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Surface processes in low-pressure capacitive radio frequency discharges driven by tailored voltage waveforms

Particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations are performed to investigate the sputtering and the secondary electron emission (SEE) in geometrically symmetric capacitively coupled Ar discharges with Cu electrodes driven by tailored voltage waveforms (TVWs). The driving voltage waveform is composed of multiple consecutive harmonics (1 ⩽ N ⩽ 4) of the fundamental frequency (13.56 MHz) and is tailored by adjusting the identical phases (θ) of the even harmonics. The simulations are based on a discharge model in which realistic approaches are implemented for the description of the SEE induced by electrons and heavy-particles at the electrodes, as well as for the sputtering of the electrodes by heavy-particles. In case of applying a single frequency, the voltage amplitude is varied (250 V ⩽ ϕ1 ⩽ 2500 V), while in case of applying multi-frequency TVWs, N and θ are varied at a fixed total voltage amplitude (ϕtot = 1000 V) at a low pressure of 0.5 Pa. By applying more than one harmonic to drive the discharge, the mean energy of Ar+ ions and fast Ar atoms at the electrodes can be controlled by changing the phase angles. Due to the dependence of the sputtering yield on the heavy-particle energies, the flux of sputtered atoms can as well be controlled by the phase angles at both electrodes. The domain over which the sputtered atom flux can be varied is enlarged by adding more harmonics to the driving voltage waveform. For all conditions investigated, electron induced SEs (δ-electrons) induce strong ionization in the α-mode and dominate the ionization dynamics at high voltage amplitudes.