Seals for Siemens Process
High Performance Seals for Polycrystalline Silicon Process Equipment
To meet growing demand in solar cells, the Siemens* process is used to produce highly purified polycrystalline silicon by thermally decomposing tri-chloro-silane (TCS). The reactors employed require three kinds of high-performance seals in the following locations: A) bell jar seals; B) electrode seals between the electrode and ceramic inlet; and C) a gasket for the view port seal (see schematic).
The process involves a very hot, dry environment with temperatures up to 1200 °C. Sealing components see up to 300 °C. Process media include TCS and aggressive by-products of its decomposition, such as HCl gas. Superior compression set performance is needed for effective sealing and extended seal life. Very low outgassing is a key requirement for preventing contamination of the process environment.
The presence of harsh chemicals in high temperature conditions often require an upgrade from standard sealing materials to PTFE or fluoroelastomers (FKM) to improve thermal stability and chemical resistance. However, PTFE can deform and creep “hot flow” in the bell jar seal increasing the risk of product loss. FKM, with its high temperature rating of 200 °C, also has deficiencies in this 300 °C environment.
Kalrez® PV8070 perfluoroelastomer parts installed as washers and gaskets in reactors for the Siemens CVD process have demonstrated outstanding performance in this demanding application. Kalrez® PV8070 parts are effective seals that provide:
- Resistance to continuous service temperatures up to 325°C and the ability to handle short-term excursions to higher temperatures.
- Superior compression set performance to maintain effective sealing over time.
- Resistance to aggressive media such as those used in the Siemens CVD process.
- Minimal outgassing, thus minimizing the potential for process contamination.
* A non-proprietary process for polysilicon production which has been in existence for nearly fifty years