JP2014037339A - Quartz glass crucible for zinc evaporation and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a quartz glass crucible for zinc evaporation, which is used for generating zinc gas which is used for a zinc reduction process in which high-purity polycrystalline silicon is manufactured by reducing silicon tetrachloride in a zinc gas atmosphere, the crucible being capable of preventing boron elution and is inexpensive, and a method for manufacturing the crucible.SOLUTION: The quartz glass crucible for zinc evaporation according to the present invention comprises a coating film for preventing boron elution on all or part of the inner surface of the quartz glass crucible.

Description

  The present invention relates to a quartz glass crucible for evaporating zinc and a method for producing the same. More specifically, the present invention relates to a crucible made of quartz glass for zinc evaporation that generates zinc gas used when producing high-purity silicon by a zinc reduction method and a method for producing the same.

  In recent years, the demand for solar cells, which are attracting attention as one of clean energy, is expanding. The Siemens method is conventionally known as a method for producing high-purity polycrystalline silicon, which is a main raw material for solar cells, and commercial production by the Siemens method is performed. However, since the Siemens method consumes a large amount of power, the manufacturing cost is high, and since it is a batch method, the production efficiency is poor.

  On the other hand, the zinc reduction method, which is expected to reduce the manufacturing cost much more than the Siemens method, has recently attracted attention. The zinc reduction method is a method of producing high-purity polycrystalline silicon by reducing silicon tetrachloride with zinc gas. In this method, a zinc evaporator that generates zinc gas by evaporating the reducing agent zinc is provided. It is installed as incidental equipment for the reduction reaction apparatus (see, for example, Patent Documents 1 and 2).

  Such a zinc evaporator contains a crucible that melts and holds metal zinc as a raw material. The material of the crucible includes graphite, silicon carbide, and quartz glass, but the crucible material such as graphite and silicon carbide elutes into the molten zinc and the carbon in the material is mixed into the polycrystalline silicon. Quartz glass is usually used because it affects the performance of solar cells.

  However, in the high-purity polycrystalline silicon obtained by dissolving metal zinc in a quartz glass crucible and reducing high-purity silicon tetrachloride using zinc gas generated by heating to the boiling point of zinc of 907 ° C. When the boron (B) concentration was measured, it was relatively high at 5 to 20 ppba, and the resistivity of the N-type silicon substrate of 0.5 to 3.5 Ω · cm required for solar cell silicon could not be satisfied. It was.

  Therefore, the present inventors diligently investigated the supply source of boron contained in high-purity polycrystalline silicon. As a result, there is a strong correlation between the boron concentration in the quartz glass crucible and the boron concentration in the high-purity polycrystalline silicon obtained by using the zinc gas generated by this crucible as a reducing agent. It has been found that the boron concentration in the high-purity polycrystalline silicon obtained by the zinc reduction method increases as the boron concentration in the quartz glass crucible for evaporating zinc increases.

  The reason why there is a strong correlation between the boron concentration in the quartz glass crucible and the boron concentration in the high-purity polycrystalline silicon is considered as follows. That is, while the metallic zinc is melted and held in the quartz glass crucible for evaporating zinc, part of the boron in the crucible elutes and dissolves in the molten zinc, and the molten zinc is used by the zinc evaporator. When zinc gas is generated, boron dissolved in molten zinc is transferred to zinc gas as it is, and as a result, silicon tetrachloride is reduced using this zinc gas in a zinc reduction reactor to produce high purity polycrystalline. When silicon is produced, it is considered that most of boron in zinc gas is transferred into high-purity polycrystalline silicon.

  Therefore, the boron concentration in high-purity polycrystalline silicon can be reduced by either reducing the boron contained in the quartz glass crucible for evaporating zinc or suppressing the elution of boron contained in the quartz glass crucible for evaporating zinc. Can be reduced.

