JPH06206719A - Purification of metal silicon - Google Patents

Abstract

PURPOSE:To simultaneously remove boron and phosphorus in the same process and provide pure metal silicon at a low purification cost by melting metal silicon containing impurities, inserting electrodes in the molten metallic silicon bath, and subsequently applying a direct electric current to the electrodes. CONSTITUTION:Silicon for producing solar batteries is purified by the below- described method. Namely, metallic silicon 1 containing impurities is melted, and electrodes 4 are inserted into the molten metallic silicon 1. A direct electric current is applied to the electrodes from a direct current electric source 6. Thereby, the impurities are electrically deposited and removed. It is preferable to coat the surfaces of the electrodes 4 with the powder of an oxide [e.g. silica (SiO2)], because the impurities gathered by their electrophoresis are reacted with the oxide and the formed compounds are evaporated off from the surface of the molten silicon to further improve the purification effect.

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 silicon used for producing solar cells.

[0002]

2. Description of the Related Art Impurity elements in silicon used for the production of solar cells need to have a low concentration of 1 ppm or less in order to ensure the required semiconductor characteristics for solar cells.
For this reason, currently excessively refined silicon for semiconductor production is used as a raw material for solar cell production. But,
In a solar cell that requires a large amount of silicon per unit as compared with a semiconductor such as an LSI or an IC, such a silicon raw material is too expensive, which increases the manufacturing cost of the solar cell and hinders its widespread use.

In order to inexpensively manufacture silicon for solar cells, a method of refining inexpensive metal silicon for metallurgy as a raw material has been proposed, but phosphorus and boron in silicon are the most difficult elements to remove. In addition, it is the most important element that determines the conductivity type of silicon depending on the content. Regarding boron, for example, JP-A-4-228414
As disclosed in the publication, there is a technique for efficiently removing boron by a method of holding molten silicon in a container containing silica or silica as a main component and injecting a plasma gas jet to the molten silicon.

Regarding phosphorus, an acid extraction method, a flux melting method, a vacuum melting method and the like have been reported, but no measures have been established as a practical technique.

[0005]

[Patent Document 1] Japanese Patent Application Laid-Open No. 4-22841
In the method disclosed in Japanese Patent Publication No. 4, the removal reaction of boron proceeds only at the plasma jet irradiation part on the surface of the silicon bath, and therefore there is a limit to the reduction of processing time to remove it to a low level of 1 ppmw or less. Therefore, it is desired to develop a method for removing boron that can be treated in a shorter time.

Regarding the phosphorus, in the acid extraction method, phosphorus is precipitated at the crystal grain boundaries at the time of solidification after melting of silicon, and then crushed at the grain boundaries, and this is extracted and washed with an acid. There is a problem that an acid is required and the cost becomes high. In the flux melting method, a high-purity flux of the same amount as that of silicon to be purified is required to remove phosphorus to 1 ppmw or less, which is very expensive. Further, in the vacuum melting method, phosphorus having a relatively high vapor pressure is removed by evaporation in a vacuum atmosphere. However, since silicon is vaporized and reduced together with phosphorus, the yield is low, resulting in high purification cost.

An object of the present invention is to solve the above problems and to provide a method for producing metallic silicon at a low refining cost by simultaneously removing boron and phosphorus in the same step.

[0008]

Means for Solving the Problems The present invention relates to a method for purifying solar cell silicon, in which metallic silicon containing an impurity element is melted, an electrode is inserted into the molten metal and a direct current is passed to deposit the impurities. It is a method for refining metallic silicon, which is characterized in that it is removed. In this method, it is preferable to apply an oxide powder to the surface of the electrode because it combines with impurities to facilitate the removal of impurities.

[0009]

Operation Pure silicon is an electrically insulating element, but it has the characteristic of being a semiconductor in which a current flows in proportion to its concentration when a doping element such as boron or phosphorus is added. Metallic silicon produced by metallurgical methods includes
Since it contains various impurity elements due to its raw material and manufacturing process, it has an electric conductivity close to that of a metal so as to show an electric resistance of 0.1 Ωcm or less in a specific resistance value. According to the present invention, the metallic silicon is heated and melted and held in the container, and the electrodes serving as the anode and the cathode are charged at a distance from each other into the molten metal so that a direct current is caused to flow. A doping element such as boron or phosphorus that makes silicon electrically conductive is electrophoresed by a current generated between the anode and the cathode and attracted to the vicinity of the electrode. In this way, the impurity element in silicon is sufficiently gathered in the vicinity of the electrode and the impurity concentration in silicon is lowered, so that the conductivity of silicon is lowered and it becomes difficult for current to flow. At this point, the electrode is cooled to solidify the silicon in the vicinity of the electrode, and the electrode is pulled out of the molten silicon, whereby the silicon enriched with impurities can be removed. Therefore, the molten silicon can be refined.

