JPH10273374A - Purification of silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon purification process that can be smoothly and inexpensively operated by combining the two steps in the production process for silicon for solar cells thereby simplifying the process to a continuous one. SOLUTION: Solid silicon is molten with electron beam 6 under vacuum to remove readily evaporating elements, then the molten silicon 4 is charged into the cast vessel by over-flowing and the molten silicon 4 is solidified from the bottom upward to remove the metallic element contaminants. At this time, a bottomless crucible 11 comprising a plurality of electroconductive pieces divided in the peripheral direction at least a part of the shaft direction and the solidified silicon 10 is continuously taken out downward.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for coagulating and refining silicon, and more particularly to a refining technique capable of efficiently removing metal impurity elements in the production of silicon for solar cells.

[0002]

2. Description of the Related Art In order for a silicon substrate for a solar cell to exhibit required semiconductor characteristics, impurity elements in silicon must be reduced to an unacceptable level. Therefore, conventionally,
As shown in FIG. 5, silicon for a solar cell is reacted with hydrochloric acid to gasify it as trichlorosilane, and the gas is rectified to remove impurity elements and to react with hydrogen gas.
It was precipitated from the gas by the VD method. Therefore, the obtained silicon was a very high-purity so-called Eleven Nine.

[0003] However, such a conventional silicon production method requires timely adjustment of components, refining and casting of an ingot which has been highly purified even for a semiconductor, so as to be suitable for a solar cell. In addition to this, the yield is low, remelting equipment and energy are separately required, and the production cost increases. So, as mentioned above,
Currently available solar cells are expensive and are an obstacle to their general spread. In addition, in purifying metallic silicon in a chemical process, there is a problem that a large amount of pollutants such as silane and chloride is inevitably generated, which hinders mass production.

Therefore, the present applicant has improved the purification of metallic silicon by the above-described chemical process, and manufactured silicon having a purity suitable for solar cells only by a metallurgical process.
We are studying a method of casting it at once to make it into a silicon substrate (see FIG. 4). According to this method, it is expected that mass production is possible and that silicon for solar cells can be produced more advantageously in terms of energy.

However, the method shown in FIG. 4 is composed of various kinds of refining steps and is complicated, so it is considered that there is still waste and there is room for improvement. In particular, if a plurality of processes can be integrated and made continuous, it is possible to improve the silicon yield and reduce the power consumption. For this reason, the present applicant disclosed in Japanese Patent Application No. 8-288219 a vaporization removal step (vacuum scouring) of P using an electron beam as a heating source, Fe, Al,
A technique for integrating a step of removing a metal impurity element such as Ti (unidirectional solidification) has been disclosed.

[0006] However, the method described in the above publication includes:
There were the following problems. When a water-cooled copper “crucible” is used as a mold for solidifying the silicon melt and the length of the ingot (hereinafter, ingot) is to be increased in order to reduce the manufacturing cost, the “crucible” needs to be deepened. as a result,
On the contrary, not only has the manufacturing cost increased due to the enlargement of the apparatus, but it has become difficult to take out the ingot, and it has become difficult to irradiate the electron beam. Therefore,
Instead of the so-called batch type `` crucible '', when using a water-cooled copper mold of a system that continuously pulls out the ingot from the bottom, the expansion during the solidification of silicon requires a significant increase in the force to pull out the ingot. There were other problems such as the fact that the ingot had to be broken and the drawing operation had to be interrupted, that the mold had been consumed and that impurity elements had been mixed in from the mold.

[0007] Deboron and decarburization process (oxidation refining)
Japanese Patent Application Laid-Open No. Hei 4-130009 also discloses a technique for achieving integration by combining the above-described directional solidification. It uses a bottomless crucible composed of conductive divided pieces that are disposed in an induction coil and at least part of the axial direction is divided into a plurality in the circumferential direction. In the bottomless crucible, a molten silicon bath surface is used. The raw silicon is continuously supplied and melted, and while the silicon is purified by spraying a hot plasma gas containing an oxygen-containing substance onto the bath surface, the solidified silicon is lowered downward from the bottomless crucible. It is a method of “continuously taking out”.