  In order to reduce the boron contained in the former crucible made of quartz glass for evaporating zinc, by using quartz glass in which the boron concentration (usually 200 to 500 ppbw) in the quartz glass, which is the raw material for crucible production, is less than the limit value It can be solved. However, such a low boron concentration quartz glass is 3 to 5 times higher in cost than a commonly used quartz glass. In the production of high-purity polycrystalline silicon for solar cells, for which cost reduction is an urgent issue due to the widespread use of solar cells, it is inappropriate from the viewpoint of cost to use such a high-priced quartz glass crucible.

JP 2004-196642 A JP 2008-184641 A

  An object of the present invention is a quartz glass crucible for evaporating zinc used in a zinc reduction method for producing high-purity polycrystalline silicon by reducing silicon tetrachloride with zinc gas and generating zinc gas as a reducing agent. Thus, an object of the present invention is to provide a quartz glass crucible for evaporating zinc that can suppress boron elution and a method for manufacturing the same.

  As a result of various investigations to solve the above problems, the present inventors have found that boron elution can be suppressed by coating a specific boron elution preventing layer on the inner surface of the fused silica glass crucible, The present invention has been completed. The present invention includes, for example, the following aspects.

[1] A quartz glass crucible for evaporating zinc, having a boron elution preventing coating layer on all or part of the inner surface of the fused silica glass crucible.
[2] The quartz glass crucible for zinc evaporation according to item [1], wherein the boron elution preventing coating layer is made of a zinc-containing oxide.

  [3] The quartz glass crucible for zinc evaporation according to [1], wherein the boron elution preventing coating layer is made of quartz glass having a boron concentration of 200 ppbw or less.

  [4] The quartz evaporation crucible for zinc evaporation according to any one of items [1] to [3], wherein the boron elution preventing coating layer has a thickness of 300 to 2000 μm.

  [5] By charging metal zinc into a fused silica glass crucible, heating and melting at a temperature of 800 ° C. or higher, holding at that temperature for 15 hours or more, and then discharging the molten metal zinc from the crucible A method for producing a quartz glass crucible for evaporating zinc, characterized in that a boron elution preventing coating layer made of an oxide is formed on all or part of the inner surface of the crucible.

[6] The method for producing a quartz glass crucible for zinc evaporation according to item [5], wherein the oxide is zinc oxide or a composite oxide of zinc and silicon.
[7] The method for producing a quartz glass crucible for zinc evaporation according to [5] or [6], wherein the boron elution preventing coating layer has a thickness of 300 to 2000 μm.

  By using the silica glass crucible of the present invention as a crucible for evaporating zinc when producing high-purity polycrystalline silicon by the zinc reduction method, high-purity polycrystalline silicon for solar cells with a significantly reduced boron concentration can be economically used. And stable production.

FIG. 1 is a schematic view showing an example of an apparatus for producing high-purity polycrystalline silicon by a zinc reduction method. FIG. 2 is a schematic view showing an example of a quartz glass crucible for evaporating zinc according to the present invention.

Hereinafter, the present invention will be described in detail.
In the production of high-purity polycrystalline silicon by the zinc reduction method, silicon tetrachloride gas and zinc gas are usually supplied from the upper part of the zinc reduction reactor 7 as shown in FIG. And in the reactor 7, as shown in the following reaction formula, silicon tetrachloride is reduced with zinc, and high purity polycrystalline silicon and exhaust gas containing zinc chloride are generated. The reaction temperature in this case is usually 950 to 1,200 ° C.
SiCl ₄ + 2Zn → Si + 2ZnCl ₂

  High-purity polycrystalline silicon grows in the shape of needles or dendrites, adheres to the tip of the silicon tetrachloride gas supply nozzle 6 and grows in a tubular shape toward the bottom, and is removed from the tip of the nozzle after the reaction is completed. It is taken out of the reactor 7 by the apparatus 9 or the like and recovered as high-purity polycrystalline silicon D. On the other hand, the product gas containing zinc chloride gas as a main component is exhausted as exhaust gas C from the exhaust gas outlet 8 to the outside of the reactor 7.

