Dynamical N-body simulations of Coulomb scattering in plasma sheaths.

Abstract

[ital N]-body simulations are used to assess the importance of discrete ion-ion Coulomb interactions in an electron-free plasma sheath, where ordinary Debye screening is ineffective. The angular dispersion [theta][sub rms] of the ion flux incident on the cathode is found to have a power-law dependence on a parameter [Omega] measuring the graininess'' of the sheath electric field, and an explanation for the observed exponent in this relation is postulated in terms of diffusion theory. In the [ital Coulomb] [ital logarithm], a lower cutoff [ital b][sub min] equal to a fraction of the mean ion-ion distance at the plasma-sheath boundary is determined, while the upper cutoff [ital b][sub max] appears to be no more than about ten times the sheath thickness [ital d]; no sustained logarithmic growth of [theta][sub rms][sup 2] with the lateral extent [ital a] over which Coulomb interactions are taken into account was observed. For sheath parameters representative of the low-pressure weakly ionized discharges used in very-large-scale-integration manufacturing processes, the dispersion due to Coulomb scattering in the sheath is typically [theta][sub rms][approx lt]0.1[degree].