In other words, the silicon melt is floated by electromagnetic force in a conductive divided crucible to be described later, and while contact with the crucible is cut off, deboron and decarburization, and further, metal impurity elements are removed. However, in this method, when using a so-called transition type power supply and heating silicon with a plasma arc,
An arc was also generated between the conductive water-cooled copper mold and the plasma torch to melt the mold, and the mold material was mixed in the molten metal or the mold was sometimes damaged. On the other hand, when silicon is heated with a high-temperature plasma gas jet stream using a non-transferred power supply, the mixing and melting problems described above can be avoided, but the power used to compensate for heat removal from the mold is significantly increased. There was a problem of doing.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been developed in consideration of the above-mentioned circumstances. It is an object of the present invention to provide a purification method of

[0010]

Means for Solving the Problems In order to achieve the above object, the inventor reviewed the above two prior arts, abolished their respective drawbacks, and achieved a great effect by combining good points. I worked hard. The present invention has been completed as a result. That is, the present invention melts solid silicon with an electron beam under vacuum,
After removing the volatile elements, the molten metal is poured into the casting vessel by overflowing, and the molten metal is unidirectionally solidified upward from the bottom to remove metal impurity elements contained in the molten metal. In addition, a solidified silicon is continuously taken out downward by using a bottomless crucible composed of conductive divided pieces arranged in an induction coil and divided into a plurality of pieces in a circumferential direction at least in a part of an axial direction. Is a method for purifying silicon.

Further, according to the present invention, the solid silicon is dissolved by a hot plasma gas to which an oxygen-containing substance is added, deboronated and decarburized, and then the molten metal is poured into a casting vessel by overflowing. In solidifying the molten metal unidirectionally upward from the bottom to remove metal impurity elements contained in the molten metal,
In the casting container, a solidified silicon is continuously taken out using a bottomless crucible composed of conductive divided pieces that are arranged in an induction coil and at least a part of which is divided in an axial direction into a plurality of pieces in a circumferential direction. This is a method for purifying silicon.

Further, the present invention is a method for purifying silicon, characterized in that the above two inventions are connected and one of them is carried out first. However, in this case, the ingot taken out by carrying out the present invention is cut and removed from the upper portion where the impurity element is concentrated, and the remaining portion is crushed to obtain a solid material for the raw material in a later step. Silicon. In the present invention, in the manufacture of silicon for a solar cell, as described above, various steps are combined, so that the entire steps are simplified and smooth operation becomes possible. Also,
Conventionally, silicon yield in each process has been a problem, but since simplification reduces the occurrence of out-of-process silicon,
The improvement in silicon yield can be achieved as compared with the conventional case. Further, for the same reason, the power consumption can be reduced.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a longitudinal section of an example of an apparatus for implementing a method for purifying molten silicon according to the present invention. That is, a holding vessel (hearth) 2 and a mold 3 are arranged in the decompression chamber 1 with a vertical difference, and molten silicon 4 (hereinafter, sometimes referred to as molten metal) melted in the holding vessel 2 sequentially overflows. You can move with. And
The heating and melting of the solid metal silicon as a raw material are performed by irradiation of an electron beam 6 from an electron gun 5 arranged above the holding container 2. This irradiation is usually performed periodically at regular time intervals (about 0.1 to 1 second).

