is a graph showing the change over time in the boron concentration in silicon when silicon is purified by the apparatus shown in FIG.

FIG. 3 is a relationship graph between a speed coefficient k and an angle θ.

[Explanation of symbols]

 1 Molten Silicon 2 Induction Heating Coil 3 Silica Stamp 4 Adiabatic Lining 5 Plasma Torch 6 Plasma Jet 7 Silicon Flow θ Angle k Velocity Coefficient

Claims (1)

[Claims] 1. A method for purifying silicon in which molten silicon is held in silica or a container containing silica as a main component, and a plasma gas jet is jetted onto a molten metal surface of the molten silicon to remove impurities. A method for purifying silicon, characterized in that when the angle formed by the plasma gas jet with respect to the surface of the molten metal is θ, 10 ° ≦ θ ≦ 60 °.