Thermodynamic Comparison of Silicon Carbide CVD Process between CH3SiCl3-H2 and C3H8-SiCl4-H2 Systems

In order to understand the difference in SiC deposition between the CH3SiCl3-H-2 and C3H8SiCl4-H-2 systems, we calculate the phase stability among beta-sic, graphite and silicon. We constructed the phase-diagram of beta-SiC over graphite and silicon via computational thermodynamic calculation considering pressure (P), temperature (T) and gas composition (C) as variables. Both P-T-C diagrams showed a very steep phase boundary between the SiC+C and SiC region perpendicular to the H/Si axis, and also showed an SiC+Si region with a H/Si value of up to 6700 in the C3H8-SiCl4-H-2, and 5000 in the CH3SiCl3-H-2 system. This difference in phase boundaries is explained by the ratio of Cl to Si, which is 4 for the C3H8-SiCl4-H-2 system and 3 for the C3H8-SiCl4-H-2 system. Because the C/Si ratio is fixed at 1 in the CH3SiCl3-H-2 system while it can be variable in the C3H8-SiCl4-H-2 system, the functionally graded material is applicable for better mechanical bonding during SiC coating on graphite substrate in the C3H8-SiCl4-H-2 system.