JPH11199382A - Silicon dissolution method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the melt yield of silicon in feeding the silicon powder having a specific dimension or smaller than it to a container arranged in a decompression chamber and melting the powder by irradiation with an electron beam, by forming silicon powder into a specific size, storing the formed material in a hopper in the decompression chamber, drying and then feeding the dried silicon material. SOLUTION: In charging a container (e.g. graphite crucible) 1 arranged in a decompression chamber 8 with silicon powder having <=1 mm and melting the silicon powder by irradiation with an electron beam 4, the silicon powder having <=1 mm is formed into a formed material having >=5 mm preferably by rotary granulation, the formed material is stored in a hopper 9 installed in the decompression chamber 8, dried by using an electric heater 10 as a heating source and fed to the container 1. Consequently the evaporation of readily volatile substances optionally occurring at the beginning of the electronic beam melting or the scattering of raw material silicon is suppressed to improve the melting yield of silicon.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for melting silicon, and more particularly, to a method for using an electron beam as a heating means.
The present invention relates to a technique for dissolving fine powder raw materials for silicon for solar cells.

[0002]

2. Description of the Related Art In recent years, photovoltaic power generation has been in the spotlight due to demands for diversification of energy sources, and research and development for practical use of low-cost power generation have been actively conducted. Under such circumstances, silicon as a raw material for solar cells is the most commonly used material, and polycrystalline silicon is regarded as the most important material used for power supply for power.

[0003] Silicon used as a raw material for solar cells must have a high purity of 99.9999% (6N) or more, and the concentration of impurity elements contained in silicon is reduced to the order of ppm or less. There is a need.
Conventionally, as a method for producing high-purity silicon of the above-mentioned degree from commercially available metallic silicon (purity 99.5%), F
e, Ti, Al and other metal impurity elements are removed by unidirectional solidification purification utilizing the small solid-liquid partition coefficient, and S
C contained in iC precipitates on the surface during solidification, and C dissolved in silicon is removed as CO gas,
Also, B is A to which H ₂ O, CO ₂ or O ₂ is added.
A technique of oxidizing and removing by r-plasma dissolution has been proposed. Furthermore, a method has been proposed in which P is dissolved and removed under reduced pressure, utilizing the fact that P has a high vapor pressure. Conventionally, there has been a problem that it takes a long time to remove P under reduced pressure. Recently, however, it has been reported that P in silicon can be removed in a short time by electron beam melting (ISIJ In).
international, vol. 32 (1992).
No. 5 p635-642), which is expected to shorten the de-P step. Further, as an advantage of this electron beam melting, it is mentioned that, in addition to P, Al and Ca are simultaneously removed.

[0004] When dissolving silicon powder using an electron beam, a phenomenon occurs in which substances (eg, water, organic substances, etc.) adhering to the surface of the powder at a low temperature evaporate at once at the start of heating. . A part of the raw material silicon powder is also involved in the evaporation and scatters from the container, which significantly lowers the dissolution yield of silicon. Further, there is also a problem that the atmospheric pressure in the decompression chamber surrounding the container increases due to the influence of the evaporant, and the generation of the electron beam stops. Therefore, it is considered that the silicon powder to be subjected to the electron beam melting preferably has a particle diameter of about 10 to 25 mm, which is difficult to be scattered, and a powder having a particle diameter of 1 mm or less is cut from the melting raw material. In this case, the raw material cost of silicon production becomes high, and the development of a utilization technology of finely divided silica has been desired.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for dissolving silicon in which fine silicon powder is efficiently dissolved by an electron beam.

[0006]

Means for Solving the Problems In order to achieve the above object, the inventor focused on pretreatment of silicon powder before electron beam melting, and worked diligently to realize it. And the result was completed as the present invention. That is, according to the present invention, a silicon powder of 1 mm or less is charged into a container placed in a decompression chamber, and when the silicon powder is melted by irradiation with an electron beam, the silicon powder of 1 mm or less is molded into 5 mm or more, and then the molded product is molded. Is stored in a hopper provided in the decompression chamber, dried, and then charged into the container.

