JP2006206392A - Method for refining polycrystalline silicon - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide refining by which high-quality polycrystalline silicon having a low impurity concentration is obtained while using simple equipment in solidification refining of silicon. <P>SOLUTION: In the method for refining polycrystalline silicon, when molten silicon held in a casting mold is solidified in one direction from the bottom toward the top of the casting mold, solidification is carried out at a solidification rate of 0.3-3.0 mm/min while agitating the molten silicon by means of a shaft with an agitating blade. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing polycrystalline silicon for producing a silicon substrate for solar cells, and more particularly to a purification method thereof.

  When the molten silicon containing impurities is solidified, most of the impurities remain in the molten silicon due to the difference in solubility between the liquid and solid, and the majority of the silicon mass after solidification is reduced in impurities. Purified. At this time, it is said that the impurity concentration distribution generally follows the Seil's formula shown by the following formula (1).

C = k · C ₀ (1-f) ^k−1 (1)
C: impurity concentration in the solidified phase, C ₀ : initial impurity concentration before solidification, k: distribution coefficient, f: ratio of the solidified phase.

  The distribution coefficient k varies depending on various parameters such as the type of impurity element, the stirring state of molten silicon, and the solidification rate. Among these, if the agitation state of the molten silicon during solidification is not sufficient, the impurity concentration C in the solidified phase increases and the purification efficiency decreases. In order to avoid this, it is necessary to adjust the coagulation speed and the stirring state. Conventionally, when the solidification rate of silicon is up to 0.3 mm / min, stirring by silicon thermal convection is sufficient, but at 0.3 mm / min or more, stirring is strengthened by using a stirring means in addition to thermal convection. It is said that there is a need to do.

Patent Document 1 discloses that the molten silicon is agitated by rotating the mold in order to control the structure of the ingot. However, in this case, there are problems in the cost and maintenance of the apparatus. . Patent Document 2 discloses a technique for generating a stirring force by the action of a magnetic field on a molten metal. However, since the equipment cost for applying a magnetic field is expensive, it is not suitable for the production of an inexpensive polycrystalline silicon raw material. Furthermore, Patent Document 3 discloses a method in which silicon is unidirectionally solidified by blowing an inert gas into the molten metal, but solidification proceeds while entraining bubbles in the silicon lump. There was a problem that bubbles would become a concentrated part of impurities.
JP 61-141612 A JP-A-5-254817 JP 10-182135 A

  In view of such circumstances, an object of the present invention is to provide a purification method for obtaining high-quality polycrystalline silicon having a low impurity concentration while being a simple facility in the solidification purification of silicon.

  The gist of the present invention is as follows.

  (1) When the molten silicon is unidirectionally solidified upward from the bottom of the mold in the mold holding the molten silicon, the solidification speed of 0.3 to 3.0 mm / min while stirring the molten silicon with the shaft with the stirring blade A method for purifying polycrystalline silicon, characterized by solidifying with

  (2) The method for purifying polycrystalline silicon according to (1), wherein the shaft with stirring blades rises in accordance with the movement of the solidification interface of silicon.

  (3) The polycrystalline silicon purification method according to (1) or (2), wherein the shaft with stirring blades is made of a material that does not react with molten silicon.

  (4) The method for purifying polycrystalline silicon according to (3), wherein the shaft with stirring blades is made of high purity carbon.

  By the purification method of the present invention, even if the solidification rate of silicon is increased, impurities can be efficiently removed without degrading the quality of the solidified silicon, and high-quality polycrystalline silicon with high productivity and low cost is provided. be able to. This polycrystalline silicon can be used as a raw material for a solar cell silicon substrate.

  Further, polycrystalline silicon can be efficiently purified by an inexpensive and simple method without using expensive equipment.