First, the silicon surface in the holding container 2 is irradiated with an electron beam 6 to a temperature of 1550 ° C. or higher. As a result, P and the like, which are easily volatile impurity elements in the molten metal 4,
It evaporates and is removed from the system due to the reduced pressure. The degree of vacuum in the decompression chamber 1 is 5 × 10 ^−3.
torr or more. After continuing this state for a predetermined time,
The solid metal silicon 7 adhered and solidified to the tap hole 17 of the holding container 2 is melted by the irradiation of the electron beam 6, and easily volatile elements such as P, Ca, and Al are removed from the holding container 2 to the lower mold 3. The molten silicon 4 moves by overflow. Then, the molten metal 4 is unidirectionally solidified in the mold 3 to purify and remove contained metallic impurity elements such as Fe, Al, and Ti. A cylindrical heater 20 is arranged above the mold to heat the molten metal.

According to a first aspect of the present invention, a short, bottomless crucible in which a plurality of conductive divided pieces 9 are combined with the mold 3 is provided.
This eliminates the problems such as the difficulty in irradiating the electron beam 5 and the difficulty in extracting the electron beam 5. FIGS. 6A and 6B show the structure of the mold 3 including the bottomless crucible 11. The bottomless crucible 11 has a plurality of conductive divided pieces 9 arranged in a circumferential direction in an induction coil 8. In this, the molten metal 4 can be sequentially pulled downward, solidified, and continuously cast into the ingot 10. According to the continuous casting method, as shown in FIG. 6A, since the bottomless crucible 11 is divided into a plurality in the circumferential direction, the current 18 flowing through the induction coil 8 causes arrows in each divided piece. The current 12 as shown occurs. Thereby, the molten metal 4 in the bottomless crucible
A current 19 as shown by an arrow is generated therein, and the molten metal 4 is heated and melted, and a repulsive force is generated between the molten metal 4 and the current 12 flowing through the divided piece, as shown in FIG. The molten metal 4 and the solidified body 13 are held in a non-contact state with respect to the divided pieces 9 of the bottomless crucible 11. That is, the molten metal 4 and the solidified body 1
3 is kept in a state having a gap with the inner surface of the bottomless crucible 11. Therefore, the force for pulling out the solidified body 13 downward may be small, and smooth pulling out can be performed without any trouble. Further, since the molten metal 4 and the solidified body 13 are not in contact with the bottomless crucible 11, they are not contaminated by the so-called impurity element from the mold material.

Next, in a second aspect of the present invention, the oxidative refining for deboron and decarburization and the above-described directional solidification are integrated. That is, as shown in FIG. 2, the holding container 2 and the above-mentioned short bottomless crucible 11 are arranged with a vertical difference. The first embodiment differs from the first embodiment in that no decompression is required, that heating is performed by a high-temperature plasma gas stream 14, and that the oxygen-containing substance 15
(Usually using steam). In the apparatus of FIG. 2, the silicon melted in the holding vessel 2 is sprayed with a plasma gas stream 14 at 160
While being heated to about 0 ° C., it preferentially oxidizes boron and carbon contained in the oxygen-containing substance 15 and evaporates as an oxide.
Will be removed. After maintaining the state for a certain period of time, the molten metal 4 is moved to the bottomless crucible 11 by overflow in the same manner as described above, and the metal impurity element is purified and removed by unidirectional solidification.

In performing the unidirectional solidification in the first and second embodiments of the present invention, a graphite cylindrical heat retaining heater 20 is disposed above the bottomless crucible 11 and the molten metal surface is heated from above. The effect of purification occurs. The third and fourth aspects of the present invention relate to the first and second aspects of the present invention.
Of the ingot (the upper 20% of the ingot is removed because impurities are concentrated) and a crushing step, and the metal silicon is continuously purified. By doing so, the purification and removal of the impurity elements required for the production of silicon for solar cells can be performed as one flow operation with labor saving and energy saving. That is, the process of FIG. 4 previously proposed by the present applicant is simplified, and the flow shown in FIG. 3 is obtained. Note that there is no reason to limit which of the first and second aspects of the present invention should be on the upstream side, so that either one may be selected. However, the one-way solidification arranged on the upstream side is rough solidification purification, and the downstream side is finish solidification purification. Further, when the second present invention is a downstream step, it is necessary to blow argon gas or a mixed gas of argon and hydrogen into the molten metal during coagulation and purification for deoxidation. When the first invention is a downstream process, degassing is performed under reduced pressure, so that the gas is not required to be blown.

