JPH10139415A - Solidification and purification of molten silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for solidification and purification of molten silicon to remove elemental impurities therefrom to an extent allowable as a preliminary purification step for production of solar cells. SOLUTION: This method passes silicon 6 molten under a vacuum, irradiated with electron beams 5 to heat the surface, to a cast container 3 while successively overflowing it at given time intervals, to solidify the molten silicon 6 unidirectionally from the bottom upwards, where an overflow rate is set to keep the quantity of the molten silicon 6 in the container 3 at <= (density (kg/cm<3> ) × surface area (cm<2> )×thickness 0.1(cm) kg and irradiation density of the electron beams 5 at 0.07 to 0.40kW/cm<2> .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for solidifying and refining molten silicon, and more particularly, to a method of producing silicon for a solar cell from metallic silicon by using a metallurgical process to remove impurity elements in molten silicon in one direction. The present invention relates to a technique of removing using coagulation.

[0002]

2. Description of the Related Art One of the techniques for increasing the purity of a metal is a solidification refining method. It consists of a metal element to be purified (here, silicon) and an impurity element to be removed (for example, A
1, Fe, Ti, etc.). That is, a certain concentration (e weight%)
When the solidus line and the liquidus line of the metal to be purified (A) containing the impurity element (B) have a relationship as shown in FIG. It is discharged into the liquid phase and concentrated in the liquid phase (the concentration of B moves from point e to point f, and the concentration of A moves from point e to point d). In particular,
When the metal to be purified held in the mold is solidified, for example, in one direction upward from the bottom, the impurity concentration decreases below the ingot, and is concentrated at the upper portion where the metal finally solidifies. Therefore, if the upper part (concentrated part) of the ingot is cut and discarded, a high-purity metal to be purified can be obtained.

On the other hand, in order for a silicon substrate for a solar cell to exhibit required semiconductor characteristics, the impurity element in silicon must be reduced to an unacceptable level. Conventionally, as shown in FIG. 3, so-called CVD method is used in which silicon for solar cells is reacted with hydrochloric acid to gasify it as trichlorosilane, and the gas is rectified to remove impurity elements and react with hydrogen gas. Had been precipitated from the gas. Therefore, the obtained silicon was a very high-purity so-called Eleven Nine.

[0004] However, such a conventional method for producing silicon requires time-consuming 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 inventor has improved the purity of metallic silicon by the above-described chemical process, manufactured silicon having a purity suitable for a solar cell only by a metallurgical process, and cast it to a silicon substrate at a stretch. (Fig. 4
See). Then, as one of the measures, an attempt is made to increase the purity of metallic silicon by utilizing the solidification purification method described above.

Meanwhile, as shown in FIG. 5, one-way solidification has been conventionally used for producing a polycrystalline silicon substrate for a solar cell. For example, Japanese Unexamined Patent Publication No. 62-260710 discloses that “a melting furnace 2 (water-cooled hearth) and a water-cooled mold 3 are provided vertically in a decompression chamber 1 and solid (metal) supplied to the melting furnace is provided.
A technique in which silicon 4 is melted by an electron beam 5 and its molten metal (molten silicon) 6 overflows from a melting furnace 2, is continuously supplied to the mold 3, and solidifies in one direction upward from the bottom of the mold 3. Is disclosed.

However, this publication mainly focuses on improving the productivity because the silicon used is of high purity and does not require refining, and there is no description of refining technology. Further, when the method described therein is performed, silicon nitride, silicon oxide, etc. applied on the inner surface of the mold are decomposed under vacuum and high temperature,
It is expected that stable electron beam irradiation cannot be performed. Furthermore, an electromagnetic field is generated by a heater (not shown) installed above the mold 3, and similarly, it is considered that irradiation of the electron beam 5 becomes unstable and stable movement of the solidification interface is not possible.

Japanese Patent Application Laid-Open No. 5-124809 discloses that impurity elements are roughly purified from silicon.
He proposed a technique to "melt and solidify part of the ingot once solidified to improve the yield of silicon." However, this publication also discloses that in the examples, the degree of vacuum is 10 ⁻³ to 10 ⁻³ .
10 ^-4 mbar or 35mm diameter ingot with 2mm speed
/ Min, or the output of the electron beam was increased from 5 kW to 6.5 kW at the time of re-melting. However, there is no description about efficient and stable removal of impurity elements. Also, regarding the concentration of the impurity element,
This is a technique in which Al is reduced from about 1800 ppm to about 360 ppm, and is not a technique to reduce Al from about 1000 ppm to 10 ppm or less, which is a problem in the production of silicon for solar cells.

