JPH0948606A - Purification of silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently obtain extremely pure silicon by irradiating electron beams and oxygen ion-containing ion beams to the surface of fused silicon in fusing and purifying silicon by electron beams in a crucible in a vacuum fusion furnace. SOLUTION: This purification process of silicon comprises putting a crucible 2 in a vacuum fusion furnace 1, inserting silicon 7 into the crucible, fusing the silicon 7 by irradiating electron beams 5 from an electron gun, irradiating ion beams 6 containing oxygen onto the fused silicon from an ion gun 4 together with the electron beams 5 and purifying the silicon. Thus, boron and carbon which have been impossible to remove can be effectively removed by oxidation and extremely pure crystal silicon for solar cells can be obtained. As an oxygen ion source, O2 , CO2 and H2 O are suitable and the beams 6 is generated by feeding a gas containing at least one above gas diluted with an inert gas to the ion gun 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to refining silicon, and more particularly to a method for refining high-purity silicon that can be used as a raw material for solar cells.

[0002]

2. Description of the Related Art In recent years, due to the demand for diversification of energy sources, photovoltaic power generation has been in the spotlight, and research and development have been actively conducted toward the practical application of low-cost power generators. Under such circumstances, silicon is the most commonly used material as a raw material for solar cells, and is most emphasized as a material used for power supply for power.

The purity of silicon used as a raw material for solar cells is required to be 99.9999 wt% (6N) or higher. Conventionally, commercially available silicon (purity 99.5 wt
%) To produce the above high-purity silicon, Al, F
The metal impurity elements such as e and Ti are removed by directional solidification refining utilizing the fact that the solid-liquid partition coefficient is small, and C
In case of SiC, it is deposited on the surface during solidification, and when it is in solid solution, it is removed as CO. On the other hand, P is removed under reduced pressure by utilizing its high vapor pressure. However, a method of removing B by Ar plasma melting with H ₂ O, CO ₂ or O ₂ added is proposed.

However, in the above-mentioned manufacturing method, the method of removing each impurity element is different, which not only complicates the process, but also causes a problem such as a decrease in yield due to a loss when the process moves to the next process. In addition, in Japanese Patent Laid-Open No. 61-232295, an attempt has been made to obtain high-purity crystalline silicon for solar cells by electron beam melting using a water-cooled mold in order to eliminate external contamination of silicon.

On the other hand, it has recently been reported that P, Ca, Al, C and B in commercially available metallic silicon can be simultaneously removed by electron beam melting (ISIJ Intrernational, vol.
32 (1992), No. 5, P635-p642), simplification of the manufacturing process is expected. However, this is because the B concentration in silicon is 1
The B concentration has been reduced to about 0 ppm, and has not reached below the B concentration targeted for solar cell silicon.

On the other hand, Japanese Patent Publication No. 5-124809 proposes a method of removing impurities by unidirectional solidification refining utilizing electron beam melting. Due to the different mechanism, it was difficult to efficiently remove impurities such as P having a solid-liquid distribution coefficient close to 1. In addition, the inventors of the present invention have filed Japanese Patent Application No.
No. 7072 describes the pressure inside the furnace during electron beam melting,
We proposed a method for refining high-purity silicon that efficiently removes impurities by controlling the atmosphere inside the furnace such as a crucible. According to this method, a sufficient purification effect can be obtained especially for P.

[0007]

In view of the state of the art as described above, the present invention is particularly advantageous in removing impurities by electron beam melting in order to avoid the complexity of the step of removing each impurity element and to simplify the steps. It is an object of the present invention to provide a method for purifying silicon, which makes it possible to efficiently, stably and easily obtain crystalline silicon for solar cells having a low B and C and high purity.

[0008]

SUMMARY OF THE INVENTION The present inventors have proposed that (1) the heating source is a clean electron beam, (2) the atmosphere is high vacuum, and (3) a water-cooled crucible is used. In addition to electron beam melting, in order to maximize the advantages of electron beam melting, such as preventing contamination of molten metal with impurities, and achieve complete removal of B and C and simultaneous removal of P, Ca, and Al. The present invention was constructed by using an ion beam as a means for supplying oxygen to molten silicon.

