Electron power absorption dynamics and ion energy distributions in capacitive discharges driven by customized voltage waveforms in argon and CF4

Abstract

The spatio-temporal electron impact excitation dynamics in a capacitively coupled RF discharge driven by tailored voltage waveforms in Ar and CF4 are 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 pressures between 3 Pa and 200 Pa. The excitation dynamics are investigated by Phase-Resolved Optical Emission Spectroscopy for different shapes of the driving voltage waveform (peaks/valleys/ sawtooth) at fixed total voltage amplitudes. The DC self bias and the ion energy distribution function (IEDF) at the electrodes are also measured. The formation of the IEDFs is understood based on a model that determines sheath voltage waveforms. It is demonstrated that the mean ion energy and the excitation dynamic can be controlled by adjusting the harmonics' phases. In CF4, strongly different excitation dynamics are observed compared to Ar at high pressures and are understood based on the simulation results. 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, the discharge asymmetry and the sign of the DC self bias are found to reverse as the pressure is increased, due to a transition of the electron heating mode from the α-to the Drift-Ambipolar heating mode.