JPH10203812A - Refining of metallic silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently refine metallic silicon in a shortened time by melting metallic silicon in a smelting vessel placed in a depressurized chamber and removing volatile impurity elements contained by evaporation where the metallic silicon is heated and molten with arc and high frequency induction to raise the surface temperature of the molten silicon and accelerate the agitation of the molten silicon to increase the evaporation rate of the impurity elements. SOLUTION: The metallic silicon 1 is molten with the induction heating coil 5 arranged to the smelting vessel 4 which is made of copper or graphite and equipped with a cooling system and with the arc generated between the upper electrode 6 and the bottom electrode 7 that are each arranged to the upper part and the bottom part of the smelting vessel placed in the depressurized chamber and the molten state is sustained for a certain time. The surface of the molten metal 1 is kept at elevated temperature by the arc 8 and stirred by the high frequency induction to accelerate the diffusion of the volatile impurity elements (for example, P, Al or Ca) in the molten metal whereby they are removed by evaporation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying metallic silicon, and more particularly, to a method of melting metallic silicon using a heating source with an electric arc or a laser in a vacuum refining step of a process for producing silicon for solar cells. This is a technique for removing impurity elements more efficiently and inexpensively than before.

[0002]

2. Description of the Related Art At present, due to the demand for diversification of energy sources,
Solar power is in the limelight, but because of its high cost,
It is not widely used for electric power. In addition, although most of the substrate material for solar cells is silicon, there is no manufacturing process dedicated to the silicon. Therefore, as shown in FIG. 7, the manufacturing of the silicon occurs in the manufacturing process of silicon for a semiconductor. It depends on scrap generated during polycrystalline silicon scraping or single crystal pulling. The polycrystalline silicon in FIG. 7 is obtained by reacting metallic silicon with hydrochloric acid to gasify it as trichlorosilane, rectifying the gas to remove impurity elements, and depositing it from the gas by a so-called CVD method of reacting with hydrogen gas. It is a thing.

In the method shown in FIG. 7, silicon which has been highly purified for a semiconductor must be adjusted (boron added), refined or cast again so as to be suitable for a solar cell. In addition, it is troublesome, the yield is low, remelting equipment and energy are separately required, and the production cost is increased. Therefore, currently available solar cells are expensive and hinder their general spread. Further, in the purification of metallic silicon mainly based on the above-described chemical process, there is a problem that a large amount of pollutants such as silane and chloride is inevitably generated, which hinders mass production. Furthermore, with the booming semiconductor industry, the amount of polycrystalline silicon for semiconductors is becoming insufficient, and the amount of silicon for solar cells is expected to decrease further in the future. Under such circumstances, it is necessary to obtain a silicon source that can be used for a solar cell at a lower cost than before by mainly using metallic silicon located further upstream than polycrystalline silicon.

Therefore, the present applicant has revised the purification of metallic silicon by the above chemical process, and
CT / JP96 / 02965), and proposes a method of producing a large amount of silicon having a purity suitable for a solar cell only by a metallurgical process as shown in FIG. 8 and casting it to a silicon substrate at a stretch. . It uses metallic silicon (purity 98-99% by weight Si) obtained by reducing silica stone as a starting material, removes easily volatile impurity elements such as P, Al, and Ca by vacuum refining and solidifies the molten metal. The resulting impurity metal elements (Fe, Ti, Al, Ca) are roughly refined, B and C are removed by oxidative refining and deoxidized, and then the above-mentioned impurity metal elements are solidified and refined by unidirectional solidification. Then, a part of the ingot is cut off, and the remaining part is sliced to produce a silicon substrate for a solar cell as a continuous flow operation.
According to such a manufacturing method, there is a prospect that silicon for solar cells can be mass-produced at a considerably lower cost than before.

