The influence of carbon precursors,on the gas-phase chemistry of titanium carbide CVD

The influence of carbon precursors, such as methane (CH4), acetylene (C2H2), and propane (C3H8), on the gas-phase chemistry of the Ti-C-CI-H system used to deposit titanium carbide (TiC) from titanium tetrachloride (TiCl4) was investigated by calculating the thermodynamic equilibrium and modeling rate equations from a proposed kinetic model. Differences in the resultant gas-phase chemistries for reactions under reduced pressure at 1500 K are discussed. Initial mixtures of TiCl4, H-2, and a hydrocarbon that produced identical equilibrium compositions were determined. Results for the Ti-C-Cl-H system for all three hydrocarbon precursors are compared to those of Ti-CI-H and C-H in order to further illustrate the differences in thermal stability of key constituents, as well as the extent of conversion. Regardless of which carbon precursor is present, species belonging to the Ti-Cl-H subsystem are near equilibrium within 5 s. Conversely, species emanating from CH4 decomposition in the Ti-C-Cl-H system are predicted to be far from equilibrium and unreactive with TiCl4- In contrast, the preponderant species under thermodynamic equilibrium, C2H2, provides a uniform source of carbon at any residence time. Finally, the stability Of C3H8 in the Ti-C-Cl-H system is significantly greater than in the C-H system. This greater stability is accompanied by a significant variation in the rate of by-product formation, suggesting that TiCl4 and its subchlorides effectively compete with C3H8 for important reactive intermediates. In the case Of C3H8, a reaction flow analysis was carried out identifying the three main reaction paths that are responsible for propane decomposition, the further links that appear between these paths, and how the paths vary over time.