Process for the production of ultra high purity polycrystalline silicon

Claim 1

A process for the formation of polysilicon crystal rods in which the formation of silicon powder is reduced comprising: (a) providing an elongated starter rod substantially in the center of a cooled enclosure which enclosure has a wall at an effective radius of from about 6 cm. to 23 cm. and an upper outlet and a lower inlet; (b) energizing the starter rod while introducing silane into the cooled enclosure, said starter rod being maintained at a temperature sufficient to effect silicon decomposition and deposition thereon and forming a thermal boundary layer around the periphery of said rod and produce by homogeneous decomposition silicon powder, said silane being provided at a rate sufficient to maintain the silane concentration in the cooled enclosure in an amount of at least about 0.5 mole percent; and (c) continuing step (b) for a time sufficient to grow a silicon rod having a diameter no greater than about 80 to 90% of the diameter of the enclosure, and while growing said rod: (i) maintaining the wall of the enclosure at a temperature below about 100° C.; (ii) removing at least a portion of an effluent comprised of unreacted silane, silicon powder and by-product hydrogen from the upper outlet of the enclosure; and (iii) recycling the effluent at a rate of at least about 50,000 cubic cm./min. to the lower inlet of the enclosure.