  FIG. 1 is a longitudinal sectional view showing a state where unidirectional solidification purification of molten silicon according to the present invention is performed. First, the process starts with filling and melting metal silicon in a graphite or quartz mold 1. Conventionally, solidification was performed immediately after the completion of the dissolution of silicon. Therefore, only a means (heating body 2) for heating and melting silicon is disposed above the mold 1. The coagulation rate was controlled by adjusting the output of the heating means.

  In the present invention, when the molten silicon 3 is solidified in one direction upward from the lower part of the mold 1 in the mold 1 holding the molten silicon 3, the molten silicon 3 is solidified while being stirred by the shaft 4 with stirring blades. Although the rotational speed of the shaft 4 with a stirring blade is not specifically limited, 50-100 rpm is preferable. If it is higher than 100 rpm, the possibility that Si (silicon) will scatter and the yield will deteriorate. If it is less than 50 rpm, it cannot counter heat convection, and the stirring effect by the shaft 4 with a stirring blade may become difficult to be obtained.

  Further, the molten silicon 3 is solidified at a solidification rate of 0.3 to 3.0 mm / min. When the solidification rate is less than 0.3 mm / min, productivity is poor and improvement in purification efficiency is not recognized. When the solidification rate exceeds 3.0 mm / min, the solidification rate of the molten silicon 3 is too high, so that the impurities are not sufficiently separated and the solidification purification efficiency is poor.

  In the present invention, after the heater is raised (after silicon is melted), solidification starts immediately at a constant solidification rate, and the shaft 4 with stirring blades is immersed and stirred. As a result, the solidification interface (the interface between the molten silicon 3 and the solidified silicon 5) advances upward, so that the tip of the shaft 4 with the stirring blade is about 10 mm above the solidification interface so that it does not contact the solidification interface. Therefore, it is necessary to increase at a speed equal to the solidification speed. Therefore, in this invention, it has a means which can raise / lower the shaft 4 with a stirring blade according to the movement of the solidification interface of silicon. Further, in order to grasp the start of solidification, the rotation of the shaft 4 with stirring blades is stopped until the start of solidification, the shaft 4 with stirring blades is immersed in the molten silicon 3, and the tip of the shaft 4 with stirring blades is the mold 1. Make sure that you touch the bottom. The start of solidification is defined as the time when the immersion depth of the shaft 4 with stirring blades starts to become shallow.

The shaft 4 with stirring blades is preferably made of a material that does not react with the molten silicon 3. For example, silica (SiO ₂ ) is less contaminated with silicon but softens. Alumina (Al ₂ O ₃ ), magnesia (MgO), etc. have a high melting point, but become a source of contamination to silicon. From this viewpoint, graphite (C) is preferable. Furthermore, it is more desirable that the graphite has a high purity. As for purity, it is preferable that ash content is 20 ppm or less. If it is more than this, graphite will become a source of impurity contamination when used as a raw material Si for solar cells.

  By carrying out such a method, the molten silicon 3 is sufficiently stirred, the boundary layer thickness of the solidification interface and the distribution coefficient are reduced, and the purification efficiency is improved.

  Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples.

Example 1
Iron (0.2%)-containing silicon prepared by adding 10 g of iron (Fe) to 5 kg of high-purity silicon (99.99999999999% or more) was used as a test material. A coagulation purification apparatus having the configuration shown in FIG. 1 was used.

  This test material was placed in a bottomed cylindrical graphite mold having a diameter of 150 mm and a height of 350 mm, and dissolved in an atmosphere of Ar (argon) gas flowing at 3 L / min (liter / min) under atmospheric pressure. The molten silicon was solidified in one direction at a solidification rate of 0.3 mm / min with a temperature gradient of 0.5 ° C./mm from the bottom to the top of the mold. When solidifying, a carbon-made shaft with stirring blades (with two wings of 30 mm × 5 mm × 2 mm, ash content of 15 ppm) was immersed in molten silicon at a height of 10 mm from the mold bottom, While rotating at 70 rpm, simultaneously with the start of solidification, it was pulled upward at the same speed as the solidification speed. A sample of silicon obtained (diameter: 148 mm × height: 125 mm) was cut from the height of 20 to 30 mm, 50 to 60 mm, 70 to 80 mm, and 90 to 100 mm from the bottom, and Fe in Si by ICP-MS method. The concentration was measured to grasp the Fe content. The results are shown in Table 1.

