Charge dynamics in capacitively coupled radio frequency discharges

J Schulze (1) , E Schüngel (1) , Z Donkó (2) , U Czarnetzki (1)

1 Institute for Plasma and Atomic Physics
2 Research Institute for Solid State Physics and Optics [Budapest]

In a capacitively coupled radio frequency (CCRF) discharge the number of positive and negative charges lost to each electrode must balance within one RF period to ensure a constant total uncompensated charge in the discharge, Q tot, on time average. This balance is the result of a compensation of electron and ion fluxes at each electrode within one RF period. Although Q tot is constant on temporal average, it is time dependent on time scales shorter than one RF period, since it results from a balance of the typically constant ion flux and the strongly time dependent electron flux at each electrode. Nevertheless, Q tot is assumed to be constant in various models. Here the dynamics of Q tot is investigated in a geometrically symmetric CCRF discharge operated in argon at 13.56 MHz and 27.12 MHz with variable phase shift θ between the driving voltages by a PIC simulation and an analytical model. Via the Electrical Asymmetry Effect (EAE) a variable DC self bias is generated as a function of θ. It is found that Q tot is not temporally constant within the low frequency period, but fluctuates by about 10 % around its time average value. This modulation is understood by an analytical model. It is demonstrated that this charge dynamics leads to a phase shift of the DC self bias not captured by models neglecting the charge dynamics. This dynamics is not restricted to dual frequency discharges. It is a general phenomenon in all CCRF discharges and can generally be described by the model introduced here. Finally, Q tot is split into the uncompensated charges in each sheath. The sheath charge dynamics and the self-excitation of non-linear Plasma Series Resonance oscillations of the RF current via the EAE at low pressures of a few Pa are discussed.

Mots clés: 52.20.-j; 52.27.Aj; 52.40.Kh; 52.50.Qt; 52.65.Rr

J Schulze, E Schüngel, Z Donkó, U Czarnetzki. Charge dynamics in capacitively coupled radio frequency discharges. Journal of Physics D: Applied Physics, 2010, 43 (22), pp.225201. ⟨10.1088/0022-3727/43/22/225201⟩. ⟨hal-00569618⟩