As the demand for polysilicon increases dramatically with the demand for solar photovoltaic cells and modules, the process design and production capacity of precursor gas plants also increase in order to capture economies of scale, reduce production cost, and produce more uniform product quality. Gas plants produce primarily trichlorosilane (TCS) gas used in the production of semiconductor-grade polysilicon in Siemens reactors for both semiconductor and solar applications and tetrahydrosilane gas (SiH4) used in the production of solar-grade polysilicon in fluidized bed polysilicon reactors for use exclusively in the solar business.
When polysilicon gas plants had production capacities below 1,000 metric tons per year, the by-product gas from both the gas plant and the Siemens deposition reactor was often incinerated, especially in nonintegrated facilities. More recently, by-product silicon tetrachloride (STC) has been recycled to the hydrochlorination reactor in an integrated plant, or distilled and sold as a separate by-product in a nonintegrated polysilicon plant. Modern polysilicon gas plants with production capacities exceeding 10,000 metric tons per year now have the economic incentive to recover the by-product gases and recycle them efficiently to the gas plant in an integrated facility. In a nonintegrated plant, current technology allows for 'closed loop' operation by converting STC back to feedstock TCS, which is recycled to the Siemens reactor for polysilicon deposition.
A significant by-product from both the Siemens deposition reactor and hydrochlorination reactors is silicon tetrachloride (STC). This study examines 3 recycle options for STC: 1) recycle the STC containing stream to the hydrochlorination fluidized bed reactor at the front of an integrated process; 2) install an on-purpose STC converter at an integrated facility, which is primarily a very-high-temperature (1,000–1,900°C) electric furnace that preferentially hydrogenates STC to TCS; and 3) install an on-purpose STC converter at a nonintegrated facility, using existing purification processing units for recycle. To assist operating companies in evaluating the merits of an on-purpose STC converter, we present preliminary process design information and corresponding economics for an STC converter system capable of making 20 kty of trichlorosilane from silicon tetrachloride and hydrogen, while producing by-product anhydrous hydrogen chloride.