JPH07315827A - Method for purifying silicon and purifying device therefor - Google Patents

Abstract

PURPOSE:To readily produce silicon for a solar cell, containing phosphorus, aluminum and calcium in low concentrations, in a short time. CONSTITUTION:Plural water-cooled crucibles 1a, 1b and 1c are used and upper melt in molten silicon 22 in the crucibles is successively poured to the lower crucibles under reduced pressure. A wall of each crucible is cooled, silicon in the each crucible is irradiated with electron beams 11 and 13, impurities (phosphorus, aluminum and calcium) in the molten silicon are attached to the walls of the higher crucibles and remain in the higher crucible bottoms to successively reduce the impurities in the molten silicon.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon refining method and a refining apparatus, and more particularly, to enhancing the effect of removing phosphorus, aluminum and calcium by an electron beam melting technique to obtain silicon having a low phosphorus, aluminum and calcium concentration. The present invention relates to a method and apparatus for reducing phosphorus, aluminum, and calcium in silicon, which has been devised in the above.

[0002]

2. Description of the Related Art In recent years, due to the demand for diversification of energy sources, photovoltaic power generation has attracted attention as an energy source, and research and development have been actively conducted toward the practical application of low-cost power generators. Under such circumstances, silicon is the most commonly used material as a raw material for solar cells, and crystalline silicon is the most important material used for power supply for power.

The purity of silicon (hereinafter abbreviated as SOG-Si) used as a raw material for solar cells is 99.9999%.
(6N) or more is required, and the concentration of impurities contained in silicon needs to be reduced to ppmw order or less. Conventionally, in order to produce the above-mentioned high-purity silicon from commercially available metallic silicon (purity 99.5%, hereinafter abbreviated as MG-Si), the solid-liquid partition coefficient of metal impurity elements such as Al, Fe and Ti is small. Is removed by unidirectional solidification refining using C, C is precipitated on the surface during solidification in the case of SiC, and is removed as CO in the case of solid solution C, and H ₂ O for B. There has been proposed a method of removing it by Ar plasma melting with addition of CO ₂ , CO ₂ or O ₂ .

On the other hand, there has been proposed a method for removing phosphorus under reduced pressure by utilizing its high vapor pressure. Conventionally, there has been a problem that it takes a long time to remove phosphorus under reduced pressure, but recently, it has been reported that phosphorus in silicon can be removed in a short time by electron beam melting (ISIJ I
international, vol. 32 (199
2). No. 5 p635-642), the shortening of the dephosphorization step is expected.

Further, as an advantage of this electron beam melting, aluminum and calcium are simultaneously removed in addition to phosphorus, and it is expected that the processing time of steps other than the dephosphorization step is shortened.

[0006]

However, in the method by electron beam melting disclosed above, the removal limits of phosphorus, aluminum and calcium in silicon are about 3 ppmw, about 470 ppmw and about 150 ppm, respectively.
w, and the phosphorus, aluminum, and calcium concentrations in the silicon are almost constant in the range where the dissolution time is 15 minutes or more, so further reduction of phosphorus, aluminum, and calcium cannot be expected, and it is required for SOG-Si. It cannot be said that the study on the electron beam melting method is sufficient to obtain high purity.

In view of the state of the art as described above, the present invention uses an electron beam as a heating source by taking advantage of the evaporation and removal of phosphorus, aluminum and calcium by electron beam melting, and uses phosphorus, aluminum and calcium in a short time. The purpose is to provide a technique for removing the.

[0008]

The present invention has been made to solve the above problems, and the method invention uses a plurality of water-cooled crucibles under reduced pressure to purify silicon by electron beam melting. While the solid raw material silicon is supplied to the crucible or after the solid raw material silicon is supplied, the solid raw material silicon is melted by irradiating with an electron beam, and the solid raw material silicon is melted or after the solid raw material silicon is melted Is poured into a lower crucible, and the molten silicon is sequentially poured into the lower crucible while irradiating with an electron beam. This is a method for purifying silicon.

