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

The mechanism for the reaction of a hydrogen atom with a silicon surfa ce trihydride species was investigated by ab initio molecular orbital techniques. This reaction is believed to be the final step in the over all mechanism for the etching of silicon surfaces by hydrogen atoms. T he gas-phase reaction of disilane with a hydrogen atom to form silyl r adical and silane was used as the model for the etching process. Two t ransition-state structures exist for this exchange reaction: (1) backs ide attack of a silyl group by the hydrogen atom and (2) frontside att ack of the SiSi bond by the hydrogen atom. Calculations show a lower a ctivation energy for frontside attack (3.0 kcal mol-1) than for backsi de 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 ex perimentally about the gas-phase reactions of hydrogen radicals with d isilane and provide new insight regarding silicon surface etching by h ydrogen atoms.