The effect of secondary electrons on the separate control of ion energy and flux in dual-frequency capacitively coupled radio frequency discharges

Dual-frequency capacitive discharges are used to separately control the mean ion energy, εion, and flux, Γion, at the electrodes. We study the effect of secondary electrons on this separate control in argon discharges driven at 2+27 MHz at different pressures using Particle in Cell simulations. For secondary yield γ≅0, Γion decreases as a function of the low frequency voltage amplitude due to the frequency coupling, while it increases at high γ due to the effective multiplication of secondary electrons inside the sheaths. Therefore, separate control is strongly limited. εion increases with γ, which might allow an in situ determination of γ-coefficients.