Ion energy distribution function in very high frequency capacitive discharges excited by sawtooth waveform

Abstract

Tailoring ion energy distribution function (IEDF) is vital for advanced plasma processing applications. Capacitively coupled plasma (CCP) discharges excited using non-sinusoidal waveform have shown its capability to control IEDF through generation of DC self-bias. In this paper, we performed a particle-in-cell simulation study to investigate the IEDF in a symmetric capacitive discharge excited by saw-tooth like current waveform at a very high frequency (VHF). At a constant driving frequency of 27.12 MHz, the simulation results predict that the ion energy symmetry scales with the discharge current amplitude and the IEDF turn into a bi-modal distribution at higher current density amplitude. Further studies at a constant current density and varying the fundamental excitation frequency, shows that the ion energy asymmetry is greatly reduced with a reduction in the driving frequency. Increase in the plasma asymmetry and significant DC self-bias at lower driving frequency is observed to be one of the principal factors responsible for the observed asymmetry in the ion energy peaks. An investigation of DC self-bias and plasma potential confirm that the powered electrode energy peak corresponds to the DC self-bias with respect to the plasma potential, and the grounded electrode peak corresponds to the plasma potential. These results suggest that although lower frequency is good for generating the asymmetry and DC self-bias in the discharge, but a narrow low energy IEDF is only possible in very high frequency driven CCP systems.