Polysilicon

For the production of polysilicon, the metallurgical silicon undergoes further treatment. In the TCS (trichlorosilane) Synthesis Reactor, the metallurgical silicon is converted to trichlorosilane (HSiCl₃) at temperatures between 300 and 350°C using gaseous hydrogen chloride (HCI). After several distillation steps, the trichlorosilane is thermally cracked into polysilicon and hydrogen chloride in the Chemical Vapour Deposition Reactor (CVD Reactor) by adding hydrogen (H₂) at 1,000 to 1,150°C. Thereby, the polysilicon is deposited on highly pure silicon rods as a result of the so-called Siemens process.

Silicon tetrachloride (SiCl₄) is a reaction by-product of the CVD process, which is converted to trichlorosilane and then re-introduced into the process. Alternatively, the silicon tetrachloride can be burnt down by a hydrogen flame to silicic acid which can be used in several other industries. Sometimes, these two processes are carried out simultaneously.

The gaseous trichlorosilane is not fully cracked in one cycle in the CVD Reactor. The remaining trichlorosilane is directed into the Vent Gas Recovery System together with the reaction by-products, where it is recycled to be re-introduced into the CVD Reactor for the production of polysilicon. This leads to a reduction in gas losses.

Instead of TCS, monosilane (SIH₄) can also be used for polysilicon deposition. It can be produced in several ways, yet is usually synthesized from TCS. While monosilane requires lower temperatures for cracking compared to TCS (~600°C), it is more complex to synthesize the raw materials for this process. Monosilane is the common raw material when a fluidized bed reactor is applied for silicon deposition.

In both processes, the polysilicon has a level of purity of 8 to 9 N (up to 99.9999999%), suitable for the production of solar cells.

Vent gases are emitted from the CVD Reactor. Our Vent Gas Recovery System recycles these vent gases into useful process gases. The trichlorosilane is re-introduced into the polysilicon production process in a closed loop. The re-use of gases leads to cost optimization in the production of polysilicon.