JPH05262512A - Silicon refining method - Google Patents

Abstract

(57) [Abstract] [Purpose] Boron, iron,
A refining method for inexpensively producing high-purity silicon for a solar cell having a low aluminum content in one step. [Composition] Irradiating plasma gas with water vapor and hydrogen chloride added from above molten metal silicon, or adding sodium chloride powder to the plasma gas if necessary,
If necessary, the treated silicon is unidirectionally solidified.

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 such as B, P, C, Fe, Al, and Ti in silicon used for manufacturing solar cells must have a low concentration of 1 ppmw or less in order to secure required semiconductor characteristics. .. Therefore, as the current silicon as a raw material for solar cell production, excessively refined silicon is used for semiconductor production. However, since silicon is expensive for solar cells in which the amount of silicon used per unit is larger than that for semiconductors, this has hindered the popularization of solar cells. In order to manufacture silicon for solar cells at low cost, a method of refining it using inexpensive metal silicon for metallurgy as a raw material has been proposed, but it is difficult to simultaneously remove boron, iron, and aluminum in silicon. The current situation is that several purification steps are carried out in sequence.

Regarding the removal of boron, for example, Japanese Patent Laid-Open No. 63-63
-218506 discloses a method of irradiating molten silicon held in a silica container with high temperature plasma so that oxygen supplied from the container into silicon reacts with boron to evaporate and remove boron oxide. It is disclosed.
It is also disclosed therein that when oxygen is further added to the plasma gas, the oxygen potential in the silicon bath is increased and the boron removal rate is increased. However, since the vapor pressures of oxides of iron and aluminum are low, there was almost no refining effect on iron and aluminum.

Regarding the removal of iron and aluminum, generally, unidirectional solidification is performed by utilizing the fact that the concentration of liquid phase impurities is higher than the concentration of solid phase impurities of silicon, and the portion where the concentration of iron and aluminum is increased by solidification segregation is cut. , Have eliminated.
However, in this method, since the solidification segregation coefficient of boron was small, it was not possible to increase the concentration in the final solidified portion of silicon, and boron could hardly be removed. Further, since a part of silicon in which impurities such as iron and aluminum are segregated to a high concentration is excluded, there is a problem that the purification yield of silicon is reduced.

[0005]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art and provides a refining method for producing high-purity silicon having a low content of boron, iron, and aluminum in one step at low cost. The purpose is that.

[0006]

In order to solve the above problems, the present invention is a method for purifying silicon in which steam and hydrogen chloride are added to plasma gas when a molten silicon surface is irradiated with thermal plasma. Further, it is a silicon refining method in which sodium chloride powder is further added to the plasma gas as the case may be, and a silicon refining method in which the refined molten silicon is subjected to unidirectional solidification treatment as it is, if necessary.

[0007]

In the present invention, the molten silicon is held in silica or a container containing silica as a main component, and the silicon liquid surface is irradiated from above with a thermal plasma of an inert gas containing water vapor and hydrogen chloride. Boron can be easily evaporated as oxides and hydroxides, and iron and aluminum as chlorides.

In the above method, by adding sodium chloride powder to the plasma gas together with water vapor and hydrogen chloride, iron and aluminum can be efficiently removed from silicon containing more iron and aluminum. Further, for silicon containing iron and aluminum at a high concentration, it is possible to easily obtain high-purity silicon by solidification segregation by gradually drawing the molten silicon out of the furnace after the above treatment and unidirectionally solidifying. it can.

According to the present invention, as described above, since steam and hydrogen chloride are supplied to the high temperature portion irradiated with the thermal plasma on the silicon bath surface, boron, oxygen and / or oxygen can be generated at a high temperature of several thousand K, which is difficult to obtain in a general furnace. Alternatively, hydrogen, iron and chlorine, aluminum and chlorine can be reacted, and boron oxide, boron hydroxide, iron chloride and aluminum chloride produced by the reaction can be removed by evaporation at high temperature of thermal plasma. become.

However, when water vapor is added to the plasma in an amount of more than 10%, a SiO ₂ film is formed on the surface of the molten silicon irradiated with the plasma, so that the supply of water vapor and hydrogen chloride to the silicon is hindered. It is not preferable because the reaction for forming boron oxide, boron hydroxide, iron chloride and aluminum chloride is stopped or suppressed. Moreover, if hydrogen chloride is added to the plasma in an amount of more than 5%,
Since a large amount of molten silicon chloride is generated, the chloride of iron and aluminum in the silicon, and the silicon itself is evaporated / reduced as a chloride as it is generated / evaporated and removed, which is not preferable because the productivity is significantly reduced.

For silicon containing more iron and aluminum, sodium chloride powder was added to the plasma gas together with water vapor and hydrogen chloride, so that the high temperature portion of the silicon bath surface irradiated with thermal plasma was heated. The chlorine ion concentration in the supplied plasma gas becomes high, and the chloride generation of iron and aluminum is promoted,
Higher iron and aluminum purification rates.

Further, for silicon containing high concentrations of iron and aluminum, it is preferable to gradually pull out the molten silicon after the above refining to the outside of the furnace at a rate of about 1 mm / min or less to unidirectionally solidify. .. As the drawing speed is slower, the iron and aluminum contained in silicon due to solidification segregation can be segregated to a higher concentration in the final solidification part of silicon. Finally, by cutting and eliminating this segregated portion, iron and aluminum in silicon can be purified to a low concentration.

