Ion energy distribution functions in a planar inductively coupled RF discharge

TLDR

Low‑pressure inductively coupled plasmas in planar geometry are increasingly used for plasma surface modification, offering high plasma densities with low ion impact energies. The study investigates the formation of ion energy distributions at a grounded wall in planar inductively coupled RF discharges, using experimental and theoretical approaches. Ion energy distribution functions were measured with a retarding field analyser and analyzed using a simple model that incorporates the non‑thermal electron energy distribution. The study found that parasitic capacitive coupling influences the distributions, which are formed in the collisional presheath and accelerated in the collisionless sheath; the observed ion impact energies correlate with non‑Maxwellian electron distributions, and models assuming Maxwellian electrons overestimate these energies.

Abstract

Low pressure inductively coupled plasmas in planar geometry have found increasing interest as a means of plasma surface modification in the last few years. These discharges provide high plasma densities at simultaneously low impact energies of the ions. In this paper the formation of an ion energy distribution at a grounded wall of such a discharge is studied experimentally as well as theoretically. The ion energy distribution functions are measured via a retarding field analyser. The effect of a parasitic capacitive coupling from the induction coil is reported. The ion energy distributions observed experimentally are interpreted as being formed in the collisional presheath and being accelerated in the collisionless sheath. A quantitative analysis is presented on the basis of a simple model, which takes into account the non-thermal character of the electron energy distribution. It is shown that observed ion impact energies are correlated to non-Maxwellian electron distributions and that models that imply the assumption of a Maxwellian overestimate the ion impact energies.