MECHANISTIC INSIGHT INTO GAS-PHASE REACTIONS OF H.+SI2H6 AND HYDROGEN-ATOM ETCHING OF SILICON SURFACES

The mechanism for the reaction of a hydrogen atom with a silicon surface trihydride species was investigated by ab initio molecular orbital techniques. This reaction is believed to be the final step in the overall mechanism for the etching of silicon surfaces by hydrogen atoms. The gas-phase reaction of disilane with a hydrogen atom to form silyl radical and silane was used as the model for the etching process. Two transition-state structures exist for this exchange reaction: (1) backside attack of a silyl group by the hydrogen atom and (2) frontside attack of the SiSi bond by the hydrogen atom. Calculations show a lower activation energy for frontside attack (3.0 kcal mol-1) than for backside attack (5.8 kcal mol-1). Hydrogen abstraction is a strong competing reaction to frontside attack, having a calculated activation barrier of 2.4 kcal mol-1. These model calculations reproduce what is known experimentally about the gas-phase reactions of hydrogen radicals with disilane and provide new insight regarding silicon surface etching by hydrogen atoms.