If oxide powder is applied to the surface of the electrode, the impurities collected by electrophoresis react with the oxide to form a compound, which is evaporated and removed from the surface of the molten silicon, further improving the purification effect. Be improved. Here, the oxide is silicon such as SiO ₂ , SiO, C ₆ H ₁₂ O ₆ , Na ₂ CO ₃ or the like, carbon, hydrogen or a compound of sodium and oxygen, which can be removed even if dissolved in silicon. Those composed of elements are desirable.

[0011]

EXAMPLES The present invention was carried out using the apparatus shown in FIG.
Silicon is put in the quartz crucible 3 and heated and melted by the heater 2. Then the electrode 4 is inserted into the molten silicon 1,
A DC voltage is applied from the DC power supply 6. When the electric resistance increases, the electrode cooler 7 rapidly cools the electrode 4 to partially fix the periphery of the electrode and pull out the electrode. It is more preferable to coat silica (SiO ₂ ) 5 or the like on the surface of the electrode.

The metallic silicon used is B: 30 pp
m, P: containing 30 ppm, this silicon 500
g was placed in a quartz crucible 3 having an inner diameter of 70 mm, melted in an Ar atmosphere, graphite electrodes 4 were inserted in parallel at intervals of 30 mm at 1500 ° C., and a current of 100 mA was applied between the electrodes to produce 1 g.
Held for hours. After that, the electrode 4 was rapidly cooled by the electrode cooler 7 and the electrode was pulled out.

In Example 1, a graphite rod made of a rubber press which has been subjected to high-purity treatment as an electrode (φ15 mm × insertion depth 30 m)
m) was used. FIG. 2 shows changes in the boron and phosphorus contents. In Example 1, both boron and phosphorus were removed to about 5 ppm, and the yield was as high as 90%. In Example 2, the ratio of SiO ₂ : C ₆ H ₁₂ O ₆ : H ₂ O was 20: 2: on the surface of the same electrode as the graphite rod of Example 1.
The result is shown in FIG. 3 using an electrode coated with the oxide of No. 1 having a thickness of 2 mm. In Example 2, the removal of boron is faster than in Example 1, and it can be seen that application of silica powder to the graphite electrode is effective. However, the yield was 85%, which is lower than that in Example 1.

In Comparative Example 1, 500 g of the above-mentioned metallic silicon was melted and held in a quartz crucible having an inner diameter of 70 mm, and a plasma jet gas containing 5% of water vapor was jetted to the surface of the molten silicon and held for 1 hour. Is shown in FIG. Comparative Example 2 is a rubber press-molded graphite crucible having an inner diameter of 70 mm, which has been subjected to high-purity treatment, and the above-mentioned metallic silicon 500.
g was melted and held at 1500 ° C and the atmospheric pressure was set to 10 ^-4 To.
The pressure was reduced to rr and held for 1 hour. The results are shown in FIG. In the silicon production by the conventional method, boron was removed in Comparative Example 1, but phosphorus was hardly removed. In Comparative Example 2, phosphorus was removed but boron was not removed. Further, the yield of silicon was as low as 80% in Comparative Example 1 and 72% in Comparative Example 2. As described above, in Comparative Example 1, only boron is removed, and in Comparative Example 2, only phosphorus is removed, but as described above, both can be removed in Examples.

As Examples 3, 4, and 5, the same graphite rods as in Example 1 were subjected to the same conditions as in Example 1, except that various oxides coated with electrodes were used. The concentrations of these oxides and boron and phosphorus after the treatment are shown in Table 1. Also in Examples 3, 4, and 5, excellent results were obtained.

[0016]

[Table 1]

[0017]

Industrial Applicability According to the present invention, it becomes possible to remove boron and phosphorus, which are particularly difficult to remove, by using inexpensive metal silicon for metallurgy as a starting material by a simple method. Therefore, Fe, Al, Ti, and By combining with a method for removing Ca, silicon for solar cells can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a silicon generator used for carrying out the method of the present invention.

FIG. 2 is a graph showing changes over time in boron and phosphorus concentrations in silicon of Comparative Example 1.

FIG. 3 is a graph showing changes over time in boron and phosphorus concentrations in silicon of Comparative Example 2.

FIG. 4 is a graph showing changes over time in boron and phosphorus concentrations in silicon of Example 1.

5 is a graph showing changes over time in boron and phosphorus concentrations in silicon in Example 2. FIG.

[Explanation of symbols]

 1 Molten Silicon 2 Heater 3 Crucible 4 Electrode 5 Silica 6 DC Power Supply 7 Electrode Cooler

Claims (2)

[Claims] 1. A method for purifying solar cell silicon, comprising:
A method for purifying metallic silicon, characterized in that, after melting metallic silicon containing an impurity element, an electrode is inserted into the molten metal and a direct current is passed to deposit and remove impurities. 2. The method for purifying metal silicon according to claim 1, wherein the surface of the electrode is coated with oxide powder.