In the above-mentioned volatile refining, as shown in FIG. 6, the melting of the metal silicon 1 (hereinafter also referred to as the molten metal) is performed because the surface of the molten metal 1 can be efficiently heated under reduced pressure. Irradiation. But,
This melting method is expensive in terms of equipment costs, and also requires the use of an electron beam 2
However, it is sensitive to the pressure of the atmosphere and the electromagnetic field, and has a drawback that stable irradiation cannot be performed even if there is a slight pressure fluctuation or the application of an electromagnetic field. Therefore, there is a problem that the stirring of the molten metal 1 cannot be performed, and a technique for accelerating the vaporization rate of the impurity element cannot be used. That is, when gas is blown into the molten metal 1, the pressure of the atmosphere fluctuates, and when a magnetic field is applied, the electron beam 2 is disturbed. This does not meet the applicant's development goal of mass-producing silicon for solar cells and further reducing the production cost.

[0006]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides a "method of purifying metallic silicon" which is more efficient (reducing the time and achieving the target purity) and inexpensive in running cost. It is intended to be.

[0007]

Means for Solving the Problems In order to achieve the above object, the inventor examined various uses of a heating source instead of an electron beam, and thought that heating by high-frequency induction was desirable in consideration of stirring of molten metal. However, since metal silicon does not melt in a solid state by high-frequency induction, the metallurgical process has focused on the use of arc melting, which is relatively frequently used. In addition, arc melting has a feature that, since the surface temperature of the molten metal is high, the surface of the molten metal, which is an advantage of electron beam melting, is high in temperature, and easily volatile impurity elements can be easily removed. Further, in addition to arc melting, heating by laser is considered to be effective for raising the temperature of the molten metal surface.

The present invention has been completed by diligent efforts to properly combine such known stirring means and dissolving means. Metal silicon is melted in a smelting vessel arranged in a decompression chamber, and the metal silicon is melted. In vaporizing and removing the easily volatile impurity element contained, the metal silicon,
This is a method for purifying metallic silicon, wherein the metal silicon is heated and melted by an arc generated between electrodes and high-frequency induction.

Further, the present invention provides a method for dissolving metallic silicon in a refining vessel placed in a decompression chamber and evaporating and removing easily volatile impurity elements contained in the metallic silicon. A metal silicon purification method characterized in that the metal silicon is dissolved by heating. Furthermore, the present invention is a method for refining metallic silicon, which comprises applying an electromagnetic field to metallic silicon in a molten state instead of the high-frequency induction heating.

[0010] In addition, the present invention is a method for purifying metallic silicon, characterized in that an inert gas is blown into metallic silicon in a molten state instead of the high-frequency induction heating. In addition, the present invention is characterized in that the refining vessel is made of a water-cooled copper or graphite vessel, or high-frequency induction heating is performed by a melting method using a cold crucible, and a molten state is formed in the crucible. This is also a method for refining metallic silicon, which comprises floating certain metallic silicon.

In the present invention, since the easily volatile impurity element of the metal silicon is removed by the above-described structure, the temperature of the surface of the molten metal is increased and the stirring of the molten metal is promoted, so that the vaporization rate is increased. As a result, it is possible to achieve the target purity in a shorter time than before with a low running cost, thereby improving the productivity of silicon for solar cells and reducing the manufacturing cost.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The contents of the present invention will be described below with reference to FIGS. FIG. 1 shows an example of an apparatus for carrying out a method for purifying metallic silicon according to the present invention. First, it is a divided type water-cooled copper or graphite smelting vessel 4 arranged in the decompression chamber 3, an induction heating coil 5 arranged therearound, and electrodes provided above and at the bottom of the vessel 4. It is formed with. According to the present invention, the metal silicon 1 is first melted by the arc 8 generated between the upper electrode 6 and the furnace bottom electrode 7 using such an apparatus. Then, the molten state is maintained for a certain period of time by the arc 8 and heating by high-frequency induction. Meanwhile, the surface of the molten metal 1 is maintained at a high temperature by the arc 8 and is stirred by high frequency induction, so that P,
It promotes the diffusion of Al and Ca and increases the contact area (reaction area) with the atmosphere. As a result, the vaporization rate of easily volatile impurity elements such as P, Al, and Ca contained in the metal silicon 1 can be increased as compared with the case of melting the electron beam 2. Normally, the temperature of the molten metal silicon 1 is 1500 to 1600 ° C., and the pressure in the decompression chamber 3 is 10 ⁻³ torr or less. However, the temperature of the molten metal surface is considered to be higher than the above.