[0013]

EXAMPLE FIG. 1 is a vertical cross-sectional conceptual view of a silicon refining apparatus used for carrying out the silicon refining method of the present invention.
Here is an example: In FIG. 1, molten silicon 1 is held in a quartz crucible 2 and placed in a graphite cylinder 4 heated by a high frequency heating coil 3. A non-transfer type plasma torch 5 is provided above the molten silicon 1, and a plasma flame 7 is applied to a bath surface 11 of the molten silicon 1 by using an argon gas 6 as a plasma gas. Near the plasma outlet 8 of the plasma torch 5, there is provided a nozzle 12 made of Teflon capable of supplying a mixed gas 9 of water vapor and hydrogen chloride and sodium chloride powder carried by argon gas toward the plasma flame 7. Further, the quartz crucible 2 is placed on a vertically movable crucible table 13 and can be lowered from the inside of the graphite cylinder 4 at a constant speed by a crucible lowering device 14.

The atmosphere 15 that surrounds all of these is maintained in an argon gas atmosphere, and the exhaust gas is discharged from the exhaust port 16 to the outside of the system. Using the above silicon refining apparatus, 3 kg of metallurgical silicon was melted at 1580 ° C. in a quartz crucible by high frequency heating, and a plasma flame was applied to the silicon bath surface at an output of 15 kW. Argon gas 20l / min was used as plasma gas,
0 to 10% of steam and 0 to 5% of hydrogen chloride were added thereto. The contents of boron, iron, and aluminum in the metallurgical silicon A, B, and C used for the raw materials are as shown in Table 1, and the purification time by plasma is 6 hours, and the solidification rate when solidification and purification is 1 mm. The silicon of the cut / excluded portion after solidification at / min is the upper 20% of the ingot height.

Table 2 shows the concentrations of boron, iron and aluminum in the silicon after the silicon purification in Comparative Examples and Examples.

[0016]

[Table 1]

[0017]

[Table 2]

Comparative Examples 1 to 5 Comparative Examples 1 to 3 are the same apparatus as the Examples, and are the results of refining by adding only steam to the plasma gas. Comparative Example 1 is 0% steam addition, and Comparative Example 2 is 5%. Comparative Example 3 is 10%. In Comparative Examples 4 and 5, after melting silicon at 1580 ° C. in the same apparatus as that of Example, only the directional solidification refining was performed without purifying by plasma, and the upper 20% was cut at the ingot height.
This is the result of exclusion.

Examples 1 and 2 Water vapor was applied to the molten silicon bath surface in Example 1 at 5%, Example 2
Is the result of plasma refining with 10% added, hydrogen chloride added with 1% in Example 1 and 5% added in Example 2, and solidified by natural cooling after the treatment. Iron and aluminum that could not be purified in Comparative Examples 1 to 3 could be removed, and boron that could not be purified in Comparative Examples 4 and 5 could be removed.

Examples 3, 4 Water vapor was applied to the surface of the molten silicon bath in Example 3 at 5%, Example 4
Is added 10%, hydrogen chloride is added 1% in Example 3 and 5% in Example 4, and sodium chloride powder is added at a feed rate of 1 g / min in Example 3 and 2 g / min in Example 4. Was added. In each of Examples 3 and 4, the treated silicon is the result of solidification by natural cooling.

Examples 5 and 6 Examples 5 and 6 were treated under the same conditions as Examples 2 and 4, respectively, and then unidirectionally solidified and refined at a solidification rate of 1 mm / min.
This is the result of cutting and removing the upper 20% of the ingot height. In Examples 3 and 4, the removal rates of iron and aluminum in silicon were faster than those in Examples 1 and 2, and in Examples 5 and 6, the concentrations of iron and aluminum in silicon could be reduced to lower levels. ing.

[0022]

INDUSTRIAL APPLICABILITY According to the present invention, a plasma gas added with water vapor and hydrogen chloride is irradiated from above molten metal silicon, and when the iron and aluminum content is high, sodium chloride powder is further added to the plasma gas. Alternatively, since the treated silicon can be unidirectionally solidified, silicon for producing solar cells having a low boron, iron and aluminum content can be produced in one step by using metallic silicon containing a large amount of boron, iron and aluminum as a starting material. It became so. In addition, according to this method, since expensive semiconductor silicon that has been conventionally used for manufacturing a solar cell can be changed from inexpensive metallurgical silicon to purified silicon,
The cost of the entire solar cell power generation system can be reduced.

[Brief description of drawings]

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

[Explanation of symbols]

 1 Molten Silicon 2 Quartz Crucible 3 High Frequency Heating Coil 4 Graphite Tube 5 Plasma Torch 6 Argon Gas 7 Plasma Flame 8 Plasma Outlet 9 Mixed Gas of Steam and Hydrogen Chloride 10 Sodium Chloride Powder 11 Molten Silicon Bath Surface 12 Nozzle 13 Crucible Stand 14 Crucible Fall Device 15 Atmosphere 16 Exhaust port

Claims (3)

[Claims] 1. A method for purifying silicon, which comprises adding steam and hydrogen chloride to a plasma gas when irradiating a molten silicon surface with thermal plasma. 2. A method for purifying silicon, further comprising adding sodium chloride powder to the plasma gas according to claim 1. 3. The method for purifying silicon according to claim 1 or 2, wherein the unidirectional solidification is performed as it is after the purification.