INVESTIGATION OF THE CHEMICAL VAPOR-DEPOSITION OF SILICON-CARBIDE FROM TETRAMETHYLSILANE BY INSITU TEMPERATURE AND GAS-COMPOSITION MEASUREMENTS

The chemical vapor deposition (CVD) of silicon carbide (SiC) from tetramethylsilane Si(CH3)4 (TMS) on a graphite susceptor at 1200-1500 K is studied in a low pressure (almost-equal-to 100 Pa) cold-wall reactor under laminar flow conditions. In addition to material characterizations (electron microscopy and chemical analysis), the gas-phase temperature distribution and composition are investigated by combining several in situ and ex situ diagnostics. Coherent anti-Stokes Raman spectroscopy (CARS) on TMS and H-2 (produced from TMS decomposition) in the hot zone of the reactor gives the rotational temperature distribution of the molecules and their concentrations. Within a few mean free paths from the surface, the H-2 gas temperature is lower than the surface temperature. This is due to a nonunity accommodation coefficient alpha of H-2 on SiC. A simple analytical model yields alpha = 0.05 for H-2 On SiC. Using gas transport coefficients and the experimental value of alpha for H-2, a two-dimensional numerical code is used to compute the gas flow and temperature profiles in the reactor. The increase of the H-2 concentration and the decrease of TMS concentration close to the surface reveals that gas-phase pyrolysis of TMS occurs within a few millimeters from the hot surface. The gas composition at the outlet of the reactor is analyzed by mass spectrometry and IR absorption spectroscopy. The global gas conversion and material balance between deposited SiC, powders, and exhaust gases is obtained. Si atoms of TMS molecules are mostly converted into solid SiC and powders. In the gaseous products a small fraction of trimethylsilane SiH(CH3)3 is detected. Other gases in decreasing order of importance are H-2, CH4, C2H4, and C2H2. These results are compared with predictions of some thermodynamic models and chemical mechanisms reported in the literature.