Two types of capacitive dual-frequency discharges, used in plasma processing applications to achieve the separate control of the ion flux, Гi, and the mean ion energy, <Ei>, at the electrodes, operated in CF4, are investigated by particle-in-cell simulations: (i) In classical dual-frequency discharges, driven by significantly different frequencies (1.937 MHz + 27.12 MHz), <Ei> and Гi are controlled by the voltage amplitudes of the low-frequency and high-frequeny components, ΦLF and ΦHF, respectively. (ii) In electrically asymmetric (EA) discharges, operated at a fundamental frequency and its second harmonic (13.56 MHz + 27.12 MHz), ΦLF and ΦHF control Гi, whereas the phase shift between the driving frequencies, θ, is varied to adjust <Ei>.
We focus on the effect of changing the control parameter for <Ei> on the electron heating and ionization dynamics and on Гi. We find that in both types of dual-frequency strongly electronegative discharges, changing the control parameter results in a complex effect on the electron heating and ionization dynamics: in classical dual-frequency discharges, besides the frequency coupling affecting the sheath expansion heating, additional frequency coupling mechanisms influence the electron heating in the plasma bulk and at the collapsing sheath edge; in EA dual-frequency discharges the electron heating in the bulk results in asymmetric ionization dynamics for values of θ around 45°, i.e., in the case of a symmetric applied
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