The present invention relates to an apparatus for refining silicon by electron beam melting, a supply apparatus for solid raw material silicon, a plurality of water-cooled crucibles equipped with means for pouring molten silicon into a lower crucible, and silicon in the crucible. An apparatus for refining silicon, comprising an electron beam device for melting and a receiver for receiving and solidifying molten silicon in a vacuum container.

[0010]

According to the present invention, (1) the heating source is a clean electron beam. (2) The atmosphere is under reduced pressure of high vacuum. (3) By using a water-cooled crucible, a film-like layer of the same metal as the molten silicon is formed on the inner surface of the crucible, thereby preventing the contamination of the molten silicon with impurities.

It is considered that phosphorus, aluminum and calcium in silicon can be further reduced by maximizing the advantage of the electron beam melting described above and promoting evaporation and removal of phosphorus, aluminum and calcium in addition to this. It was completed by. As described above, according to electron beam melting, the melting time (beam irradiation time) is 15
In the range of more than minutes, phosphorus in the silicon, aluminum,
Calcium concentration is about 3ppmw, about 470p
It became almost constant at pmw and about 150 ppmw, and it was impossible to achieve further purification.

The inventors of the present invention have conducted extensive studies based on the above-mentioned idea and have clarified the reason why it is difficult to purify silicon as follows. That is, since electron beam melting is a melting method by heating the substance to be melted from one direction to the top, an undissolved raw material containing a large amount of phosphorus, aluminum, and calcium remaining at the bottom of the melted silicon, and Immediately after melting, silicon, which contains a large amount of phosphorus, aluminum, and calcium fixed to the crucible, gradually diffuses phosphorus, aluminum, and calcium into the molten silicon. Actually, the silicon melted by the electron beam was solidified in the crucible and the concentration distribution of phosphorus, aluminum and calcium in the silicon in the crucible was investigated. In comparison, it was confirmed to be 10 times higher. Therefore, if you prevent this pollution,
It became clear that phosphorus, aluminum, and calcium in silicon can be further reduced. By the way, a report example in the case where Ti is melted by electron beam (ISIJ
International, vol. 32 (199
2), No. 5, p607-615), the result is that the inside of the container in which Ti is dissolved is in a completely mixed state, and the Al concentration in Ti in the container is almost uniform. The non-uniformity of phosphorus, aluminum and calcium concentrations in silicon is considered to be unique to silicon.

From the above, it was concluded that a plurality of water-cooled crucibles are required. This is because if a plurality of crucibles are used and molten silicon is sequentially supplied to other crucibles,
For the crucibles after the first, only silicon with phosphorus, aluminum, and calcium removed to some extent in the crucible in the previous step of the crucible was supplied, and the phosphorus remaining on the bottom of the molten silicon or the crucible wall surface in the previous step was supplied. This is because silicon with high aluminum, aluminum and calcium concentrations is not supplied.

If only one water-cooled crucible is used, the undissolved raw material remaining at the bottom of the melted silicon and the contamination of the silicon containing a large amount of phosphorus, aluminum and calcium fixed to the crucible immediately after the melting. Therefore, phosphorus, aluminum, and calcium in silicon cannot be completely removed. In this case, even if phosphorus, aluminum, and calcium can be removed, it will take a long time and it will be unsuitable for mass production.

The crucible of the present invention is preferably made of a material having a high thermal conductivity from the viewpoint of enhancing the water cooling effect. As the material of the crucible, a metal having a high thermal conductivity, such as pure copper or copper, is generally used as a main component. Alloys, pure iron, alloys containing iron as a main component, and the like. The molten silicon may be fed from one crucible to another crucible continuously or batchwise.

