Translationally hot neutrals in etching discharges

Abstract

The presence and influence of translationally energetic ions in low-pressure etching discharges is well known. Neutral atoms and molecules, though known to be chemically reactive, are not generally considered to be otherwise activated in these plasmas. Neutral species may, however, become translationally hot through either charge exchange collisions or by dissociative excitation caused by electron impact. These species are important in etching discharges because they may bring an isotropic source of activation energy to the substrate which may compromise anisotropic etching mechanisms. In this paper we present a theoretical study of the sources and effects of translationally hot neutral atoms and molecules in CF4 etching plasmas. We find that ballistically hot F atoms comprise a significant fraction of the radical flux striking the substrate at pressures of <100 mTorr. In CF4 plasmas, the maximum flux of translationally hot F incident on the substrate of a parallel-plate rf etching discharge occurs between 10 and 100 mTorr. At these pressures the hot atom and ion fluxes to the substrate are comparable. The effects of translationally hot species on gas-phase plasma chemistry and surface reactions are discussed.