Understanding the Reaction Chemistry of 1,1,3,3-Tetramethyldisilazane as a Precursor Gas in a Catalytic Chemical Vapor Deposition Process.

The reaction chemistry of 1,1,3,3-tetramethyldisilazane (TMDSZ) in catalytic chemical vapor deposition (Cat-CVD), including its primary decomposition on a heated W filament and secondary gas-phase reactions in a Cat-CVD reactor, was studied using 10.5 eV vacuum ultraviolet single-photon ionization and/or laser-induced electron ionization in tandem with time-of-flight mass spectrometry. It has been demonstrated that TMDSZ initially breaks down to form various species, including methyl radical (•CH3), ammonia (NH3), and 1,1-dimethylsilanimine (DMSA). The activation energies (Ea) for the formation of •CH3 and NH3 were determined to be 61.2 ± 1.0 and 42.1 ± 0.9 kJ mol-1, respectively, in the temperature range of 1400-2000 and 900-2400 °C. It was found that the formation of DMSA may have two different contributing routes, i.e., a concerted one (Ea = 33.6 ± 2.3 kJ mol-1) at lower temperatures of 900-1500 °C and a stepwise one (Ea = 155.0 ± 7.8 kJ mol-1) at higher temperatures of 2100-2400 °C. The secondary gas-phase reactions occurring in the Cat-CVD reactor environment were found to stem from two competing processes. The first one, free-radical short-chain reactions initiated by •CH3 formation and propagated by H abstraction reactions, is the dominating chemical process, producing many high-mass stable alkyl-substituted or silyl-substituted disilazane or trisilazane products via radical recombination reactions. Head-to-tail cycloaddition of unstable DMSA is the second contributing chemical process, which forms cyclodisilazane species. In addition, evidence was found for the conversion of NH3 into H2 and N2 in the Cat-CVD reactor.

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.