The decomposition pathways of hexamethyldisilazane (HMDSZ) and tetramethyldisilazane (TMDSZ) in the gas phase were studied theoretically. This was done in pursuit of a potential single source precursor for the chemical vapor deposition of silicon carbonitride (Si_xC_yN_z) thin films. Using the CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory, the decomposition kinetics and thermochemistry were predicted for both chemicals. The major bond cleavages studied were the Si-CH₃,Si-N, N-H, and in the case of TMDSZ, Si-H bonds. It was found that the cleavages of Si-CH₃, Si-N and N-H bonds in TMDSZ had lower enthalpies than the values for the same homolytic cleavages in HMDSZ. The enthalpy required to break the Si-H bond in TMDSZ is slightly higher than that for Si-CH3, but significantly lower than those for Si-N and N-H. The reactivity of TMDSZ is further exemplified when comparing the most kinetically and thermodynamically favorable reaction in TMDSZ with its HMDSZ analog. The most favorable reaction in TMDSZ is the formation of 1-dimethylsilylaminosilene via the elimination of CH₄, with an activation barrier of 48.5 kcal/mol and a reaction enthalpy of 11.6 kcal/mol. The analogous reaction in HMDSZ leading to the release of methane and the formation of 1-trimethylsilylamino-1-methylsilene has a much higher activation barrier of 88.9 kcal/mol, making this reaction no longer the most kinetically favorable reaction within the decomposition of HMDSZ. This lowering of the activation barrier in TMDSZ for the elimination of CH₄ is likely due partly to the reduced steric hindrance in TMDSZ as compared to HMDSZ. Seven stable products are made from the decomposition of HMDSZ; two reactive silene species, two silanimine, tetramethylsilane (TMS), trimethylsilylamine, and CH₄. With the Si-H bond introduction TMDSZ produces eleven stable products during decomposition which include two reactive silene species, four silanimine, trimethylsilane (TriMS), dimethylsilane (DMS), dimethylsilylamine, and CH₄. TMDSZ can produce CH₄ and a nitrogen-containing silene much more efficiently than HMDSZ. It is known that TriMS and DMS are much more efficient precursors for Si-C film deposition than TMS. These, along with the fact that more silanimine species were produced from TMDSZ, suggest that TMDSZ could be a better single-source precursor than HMDSZ for a C-rich Si_xC_yN_z thin film formation.

Eric Ampong

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