Plasma sampling diagnostics using ion kinetic energy distributions

Abstract

Kinetic energy distributions of ions extracted from a microwave discharge in nitrogen are used to diagnose the influence of the plasma sheath on the sampling process. Nitrogen plasmas having n e ≅ 10 9–10 10 cm −3 and kT e ≅ 1 eV were examined over the pressure range 0.02 2–0.42 torr. Over this range N 2 + and N + are the major ion species with N 2 + predominant by approximately one order of magnitude. The ions are sampled from the bulk phase through the plasma sheath which forms over a tiny platinum leak. The leak itself acts as a circular planar Langmuir probe. After passage through the leak, the ions are analysed for kinetic energy by a series of planar retarding grids and them mass-analysed using a quadrupole mass spectrometer. Analysis of the ion kinetic energy distributions indicates that a model of the plasma sheath dominated by mobility considerations is applicable for pressures > 0.42 torr. On the other hand, a collisionless free fall sheath model is applicable at pressures < 0.02 2 torr. However, the sampling is complicated by the occurrence of ion—molecule reactions in the sheath. The evidence suggests that the cross section for dissociative charge transfer N 2 ++N 2  N ++N+N 2 exceeds that for symmetric charge transfer N 2 ++N 2  N 2+N 2 + at low pressure; the reaction N 2 ++N 2  N ++N 3, if present at all, does not play a major role.