Next, in the second invention, as shown in FIG. 2, a laser beam 15 emitted from a laser oscillator 14 disposed above the refining vessel 4 is used instead of the arc. . Then, the third invention is carried out by the apparatus shown in FIG. It is provided with an electromagnetic coil 9 for applying a magnetic field instead of heating by the high frequency induction. In this case, only the arc 8 is heated, but the molten metal 1 is stirred by the electromagnetic field, and the vaporization rate of the easily volatile impurity element similarly increases.

The fourth invention can be implemented by the apparatus shown in FIG. That is, an inert gas 10 such as argon or helium is blown into the molten metal 1 instead of the heating by the high frequency induction. In FIG. 4, the gas 10 is blown through a so-called porous plug 11 provided at the bottom of the refining vessel 4. However, the lance may be immersed in the molten metal 1, but is not limited thereto. Absent.

According to a fifth aspect of the present invention, as shown in FIG. 5A, a smelting vessel 4 used in the first aspect of the present invention is a copper water-cooled crucible 12 (commonly called cold crucible) divided into segments. The metal silicon 1 is melted without being brought into contact with the container wall while being levitated by magnetic force (see FIG. 5B). As a result, the metal silicon 1 can remove impurity elements while avoiding contamination from the refining vessel 4. In the fifth aspect of the present invention, the initial melting of the metal silicon 1 and the heating of the surface of the molten metal 1 are performed by the arc 8 or the laser beam 15.

Further, in the sixth aspect of the present invention, the refining vessel 4 used for each of the above is made of water-cooled copper or graphite, whereby contamination of the molten metal from the refining vessel 4 is prevented, and long-term operation is possible. Becomes possible.

[0017]

EXAMPLES Ingots of commercially available metallic silicon 1 were melted in vacuum using the apparatuses shown in FIGS. In Table 1, I
CP (Inductively coupled pl)
asma) shows the content of impurity elements before dissolution analyzed by emission spectroscopy.

[0018]

[Table 1]

Example 1 3 kg of the above metallic silicon
Was charged into the refining vessel 4 shown in FIG. 1 in a reduced-pressure chamber 3 at a pressure of 10 ⁻⁴ torr, and a current of 200 amperes was passed between the upper electrode 6 and the furnace bottom electrode 7 to be melted by the arc 8 having an output of 20 kW. . Thereafter, the temperature inside the molten metal 1 was maintained at 1700 ° C. by high-frequency induction heating at an output of 20 kW (total output of 40 kW).
After a lapse of 30 minutes, an analysis sample was taken from the molten metal 1 and P, Al, and Ca, which are volatile impurity elements, were analyzed by the IPC method. As a result, the target value was reached. Example 2 3 kg of metallic silicon was melted in the apparatus shown in FIG. That is, a laser beam 15 generated by a CO ₂ gas from a laser oscillating device 14 with an output of 20 kW was applied to the metal silicon to dissolve the metal silicon, and then heating by high-frequency induction with an output of 20 kW was also applied. The laser beam 15 scans the surface of the molten metal horizontally by using a stepping motor (not shown) so as to prevent the splash of silicon.

