Anion reactions in silane plasma

摘要

Previous measurements of Si_(x)H_(m)~(-) anions and electrons in the afterglow of silane discharges are analyzed here. A model of plasma anion chemistry includes two sources of anions, two possible causes of x→x+1 anion growth, and anion loss by mutual neutralization. Comparison to pulsed-discharge measurements for x=1-6, for a range of discharge times, clearly establishes radical reactions as the primary cause of anion growth; prior estimates generally suggested silane reactions. This comparison also indicates that electron attachment to radicals, not silane, is the largest source of SiH_(m)~(-) anions. Measured afterglow electron decay and SiH_(m)~(-) signals are also consistent with efficient, low energy electron attachment to radicals. Reasonable agreement with the observations is obtained using realistic estimates of the radical density, cation density, and the values of radical-anion (k_(x)) and mutual neutralization rate coefficients. However, the x dependence of the k_(x) required to fit the data is surprising. These conclusions have major consequences for silane-discharge, particle-nucleation models.