Charge dynamics in capacitively coupled radio frequency discharges

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 and 27.12 MHz with variable phase shift theta 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 theta. 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 pascals are discussed.