After dissolution for 30 minutes, a sample was taken and the ICP
Was analyzed by the method. (Embodiment 3) Using the apparatus shown in FIG. 3, metal silicon 1 was melted with an arc 8 having an output of 40 kW under the same conditions as in Embodiment 1. At that time, an electromagnetic field of about 100 Oersted was applied to the molten metal 1. After a lapse of 30 minutes, a sample was collected and analyzed in the same manner as described above. (Embodiment 4) Using the apparatus of FIG.
The metal silicon 1 was melted by the kW arc 8. After being in a molten state, argon gas 10 was blown in at a flow rate of 0.1 Nl / min from a porous plug 11 arranged at the bottom of the refining vessel 4 and melted for 30 minutes in that state. (Embodiment 5) Using the apparatus shown in FIG.
Dissolved by the crucible method. Thereafter, the surface of the molten metal was heated by the arc 8 having an output of 20 kW. After dissolution for 30 minutes, the molten metal was analyzed. (Comparative Example) In the apparatus shown in FIG. 6, the same ingot of metallic silicon 1 as in the example was charged into a graphite refining vessel 4 and melted by the electron beam 2.

The results of the vacuum refining are evaluated based on the time required for the refining and the content of impurity elements. The results are collectively shown in Table 2.

[0022]

[Table 2]

It is clear from Table 2 that the performance of the present invention is much better than the case where the conventional electron beam 2 is used. The melting apparatus used in the present invention is much cheaper than the electron beam melting apparatus.

[0024]

As described above, according to the present invention, metal silicon can be stably manufactured in a short time at a running cost lower than the conventional one. As a result, the cost of refining metallic silicon has been reduced, and inexpensive silicon substrates for solar cells have become available.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of an apparatus for performing a method for producing metal silicon according to a first invention.

FIG. 2 is a longitudinal sectional view of an apparatus for performing a method for producing metal silicon according to a second invention.

FIG. 3 is a longitudinal sectional view of an apparatus for performing a method for producing metal silicon according to a third invention.

FIG. 4 is a longitudinal sectional view of an apparatus for performing a method for producing metal silicon according to a fourth invention.

FIG. 5 is a view of a refining vessel of an apparatus for performing the method for producing metal silicon according to the fifth invention, (a) is a perspective view,
(B) is a longitudinal sectional view.

FIG. 6 is a view showing a conventional vacuum refining apparatus for performing electron beam melting.

FIG. 7 is a flow chart showing a conventional method for producing silicon for solar cells.

FIG. 8 is a flow chart showing a method for manufacturing solar cell silicon by a metallurgical process proposed by the applicant.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Metal silicon (molten metal) 2 Electron beam 3 Decompression chamber 4 Refining vessel 5 Induction heating coil 6 Upper electrode 7 Bottom electrode 8 Arc 9 Electromagnetic coil 10 Inert gas (argon gas) 11 Porous plug 12 Water-cooled crucible (cold Crucible) 13 electron gun 14 laser oscillation device 15 laser beam

 ──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Yasuhiko Sakaguchi 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Engineering Co., Ltd. (72) Inventor Yoshihide Kato 1-Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Technical Research In-house (72) Inventor Kenshi Saito 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Corporation Technical Research Institute

Claims (6)

[Claims] 1. An arc generated between electrodes and a high-frequency wave formed by dissolving metallic silicon in a refining vessel disposed in a decompression chamber and vaporizing and removing volatile impurity elements contained in the metallic silicon. A method for purifying metallic silicon, characterized by heating and melting by induction. 2. Dissolving metallic silicon in a smelting vessel arranged in a decompression chamber and evaporating and removing volatile impurities contained in the metallic silicon by heating the metallic silicon with a laser and high-frequency induction. A method for purifying metallic silicon, characterized by dissolving. 3. The method for purifying metallic silicon according to claim 1, wherein an electromagnetic field is applied to the metallic silicon in a molten state instead of the high-frequency induction heating. 4. The method for purifying metal silicon according to claim 1, wherein an inert gas is blown into the metal silicon in a molten state instead of the high-frequency induction heating. 5. The method according to claim 1, wherein the high-frequency induction heating is performed by a melting method using a cold crucible, and the metal silicon in a molten state is floated in the crucible. The method for purifying metallic silicon according to the above. 6. The method for purifying metallic silicon according to claim 1, wherein the refining vessel is a vessel made of water-cooled copper or graphite.