That is, the present invention is characterized in that when melting and refining silicon with an electron beam in a crucible provided in a vacuum melting furnace, the surface of the molten silicon is irradiated with an electron beam and an ion beam containing oxygen ions. The present invention also provides a method for purifying silicon, wherein the ion beam does not contain one or more selected from O ₂ , CO ₂ , and H ₂ O gas as an oxygen ion supply source. It is preferable that the silicon is obtained by using a gas diluted with an active gas, and the crucible is a water-cooled crucible.

[0010]

In the present invention, when melting and refining silicon by an electron beam in a crucible arranged in a vacuum melting furnace, the surface of the molten silicon is irradiated with an electron beam and an ion beam containing oxygen ions. As a result, B and C which could not be removed until now can be effectively removed by oxidation.
The ion beam may be irradiated from the beginning of melting, but it is preferable to irradiate after the silicon is melted. As the oxygen ion source, any gas can be used as long as it can supply oxygen ions. Among them, O ₂ , H ₂ O, CO
_Two gases are preferred. These gases may be used alone, or two or more kinds selected from these gases may be mixed. Oxygen concentration (wt%) in the inert gas is 1 to 1
0% is preferable. Excessive oxygen content exceeding 10% is
Not only accelerates the generation of O, but also Si generated on the surface
O ₂ inhibits removal of B and C. Also, 1%
If the amount is less, sufficient O cannot be obtained to oxidize and remove B and C. Therefore, the gas of the oxygen ion source is diluted with an inert gas so as to have the above-mentioned amount of O and supplied to the ion gun.

The type of ion gun is not particularly limited,
Among them, duoplasma type, microwave discharge type, high frequency discharge type, and beam plasma type are effective. In the present invention, the silicon is melted and refined by an electron beam in a vacuum melting furnace, but the furnace pressure is not particularly limited as long as it is within the range of the furnace pressure at which the electron beam can be stably irradiated. The pressure is preferably 1 × 10 ^{−4 to} 5 × 10 ⁻³ Torr. If the pressure in the furnace exceeds 5 × 10 ⁻³ Torr, the effect of removing impurities is not sufficiently exerted, and the electron beam shape is disturbed, which may cause discharge and the like, so that the electron beam can be stably irradiated. difficult. Impurity removal rate is 5 × 10 ^-3 T
It remains unchanged below orr. In the range of less than 1 × 10 ⁻⁴ Torr, the loss due to evaporation of silicon is large. From the viewpoint of loss due to evaporation of silicon, the furnace pressure is 5 x
10 ^-4 Torr or more is desirable.

Next, the crucible for holding the melted silicon is preferably a water-cooled crucible, and further, a silicon having a higher purity than the desired purity of silicon,
For example, like semiconductor silicon (purity 11N),
It is desirable to use a water-cooled crucible in which high-purity silicon is adhered and solidified in a film shape. The crucible is preferably made of copper, but is not limited to this. It may have a high thermal conductivity, for example, a copper alloy, iron, or an iron alloy.

It is also important to remove beforehand C and the like in the atmosphere in the furnace which may enter molten silicon. Therefore, the pressure in the melting furnace is 1 × 10 ^-6 to 1
After vacuum evacuation to × 10 ^-5 Torr, Ar gas was introduced into the furnace and then evacuated to a furnace pressure of 1 × 10 ^-4.
It is desirable to perform the electron beam melting while maintaining the pressure at 5 × 10 ⁻³ Torr. The reason why Ar gas is introduced is that the cost is low and a decrease in vacuum exhaust capability due to the adhesion of gas components to the furnace wall can be avoided.

FIG. 1 is a schematic diagram showing an example of the melting apparatus used in the present invention. A water-cooled crucible 2 is placed on the central floor of a steel vacuum melting furnace 1, and an electron gun 3 of maximum 150 kW class and an ion gun 4 are installed above the crucible 2, and an electron beam 5 and an ion beam 6 are formed of molten silicon 7. Irradiate the surface of the molten metal. Gas serving as an ion source is introduced through the gas inlet 4A. Both the electron beam and the ion beam can change the irradiation direction to irradiate the molten metal surface.