  The quartz glass crucible for zinc evaporation of the present invention (hereinafter also referred to as “zinc evaporation crucible”) is installed in the zinc evaporator 2 shown in FIG. Accept and hold fluid. This zinc melt is heated to the boiling point of zinc (907 ° C.) by an external heating device to generate zinc gas. The generated zinc gas is heated to a predetermined temperature through the superheater 3, led to the zinc gas supply nozzle 5 installed at the upper part of the reduction reactor 7, and used for the reduction reaction.

  Here, the quartz glass includes a fused silica glass manufactured by flame melting or electric melting using quartz powder which is natural quartz as a raw material, and flame hydrolysis (CVD method, using ultra high purity silicon tetrachloride as a raw material). And synthetic quartz glass produced by the DVD method).

  Synthetic quartz glass has a very high purity, metal components such as Al, Fe, and Ti, which are impurities, are 100 ppbw or less, alkali metals such as K, Li, and Na are 10 ppbw or less, and B is 10 ppbw or less. On the other hand, fused silica glass has a higher impurity concentration than synthetic quartz glass, and typical impurity concentrations include Al of 7,000 to 10,000 ppbw, Fe of 2,000 to 3,000 ppbw, and K of 60 to 100 ppbw. , Li is 20 ppbw or less, Na is 600 to 800 ppbw, and B is 200 to 500 ppbw.

  In this way, synthetic quartz glass and fused silica glass are not only greatly different in impurity concentration, but due to differences in raw materials and manufacturing methods, synthetic quartz glass is more expensive and more expensive than fused (natural) quartz glass. Is 3 to 5 times more expensive.

  The crucible for evaporating zinc according to the present invention is a crucible prepared by using fused silica glass that is cheaper than synthetic quartz glass, and generates zinc gas by melting and evaporating zinc.

  However, high-purity polycrystalline silicon obtained by reducing silicon tetrachloride with zinc gas generated using a fused silica glass crucible as it is is manufactured using this as a raw material because of its high boron concentration as described above. It adversely affects the electrical characteristics of solar cells.

As a result of examining the relationship between the boron concentration in the high-purity polycrystalline silicon and the resistivity of the silicon single crystal for solar cells in detail, the present inventors have found that the boron concentration in the high-purity polycrystalline silicon is 5 ppba (2.5 If it is less than × 10 ¹⁴ atoms / cc), it has been found that an N-type silicon single crystal having a resistivity of 0.5 to 3.5 Ω · cm generally required for solar cell silicon can be stably produced. It was.

  As shown in FIG. 2, the zinc evaporation crucible of the present invention has a coating for preventing boron elution on the whole or part of the inner surface of the fused silica glass crucible 21 as shown in FIG. A layer 22 (hereinafter also simply referred to as “coating layer”). Examples of the coating layer include a first aspect and a second aspect described later.

<First aspect>
As the first aspect of the coating layer, a zinc oxide such as zinc oxide (ZnO), a zinc-silicon composite oxide such as zinc silicate (Zn ₂ SiO ₄ ), or a zinc-containing oxide in which these are mixed or combined. It is formed of objects.

The zinc evaporation crucible having the coating layer of the first aspect can be produced, for example, as follows.
Metal zinc is charged into a crucible made of fused silica glass having a predetermined size, and the metal zinc is 800 ° C. or higher, more preferably 850 ° C. or higher, particularly with an external heater (resistance heating or induction heating) provided around the crucible. Preferably, it dissolves by heating to 890 ° C to 907 ° C. In this state, after melting and holding for 15 hours or more, preferably 18 to 50 hours, the molten zinc is discharged from the crucible. As a result, a coating layer for preventing boron elution of a zinc-containing oxide made of zinc oxide, zinc silicate (Zn ₂ SiO ₄ ), or a mixture or combination thereof is thinly formed on the inner surface of the crushed crucible. It is formed.

As the metal zinc, electrolytic zinc, dry refined zinc, recycled zinc, molten salt electrolytic zinc regenerated from exhaust gas of zinc reduction method, or the like can be used. The boron concentration in the metallic zinc is preferably 1 ppbw or less.
The amount of the metallic zinc charged into the fused silica glass crucible is preferably 80 to 95% of the internal volume of the crucible.

