Sheath resistance measurements in the GEC reference reactor

Abstract

A Langmuir probe biased above or below a floating potential draws net electron or ion current. As a result, a different electron current must flow through the grounded electrode sheath to maintain the plasma's time-averaged quasi-neutrality. The direct current components of the sheath, plasma and floating potentials must change from their values at zero probe current in order to change the electron current to the grounded electrode. The rate at which the floating potential changes with probe (and sheath) current is called the sheath resistance. Analysis of probe data could give incorrect values for the plasma potential, the electron temperature and the electron density (ne), and give a distorted electron energy probability function if sheath resistance is ignored. The sheath resistance was measured using a Langmuir probe and a microwave interferometer in inductively coupled plasmas in argon, nitrogen and an argon(95%)/chlorine(5%) mixture in the Gaseous Electronics Conference reactor. The sheath resistance was found to be small for these measurements, consequently if it had been ignored, the electron density would have been underestimated by up to 42% and the electron temperature overestimated by up to 31%. The sheath resistance was found to have an ne-1 dependence in argon as expected, but an ne-1.4 dependence in nitrogen and an ne-0.6 dependence in the argon/chlorine mixture.