To prevent the failure of a casting and a casting mold even if the casting is flat by managing the cooling rate in the bottom of the casting mold to specific conditions at the time of casting polycrystalline silicon by unidirectionally solidifying the silicon in the casting mold from the lower part to the upper part by a prescribed method.
The cooling rate in the bottom of the casting mold 40 is managed to ≤5°C/min at the time of casting the polycrystalline silicon by lowering the casting mold 40 from the upper part to the lower part, thereby unidirectionally solidifying the silicon 30 in the casting mold 40 from the lower part to the upper part within a solidifying furnace 20 provided with a heating section 21 in the upper part and a cooling section 22 in the lower part. At the time of casting, the front surface of the silicon 30 in the casting mold 40 is preferably maintained at a molten state until the time of the final solidification by increasing the areas of heaters 23a to (c), 24 contributing to the front surface heating of the silicon in the casting mold 40 accompanying with the descending of the casting mold 40. Namely, the failure of the casting and the casting mold by the volumetric expansion occurring in the earlier solidification of the upper part, the confinement of the molten part in the casting and the solidification of the molten part is prevented.
OGASAWARA TADASHI
OKAMOTO NOBORU
FUJITA KENJI
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