Positive Ion Chemistry of SiH4/NF3 Gaseous Mixtures Studied by Ion Trap Mass Spectrometry

Abstract

The positive ion chemistry occurring in silane/nitrogen trifluoride gaseous mixtures has been investigated by ion trap mass spectrometry. Reaction sequences and rate constants have been determined for the processes involving the primary ions SiH n+ ( n = 0–3) and NF x+ ( x = 1–3) and the secondary ions obtained from their reactions with SiH4 and NF3. The SiH n+ efficiently react with NF3 and undergo cascades of abstraction and scrambling reactions which form the fluorinated and perfluorinated cations SiHF m+ ( m = 1, 2), SiH2F+ and SiF x+ ( x = 1–3). Fluorinated Si2-clusters such as Si2H2F+, Si2H3F+ and Si2H5F+ were also observed. The reaction of both SiH3+ and SiH2F+ with NF3 produces the elusive fluoronitrenium ion NHF+. Any NF x+ reacts with SiH4 mainly by charge transfer. Additional ionic products are, however, observed which suggest intimate reaction complexes. Worth mentioning is the formation of SiNH2+ from the reaction of both NF+ and NHF+ with SiH4. The primary ions NF2+ and SiH3+ are also “sink” species in our observed chemistry.