The electrical asymmetry effect in electronegative CF4 capacitive RF plasmas operated in the striation mode

Abstract

Abstract The Electrical Asymmetry Effect (EAE) provides control of the mean ion energy at the electrodes of multi-frequency capacitively coupled radio frequency plasmas (CCP) by tuning the DC self-bias via adjusting the relative phase(s) between the consecutive driving harmonics. Depending on the electron power absorption mode, this phase control affects the ion flux in different ways. While it provides separate control of the mean ion energy and flux in the α-mode, limitations were found in the γ- and Drift-Ambipolar modes. In this work, based on experiments as well as kinetic simulations, the EAE is investigated in the striation-mode, which is present in electronegative CCPs driven by low frequencies. The discharge is operated in CF4 and is driven by two consecutive harmonics (4/8 MHz). The simulation results are validated against measurements of the DC self-bias and the spatio-temporally resolved dynamics of energetic electrons. To include heavy particle induced secondary electron emission realistically, a new computationally assisted diagnostic is developed to determine the corresponding secondary electron emission coefficient from a comparison of the DC self-bias obtained experimentally and from the simulations. Based on the validated simulation results, the EAE is found to provide separate control of the mean ion energy and flux in the striation mode, while the axial charged particle density profiles and the number of striations change as a function of the relative phase. This is understood based on an analysis of the ionization dynamics.