Thermal Converters (Silicon Tetrachloride [STC] Conversion Reactors)

Recycle Processes - Since about 8 kg of silicon tetrachloride are formed when depositing 1 kg of polysilicon, it is necessary to design a cost-effective process for recycle of STC and the other co-products. In the Siemens process, the large volumes of silicon tetrachloride are reacted with excess hydrogen at high temperatures (1400 degrees C) to form TCS.

Other processes have been proposed in the past. Both Texas Instruments and Motorola patented two-stage processes for recycled compounds. The Texas Instruments patent describes a two-stage reaction of silicon tetrachloride, MG silicon, hydrogen and hydrogen chloride to produce TCS. Hydrogen is added in the first stage with temperature at 1327 degrees C; hydrogen chloride is added in the second stage with temperature at 800 degrees C.

The Motorola patent is similar, with lower temperatures, but uses a copper catalyst and generates large amounts of dichlorosilane. Dichlorosilane can be disproportionated with silicon tetrachloride to form TCS. DCS can also be reacted with MG silicon and hydrogen chloride to form TCS.

Another process uses a catalytic converter to convert STC to TCS. 

Another design uses the hydrochlorination process to convert STC to TCS. In this design, trichlorosilane is produced by the reaction of silicon tetrachloride, MG silicon, and hydrogen according to:

MG Si  +  2 H₂  +  3 SiCl₄  =>  4 HSiCl₃ at about 500 degrees C and 500 psi.  Small particles of MG silicon are fluidized with hydrogen in a fluid bed to maximize the reaction rate.  A large reactor can be built to process all of the STC gases from the deposition reactors. The disadvantage of this process is the high operating temperature and pressure, increasing safety risk.

STC Conversion Reactor Design and Performance - The thermal conversion reactor consists of a large specialty steel vessel with internal heater rods and electrical, gas, and cooling water utilities. A reactor design is available commercially, sized to process the STC generated by two of the fast-rate trichlorosilane deposition reactors. 

Key Words: STC reactors, thermal converters, STC to TCS conversion, trichlorosilane recycle, silicon tetrachloride recycle, conversion of silicon tetrachloride to trichlorosilane