Further, the installation position of the water-cooling crucible of the present invention and the supply of the molten silicon between the water-cooling crucibles are as long as the molten silicon can be supplied from the water-cooling crucible to which the raw materials are supplied to the water-cooling crucible to be finally purified. Anything will do.
For example, as shown in FIG. 1, three water-cooled crucibles 1a, 1b, 1c having a water-cooled portion 2 are arranged, solid silicon 21 as a raw material is supplied to the uppermost water-cooled copper crucible 1a, and an electron gun is used. The molten silicon 22 is melted by the electron beam 11 irradiated from 10 to obtain molten silicon 22, and after melting for a predetermined time, that is, after removing phosphorus, aluminum, and calcium to some extent, the crucible 1a is tilted to melt the molten silicon 22 in the crucible 1b.
To supply. Here, the electron beam 13 emitted from the electron gun 12 further removes phosphorus, aluminum, and calcium, and then the molten silicon 22 is transferred to the crucible in the subsequent stage, subjected to the same treatment, and finally in the bottom stage. From the overflow port 3 of the water-cooled copper crucible 1c to the graphite mold (receiver) 3
Hot water is poured into 1 to become silicon 32 with reduced phosphorus, aluminum and calcium.

Preferably, as shown in FIG. 3, water cooling crucibles 1a, 1b and 1c are installed in a staircase pattern, and while the solid raw material silicon 21 is continuously supplied, the water cooling crucibles 1a, 1b and 1c.
It is desirable to overflow the molten silicon 22 from the overflow port 3 of c and continuously take out the silicon 32 with reduced phosphorus, aluminum, and calcium in a molten state.

In this case, there is also an advantage that it is possible to preferentially overflow the molten silicon upper melt in which phosphorus, aluminum and calcium are most easily reduced. Further, as impurities that can be reduced by the present invention, in addition to phosphorus, aluminum, and calcium described above, Ni, Ge, Cu, S having a higher vapor pressure than silicon can be used.
Examples thereof include n, Ag, In, Mn, Pb, Sb and Tl.

[0019]

【Example】

[Prior Art 1] FIG. 1 shows a schematic view of a melting apparatus of the present invention.
Conventional example 1 is made by using a part of the apparatus shown in FIG. Water-cooled copper crucibles 1a, 1b, 1c are placed in the vacuum chamber 40.
3 are installed in a staircase shape, and two electron guns 10 and 12 having a maximum output of 150 kW class are provided on the copper crucibles 1a, 1b, and 1c. Further, the purified silicon 32 was made to be able to be collected in a graphite template (receiver) 31. here,
The shape of the water-cooled copper crucibles 1a, 1b, 1c is 150 on the surface of the molten metal.
× 200 mm, depth 60 mm, vacuum chamber 40
The internal pressure is 1 × 10 ⁻⁴ Torr, the electron beams 11 and 13
The output of each is 50 kW.

Using the above apparatus, phosphorus, aluminum and calcium were added at 25 ppmw, 700 ppmw,
MG-Si containing 650 ppmw (purity 99.5%,
2 kg of powder having a diameter of 1 to 3 mm) is inserted into the water-cooled copper crucible 1a, the silicon is melted by irradiating the electron beam 11 from the electron gun 10, and after melting for a predetermined time, the water-cooled copper crucible 1a is tilted and water-cooled. Purified silicon 32 was injected into a graphite mold (receiver) 31 installed on the copper crucible 1b.

Silicon 32 obtained under the above conditions
The phosphorus, aluminum and calcium in the ICP (Induct
The result is shown in Table 1. According to this result, when only one water-cooled copper crucible was used, the phosphorus, aluminum, and calcium concentrations in the silicon hardly changed after the dissolution time of 30 minutes, respectively, similar to the results in the above-mentioned literature. 3ppmw, about 290ppmw, about 1
It can be seen that it can only be reduced to 50 ppmw.

[0022]

[Table 1]

[Embodiment 1] Using the same electron beam melting apparatus as in Conventional Example 1, phosphorus of 25 ppmw, aluminum,
MG-S containing 700 ppmw of calcium each
2 kg of i (purity 99.5%, powder having a diameter of 1 to 3 mm) was inserted into a water-cooled copper crucible 1a, the silicon was melted by irradiating an electron beam 11 from an electron gun 10 and melted for a predetermined time, and then water-cooled. The copper crucible 1a was tilted, and the molten silicon 22 was injected into the water-cooled copper crucible 1b. And the electron gun 12
After irradiating the water-cooled copper crucible 1b with the electron beam 13 for a predetermined time, the water-cooled copper crucible 1b is tilted to move the water-cooled copper crucible 1b.
Purified silicon 32 was injected into a graphite mold (receiver) 31 placed on c.