[0015]

EXAMPLES Silicon was purified using the melting apparatus shown in FIG. (Example 1) 150 m inside diameter installed in a vacuum melting furnace
In a water-cooled copper crucible with a depth of m and a depth of 60 mm, 1500 g of commercially available metal-grade silicon (purity: 99.5 wt%, P: 25 ppm)
w, Al: 900 ppmw, Ca: 100 ppmw,
C: 1000 ppmw, B: 25 ppmw)
While maintaining the furnace pressure at 5 × 10 ^{−5 to} 5 × 10 ⁻³ Torr, an electron beam (output 30 kW) was applied to melt the silicon, and the surface of the molten silicon was irradiated with an ion beam together with the electron beam. As shown in the table, the ion beam used as an oxygen ion source was O ₂ , CO ₂ , H ₂ O.
A gas, which was a combination of Ar and He selected as a gas and a diluent gas, was introduced into an ion gun and generated. The ion gun used was a duoplasma type, and the extraction voltage was 30 kV and the ion current was 5 mA. Irradiation time is 10min-60min
And

Impurities in silicon obtained by melting and refining under the above-mentioned conditions are changed to ICP for P, Al, Ca and B.
(Inductively Coupled Plasma) Tables 1, 2 and 3 show the results of analysis by an emission analysis method and by a combustion method for C.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

Within the scope of the present invention, B, C, and P are irradiated by irradiating the surface of the molten silicon with an electron beam and an ion beam containing oxygen ions within the scope of the present invention.
It can be seen that Al and Ca can be reduced at the same time. (Example 2) 150m inner diameter installed in a vacuum melting furnace
In a water-cooled copper crucible having a depth of 60 mm and a depth of 60 mm (on the inner wall, a silicon layer 8 in which commercially available high-purity silicon (11N) for electronic materials was adhered and solidified in a film shape in advance) was formed in the same manner as in Example 1. 1500g of commercially available metallic grade silicon (purity 99.5w
t%) was charged, and while maintaining the furnace pressure at 5 × 10 ⁻⁴ Torr, an electron beam (output 30 kW) was applied to melt the silicon, and the molten silicon surface was irradiated with the ion beam and the ion beam. . For the ion beam, as shown in Table 4, O ₂ , CO ₂ gas was used as the oxygen ion source, and Ar was used as the diluting gas. The ion beam irradiation conditions were the same as in Example 1. The irradiation time was 60 min. Impurities in silicon obtained by dissolution purification under the above conditions,
For P, Al, Ca and B, ICP (Inductively Co
Table 4 shows the results of the upled Plasma) emission analysis and the combustion analysis for C.

[0021]

[Table 4]

By forming a high-purity silicon layer on the inner wall of the water-cooled crucible in advance and irradiating the molten silicon surface with an electron beam and an ion beam containing oxygen ions within the scope of the present invention, B and C can be more effectively. , P, A
It can be seen that l and Ca can be reduced at the same time.

[0023]

According to the present invention, the advantages of removing impurities by electron beam melting are utilized, the complexity of the process is avoided and simplification is achieved, and the process is efficient and stable, especially with high purity with low B and C. Crystalline silicon for solar cells can be obtained.

[Brief description of drawings]

FIG. 1 is a side sectional view showing an outline of a melting apparatus used in the present invention.

[Explanation of symbols] 1 vacuum melting furnace 2 water-cooled crucible 3 electron gun 4 ion gun 5 electron beam 6 ion beam 7 molten silicon 8 silicon layer

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Fumio Araya, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba, Kawasaki Steel Corporation Technical Research Laboratory (72) Inventor Michio Shimotome 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Center

Claims (3)

[Claims] 1. When melting and refining silicon with an electron beam in a crucible provided in a vacuum melting furnace, the surface of the molten silicon is irradiated with an electron beam and an ion beam containing oxygen ions. Purification method. 2. The ion beam is obtained by using, as an oxygen ion supply source, a gas obtained by diluting one or more kinds selected from O ₂ , CO ₂ and H ₂ O gas with an inert gas. The method for purifying silicon according to claim 1, wherein the method is for refining silicon. 3. The method for refining silicon according to claim 1 or 2, wherein the crucible is a water-cooled crucible.