The present invention also provides a method for dissolving silicon, characterized in that the silicon powder is formed by rotary granulation 1. Further, the present invention provides
The present invention also provides a method for dissolving silicon, wherein an electric heater is used as a heating source for the drying. According to the present invention, even if the silicon powder is continuously melted using an electron beam, the silicon powder is formed in advance to have a size of 5 mm or more, so that it is difficult to be scattered. Since the material is removed, evaporation of the substance and scattering of dust at the start of electron beam irradiation are reduced. As a result, not only is the dissolution yield of silicon improved, but also inexpensive silicon powder such as silicon powder and cutting scrap generated in the currently discarded semiconductor manufacturing process can be dissolved.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. First, there is an increase in the particle size of the silicon powder as a raw material for melting, which is performed by a known means for treating a powdery material. That is, a so-called rotary granulation in which the powder having a particle size of 1 mm or less is put into a rotating drum or a tara-like container (both not shown) and the particles are aggregated while adding a small amount of water, or an appropriate container. (Not shown), various particle size enlargement techniques, such as so-called pressure molding, in which powder is filled, pressurized from the upper portion to form a briquette, and then crushed to a desired size, can be used. In the present invention, among these techniques, the use of rotary granulation is preferred. This is because a large amount of granulation can be performed in a short time.

In the present invention, the particle diameter is set to 5 mm or more because silicon particles are not scattered by electron beam irradiation if it is 5 mm or more. In addition, the reason why the upper limit is not set is that the dissolution can take place at most if it takes time. However, in practice, the upper limit is preferably about 30 mm. Next, as for drying, as shown in FIG. 1, a part of a dissolving device for silicon powder is used.

That is, the apparatus is provided with a container 1 for holding dissolved silicon 5 and an electron gun 3 as a heating source inside a decompression chamber 8. In the decompression chamber 8, a hopper 9 for storing and drying the silicon 5, which is a molten raw material whose particle diameter has been expanded by the granulation, is newly provided. Then, the silicon 5 is supplied into the container 1 via the hopper 9, and is melted by heating with the electron beam 4 from the electron gun 3 provided above.

In the case of ordinary metal-grade silicon raw material, it is sufficient that the temperature required for drying is 100 to 200 ° C., which is the temperature of silicon 5 in hopper 9. The substance evaporated by drying is mainly water. It should be noted that cutting powders and the like include oils, but in this case, it is necessary to select a drying temperature according to the evaporation temperature of the oils used. The same applies to the case where a binder is used during granulation. In addition to the relatively long residence time of the silicon 5 in the hopper 9, which is one hour or more, the silicon 5 can be dried under reduced pressure by effectively using a vacuum pump attached to the furnace body. No large heating source is required. For example, the electric heater 10 or the like may be used.

[0012]

FIG. 2 shows an apparatus used in the prior art. It is provided with one graphite crucible 1 as a container 1 in a decompression chamber 8 and two electron guns 3 with a maximum output of 150 kW above the graphite crucible 1. The silicon 5 refined in the graphite crucible 1 is solidified and refined in a water-cooled copper mold 6 (receiver). Here, the shape of the graphite crucible 1 is 150 × 480 mm and the depth is 60 mm on the surface of the molten metal.

First, metal silicon 5 (purity 99.5%,
5 kg of a powder having a diameter of 1 mm or less was charged into the graphite crucible 1 and the metal silicon 5 was melted for 30 minutes while scanning the surface of the molten metal with the electron beam 4 from the electron gun 3. Thereafter, the same metallic silicon 5 was flowed from the raw material supply device 11 at a predetermined speed of 5 kg / hr until the silicon ingot in the graphite crucible 1 reached about 20 kg. In this case, the degree of vacuum in the decompression chamber 8 was 4 × 10 ⁻⁴ Torr, and the density of the electron beam 4 irradiating the molten silicon 5 in the graphite crucible 1 was 0.12 kW / cm ² .

