Spatially resolved measurements in r.f. capacitive discharges in argon and nitrogen

Abstract

Plasma potential, electron concentration and energy distribution of ions collected by grounded surface in contact with plasma were measured in argon and nitrogen capacitively coupled r.f. discharges. Both diagnostics tools used, Langmuir probe and Balzers Plasma Process Monitor (PPM 421), were oriented parallel to the movable electrodes allowing a perpendicular spatially resolved measurements. The asymmetry of capacitive coupling was estimated from the plasma potential and d.c. self-bias. It increased for increasing power since the discharge extended towards the grounded PPM orifice, its tubular housing as well as chamber walls. The IEDs measured at the high power with PPM therefore reflected the situation at the grounded electrode. It exhibited only one peak which was typically found for low voltage modulated r.f. sheaths. The mean energy of Ar+ and N2+ ions was about 92% of the energy related to the plasma potential. Moreover, low energy electron tail appeared in their IEDs. These both facts are explained by collisions in the sheath. Since the mean free path of N+ ions is about one order higher we did not observed effect of collisions in their IEDs. The ion mean energies decreased towards the grounded electrodes in agreement with the plasma potential. The ion and electron concentrations showed similar profiles with the maximum about 12 mm above the grounded electrode.