Electron heating and the electrical asymmetry effect in dual-frequency capacitive CF4 discharges

The electron heating and the electrical asymmetry effect (EAE) in electronegative dual-frequency capacitive CF4 discharges are investigated by particle-in-cell simulations and analytical modeling. One electrode is driven at 13.56 and 27.12 MHz with fixed but adjustable phase shift, theta, between the driving harmonics. First, the electron heating and ionization rates are studied, space and time resolved, for different phase shifts and pressures. The results are compared with those obtained for an electropositive gas (argon). In contrast to classical alpha- or gamma-mode operation, we observe the electron heating and ionization rates to be high inside the bulk. This bulk heating mode is a consequence of the high electronegativity of CF4 discharges, where the conductivity in the bulk is low due to the low density of electrons. Thus, a high electric field builds up to drive the RF current through the bulk causing a high electron mean energy and ionization rate in the discharge center. Second, we investigate the consequences of the bulk heating on the EAE. We focus on the electrical generation of a dc self-bias as a function of theta and the quality of the separate control of the ion mean energy and flux at the electrodes by tuning theta. Compared with argon discharges the high voltage drop across the plasma bulk and the specific ionization dynamics affect the bias generation and the separate control of ion properties. These effects are described and explained by an analytical model.