Gas-phase Chemistry of Methyl-Substituted Silanes in a Hot-wire Chemical Vapour Deposition Process
Date
2013-08-27
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Abstract
The primary decomposition and secondary gas-phase reactions of methyl-substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS) and tetramethylsilane (TMS), in hot-wire chemical vapour deposition (HWCVD) processes have been studied using laser ionization methods in combination with time of flight mass spectrometry (TOF-MS). For all four molecules, methyl radical formation and hydrogen molecule formation have been found to be the common decomposition steps on both tungsten (W) and tantalum (Ta) filaments. The apparent activation energy ranges from 51.1 to 84.7 kJ ∙ mol-1 for the methyl radical formation and 55.4 to 70.7 kJ ∙ mol-1 for the formation of H2. Both activation energy values increase with the number of methyl substitutions in the precursor molecules on W and Ta filaments. The formation of these two species is initiated by Si-H bond cleavage. This cleavage is then followed by Si-CH3 bond breakage producing methyl radical, whereas two H adsorbates on the surface of the filament recombine releasing H2 into the gas phase, following Langmuir-Hinshelwood mechanism.
The secondary gas-phase reactions of MMS and DMS in a HWCVD reactor have also been investigated. For DMS as a precursor gas, a competition between silene/ silylene chemistry occurring at low temperature and radical chain mechanism present at high temperature is observed. For MMS, its gas-phase chemistry involves exclusively silylene species, characterized by its insertion and dimerization reactions. It is concluded that both free-radical and silenes/silylenes intermediates play important roles in the gas-phase chemistry of methyl-substituted silanes. A comparison of the secondary gas-phase reactions of TMS, TriMS, DMS, and MMS revealed a switch in dominance from free-radical chemistry to silene/silylene chemistry as the number of methyl substitution on the precursor molecule is decreased. A study of the effect of deposition parameters, including precursor pressure and filament material, has shown that these parameters influenced significantly the gas-phase chemistry of TriMS and DMS. This is due to the competition between free-radical reactions and silylene/silene reactions. However, these parameters do not affect the gas-phase chemistry of MMS since it involves only one type of intermediate.
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Toukabri, R. (2013). Gas-phase Chemistry of Methyl-Substituted Silanes in a Hot-wire Chemical Vapour Deposition Process (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/26257