Control of charged particle dynamics in capacitively coupled plasmas driven by tailored voltage waveforms in mixtures of Ar and CF4

Abstract

The charged-particle power absorption dynamics in capacitively coupled plasmas operated in different CF4-Ar gas mixtures and driven by tailored voltage waveforms is experimentally investigated by phase-resolved optical emission spectroscopy in conjunction with kinetic simulations and an analytical model. Single- and triple-frequency ‘peaks’- and ‘valleys’-type waveforms (generated as a superposition of multiple consecutive harmonics of 13.56 MHz) are studied at pressures of 20 and 60 Pa with 25 mm electrode gap and 150 V total driving voltage amplitude to determine the effects of the tailored driving voltage waveform in different gas mixtures on the density profiles of the particle species, the electronegativity, the DC self-bias, and the excitation/ionization dynamics. As the argon content in the buffer gas is increased, the discharge switches from the drift-ambipolar (DA) power absorption mode to the α-mode. This transition occurs due to the disappearance of the bulk and ambipolar electric fields as the electronegativity of the plasma decreases with increasing argon content. This effect is more pronounced at higher pressures, where the negative ion density is higher. We observe a significant change in the plasma’s symmetry, DC self-bias, and mean electron energy as a result of the DA- to α-mode transition. At 60 Pa the simulation reveals a drastic increase of the spatially averaged electronegativity induced by increasing the argon admixture from 20% to 30%. This counterintuitive finding is explained by the effect of this admixture on the spatio-temporal electron dynamics. Finally, the generation of the DC self-bias as a function of the argon content is understood by the analytical model based on these fundamental insights into the plasma physics.