Process for preparing polysilicon with diminished hydrogen content by using a two-step heating process

In a process for preparing particulate polysilicon, which process comprises:

• (a) a productivity mode in which silicon seed particles are contacted, in a fluidized bed, with a gas comprising from about 10 to about 100 mol % silane at a temperature which causes the thermal decomposition of the silane whereby silicon metal resulting from such decomposition is deposited on the seed particles to yield intermediate silicon particles and whereby silicon dust is formed which deposits on the surfaces of such intermediate silicon particles;
• and (b) a quality mode which comprises contacting, in a fluidized bed, the intermediate particle having surface dust with a gas comprising from about 1 to about 5 mol % silane at a temperature above the thermal decomposition temperature of the silane whereby a silicon metal layer is deposited on the intermediate particles which cements at least a portion of the silicon dust to the intermediate particles, the improvement which comprises subjecting the intermediate particles which have at least a portion of the silicon dust cemented thereto to heat in a fluidized bed in the presence of a non-contaminating inert gas for a time and at a temperature which exceeds the fluidized bed temperatures used in forming such cemented intermediate particles so that the hydrogen content of such cemented intermediate particles is reduced, the temperature however being insufficient to melt-consolidate the cemented intermediate particles.