Electron-energy distribution function measurements in capacitively coupled rf discharges

Measurements by Langmuir probes have been performed in a capacitively coupled, parallel- plate rf discharge of He and Ar with pressures ranging from 0.1 to 2 Torr and discharge power from 10 to 50 W at 27 MHz. The first part of this paper is devoted to a discussion of the preparation of the electrostatic probe technique, for which a rf discharge is well known to be a very hostile environment, and of its intrinsic limitations, in order to assess the reliability of the measurements. Results showing a marked non-Maxwellian character of the electron-energy distribution function are then presented and discussed in the light of the most recent theories about the sustainment of this kind of gas discharge.