The effect of the driving frequencies on the electrical asymmetry of dual-frequency capacitively coupled plasmas

Abstract

In capacitively coupled radio frequency discharges driven by two consecutive phase-locked harmonics, the electrical asymmetry effect (EAE) allows one to generate a dc self-bias as a function of the phase shift, θ, between the driving harmonics. If the two frequencies are chosen to be 13.56 and 27.12 MHz, the mean ion energy at both electrodes can be varied by a factor of about 2 by tuning θ at nearly constant ion flux. Until now the EAE has only been investigated in discharges operated at a fundamental frequency of f = 13.56 MHz. Here, we study the effect of changing this fundamental frequency on the performance of the EAE, i.e. on the electrical generation of a dc self-bias, the control range of the mean ion energy, and on the ion flux at both electrodes as a function of θ, by kinetic particle-in-cell/Monte Carlo simulations and theoretical modelling. We use argon gas and cover a wide range of fundamental frequencies (0.5 MHz ⩽ f ⩽ 60 MHz) and secondary electron yields. We find that the performance of the EAE is significantly worse at lower frequencies, i.e. the control range of the dc self-bias and, thus, the control range of the mean ion energy are strongly reduced. Based on the analytical model (i) the enhanced charged dynamics at lower frequencies and (ii) the transition of the electron heating mode induced by changing f are found to be the reasons for this effect.