JPH0748114A - Method for purifying silicon - Google Patents

Abstract

PURPOSE:To efficiently remove boron contained in silicon by disposing a plasma- generating device in the bottom of a container holding the melted silicon and blowing a plasma gas mixed with a specific amount of steam into the melted silicon from the device. CONSTITUTION:A plasma-generating device 2 equipped with an anode 22, a cathode 23, etc., is disposed in the bottom of a container 3 (e.g. quartz crucible) for purifying silicon containing impurities. Solid silicon is supplied into the container 3 and subsequently melted with a plasma 21 generated from a plasma- generating device 2. <=10vol.% of steam is supplied into the plasma 21 from a steam-adding pipe 25. The high temperature steam-added plasma 13 is blown into the melted silicon 12 to remove impurities contained in the melted silicon 12. Thus, the contact area of the plasma with the melted silicon is widened, and the plasma gas 13 vigorously stirs the melted silicon 12, thereby enhancing the probability of the reaction of the plasma 13 with boron contained in the melted silicon 12.

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 high-purity silicon that can be used as a raw material for solar cells and the like.

[0002]

2. Description of the Related Art As a raw material used for solar cells, generally, high-purity silicon having a specific resistance value of 0.1 Ωcm or more and p-type conductivity is used. Impurities contained in silicon are in the ppm order. Must have been removed. On the other hand, various methods have been studied so far, but it is known that boron as an impurity is the most difficult element to remove.

For example, in Japanese Patent Laid-Open No. 63-218506, silicon is melted by using thermal plasma excited by high frequency,
A method of removing boron in silicon is disclosed.
In this method, a part of solid silicon is irradiated with plasma generated by high-frequency excitation of a mixed gas of hydrogen and oxygen to cause zone melting, and boron is removed at the melted part. However, in this method, part of the solid silicon is melted only by the heat of the plasma and the purification is performed, resulting in poor heat utilization efficiency and low productivity, and the temperature of the silicon melting part is excessive. Therefore, there is a drawback that the loss of silicon due to evaporation increases.

Further, since a mixed gas of hydrogen and oxygen is excited at a high frequency, the degree of activation of oxygen is high, which is a condition that an oxide film is easily formed on the surface of silicon. Therefore, the upper limit of the concentration of oxygen added to the plasma gas is as low as 0.05%, and it is difficult to improve the removal rate of boron by oxidation.

[0005]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems, and even when silicon having a high boron content is used as a raw material, it can be efficiently and sufficiently refined to a concentration at which it can be used as silicon for solar cells. It is intended to provide a method for removing boron.

[0006]

That is, according to the present invention, a plasma generator is provided at the bottom of a container for holding molten silicon, and a plasma gas added with 10 vol% or less of steam from the device is blown into the silicon. It is a method of purifying silicon that is a feature, and as a container for holding molten silicon, there is no particular problem as long as it can hold silicon in a temperature range of 1600 to 1700 ° C, but a quartz crucible, a silica stamp material or a water-cooled copper crucible is advantageous. Can be used.

[0007]

As described above, according to the present invention, the plasma generator is provided at the bottom of the container for holding the molten silicon, and the plasma gas to which 10 vol% or less of steam is added from the device is introduced into the molten silicon from the bottom of the container. Since it is blown in, the area where the high temperature steam-added plasma contacts the silicon becomes wider, the probability that the boron present in the molten silicon and the steam-added plasma will react becomes higher, and the evaporation removal rate of boron by oxidation can be improved. Became.

Further, since the plasma gas blown from the bottom of the container violently agitates the molten silicon, the boron dissolved in the portion where the steam-added plasma and the silicon come into contact with each other moves rapidly, and the boron and the steam-added plasma existing in the molten silicon. Has a higher probability of reacting with, and the evaporation removal rate of boron due to oxidation can be improved. Here, if the water vapor concentration to be added exceeds 10%, the production of SiO ₂ is rapidly increased by the oxidation of silicon, and the purification yield of silicon is significantly deteriorated. Therefore, in the present invention, the water vapor addition concentration is set to 10% or less. did.

Further, when a quartz crucible, a silica stamp material or a water-cooled copper crucible is used as a container for holding the molten silicon, not only boron can be removed but also dissolved carbon can be removed and contamination with other impurities can be prevented.

[0010]

1 and 3 are longitudinal sectional views showing an example of an apparatus used for carrying out the present invention. 150kW inner diameter with a 30kW class non-transfer type argon plasma generator 2 installed at the bottom
mm plasma quartz crucible [Figs. 1 and 3 (a)], silica stamp material [Fig. 3 (b)], water-cooled copper crucible [Fig. 3 (c)] or graphite crucible [Fig. 3 (d)] After that, 2 kg of solid metallic silicon was charged under an argon atmosphere, and 5% of water vapor was supplied from the water vapor addition pipe 25 into the plasma at the time when the entire amount of silicon melted down.