[0009]

SUMMARY OF THE INVENTION In view of the foregoing, the present invention provides a method for solidifying and refining molten silicon in which impurity elements in the molten silicon are removed to an extent acceptable as coarse refining of silicon for solar cells. It is intended to be.

[0010]

Means for Solving the Problems In order to achieve the above-mentioned object, the inventor conducted intensive studies with a focus on stable movement of the solidification interface, and completed the present invention. That is, according to the present invention, when molten silicon melted under reduced pressure is poured into a casting vessel at intervals of a predetermined time, and the surface is heated by an electron beam, the molten silicon is solidified in one direction upward from a bottom. The amount of molten silicon per overflow was determined by the density (kg / cm ³ ) of silicon in the casting vessel.
× surface area (cm ² ) × thickness 0.1 (cm)) kilograms or less, and the electron beam irradiation density is 0.07
A method for solidifying and refining molten silicon, characterized in that the pressure is set to 0.40 kW / cm ² .

Further, the present invention provides that the pressure in the decompression chamber is 5 × 1
A method for solidifying and refining molten silicon, comprising maintaining the pressure at 0 ^-3 torr or less and vaporizing and dephosphorizing the molten silicon. In the present invention, since the solidification purification of the molten silicon is performed with the above-described configuration, the temperature gradient in the solid-liquid coexistence region at the solidification interface approaches an ideal (as shown in the state diagram) and becomes uniform over the entire interface. As a result, the interface stably moves upward. As a result, the impurity element is sufficiently concentrated in the portion that finally solidifies, and the purity of the molten silicon is improved to an extent that can be accepted as a rough refinement for obtaining silicon for solar cells.

[0012]

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 container 2 (hearth) and a copper water-cooled crucible (casting container, see FIG. 7 (a)) 3 are vertically arranged in a decompression chamber 1 so that the molten silicon 6 that has been melted can move sequentially by overflow. ing. And the raw metal silicon 4
Is melted and heated by irradiation of an electron beam 5 from an electron gun 8 arranged above. 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
The pattern of irradiating with the electron gun 8 so as to have the locus shown in FIG. 6 and then irradiating the solidified silicon to the tap hole 11 of the holding container 2 is repeated. Therefore, tap hole 11
Is irradiated, the molten silicon overflows from the holding container 2 to the copper water-cooled crucible 3 below. The solidification of the silicon at the tap hole 11 is caused by the remaining hot water at the time of the previous overflow since the tap hole is also water-cooled.

According to the present invention, in such an apparatus, the dissolution of the metal silicon 4, the vaporization dephosphorization from the molten silicon 6 and the solidification purification by one-way solidification are performed as a series of continuous treatments. As important points in this case, the beam irradiation density and the overflow amount of the molten silicon to the mold are limited as follows. That is, in the present invention, the amount of molten silicon per overflow from the holding container 2 is reduced to (density × surface area × thickness 0.1) kilograms or less because a large amount of overflow occurs at once. This is because the solidification rate varies depending on the position and component segregation occurs in the product. The reason why the irradiation density of the electron beam 5 is set to 0.07 to 0.40 kW / cm ² is as follows.
If it is less than 0.07, the temperature of the molten metal surface is too low, the temperature gradient in the solid-liquid region is too small, and the solidification interface is not stable. On the other hand, if it exceeds 0.40, the surface temperature rises too much and the evaporation amount of silicon increases, and at the same time, the surface of the surface vibrates to disturb the solidification interface and the impurity element enrichment phenomenon becomes insufficient. .

