1. Field of the Invention
The present invention relates to a vitreous silica crucible and a method of manufacturing a silicon ingot.
2. Description of Related Art
In general, silicon single crystal is manufactured by melting high-purity polycrystalline silicon in a vitreous silica crucible to obtain silicon melt, dipping an end of a seed crystal to the silicon melt, and pulling the seed crystal while rotating it.
A vitreous silica crucible is a crucible made of vitreous silica, and, in general, has two-layer structure of an inner layer of a synthetic vitreous silica layer (hereinafter, referred to as “synthetic layer”), and an outer layer of a natural vitreous silica layer (hereinafter, referred to as “natural layer”). The synthetic layer is a vitreous silica layer formed by fusing and solidifying chemically synthesized amorphous or crystalline silica (silicon oxide) powder, and the natural layer is a vitreous silica layer formed by fusing and solidifying silica powder obtained from natural mineral whose main component is a-quartz. The synthetic layer has very high purity, and thus it is possible to reduce the amount of impurities mixed in the silicon melt by providing the synthetic layer as the inner layer. The natural layer has high strength, and thus it is possible to enhance strength of the crucible by providing the natural layer as the outer layer.
The melting point of silicon is 1410 deg. C., and thus the temperature of silicon melt is kept at a temperature higher than 1410 deg. C. At such temperature, a vitreous silica crucible reacts with silicon melt, and the thickness of the crucible wall gradually decreases. When the thickness of the crucible wall decreases, the strength of the crucible is lowered. This leads to problems such as buckling and sidewall lowering of the crucible.
In order to solve such problems, there is known a technique to provide a layer, between the natural layer and the synthetic layer, for promoting crystallization of vitreous silica (e,g, JP Patent 3798907). When such a layer is provided and the crucible is heated for a long time, crystallization of the synthetic layer, which is in contact with silicon melt, is promoted, and eventually the entire portion of the inner layer of the crucible is crystallized. Crystalline silica has higher strength per unit thickness than vitreous silica, and, in addition, has lower reactivity with silicon melt than vitreous silica. Therefore, the crystallization enhances strength per unit thickness and suppresses reduction of the wall thickness of the crucible.
Furthermore, there is known a technique to provide a layer, on the outside of the crucible, to promote crystallization. When such a layer is provided, the outer layer of the crucible is crystallized and the crucible strength is improved (e.g. JP-A-2000-247778).
By use of any of these techniques, the crucible strength is improved, and thus problems such as buckling and sidewall lowering of the crucible can be solved to some extent.