Positive and negative ions in silane and disilane multipole discharges

Abstract

Mass spectra have been obtained for positive and negative ions extracted from a multipole discharge where thin films of hydrogenated amorphous silicon are produced from silane and disilane plasma decomposition. Highly polymerized ions of both signs are observed at high pressure. In silane plasmas, the pressure dependence of the positive ion composition is described according to known reaction schemes. The first steps of negative ion formation and reaction are discussed. Hydrogen depletion of large oligomeric negative ions is attributed to increasing electron affinities from Si n H 4–5 to Si n H 0–1. An upper limit for the total reaction rate coefficient, k, for Si n − and Si n H − ( n = 2 to 6) on SiH 4 is found to be ≤5×10 −13 cm 3 molecule −1 s −1 which is consistent with the similar upper limit for the total reaction rate coefficients for C 2 − and C 2H − on CH 4. The influence of increasing discharge currents on the positive and negative ion chemistry is also analyzed.