In the present invention, the pressure in the decompression chamber is set to 5 × 1
Although it is set to 0 ^-3 torr or less, when it exceeds this value, the vapor dephosphorization from silicon is significantly reduced. In the present invention, the molten silicon is melted in the melting vessel or holding vessel 2 for 15 minutes.
Since the temperature is maintained at 00 ° C. or higher, a heating source other than the electron gun 8 may be additionally provided if necessary. However, in this case, when utilizing the electric heat, it is necessary to pay sufficient attention to the disturbance of the traveling direction of the electron beam due to the electromagnetic field. Also,
The holding container 2 is usually formed of a copper water-cooled jacket. However, in the case where efficient vaporization and dephosphorization is performed there, a graphite container which is separately filed by the inventor may be used. The material of the copper water-cooled crucible 3 is the same as that of the holding container 2. Further, in the present invention, instead of the copper water-cooled crucible 3, it is possible to use a mold whose bottom can be moved up and down as shown in FIG.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the decompression chamber 1, an output of 300 kW provided above
An electron beam 5 of 100 kW was emitted from the electron gun 8 of the above, and the metal silicon 4 continuously supplied to the melting vessel 7 at 6 kg / hour was melted. A part of the melted molten silicon 6 overflowed from above the melting vessel 7 at regular intervals, and was poured into the graphite holding vessel 2 located below. During this time, since the pressure in the decompression chamber 1 was maintained at 1 × 10 ⁻⁴ torr, a part of phosphorus, Al and Ca was vaporized and removed from the molten silicon 6. Next, a part of the molten silicon 6 is caused to overflow into the copper water-cooled crucible 3 at regular intervals from above the holding container 2 so that the solidification rate becomes 0.6 mm / min from the bottom of the crucible upward. Then, the supply speed of metallic silicon was adjusted to solidify in one direction. An electron beam 5 was emitted from above the copper water-cooled crucible 3 at an irradiation density of 0.23 kW / cm ² to heat the surface of the molten silicon 6 according to the present invention. The amount of overflow to the copper water-cooled crucible 3 per one time was about 100 on average because the diameter of the crucible 3 was 300 mm.
g.

When the ingot 9 of 50 kg was obtained, the operation was stopped and the ingot 9 was cooled to 1500 ° C.
To 200 ° C. over 1 hour. Then, the upper 20% of the ingot 9 was cut and removed, and a sample was taken from the remaining portion. Table 1 shows the analysis results of the sample. Table 1
It is clear from the above that the impurity element of silicon produced according to the present invention is significantly reduced from the value of the starting material, which is a sufficient value for rough purification of silicon for solar cells.

[0017]

[Table 1]

[0018]

As described above, according to the present invention, impurity elements in molten silicon can be removed 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 performing a method for solidifying and refining molten silicon according to the present invention.

FIG. 2 is a diagram showing a part of an equilibrium diagram of a two-component system.

FIG. 3 is a flowchart showing a conventional method for manufacturing a silicon substrate for a solar cell.

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 an apparatus for unidirectionally solidifying molten silicon described in JP-A-62-260710.

FIG. 6 is a diagram illustrating a pattern in which silicon in a holding container is irradiated with an electron beam.

FIG. 7 is a view of a container used for solidifying silicon,
(A) is a water-cooled copper crucible, and (b) is a water-cooled mold whose bottom can be raised and lowered.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Decompression room 2 Holding vessel (hearth) 3 Water-cooled crucible made of copper (casting vessel) 4 Solid (metal) silicon 5 Electron beam 6 Melted silicon 7 Melting vessel (Heath) 8 Electron gun 9 Ingot 10 Cooling water 11 Outlet

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Yasuhiko Sakaguchi 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Engineering Co., Ltd. (72) Inventor Yoshihide Kato 1-Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Technical Research In-house (72) Inventor Kazuo Aratani 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Corporation Technical Research Institute

Claims (2)

[Claims] 1. A method in which molten silicon melted under reduced pressure is poured into a casting vessel in such a manner that the molten silicon is sequentially overflowed at predetermined time intervals and the surface thereof is heated by an electron beam while solidifying the molten silicon in one direction upward from a bottom. The amount of molten silicon per overflow is calculated as (density (kg / cm ³ ) × surface area (c) of silicon in the casting container.
m ² ) × thickness 0.1 (cm)) kilogram or less, and the electron beam irradiation density is 0.07 to 0.40 k.
A method for solidifying and refining molten silicon, wherein the method is W / cm ² . 2. The method for solidifying and refining molten silicon according to claim 1, wherein the pressure in the decompression chamber is maintained at 5 × 10 ⁻³ torr or less, and the molten silicon is vaporized and dephosphorized.