Theoretical Study on the Decomposition Kinetics and Thermochemistry of Tetramethyldisilazane and HexamethyldisilazaneFormation of Silanimine and Silene Species

The gas-phase decomposition kinetics and thermochemistry of 1,1,1,3,3,3-hexamethyldisilazane (HMDSZ) and 1,1,3,3-tetramethyldisilazane (TMDSZ), two potential single-source precursors for the chemical vapor deposition of silicon carbonitride thin films, were systematically investigated using ab initio calculations at the B3LYP/6-311++G­(d,p)//CCSD­(T)/6-311++G­(d,p) level of theory. Both concerted and stepwise decomposition routes for each molecule were examined, allowing for a comparison of the reactions involving the cleavages of common bonds of Si–C, Si–N, and N–H for the two molecules. A new set of reaction pathways open to TMDSZ due to the presence of a Si–H bond was also explored. It was found that all three bonds of Si–N, Si–C, and N–H could be broken more easily in TMDSZ than HMDSZ. Both HMDSZ and TMDSZ are capable of producing silene and silanimine species upon decomposition. In fact, the most kinetically and thermodynamically favorable pathways fall in the formation of these species. The concerted formation of 1-dimethylsilylaminosilene via the elimination of methane from TMDSZ is the most kinetically and thermodynamically favorable route between the two molecules with an activation barrier (ΔH₀^⧧) of 48.5 kcal mol^–1 and reaction enthalpy (ΔH₀) of 11.6 kcal mol^–1, respectively. These values are lower than the corresponding lowest values in HMDSZ of ΔH₀^⧧ = 66.4 kcal mol^–1 for the concerted production of 1,1-dimethylsilene and trimethylsilylamine and ΔH₀ = 41.7 kcal mol^–1 for the formation of CH₄ and N-trimethylsilyl-1,1-dimethylsilanimine. Overall, this work has provided insights into the reactivity of the two molecules. It has been shown that TMDSZ is more reactive than its analog HMDSZ due to the presence of the Si–H bonds and reduced steric hindrance.