  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.