The results shown in Table 2 were obtained for the concentrations of phosphorus, aluminum and calcium in silicon obtained under the above conditions. According to this result, when two water-cooled copper crucibles are used, the phosphorus, aluminum, and calcium concentrations in silicon can be reduced more easily and in a shorter time than in the conventional example. Especially, the total dissolution time is 60 minutes, and the phosphorus, aluminum, and calcium concentrations are 0.2 ppm each.
w, reduced to 10 ppmw, 0.5 ppmw or less.

[0025]

[Table 2]

Example 2 Using the same electron beam melting apparatus as in Example 1, phosphorus, aluminum and calcium were added at 25 ppmw, 700 ppmw and 650 pp, respectively.
MG-Si containing mw (purity 99.5%, diameter 1-3
mm powder) is inserted into a water-cooled copper crucible 1a, the silicon is melted by irradiating an electron beam 11 from an electron gun 10 and melted for a predetermined time, and then the water-cooled copper crucible 1a is tilted to form a water-cooled copper crucible 1b. Molten silicon 22 was injected into. Then, after irradiating the electron beam 13 from the electron gun 12 toward the water-cooled copper crucible 1b for a predetermined time, the water-cooled copper crucible 1b was tilted and the molten silicon 22 was injected into the water-cooled copper crucible 1c. After irradiating the electron beam 13 from the electron gun 12 toward the water-cooled copper crucible 1c for a predetermined time, the water-cooled copper crucible 1c is tilted, and the purified silicon 3 is put on the graphite mold (receiver) 31.
2 was injected.

As a result of chemical analysis of the silicon obtained under the above conditions, the results shown in Table 3 were obtained for the concentrations of phosphorus, aluminum and calcium. According to this result, when three water-cooled copper crucibles are used, compared with the case where two water-cooled copper crucibles are used, it is easier and quicker.
The phosphorus, aluminum, and calcium concentrations in silicon are 0.2 ppmw, 10 ppmw, and 0.5 ppm, respectively.
It can be seen that it can be reduced to w or less.

Considering the above results together, it can be easily inferred that the more the number of crucibles 1a, 1b, 1c used, the more effectively phosphorus, aluminum and calcium in silicon are reduced.

[0029]

[Table 3]

[Prior Art Example 2] FIG. 4 is a schematic view of the dissolving apparatus used. Water-cooled copper crucible 1 in vacuum chamber 40
One electron gun a with a maximum output of 150 kW,
Two 12 are provided on the upper part of the copper crucible 1a, and have the same capability as the melting apparatus shown in FIG. Further, the purified silicon was made to be able to be recovered in the graphite mold (receiver) 31. Here, the shape of the water-cooled copper crucible 1a is 150 × 480 mm on the surface of the molten metal and 60 mm in depth, the pressure in the vacuum chamber 40 is 1 × 10 ⁻⁴ Torr, the electron gun 10,
Output of electron beams 11 and 13 from 12 is 50 kW each
Is.

Using the above equipment, phosphorus, aluminum and calcium were added at 25 ppmw, 700 ppmw,
MG-Si containing 650 ppmw (purity 99.5%,
5 kg of powder having a diameter of 1 to 3 mm) is inserted into the water-cooled copper crucible 1a, and electron beams 11 and 13 having an output of 50 kW are emitted from the electron guns 10 and 12 while scanning the surface of the molten metal to melt the silicon. After melting for 5 minutes, MG-Si as a raw material was fed from the raw material feeding device 15 at a predetermined rate. Then, the purified molten silicon 22 overflowing from the overflow port of the water-cooled copper crucible 1a was received in a graphite mold (receiver) 31, and electron beam melting was performed until 10 kg of purified silicon was accumulated in the graphite mold 31.

