JPH10182133A - Refining of silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To repeatedly use a graphite crucible useful for silicon refining and to prolong the life of the graphite crucible by applying or depositing SiC or C as a releasing agent to or on the inside of a graphite container in refining silicon by melting by an electron beam. SOLUTION: SiC or C as a releasing agent 2 is applied to or deposited on the inside of a silicon refining container 1 made of graphite for purifying silicon 3 in vacuum by using an electron beam as a heating source. A material having the concentration of impurities lower than those of the target concentration of impurities of silicon is preferable as the releasing agent 2. In order to provide the inside of the graphite container 1 with the releasing agent 2, for example, a solution obtained by adding 1% PVA to pure water is mixed with carbon particles having 0.5μm average particle diameter to give a slurry having 30% water content, which is applied to the inner wall of the container 1 by spraying. After stamping, the coated film is dried together with the graphite container 1 in an Ar atmosphere at about 1,500 deg.C for about 3 hours.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to silicon for photovoltaic cells, which is obtained by heating P, Al,
The present invention relates to a silicon purification method used for removing impurities such as Ca.

[0002]

2. Description of the Related Art In recent years, photovoltaic power generation has been in the spotlight as an energy source due to the demand for diversification of energy sources, and research and development are being actively conducted toward the practical use of low-cost power generation devices. Under these circumstances, silicon is the most commonly used material as a raw material for solar cells, and crystalline silicon is regarded as the most important material used for power supply for power.

[0003] The purity of silicon used as a raw material for solar cells must be 99.9999% (6N) or more.
It is necessary to reduce the concentration of impurities contained in silicon to the order of ppm or less. Conventionally, in order to manufacture the high-purity silicon from commercially available metal silicon (purity: 99.5%), it has been known that impurity elements such as P, Al, and Ca can be simultaneously removed in a short time by electron beam melting. I have.

A technique for removing impurities such as P, Al, and Ca in a short time by using an electron beam as a heat source has been developed, taking advantage of the advantage of evaporating and removing impurities such as P, Al, and Ca by electron beam melting. Conventionally, a water-cooled crucible has been used as a silicon refining vessel. However, since the heat efficiency is low, a technique has been developed to improve the efficiency and use a graphite vessel to raise the temperature of molten silicon, that is, to further improve the impurity removal rate.

[0005]

The use of such a graphite container has the advantage that the rate of removal of impurities (P, Al, Ca) can be greatly improved as compared with a water-cooled container, but the graphite container can be reused. When considered, there was a problem that silicon adhered to the graphite container and could not be separated, so that it could not be separated. In particular, since the impurities adhering to the upper portion of the inner surface of the container from the hot water surface are concentrated, if not removed, there is also a problem caused by contamination from the adhering substances such as a variation in silicon impurity concentration.

The present invention has been developed to solve such a problem, and has as its object to repeatedly use a graphite container used for refining silicon by electron beam melting of silicon and prolong its life.

[0007]

According to the present invention, an electron beam is used as a heating source, and a high temperature, high temperature necessary for melting an electron beam as a mold release agent is provided on the inner surface of a graphite silicon purification vessel for purifying silicon in a vacuum. A silicon refining method characterized by applying or depositing SiC or C powder that withstands vacuum and does not contaminate silicon.

Preferably, the release agent is a material having an impurity concentration lower than the target impurity concentration of silicon.

[0009]

FIG. 1 is a sectional view of a graphite container used for purifying silicon by electron beam melting of silicon.
In the present invention, a release agent is applied to the inner surface of a graphite container to facilitate the peeling of silicon, prevent breakage of the graphite container, and enable reuse of the container. SiC or C is used as a release agent. These release agents are stable at high temperature and high vacuum, and the dissolved amount of carbon in silicon is about 100 ppm.
Easy to remove by oxidation.

The release agent is applied by using water and P as a binder.
Brush or spray the inside of the graphite container using VA and then stamp and densify. Stamping is performed to prevent silicon from seeping into the release agent layer. By applying the release agent, adhesion between the silicon and the container is prevented, and it becomes possible to reuse the graphite container after separating the container and the silicon ingot and to separate deposited silicon which causes contamination. Therefore, the graphite container can be used repeatedly and the life can be extended.

[0011]

EXAMPLE A solution obtained by adding 1% of PVA to pure water was added to carbon particles having an average particle size of 0.5 μm to prepare a slurry having a water content of 30%. This slurry has an inner size of 300 mm x 150 mm x
It was applied to the inner wall of a 5 mm graphite container by spraying, and after stamping, dried together with the graphite container in an Ar atmosphere at 1500 ° C. for 3 hours. The thickness of the carbon particle layer after drying is about 1 m
m.

After charging 2000 g of commercially available metal-grade silicon into the above container and dissolving the silicon with an electron beam output of 30 kW for 1 hour, separation of the silicon ingot from the container was attempted. It turns out.
Similarly, the same experiment was performed on silicon carbide particles having an average particle diameter of 0.4 μm, and similar results were obtained.

The carbon concentration in the silicon ingot after separation was measured by a combustion method and found to be 55 ppm. The surface of the graphite container was coated with a dense silicon carbide film having a thickness of about 100 μm by a chemical vapor deposition method, and the same dissolution test was performed. The same result was obtained.

[0014]

According to the present invention, in a silicon device in a graphite container using an electron beam, a graphite container which has been replaced with a new one by using once or twice can be used repeatedly.

[Brief description of the drawings]

FIG. 1 is a sectional view of a graphite container.

[Explanation of symbols]

 1 Graphite container 2 Release agent 3 Silicon

Claims (2)

[Claims] 1. A method for refining silicon, comprising applying or vapor depositing SiC or C as a release agent on the inner surface of a silicon refining container made of graphite for refining silicon in a vacuum using an electron beam as a heating source. 2. The silicon refining method according to claim 1, wherein the release agent is a material having an impurity concentration lower than a target impurity concentration of silicon.