Ion energy distribution function in very high frequency capacitive discharges excited by saw-tooth 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.

High frequency sheath modulation and higher harmonic generation in a low pressure very high frequency capacitively coupled plasma excited by sawtooth waveform

“…High-frequency sheath modulation and higher harmonics were observed in low-pressure CCP discharges excited by a current driven saw-tooth like waveform [57]. The study of IEDF by varying the current density amplitude of saw-tooth like waveform and driving frequency presented narrow IEDFs at lower current density amplitudes and higher driving frequencies [59].…”

“…In the past decade, the use of non-sinusoidal voltage waveforms has emerged as a promising way to control the asymmetry and ion flux/ion energies in CCP discharges [49][50][51][52][53][54][55][56][57][58][59]. Such waveforms are known as "Tailored Waveform" that are produced by incorporating higher harmonics in the sinusoidal waveform.…”

“…The present study is focused on the temporal asymmetry in a symmetric CCP discharge excited by a sawtooth-like current waveform. We extended our previous studies to investigate the effect of the number of harmonics (N) on plasma density, ionization, electron heating, ion flux, and ion energy asymmetry in a low-pressure geometrically symmetric CCP discharge [57,59]. One of the previous simulation studies performed in collisional CCP discharge by varying a number of harmonics up to 5 [52] showed ionization, density and flux asymmetry increases with N. We followed the earlier studies to systematically investigate the effect of N on plasma asymmetry in collisionless CCP discharge.…”

Radio-frequency thermal-sheaths containing kappa distribution electrons and dust particles

“…The interaction between the plasma bulk and the sheath may produce a plasma series resonance which excites harmonics that not be driven by the power supply [17,18,19,20]. Also, substrates in the plasma are driven with tailored voltages to optimize the ion flux and ion energy to control plasma processing rates and process selection [21,22,23,24,25,26,27,28]. Tailoring the applied voltages may lead to a non-sinosoidal current.…”

A Harmonic Study of Electric Field Nonlinearity and Field Reversal in Collisionless Capacitive Discharges Driven by Sawtooth-Like Waveforms

“…In the last decade, CCP systems excited by non-sinusoidal waveforms [52][53][54][55][56][57][58][59][60][61][62][63][64][65] have shown a promising way to control the electron and ion heating asymmetry in the discharge that is useful in many plasma processing applications. These waveforms are known as "tailored waveform" produced by superimposing higher harmonics in the sinusoidal waveform.…”