Spatial flux and energy asymmetry in a low pressure capacitively coupled plasma discharge excited by sawtooth waveform: A harmonic study

Abstract

Plasma asymmetry generation in capacitively coupled plasma (CCP) discharges provides control over vital parameters that are useful in many plasma processing applications. In this article, using the particle-in-cell simulation technique, we investigated the spatial plasma asymmetry creation by a temporally asymmetric (sawtooth) waveform by varying its number of harmonics (N). The simulation results predict a non-linear increase in the plasma density and ion flux with an increase in N, i.e., it first decreases, reaching to a minimum value for a critical value of N and then increases almost linearly with a further rise in N. The ionization asymmetry increases with N, and higher harmonics on the instantaneous sheath position are observed for higher values of N. These higher harmonics generate multiple ionization beams that are generated near the expanding sheath edge and are responsible for an enhanced plasma density. The ion energy distribution function (IEDF) depicts a bi-modal shape for different values of N. DC self-bias is observed on the powered electrode, its magnitude with respect to the plasma potential decreases with increasing N, and therefore, the corresponding ion energy on the powered electrode decreases. The results conclude that by changing N of a sawtooth waveform in a collisionless CCP system, the ion flux asymmetry is not altered, whereas sheath symmetry could be significantly affected causing a systematic variation in the ion energy asymmetry. Due to an increase in the higher harmonic contents in the sawtooth waveform, a transition from broad bi-modal to narrow-shaped IEDFs is found.