During main decomposition, the silane flow and filament temperature are controlled to follow parameters defined by a particular growth curve. The efficiency changes throughout the run, but the gross chemical efficiency is between 95 and 98% depending upon the growth curve. Generally, the exiting silane concentration is below 1% to provide an overall yield of 98 mol% (2 moles of hydrogen being produced per mole of silane).
A fraction of the silane decomposes to silicon but does not become incorporated into the growing filament/rod. This amorphous silicon is a very fine powder having an average particle size of 20 to 50 nanometers.
The polysilicon reactors have large internal cooling surfaces which become coated with the fine silicon powder. Reactor cooling water is supplied to the tube bundle, bell jar, base plate, and the Reactor Internal Heat Exchanger (E-14XX). The internal heat exchanger allows the reactor to run with a faster growth rate by cooling the bulk gas and providing a large surface area to collect powder. Production runs are stopped prematurely when the powder collected on the walls gets thick enough to fall from the walls.
All reactors have exhaust gas filters, Reactor Gas Filter GF-1 (F-14XX-1) and Reactor Gas Filter GF-2 (F-14XX-2) to remove silicon powder from the hydrogen exhaust gas being recycled to the FBR Unit.