As a result, the dissolution was completed in about 8 hours in total. However, on the ceiling of the decompression chamber 8 and around the graphite crucible 1,
A large amount of scattered silicon was attached. About 40 kg of the total amount of the metal silicon 5 powder supplied to the graphite crucible 1 was overflowed from the graphite crucible 1,
Was about 20 kg.
This is equivalent to 50% as the dissolution yield of silicon 5,
A considerable amount has been scattered.

[Inventive Example] Using a melting apparatus (see FIG. 1) equipped with a hopper according to the present invention, metal silicon 5 (purity 9) was used.
9.5%, powder having a diameter of 1 mm or less). However, in this case, approximately 5% by weight of the metal silicon 5
Of water and rotary granulation, the particle size of which is 5 mm to 25 m.
m. Here, the degree of vacuum in the decompression chamber 8, the shape of the graphite crucible 1, the density of the electron beam 4, and the maximum output of the electron gun 3 are the same as those in the conventional example. The metal silicon 5 having the adjusted particle size is stored in the hopper 9 and dried using an electric heater 10 as a heating unit.

First, 5 kg of the dried metal silicon 5 is charged into the graphite crucible 1 and the electron beam 4 from the electron gun 3 is charged.
While scanning the surface of the molten metal with
Dissolved in minutes. Thereafter, the same dried metal silicon 5 is supplied from the hopper 9 via a chute at a predetermined speed of 5 kg / kg.
hr, the silicon ingot in the graphite crucible is about 20k
g.

As a result, the dissolution was completed in about 4.5 hours in total. However, in this case, unlike the conventional example, the decompression chamber 8
Scattered matter attached to the ceiling and the periphery of the graphite crucible 1 was very small. The amount of the molten silicon that overflowed from the graphite crucible 1 and was injected into the mold 6 was about 20 kg with respect to the total amount of the metal silicon 5 powder supplied to the graphite crucible 1 of about 22.5 kg. This is equivalent to 89% as the dissolution yield of silicon 5, which is considerably improved as compared with the conventional example.

[0018]

As described above, according to the present invention, the evaporation of easily volatile substances at the beginning of the electron beam melting, which has conventionally occurred,
Alternatively, the scattering of the raw material silicon has been suppressed. As a result, not only is the dissolution yield of silicon improved, but also inexpensive silicon powder, such as silicon powder and cutting scrap generated in the semiconductor manufacturing process that is currently discarded, can be melted.

[Brief description of the drawings]

FIG. 1 is a view showing an apparatus in which a silicon melting method according to the present invention is performed.

FIG. 2 is a diagram showing a conventional silicon melting apparatus.

[Description of Signs] 1 Container (graphite crucible) 2 Overflow port 3 Electron gun 4 Electron beam 5 Silicon (metallic silicon, molten silicon) 6 Water-cooled copper mold (receiver) 7 Deposit 8 Decompression chamber 9 Hopper 10 Electric heater 11 Raw material Supply device

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hiroyuki Baba 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. (72) Inventor Naomichi Nakamura 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Iron and Steel Corporation (72) Inventor Kenkichi Yushita 1 at Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Center at Kawasaki Steel Corporation (72) Yoshihide Kato 1, Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corp.

Claims (3)

[Claims] 1. A silicon powder of 1 mm or less is charged into a container placed in a decompression chamber, and when the silicon powder is melted by irradiation with an electron beam, the silicon powder of 1 mm or less is formed into a size of 5 mm or more. Stored in a hopper provided in the decompression chamber,
A method for dissolving silicon, comprising drying and then charging the container. 2. The method for dissolving silicon powder according to claim 1, wherein the molding of the silicon powder is performed by rotary granulation. 3. The method for dissolving silicon according to claim 1, wherein an electric heater is used as a heating source for the drying.