JP3138003B2 - Method and apparatus for purifying silicon - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for producing high-purity silicon which can be used as a solar cell material.

[0002]

2. Description of the Related Art As high-purity silicon used in solar cells, for example, P-type silicon having a specific resistance of 0.1 .OMEGA.cm or more is used. In such silicon, impurities contained therein are removed to the order of ppm. There is a need to. Conventionally, various techniques have been studied, but boron as a doping element and carbon as a neutral element are the elements which are most difficult to remove.

As a method for removing boron and carbon in silicon, Japanese Patent Application Laid-Open No. 63-218506 discloses that boron can be removed by melting silicon under thermal plasma obtained by high-frequency excitation. ing. In this method, in dissolving silicon with plasma excited by high frequency, in the first step, 1 to 10
It was treated with 0% H ₂ and 99 to 0% mixed gas of Ar, in the second step from 0.005 to 0.05 percent oxygen and 1 to 99.99
It is described that the plasma treatment is performed using a mixed gas with Ar containing 5% of H ₂ as a plasma generation gas.

[0004] In such a method, (a) dissolving silicon with plasma having a poor heat utilization efficiency;
(2) When melted by plasma, the area of the melted silicon is limited to a relatively small area, resulting in poor productivity and use for solar cells. (C) The silicon temperature during local refining is too high, so there is a lot of loss (scattering and evaporation) of silicon during refining, and the oxygen concentration in the plasma gas cannot be increased. There were drawbacks such as.

[0005] The present inventors have responded to this by applying to Japanese Patent Application No.
As shown in Japanese Patent Application No. 138266 and Japanese Patent Application No. 138268/1990, molten silicon is held in a container having a heating device for applying heat from the outside of the container, and a plasma jet gas is injected onto the molten metal surface to form boron. And a method by which carbon is removed. However, it has been desired to develop a method and an apparatus for removing boron and carbon, which have a shorter processing time than this technology and have a low production cost in terms of silicon yield and power consumption.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method and an apparatus for efficiently removing boron and carbon in order to solve the above-mentioned demands in the prior art.

[0007]

According to the present invention, in order to solve the above-mentioned problems, molten silicon is housed in a container having a heating device for applying heat from the outside of the container, and the molten silicon is emitted from a plasma torch on the molten metal surface. In the method of purifying silicon by injecting plasma jet gas, a method in which a plasma torch is moved above the surface of the molten metal horizontally, and the amount of plasma jet gas injected from a plurality of plasma torches at the same height from the molten metal surface is periodically adjusted. And a method of sequentially moving peaks of the injection amount, and a method of purifying silicon for electrically connecting the plasma torch and the molten silicon in these methods. And a container for containing molten silicon and having a heating device for supplying plasma, and a plasma jet gas jetted toward the molten metal surface In a silicon refining device equipped with a torch, a device in which the plasma torch can move above the surface of the molten metal in the horizontal direction, the injection amount of the plasma jet gas changes periodically, and the peak of the injection amount can move sequentially An apparatus in which a plurality of plasma torches are provided at the same height from the surface of the molten metal,
Another object of the present invention is to provide a silicon purifying apparatus provided with a circuit for electrically connecting a plasma torch and molten silicon in these apparatuses.

[0008]

FIG. 2 is a schematic longitudinal sectional view of a main part of an embodiment of the present invention. In FIG. 2, 3 is a torch rotation device, 31 is a motor, 32 is a rotation shaft, 33 is a rotation direction, 34 is a rotation radius, and the torch 2 makes a circular motion and moves horizontally above the molten metal surface. The plasma gas 26 is Ar, He and / or H ₂
Gas is used. With the above movement of the torch 2,
The molten silicon also moves on the molten silicon surface 51 in the horizontal direction, the agitation of the molten silicon 5 is enhanced, and the removal of boron and carbon that can be organized by the primary reaction of the concentration is promoted.

The plasma gas used here is preferably mixed with an oxidizing gas such as H ₂ O and O ₂ and / or an oxide powder such as a silica powder in a plasma jet at a plasma torch outlet. The gas and oxide powder remove boron and carbon in the molten silicon as an oxide gas in the gas phase. In this case, when the oxygen content in the plasma jet is high, a large amount of slag such as SiO ₂ is generated on the surface of the molten silicon not exposed to the plasma jet, and the slag takes in boron and carbon in the molten silicon. As a result, the plasma jet gas is injected toward the slag by the movement of the torch, and the slag sublimates into the gas phase, thereby efficiently removing boron and carbon, thereby reducing the processing time and reducing the silicon yield. improves.

