Control of ion flux-energy distributions by low frequency square-shaped tailored voltage waveforms in capacitively coupled plasmas

Abstract

Capacitively coupled plasmas are routinely used in an increasing number of technological applications, where a precise control of the quantity and the shape of the energy distribution of ion fluxes impacting boundary surfaces is required. Oftentimes, narrow peaks at controllable energies are required, e.g. to improve selectivity in plasma etching, which cannot be realized in classical discharges. We combine experimental ion flux-energy distribution measurements and PIC/MCC simulations to provide insights into the operation and ion acceleration mechanisms for discharges driven by square-shaped tailored voltage waveforms composed of low-frequency (100 kHz) pulsed and high-frequency (27.12 MHz) signals. The formation of ion flux-energy distributions with a narrow high energy peak and strongly reduced ion fluxes at intermediate energies is observed. The position of the high energy peak on the energy axis can be controlled by adjusting the low-frequency voltage pulse magnitude and duty cycle. The effects of tailoring the driving voltage waveform by adjusting these control parameters as well as its repetition rate on the plasma operation and the ion flux-energy distribution are analysed in depth. We find, e.g. that the duty cycle regime (% or %) determines if the high energy ions form at the grounded or the powered electrode and that the duration of the pulse must exceed the ion energy relaxation time, on the order of 0.5 μs.