Further, in the present invention, since the molten silicon is held at 1500 ° C. or higher in the holding container 2, if necessary, a heating source other than the electron gun 5 and the plasma torch 16 is additionally provided. May be. However, in this case, when utilizing electric heat, it is necessary to pay sufficient attention to disturbance in the traveling direction of the electron beam 6 due to the electromagnetic field. In addition, holding container 2
Usually, a silica container is used for deboron and decarburization, and a water-cooled copper container is used for dephosphorization. In order to perform efficient vaporization and dephosphorization there, the inventor has filed a separate application. A certain graphite container may be used.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first invention (see FIG. 1) is connected upstream and the second invention (see FIG. 2) is connected downstream, and the third invention (see FIG. 3) is applied. To produce silicon for solar cells. First, in the decompression chamber 1, the maximum output 10
An electron beam 6 of 80 kW was emitted from an electron gun 5 of 0 kW to dissolve solid metal silicon continuously supplied at a rate of 6 kg / hour into a graphite holding container 2 having an inner area of 20 cm × 30 cm and a depth of 6 cm. . Melted molten silicon 4
Was overflowed from above the holding vessel 2 at regular intervals, and poured into the bottomless crucible 11 located below. Meanwhile, the pressure in the decompression chamber 1 is 1 × 10 ⁻⁴ torr.
, A part of phosphorus, Al, and Ca was vaporized and removed from the molten metal 4. In the bottomless crucible 11, one-way solidification for roughly refining metal impurity elements was performed by adjusting the supply rate of solid metal silicon so that the solidification rate was 0.6 mm / min upward from the bottom. . The surface of the molten metal 4 was heated from above the bottomless crucible 11 by using a cylindrical heat retaining heater 20 made of graphite. The amount of overflow per one time into the bottomless crucible 11 was about 100 g on average because the diameter of the bottomless crucible 11 was 300 mm. This is the part corresponding to the first present invention.

Next, when the ingot 10 of 50 kg was pulled out, the ingot 10 was cooled from 1500 ° C. to 20 ° C.
Cooled to 0 ° C over 1 hour. Then, the upper 20% of the ingot 10 was cut and removed, and the remaining portion was pulverized. And this pulverized material was used as a raw material of the second present invention. Here, direct melting may be performed while gradually feeding the rod-shaped raw material to the plasma spraying unit without pulverizing. The pulverized material was continuously supplied into the holding container 2 shown in FIG. 2 at a rate of 6 kg / hour, and a plasma gas stream 14 containing 10 vol% of steam 15 was blown from a plasma torch 16 disposed above and melted. Set the temperature of molten metal 4 to 1550
C., and boron and carbon contained in the molten metal 4 were evaporated and removed as oxides. Then, the molten metal 4 was overflowed into the bottomless crucible 11 in the same manner as described above, and unidirectional solidification for finishing was performed while deoxidizing by blowing argon gas into the molten metal.

Approximately 20% of the upper part of the pulled ingot 10
Was cut and removed, and an analysis sample was collected from the remainder. Table 1 shows the analysis results of the samples. From Table 1, it is clear that the impurity element of silicon produced according to the present invention is significantly reduced from the value of the starting material, and is a sufficient value for the purification of silicon for solar cells. According to the flow shown in FIG. 4, this operation took a long time until this stage, but according to the simplified present invention, about 75%
It took no time. In addition, the power consumption per unit was about 80% of that of the prior art, and the silicon yield was 35%, which was 35% of the conventional one.

[0022]

[Table 1]

[0023]

As described above, according to the present invention, impurity elements in molten silicon can be removed smoothly to an extent acceptable as silicon for solar cells. As a result, with the adoption of this technology, the production of inexpensive silicon substrates for solar cells can be expected.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an example of an apparatus for carrying out a method for purifying molten silicon according to the first invention.