The number of torches, their rotation speed, etc. are not particularly limited. FIG. 3 is a schematic cross-sectional view of a main part of another embodiment of the present invention, and FIG.
Shown in In FIGS. 3 and 4, 2a, 2b, 2c and 2d are provided at the same height from the molten metal surface, change the injection amount of the plasma jet gas periodically, and sequentially move the peak of the injection amount. In this plasma torch, reference numerals 21a and 21c denote plasma jet gases injected from the plasma torches 2a and 2c, respectively, and reference numerals 52a and 52c denote fire points generated on the molten metal surface 51 by the plasma jet gases 21a and 21c, respectively.

The cathode and anode of each torch are individually connected to a plasma power source as shown in FIG. 1 by an electric circuit (not shown), and reference numeral 35 denotes an arrangement radius of the center of the torch. FIG. 5 shows an example of the relationship between the ratio of the amount of ejected plasma gas and the time in the apparatus shown in FIG. The ratio of the plasma gas ejection amount of each torch is periodically changed by a control device (not shown), and the peak of the ejection amount can be sequentially moved, so that the plasma ejection gas having the peak ejection amount is ejected from the same height. Since the position of the molten metal surface moves with the passage of time, carbon and boron can be efficiently removed as described above.

The number of plasma torches, the cycle of changing the amount of plasma jet gas injected, and the like are appropriately selected. In the above-described method of the present invention, the energy loss (due to torch cooling) is usually about 50% due to the arc flying in the plasma torch. However, when the plasma torch is electrically connected to the molten silicon, the electron beam from the plasma torch cathode is (Arc) passes through the plasma and strikes the silicon bath surface and flows as a current through the molten silicon, so that energy loss is reduced and the current in the silicon enhances the stirring of the bath. The speed at which carbon moves to the flash point 52 is increased, and as a result, the purification time can be reduced.

FIG. 6 is a schematic vertical sectional view of a main part of still another embodiment of the present invention. In the apparatus shown in FIG. 2, a conductive hole 41 is formed at the bottom of the silica crucible 11 and the silicon side electrode 4 is electrically conductive. Hole 4
Reference numeral 42 denotes an electrode terminal, 43 denotes a water cooling jacket, 44 denotes cooling water, and 45 denotes cooling drainage. The water cooling jacket 43 cools and solidifies the silicon in the conductive holes 41, so that the plasma torch 2 and the molten silicon can be electrically connected without leaking the molten silicon 5 from the crucible 11.

FIG. 7 is a schematic vertical sectional view of a main part of a further different embodiment of the present invention, in which a silicon-side electrode 4 is provided in the device shown in FIG. 3 in the same manner as in FIG. The effect is the same as in the device shown in FIG.

[0015]

EXAMPLE 2 kg of silicon was melted in a resistance heating furnace having an inner diameter of 150 mm in an Ar atmosphere using a silica crucible, and a plasma jet gas was injected from above into the molten metal surface.
The plasma gas was Ar 20 liter / min, the plasma added gas was H ₂ O (gas) 1 liter / min, and the plasma input power was 20 kW.

The change in the concentration of boron and carbon in the molten silicon can be obtained by collecting molten silicon in a quartz tube at regular intervals, and the boron can be collected by an ICP (Inductively Coupled).
dPlasma: inductively coupled plasma) emission spectroscopy and carbon by combustion. Comparative Example The device shown in FIG. 1 was used. Example 1 Using the apparatus according to FIG. 2, the turning radius of the torch was 40 mm,
The number of revolutions was 4 times / min. Example 2 Example 2 was the same as Example 1 except that the number of rotations of the torch was 8 times / minute. Example 3 Example 3 was the same as Example 1 except that the number of rotations of the torch was 16 times / minute. Example 4 Using the apparatus shown in FIGS. 3 and 4, the arrangement radius of the center of the torch was set to 40 mm, and the cycle of the peak of the jet amount of the plasma jet gas was set to 8 times / min. FIG. 5 shows the ratio of the ejection amount of the plasma jet gas from each torch. Example 5 The peak cycle of the plasma jet gas ejection amount was set to 16 times /
Other than the above, it was the same as Example 4. Example 6 Example 6 was the same as Example 1 except that the apparatus shown in FIG. 6 was used. Example 7 Example 7 was the same as Example 6, except that the number of rotations of the torch was 8 times / minute. Example 8 Example 8 was the same as Example 6, except that the number of rotations of the torch was 16 times / min. Example 9 Example 9 was the same as Example 4 except that the apparatus shown in FIG. 7 was used. Example 10 Example 9 was the same as Example 9 except that the cycle of the peak of the ejection amount of the plasma jet gas was set to 16 times / min. Tables 1 and 2 show the test results of Comparative Examples and Examples.

