CHLORINATION AND DISTILLATION
The manufacturing of high-purity semiconductor grade polysilicon begins with the raw material, metallic silicon, of greater than 98% purity.
The metallic silicon is reacted with hydrogen chloride gas (HCI) in a fluidized bed reactor, producing trichlorosilane gas (TCS) and a side stream product, silicon tetrachloride (STC). The STC is sent to a co-product sharing customer.
The trichlorosilane gas is cooled in a chain of low-temperature heat exchangers and liquefied. Impurities with higher or lower boiling points than the trichlorosilane liquid - such as iron, boron, phosphorous and aluminum compounds - are then removed in a series of distillation columns. The refined trichlorosilane is stored in tanks and tested for chemical and physical performance before further processing.
REDUCTION
Once it has been analyzed, the trichlorosilane liquid is vaporized, mixed with hydrogen gas and sent to a reduction or Chemical Vapor Deposition (CVD) furnace. During the chemical vapor deposition process, silicon from the gas stream is deposited on the surface of electrically heated polysilicon seed rods. The reduction furnace is precisely controlled to grow the polysilicon rod to a predetermined diameter.
Exhaust gases which are released during this process are cooled, liquefied and distilled, yielding the co-products silicon tetrachloride (STC) which is marketed separately and trichlorosilane which is recycled.