FIG. 2 is a longitudinal sectional view showing an example of a conventional type silicon refining apparatus used for comparison of the present invention. The non-transfer type argon plasma generator 2 is the same as that of the above embodiment. Put 2 kg of solid metal silicon into a quartz crucible with an inner diameter of 150 mm, a silica stamp material, a water-cooled copper crucible or a graphite crucible, and blow argon gas onto the silicon from a height of 40 mm above the crucible to melt it, and the entire amount of silicon will melt down. At that time, 5% steam was supplied from the steam addition tube 25 into the plasma.

In each of the examples and comparative examples, the plasma working gas was supplied with argon gas of 19 liters / minute, the plasma additive gas was supplied with water vapor of 1 liters / minute (5.0%), and the input power to the plasma was 20 kW. The processing time from the start of steam supply to the end of supply is 2 hours. In addition, in order to observe the effect of the concentration of water vapor added, using a quartz crucible with an inner diameter of 150 mm in the refining equipment of Fig. 1, 2 kg of solid metallic silicon was charged in the argon atmosphere after plasma generation, and the total amount of silicon melted down. At that time, the water vapor was supplied into the plasma from the water vapor addition pipe 25 while changing the water vapor concentration in the range of 0 to 15%, and changes in the removal rate of boron and carbon and changes in the silicon yield were investigated.

Concentration changes of boron and carbon were measured by ICP emission spectroscopy after the silicon was sucked and sampled by a quartz tube immediately after the silicon melted down, 1 hour after the start of the steam addition, and after the end of the steam supply. , Carbon was also determined by activation analysis. The results of removing boron in silicon according to Examples and Comparative Examples are shown in Table 1, the results of removing carbon and the yield of silicon are shown in Table 2, the results of changing the concentration of added steam are shown in Table 3, and are shown in FIG.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

[Table 3]

The reaction rate coefficients k _B and k c in the table are d [B] / dt = -k _B [B] and d [C] because boron and carbon are removed by the first-order reaction of the concentration in silicon. / Dt =
It is a coefficient obtained from −k _C [C]. As can be seen from Table 1, in each case, boron is removed about twice as fast in the example as compared with the comparative example. In addition, it can be seen from Table 2 that when the quartz crucible and the water-cooled copper crucible are used, carbon can be removed at the same time as boron, and in this case as well, the removal rate is about twice as fast in the example.

From Table 2, it can be seen that no difference in silicon yield is observed between the example and the comparative example. Since the yield decreases with the extension of the treatment time, when removing boron to the same concentration, the present invention capable of removing boron in a short treatment results in a higher yield. The yield is highest when a water-cooled copper crucible is used. However, since the silicon in the vicinity of the crucible is not dissolved, purification of this portion cannot be expected.

Further, as shown in Table 3 and FIG. 4, 10%
If steam is added beyond the temperature range, the oxidation of silicon will become more vigorous and the silicon yield will drop sharply due to the large amount of SiO ₂ produced.Since the unevaporated SiO ₂ film will remain in the silicon melt, it will be removed by filtration after purification. It is necessary to remove this by means. In the above embodiments, the case of non-transfer type argon plasma has been described, but the present invention is not limited to this, and a transfer type plasma device or a device using helium, hydrogen, or nitrogen as a working gas or those You may use the mixed gas of.

[0020]

According to the present invention, it becomes possible to easily and efficiently remove boron that is difficult to remove from silicon. Further, if the crucible material is selected, the dissolved carbon can be removed at the same time as boron, so that the carbon removal step in the metallurgical production process of silicon for solar cells can be simplified or omitted.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing an example of an apparatus used for carrying out the present invention.

FIG. 2 is a vertical sectional view showing an example of a conventional device.

FIG. 3 is a vertical cross-sectional view showing several examples of devices used when carrying out the present invention.

FIG. 4 is a graph showing the relationship between the concentration of water vapor added, reaction rate coefficient, and Si yield.

[Explanation of symbols]

 1 Silicon 11 Solid Silicon 12 Molten Silicon 13 Plasma Gas Bubbles 2 Plasma Torch 21 Steam Addition Plasma Flow 22 Plasma Torch Anode 23 Plasma Torch Cathode 24 Plasma Working Gas Supply Pipe 25 Steam Addition Pipe 3 Silicon Holding Container 31 Quartz Crucible 32 Silica Stamping Material 33 Water-cooled copper crucible 34 Graphite crucible

Claims (2)

[Claims] 1. A method for purifying silicon, characterized in that a plasma generator is provided at the bottom of a container holding molten silicon, and a plasma gas to which 10 vol% or less of steam is added is blown into the silicon from the device. 2. The method for purifying silicon according to claim 1, wherein a quartz crucible, a silica stamp material, or a water-cooled copper crucible is used as the container for holding the molten silicon.