When the silicon obtained under the above conditions was chemically analyzed, the results shown in Table 4 were obtained. According to this result, even when the raw materials are continuously supplied, if only one water-cooled copper crucible is used, the phosphorus, aluminum, and calcium in the silicon are similar to the results in the above-mentioned literature and the conventional example 1. The concentration hardly changed even if the inflow rate of the raw material was slowed down, and each was about 3 ppm.
It can be seen that w can be reduced only to about 290 ppmw and 150 ppmw.

[0033]

[Table 4]

[Embodiment 3] FIG. 2 shows a schematic view of an apparatus similar to the prior art 2 except that two crucibles are used. Two water-cooled copper crucibles 1a and 1b are installed in the vacuum chamber 40,
A copper crucible 1 with electron guns 10 and 12 with a maximum output of 150 kW
Two units are provided on top of a and 1b. Further, the purified silicon 22 was made to be able to be collected in a graphite template (receiver) 31. Here, the shapes of the water-cooled copper crucibles 1a and 1b were 150 × 240 mm on the surface of the molten metal and 60 mm in depth, and had a surface area of the molten metal which was half that of the crucible used in Conventional Example 2. That is, the total of the two pieces has the same crucible surface area as in Conventional Example 2. The pressure inside the vacuum chamber 40 was set to 1 × 10 ⁻⁴ Torr.

Using the above equipment, phosphorus, aluminum and calcium were added at 25 ppmw, 700 ppmw,
MG-Si containing 650 ppmw (purity 99.5%,
2.5k in powder form with a diameter of 1-3 mm) in a water-cooled copper crucible 1a
electron beam 1 with 50 kW output from electron gun 10
By irradiating 1 while scanning the surface of the molten metal,
After melting silicon for 5 minutes, raw material supply device 1
Starting material silicon 21 (MG-Si) was introduced from 5 at a predetermined rate. After that, the molten silicon 22 overflowing from the overflow port 3 of the water-cooled copper crucible 1a is received by the water-cooled copper crucible 1b, and the molten silicon 22 in the water-cooled copper crucible 1a and the water-cooled copper crucible 1b has an output of 50 kW from the electron guns 10 and 12, respectively. The electron beam 11, 13 is irradiated while scanning the surface of the molten metal, and the purified molten silicon 22 overflowed from the overflow port 3 of the water-cooled copper crucible 1b is received by the graphite mold (receiver) 31 and the graphite mold (receiver). Electron beam melting was performed until 10 kg of purified silicon 32 accumulated in 31.

When the silicon obtained under the above conditions was chemically analyzed, the results shown in Table 5 were obtained. According to this result, even when the raw material is continuously supplied, if two water-cooled copper crucibles are used and an appropriate inflow rate of the raw material is selected, as easily as the result in Example 2,
In a short time, the concentrations of phosphorus, aluminum and calcium in silicon were 0.2 ppmw, 10 ppmw,
It can be seen that it can be reduced to 0.5 ppmw or less.

[0037]

[Table 5]

[Embodiment 4] FIG. 3 shows a schematic view of an apparatus similar to that of Embodiment 3 except that the number of crucibles 1a, 1b and 1c is changed to three. Three water-cooled copper crucibles are installed in the vacuum chamber 40, and two electron guns 10 and 12 with a maximum output of 150 kW class are provided above the copper crucibles 1a, 1b and 1c. Further, the purified silicon can be collected in the graphite template 31. Here, the shape of each of the water-cooled copper crucibles 1a, 1b, and 1c is 150 × 160 mm on the surface of the molten metal, and the depth is 6
0 mm, which is one third of that used in Example 3.
With the surface area of the molten metal. That is, the total of three was set to be the same as in Example 3. The pressure inside the vacuum chamber 40 was set to 1 × 10 ⁻⁴ Torr.