(Comparative Example 1)
An iron (0.2%)-containing silicon obtained by adding 10 g of iron to 5 kg of high-purity silicon (99.99999999999% or more) was used as a test material. As the coagulation purification apparatus, a device obtained by removing the shaft with stirring blades, the rotation mechanism of the shaft with stirring blades, and these lifting mechanisms from the device used in Example 1 was used.

  This specimen is placed in a bottomed cylindrical graphite mold having a diameter of 150 mm and a height of 350 mm, dissolved in an atmosphere of Ar gas flowing at 3 L / min under atmospheric pressure, and then from the mold bottom to the top. With a temperature gradient of 0.5 ° C./mm, the molten silicon was solidified in one direction at a solidification rate of 0.3 mm / min. However, the molten silicon was stirred only by thermal convection. A sample of silicon obtained (diameter: 148 mm × height: 125 mm) was cut from the height of 20 to 30 mm, 50 to 60 mm, 70 to 80 mm, and 90 to 100 mm from the bottom, and Fe in Si by ICP-MS method. The concentration was measured and the Fe content was measured. The results are shown in Table 1.

(Example 2)
Coagulation purification was performed in the same manner as in Example 1. However, the solidification rate was 1.5 mm / min. The obtained silicon lump was subjected to component analysis in the same manner as in Example 1 to calculate the Fe content. The results are shown in Table 1.

(Comparative Example 2)
Coagulation purification was performed in the same manner as in Comparative Example 1. However, the solidification rate was 1.5 mm / min. The obtained silicon lump was subjected to component analysis in the same manner as in Example 1 to calculate the Fe content. The results are shown in Table 1.

(Example 3)
Coagulation purification was performed in the same manner as in Example 1. However, the solidification rate was 3.0 mm / min. The obtained silicon lump was subjected to component analysis in the same manner as in Example 1 to calculate the Fe content. The results are shown in Table 1.

(Comparative Example 3)
Coagulation purification was performed in the same manner as in Comparative Example 1. However, the solidification rate was 3.0 mm / min. The obtained silicon lump was subjected to component analysis in the same manner as in Example 1 to calculate the Fe content. The results are shown in Table 1.

  As is apparent from the results in Table 1, in contrast to the comparative example solidified by stirring only with thermal convection, the example in which forced stirring was performed with a shaft with a stirring blade had less iron content at any solidification speed, and solidification purification It can be seen that this is done efficiently.

  In addition, the silicon mass obtained in each example was vertically divided and the solidified structure was observed. In any of the examples, defects such as bubble entrainment were not observed, and impurities were concentrated except for the uppermost part of the silicon mass. No site of chemical segregation was observed.

  This result also confirms that the manufacturing cost can be kept low because the equipment cost can be kept low compared with the previously disclosed mold rotation and stirring of molten silicon by electromagnetic force.

It is a schematic diagram of the apparatus used for the purification method of the present invention.

Explanation of symbols

1 mold,
2 heating elements,
3 Molten silicon,
4 Shaft with stirring blade,
5 Solidified silicon.

Claims (4)

  When the molten silicon is unidirectionally solidified upward from the lower part of the mold in the mold holding the molten silicon, the molten silicon is solidified at a solidification rate of 0.3 to 3.0 mm / min while stirring with a shaft with a stirring blade. A method for purifying polycrystalline silicon.   The method for purifying polycrystalline silicon according to claim 1, wherein the shaft with stirring blades rises in accordance with movement of a solidification interface of silicon.   The method for purifying polycrystalline silicon according to claim 1, wherein the shaft with stirring blades is made of a material that does not react with molten silicon.   The method for purifying polycrystalline silicon according to claim 3, wherein the shaft with stirring blades is made of high purity carbon.