In-situ temperature and concentration measurements of organosilicon decomposition in a chemical vapor deposition reactor

This work reports in-situ CARS (Coherent Anti-Stokes Raman Scattering) temperature and concentration measurements in a cold-wall LPCVD (Low Pressure Chemical Vapor Deposition) vertical reactor. Silicon Carbide (SiC) is deposited from a pure low velocity Tetramethylsilane (TMS) flow. Temperature and concentration maps were measured in the gas phase near a graphite substrate heated by the Joule effect. In-situ concentration measurements were made on TMS and Hydrogen which is the main product of TMS thermal decomposition. Ex-situ partial pressure measurements were made and a temperature discontinuity was observed between the hot surface and the nearby gas in the TMS flow where thermal exchanges and chemical reactions take place simultaneously. This discontinuity is larger in the case of a pure Hydrogen flow. These measurements prove incomplete energy transfer between the rotation of molecular species and a hot surface. This is explained using an accommodation coefficient which ranges from 0.05 for Hydrogen on SiC to 0.15 for Hydrogen on graphite. Thermal exchanges have also been studied in the case of Hydrogen or Nitrogen on Tungsten. This last case exhibits a non-Boltzmann rotational distribution when the distance from the hot wall is of the same order of magnitude as the mean free path. An explanation is proposed for this effect. (author)