Sheath collision processes controlling the energy and directionality of surface bombardment in O2 reactive ion etching

Abstract

The effect of sheath collision processes on the energy and directionality of surface bombardment in reactive ion etching is modeled. Although the methods used are generally applicable, all the numerical examples are for a low-pressure high-frequency oxygen plasma. Charge transfer is shown to be the dominant process controlling bombardment energies. The effect of momentum-transfer collisions on ion bombardment energies is shown to be negligible. Equations are derived for the average energy of ions and neutrals, the average ion energy, the average neutral energy, and the ion energy distribution function. The ion drift velocity at a point in the sheath is related to the voltage distribution by an equation that provides a rigorous basis for a self-consistent theory of the sheath voltage distribution. These equations are generally applicable to high-frequency, low-pressure plasmas where charge transfer is the dominant collision process. The angular distribution of energetic species is modeled using elastic scattering theory. These angular distributions can be used as input to etching models that calculate profiles and process latitudes.