Using the above equipment, phosphorus, aluminum and calcium were added at 25 ppmw, 700 ppmw,
MG-Si containing 650 ppmw (purity 99.5%,
1.6k in powder form with a diameter of 1-3 mm) in a water-cooled copper crucible 1a
electron beam 1 with an output of 30 kW from the electron gun 10
By irradiating 1 and 13 while scanning the surface of the melt, the silicon was melted and melted for 5 minutes, and then commercially available high-purity silicon as a raw material was supplied from the raw material supply device 15 at a predetermined rate. Then, the molten silicon 22 overflowing from the overflow port 3 of the water-cooled copper crucible 1a is received by the water-cooled copper crucible 1c via the water-cooled copper crucible 11b, and the silicon of the water-cooled copper crucible 1a is supplied from the electron gun 10 to the water-cooled copper crucible 1a.
b and the molten silicon 22 in the water-cooled copper crucible 1c, an electron beam 11 with an output of 50 kW from the electron gun 12
13 was irradiated while scanning the surface of the molten metal, and the purified molten silicon 22 overflowing from the overflow port 3 of the water-cooled copper crucible 1c was received by a graphite mold (receiver) 31, and 10 kg of the purified graphite in the graphite mold 31 was purified. Silicon 3
Electron beam melting was performed until 2 was accumulated.

When chemical analysis of the silicon obtained under the above conditions was performed, the results shown in Table 6 were obtained.
According to this result, even when the raw material is continuously supplied, when three water-cooled copper crucibles are used, an appropriate inflow rate of the raw material is selected as compared with the case where one or two water-cooled copper crucibles are used. Then, similar to the result in Example 3, phosphorus in the silicon, aluminum, and
Calcium concentration is 0.2ppmw, 10pp
It can be seen that mw can be reduced to 0.5 ppmw or less.
From this result, it can be easily inferred that the more crucibles 1a, 1b, 1c used, the more effectively phosphorus, aluminum, and calcium in silicon are reduced.

[0041]

[Table 6]

[0042]

According to the present invention, in the electron beam melting of silicon, phosphorus remaining on the bottom of the melted silicon,
Since aluminum and calcium are contained in a large amount of undissolved raw material, as well as phosphorus that adheres to the water-cooled crucible immediately after melting, aluminum, and silicon that contains a large amount of calcium to prevent contamination, phosphorus, aluminum, and calcium in the silicon The amount of phosphorus, aluminum, and calcium in silicon can be reduced to 0.2 ppmw, 10 ppmw, and 0.5 p, respectively, easily and in a short time.
Silicon that is below pmw can now be obtained.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a silicon refining method and a refining apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory view showing a silicon refining method and a refining apparatus according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a silicon refining method and a refining apparatus according to an embodiment of the present invention.

FIG. 4 is an explanatory view showing a conventional silicon refining method and refining apparatus.

[Explanation of symbols]

1a, 1b, 1c Crucible 2 Water cooling part 3 Overflow port 10, 12 Electron gun 11, 13 Electron beam 15 Raw material supply device 21, 22, 32 Silicon 31 Graphite template (receiver) 40 Vacuum chamber

Claims (2)

[Claims] 1. When purifying silicon by electron beam melting, a plurality of water-cooled crucibles are used under reduced pressure, and while supplying solid raw material silicon to an upper crucible or after supplying solid raw material silicon, electron beam irradiation is performed. The solid raw material silicon is melted, and while the solid raw material silicon is melted or after the solid raw material silicon is melted, molten silicon is poured into a lower crucible, and the molten silicon is sequentially subordinated while being irradiated with an electron beam. A method for purifying silicon, which comprises repeatedly pouring molten metal into a crucible. 2. In an apparatus for purifying silicon by electron beam melting, a solid raw material silicon supply device, a plurality of water-cooled crucibles equipped with means for pouring molten silicon into a lower crucible, and melting silicon in the crucible. An apparatus for refining silicon, comprising: an electron beam device for performing a heat treatment; and a receiver for receiving and solidifying molten silicon in a vacuum container.