<Second aspect>
As a second aspect of the coating layer, the coating layer is formed of high-purity fused silica glass having a low boron concentration or synthetic quartz glass. The boron concentration in the fused silica glass or synthetic quartz glass constituting the coating layer in the second aspect is preferably 200 ppbw or less, more preferably 180 ppbw or less, and still more preferably 10 to 150 ppbw. If the boron concentration exceeds the above range, the boron concentration in the finally obtained silicon single crystal becomes 5 ppba or more, which is not suitable as high-purity silicon for solar cells.

The zinc evaporation crucible having the coating layer of the second aspect can be produced, for example, as follows.
The fused silica glass powder or synthetic quartz glass powder having a low boron concentration is attached to a predetermined thickness on the inner surface of a fused silica glass crucible having a predetermined size, and arc discharge is performed while rotating the crucible. Thereby, a thin coating layer of synthetic quartz glass is formed on the inner surface of the crucible. In addition, as temperature at the time of arc discharge, Preferably it is 1700 degreeC or more, More preferably, it is 1730-1780 degreeC.

  The thickness of the coating layer is preferably 300 to 2000 μm, more preferably 400 to 1500 μm in both the first and second aspects. When the coating layer is in the above range, a boron elution preventing effect can be sufficiently obtained. Moreover, since the boron elution prevention effect does not change even when the coating layer becomes thicker than the above range, the above range is preferable from the economical viewpoint.

EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example, this invention is not limited to these Examples at all.
In the following, the boron concentration in the quartz glass constituting the crucible was measured using ICP-MS. The thickness of the coating layer was measured by observing the cross section of the coated crucible with SEM / EDX.

[Example 1]
A fused silica glass having a boron concentration of 300 ppbw was used to produce a crucible having an outer diameter of 460 mm, an inner diameter of 400 mm, and a height of 850 mm. The crucible was charged with 300 kg of electrolytic zinc having a boron concentration of 1 ppbw or less, heated to 907 ° C. to dissolve the metallic zinc, and kept in the molten state for 20 hours. Thereafter, molten zinc was discharged from the crucible to obtain a zinc evaporation crucible having a coating layer made of a zinc-containing oxide. When the thickness of the coating layer formed on the inner surface of the obtained zinc evaporation crucible was measured, the average was 500 μm.

Next, the obtained zinc evaporation crucible was installed in a zinc evaporator, and high purity polycrystalline silicon was produced by a zinc reduction method using an apparatus as shown in FIG.
Using the obtained high-purity polycrystalline silicon, crystal growth was performed by the Czochralski method (CZ) method to produce single crystal silicon. When the obtained single crystal silicon was cooled from 12K to 14K and analyzed and evaluated by the FT-IR method, the boron concentration in the single crystal silicon was 2.5 ppba.

[Example 2]
A zinc evaporation crucible was prepared by the same method as in Example 1 except that the holding time in the molten state of metallic zinc was 30 hours, and the high purity polycrystalline silicon was obtained using the obtained zinc evaporation crucible. Was manufactured, and single crystal silicon was manufactured and evaluated using the obtained high-purity polycrystalline silicon.

  The average thickness of the coating layer formed on the inner surface of the obtained zinc evaporation crucible was 800 μm. Moreover, the boron concentration in the obtained single crystal silicon was 0.6 ppba.

[Example 3]
A crucible having the same size as in Example 1 was prepared except that fused silica glass having a boron concentration of 230 ppbw was used. A fused silica glass powder having a boron concentration of 90 ppbw was attached to the inner surface of the crucible so that the thickness was about 2000 μm. Arc discharge was performed while rotating the crucible, and a coating layer was formed at 1750 ° C., thereby obtaining a zinc evaporation crucible having a coating layer made of fused silica glass having a boron concentration of 90 ppbw. The average thickness of the coating layer formed on the inner surface of the obtained zinc evaporation crucible was 1000 μm.

  Except for using the obtained zinc evaporation crucible, high-purity polycrystalline silicon was produced in the same manner as in Example 1, and single-crystal silicon was produced using the obtained high-purity polycrystalline silicon. evaluated. The boron concentration in the obtained single crystal silicon was 2.0 ppba.

