Improvements in or relating to methods of producing silicon of high purity

摘要

Pure silicon is prepared by the thermal decomposition of silane (SiH4) by heating to temperatures between the decomposition temperature (about 500 DEG C.) and 800 DEG C. and with molecular concentrations about 0.3 per cent. The silane may be decomposed so as to form a coherent body around a seed of pure silicon. In Fig. 2 pure silane, preheated by heater 28, is drawn through aperture 27 via valve 25 and flow meter 24 into decomposition chamber 12 by a vacuum pump connected to outlet 29. The gas flow is directed on to the silicon seed 16 supported on rod 17 and heated by copper current concentrator 18, itself cooled by water in coil 19, and primary coil 22 which form the tuned circuit of an induction heater (see Group XI). The rod 17 passes through a vacuum seal 31 and is connected to a mechanism 32 which alters its position so as to give optimum crystal growth. Pure silane is prepared from silicon tetrachloride and lithium aluminium hydride. Silicon tetrachloride is refluxed with copper turnings and the arsenic content reduced to less than 0.01 micrograms per mililitre by fractional distillation of the chloride. The silicon tetrachloride is then mixed with pure dry ether or tetrahydrofuran, and reacted with lithium aluminium hydride which has also been refluxed with pure dry ether or tetrahydrofuran. The resulting silane after fractional freezing and distillation may be stored in cylinders or swept straight into the decomposition chamber with an inert gas.