The reaction rate coefficients k ′ and k ″ are d [B] / dt = − since B and C are removed by the primary reaction of the concentration.
k ′ [B] and d [C] / dt = −k ″ [C].

[0018]

[Table 1]

[0019]

[Table 2]

As can be seen from the table, it can be seen that both the boron and carbon were removed faster in the example than in the comparative example. It can also be seen that the removal speed of both boron and carbon increases as the rotation speed of the torch or the peak moving speed of the plasma jet gas ejection amount is increased, and further, as the torch is electrically connected to the molten silicon. Regarding the silicon yield, Examples 1 to 5 were 94%, 92% and 93%, respectively.
%, 96%, and 95%.
%, 94%, 92%, 95%, and 93%.
%Met. Therefore, in order to remove boron and carbon to the same level, the process according to the present invention requires a shorter time, so that boron and carbon can be removed at a higher yield.

[0021]

According to the present invention, the processing time for removing boron and carbon is reduced, and the yield of silicon can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view of a main part of a conventional apparatus.

FIG. 2 is a schematic longitudinal sectional view of a main part of an embodiment of the present invention.

FIG. 3 is a schematic longitudinal sectional view of a main part of another embodiment of the present invention.

FIG. 4 is a sectional view taken along the line AA in FIG. 3;

FIG. 5 is a graph showing a relationship between a plasma jet gas ejection amount ratio and time.

FIG. 6 is a schematic longitudinal sectional view of a main part of still another embodiment of the present invention.

FIG. 7 is a schematic vertical sectional view of a main part of still another embodiment of the present invention.

[Explanation of symbols]

 2, 2a, 2b, 2c, 2d Plasma torch 3 Torch rotating device 4 Silicon side electrode 5 Molten silicon 6 Resistance heating furnace 11 Silica crucible 21, 21a, 21c Plasma jet gas 22 Plasma cathode 23 Plasma anode 24 Plasma power supply 25 Electric resistance 26 Plasma gas 27 Oxidizing substance 31 Motor 32 Rotation axis 33 Rotation direction 34 Rotation radius of torch center 35 Arrangement radius of torch center 41 Conducting hole 42 Electrode terminal 43 Water cooling jacket 44 Cooling water 45 Cooling drainage 51 Molten silicon surface 52, 52a, 52c Fire point caused by plasma jet

──────────────────────────────────────────────────の Continued on front page (56) References Baba et al "REM OVAL OF BORON FROM MOLTEN SILICON BY argon-plasma mixed with water vapor" Tenth E. C. Photovoltaic Solar Energy Conference (April 12, 1991) p. 286-289 (58) Field surveyed (Int. Cl. ⁷ , DB name) C01B 33/00-33/039 CA (STN) JICST file (JOIS)

Claims (6)

(57) [Claims] 1. A method for purifying silicon by storing molten silicon in a container provided with a heating device for applying heat from the outside of the container and injecting a plasma jet gas emitted from a plasma torch onto a molten metal surface. A method for purifying silicon, comprising: moving a plasma torch horizontally above a surface of a molten metal. 2. A silicon refining apparatus comprising: a container having a heating device for applying heat from the outside of the container and containing molten silicon; and a plasma torch for injecting a plasma jet gas toward the surface of the molten silicon. An apparatus for refining silicon, wherein the plasma torch is movable horizontally above the surface of the molten metal. 3. A method for purifying silicon by storing molten silicon in a container provided with a heating device for applying heat from the outside of the container and injecting a plasma jet gas emitted from a plasma torch onto a surface of the molten silicon. A method for purifying silicon, comprising: periodically changing an injection amount of a plasma jet gas from a plurality of plasma torches having the same height from a surface, and sequentially moving a peak of the injection amount. 4. A silicon refining apparatus comprising: a container for storing molten silicon having a heating device for applying heat from the outside of the container; and a plasma torch for injecting a plasma jet gas toward the surface of the molten silicon. A plurality of plasma torches which periodically change an injection amount of a plasma jet gas and whose peak of the injection amount is sequentially movable are provided at the same height from a molten metal surface. 5. The method for purifying silicon according to claim 1, wherein the plasma torch and the molten silicon are electrically connected. 6. The silicon refining apparatus according to claim 2, further comprising a circuit for electrically connecting the plasma torch and the molten silicon.