A TUNED LANGMUIR PROBE FOR MEASUREMENTS IN RF GLOW-DISCHARGES

Measurements of charged-particle concentrations and the electron energy distribution function (EEDF) have been made in Ar and SF6 glow discharges using a tuned Langmuir probe technique. A simple passive circuit connected to the probe when properly tuned increases the impedance between the probe and ground, thereby forcing the probe to follow the instantaneous plasma potential. In this manner, rf-induced distortion of the probe characteristic is mitigated. At 13.56 MHz the electron collection characteristic of a detuned probe is distorted by rf interference; the ion collection characteristic is unaffected. The EEDF is highly non-Maxwellian in argon discharges, but quite Maxwellian in SF6 discharges. The mean electron energy increases with decreasing pressure and increasing power in argon discharges, but is independent of pressure and power in SF6 discharges. The measured distribution functions and charged particle concentrations are in good agreement with calculations.