[Comparative Example 1]
A fused silica glass having a boron concentration of 230 ppbw was used to produce a crucible having an outer diameter of 460 mm, an inner diameter of 400 mm, and a height of 850 mm. This crucible was installed in a zinc evaporator, and high purity polycrystalline silicon was produced by a zinc reduction method using an apparatus as shown in FIG.

  Single crystal silicon was produced and evaluated in the same manner as in Example 1 except that the obtained high-purity polycrystalline silicon was used. The boron concentration in the obtained single crystal silicon was 5.5 ppba.

[Comparative Example 2]
Except for using a fused silica glass having a boron concentration of 700 ppbw, a crucible was produced in the same manner as in Comparative Example 1, and high-purity polycrystalline silicon was produced using the obtained crucible. Single crystal silicon was manufactured and evaluated using crystalline silicon. The boron concentration in the obtained single crystal silicon was 15.0 ppba.

[Comparative Example 3]
A zinc evaporation crucible was prepared in the same manner as in Example 1 except that the holding time in the molten state of metallic zinc was 10 hours, and the high purity polycrystalline silicon was obtained using the obtained zinc evaporation crucible. Was manufactured, and single crystal silicon was manufactured and evaluated using the obtained high-purity polycrystalline silicon.

  The average thickness of the coating layer formed on the inner surface of the obtained zinc evaporation crucible was 200 μm. Moreover, the boron concentration in the obtained single crystal silicon was 6.8 ppba.

[Reference example]
Except that synthetic quartz glass having a boron concentration of 90 ppbw was used, a crucible was produced in the same manner as in Comparative Example 1, and high-purity polycrystalline silicon was produced using the obtained crucible. Single crystal silicon was manufactured and evaluated using crystalline silicon. The boron concentration in the obtained single crystal silicon was 2.0 ppba.
The results of the above Examples, Comparative Examples and Reference Examples are shown in Table 1.

  As shown in Table 1, the boron concentrations in the single crystal silicon of Examples 1 to 3 were all 5.0 ppba or less, and were sufficiently satisfactory as a material for solar cell silicon.

A Metallic zinc raw material B Silicon tetrachloride raw material C Exhaust gas D High-purity polycrystalline silicon 1 Zinc dissolution tank 2 Zinc evaporator 3 Superheater 4 Silicon tetrachloride evaporator 5 Zinc gas supply nozzle 6 Silicon tetrachloride gas supply nozzle 7 Reduction reactor 8 Exhaust gas outlet 9 Silicon extraction device 10 High-purity polycrystalline silicon production device 20 Zinc evaporation quartz glass crucible 21 Fused silica glass crucible 22 Coating layer

Claims (7)

  A quartz glass crucible for evaporating zinc, comprising a coating layer for preventing boron elution on all or part of the inner surface of a fused silica glass crucible.   The crucible made of quartz glass for zinc evaporation according to claim 1, wherein the boron elution preventing coating layer is made of a zinc-containing oxide.   The crucible made of quartz glass for zinc evaporation according to claim 1, wherein the boron elution preventing coating layer is made of quartz glass having a boron concentration of 200 ppbw or less.   4. The quartz glass crucible for zinc evaporation according to claim 1, wherein the boron elution preventing coating layer has a thickness of 300 to 2000 μm.   Metal crucible is charged into a fused silica glass crucible, heated to a temperature of 800 ° C. or higher and melted, held at the temperature for 15 hours or more, and then the molten metal zinc is discharged from the crucible. A method for producing a quartz glass crucible for evaporating zinc, comprising forming a boron elution preventing coating layer made of an oxide on all or a part of the inner surface of the glass.   6. The method for producing a quartz glass crucible crucible for zinc evaporation according to claim 5, wherein the oxide is zinc oxide or a composite oxide of zinc and silicon.   The method for producing a quartz glass crucible for zinc evaporation according to claim 5 or 6, wherein the boron elution preventing coating layer has a thickness of 300 to 2000 µm.

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