Influence of Excitation Frequency on the Electron Distribution Function in Capacitively Coupled Discharges in Argon and Helium

The effect of excitation frequency on the electron distribution function (EDF) in the 13.56-60 MHz range capacitive discharge at a fixed voltage of 80 V (peak-to-peak) in the 50-200 mTorr range of argon and helium was investigated. A marked difference in the frequency dependence of the EDF was observed between the argon and the helium discharge. In the case of argon, the EDF at low pressures (50 mTorr) is bi-Maxwellian type irrespective of the frequency while the EDF at high pressures (100 and 150 mTorr) changes from Druyvesteyn type at low frequencies to bi-Maxwellian type in the VHP range (> 30 MHz). In the case of helium, however, the EDF resembles Maxwellian type over a wide frequency range, although the EDF is weakly bi-Maxwellian at frequencies higher than 40 MHz. The observed results are discussed taking into account surface heating, Ramsauer effect and two different electron heating modes (collisional ohmic heating and collisionless surface heating).