Heating dynamics and control of energy distribution functions in capacitive discharges driven by tailored voltage waveforms
Résumé
The spatio-temporal electron impact excitation dynamics in capacitive RF discharges driven by tailored voltage waveforms in Ar and CF4 is investigated experimentally and by PIC simulations. In the experiment, the discharge is driven by up to three consecutive harmonics of 13.56 MHz with individually adjustable harmonics amplitudes and phases based on a novel RF supply system at different pressures (3 Pa - 200 Pa)1-3. The excitation dynamics is investigated by Phase Resolved Optical Emission Spectroscopy for different shapes of the driving voltage waveform (peaks/valleys/sawtooth). The DC self bias and the ion energy distribution function at the electrodes are measured. It is demonstrated that the mean ion energy can be controlled and the excitation dynamics can be tailored on a nanosecond timescale by adjusting the harmonics phases. In CF4, strongly different excitation dynamics are observed compared to Ar and understood based on the simulation results3,4. The plasma is divided spatially into two different halves of strongly different electronegativity for specific driving voltage waveforms. This asymmetry can be reversed by inverting the driving waveform. For Sawtooth waveforms, an electron heating mode transition occurs as the pressure is increased, leading to a reversed discharge asymmetry and sign of the DC self bias. References: 1. J. Franek et al., Rev. Sci. Instr., 86, 053504, (2015). 2. B. Berger et al., J. Appl. Phys., 118, 223302, (2015) 3. S. Brandt et al., Plasma Sourc. Sci. Technol., submitted 4. B. Bruneau et al., Plasma Sourc. Sci. Technol., 25, 01LT02 (2016)