GAS-PHASE N-SI ION CLUSTERS IN AMMONIA/SILANE MIXTURES

Ammonia/silane mixtures have been studied by ion trap mass spectrometry, and variations of ion abundances with reaction time in 1:5, 1:1, and 5:1 mixtures have been reported. Mechanisms of ion-molecule reactions have been elucidated by single and multiple isolation steps, and exact mass measurements of isobaric ions have been carried out by Fourier transform mass spectrometry. The SiHn+ (n = 0-3) primary ions give self-condensation processes in which SinHm+ species are formed. These ions react with NH3 and give Si2NHn+ (n = 3-6) and Si2N2Hn+ (n = 4-7) ions in successive steps with elimination of H-2. In parallel processes, the SiHn+ (n = 0-3) primary ions react with NH3 to give ions belonging to the SiNHn+ (n = 2-4) and SiN2Hn (+) (n = 4-7) families, which do not react with SiH4 furtherly. A number of precursors give SiNH6+ and NH4+ through different pathways, the last ion being the most abundant one also after short reaction times in all the mixtures examined. The rate constants of the gas-phase reactions of the most important ions have been determined by ion trap mass spectrometry, compared with calculated collision rate constants, and the efficiencies have been determined. Formation of Si2N2Hn+ (n = 3-7) and Si3N2Hn+ (n = 7, 8) ions, even if with rather low efficiencies, suggests that ionic species in addition to radicals can give a contribution to the deposition of solid silicon nitride from ammonia/silane mixtures by radiolytical methods.