Dissociative attachment in silane

Cross sections for production of SiH−n from silane by electron impact from 6–12 eV have been measured with Fourier transform mass spectrometry techniques. The peak cross sections decrease and thresholds increase for the more extensively dissociated silicon hydride anions: σa(SiH−3)=1.9×10−18 cm2; σa(SiH−2)=0.8×10−18 cm2; σa (SiH−)=0.2×10−18 cm2. The cross sections for production of SiH−4 and H− are less than 8×10−21 cm2 over the 6–12 eV energy range. Ab initio calculations of the anion thermochemistry show that if the neutral products include molecular hydrogen substantial excitation of the anion or neutral fragments is required. The rates of anion production calculated from dissociative attachment cross sections show that attachment of electrons to radicals or excited species are the most important source of negative ions in silane plasmas.