FIG. 2 is a longitudinal sectional view showing an example of an apparatus for implementing a method for purifying molten silicon according to the second invention.

FIG. 3 is a flowchart showing a method for purifying molten silicon according to a third invention.

FIG. 4 is a flowchart showing a method of manufacturing a silicon substrate for a solar cell, which was researched and developed by the inventors.

FIG. 5 is a flowchart showing a conventional method for manufacturing a silicon substrate for a solar cell.

FIG. 6 is a view showing a mold made of a bottomless crucible,
(A) is a plane, (b) is a longitudinal section.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 decompression chamber 2 holding vessel (hearth) 3 mold 4 molten silicon (molten metal) 5 electron gun 6 electron beam 7 solid metal silicon 8 induction coil 9 split piece 10 ingot 11 bottomless crucible 12 current flowing in split piece 13 solidified body 14 Plasma gas flow 15 Oxygen-containing substance (water vapor) 16 Plasma torch 17 Outlet 18 Current flowing in induction coil 19 Current flowing in molten metal 20 Cylindrical heat retention heater (made of graphite) 21 Raw material hopper 22 Argon gas injection Nozzle

Continuing from the front page (72) Inventor Hiroyuki Baba 1 Kawasaki-cho, Chuo-ku, Chiba City, Kawasaki Steel Engineering Co., Ltd. (72) Inventor Naomichi Nakamura 1 Kawasaki-cho, Chuo-ku, Chiba City, Kawasaki Steel Co., Ltd. Inventor Kenkichi Yushita 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Kawasaki Steel Engineering Co., Ltd. (72) Inventor Yasuhiko Sakaguchi 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Kawasaki Steel Co., Ltd. 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Engineering Co., Ltd.

Claims (4)

[Claims] 1. After melting solid silicon by an electron beam under vacuum to remove volatile elements, the molten metal is poured into a casting vessel by overflowing, and the molten metal is unidirectionally upward from the bottom. Upon solidification, when removing the metal impurity element contained in the molten metal, the casting container is disposed in an induction coil, and is formed of a plurality of conductive divided pieces that are divided at least partially in the axial direction in the circumferential direction. A method for purifying silicon, wherein solidified silicon is continuously taken out downward using a bottomless crucible. 2. After dissolving solid silicon with a plasma gas to which an oxygen-containing substance has been added, removing boron and decarburizing,
The molten metal is poured into the casting container by overflowing, and the molten metal is unidirectionally solidified above the bottom to remove metal impurity elements contained in the molten metal. A method for purifying silicon, wherein solidified silicon is continuously taken out downward using a bottomless crucible made of a plurality of conductive divided pieces at least partly of which is divided in a circumferential direction in an axial direction. 3. Melting the solid silicon with an electron beam under vacuum to remove volatile elements, and then pouring the molten metal into a casting vessel by overflowing the molten metal in one direction upward from the bottom. Upon solidification, when removing the metal impurity element contained in the molten metal, the casting container is disposed in an induction coil, and is formed of a plurality of conductive divided pieces that are divided at least partially in the axial direction in the circumferential direction. A method for refining silicon, comprising using a bottomless crucible, continuously taking out solidified silicon downward, and subsequently performing the process according to claim 2. 4. Melting solid silicon with a hot plasma gas to which an oxygen-containing substance is added, deboroning and decarburizing, and then pouring the molten metal into a casting vessel by overflowing the molten metal.
When the molten metal is solidified in one direction upward from the bottom to remove metal impurity elements contained in the molten metal, the casting container is disposed in an induction coil, and at least a part in the axial direction is formed in a plurality in the circumferential direction. 2. A method for refining silicon, comprising using a bottomless crucible made of divided conductive pieces to continuously remove solidified silicon downward, and subsequently performing the process according to claim 1.