Plasma dynamics in a capacitively coupled discharge driven by a combination of a single high frequency and a tailored low frequency rectangular voltage waveform

Abstract

Spatiotemporal dynamics in a capacitively coupled plasma discharge generated using a combination of 13.56 MHz sinusoidal voltage and 271.2 kHz tailored rectangular voltage is examined both experimentally and computationally. In the experiments, a fast-gated camera is used to measure the space and time-resolved emission at a wavelength of 750.39 nm from the Ar 2p1 1s2 transition. A particle-in-cell model is used to simulate the Ar plasma. The rectangular waveform is formed using 20 consecutive harmonics of 271.2 kHz, and the waveform duty cycle (DC) is varied between 5%–50%. The experiments and simulation show that excitation and argon metastable (Ar) production are primarily caused by electrons accelerated by the expanding sheath. Species generation occurs asymmetrically with more production happening adjacent to the powered electrode when the low frequency (LF) voltage is positive and vice versa. Densities of charged and excited-state neutral species decrease with increasing LF voltage due to the thinning of the plasma region and enhanced charged species loss at surfaces. At DC = 10%, the plasma responds strongly when the LF rectangular voltage switches from a small negative to a large positive voltage. Emission from the plasma and Ar production decrease considerably during this phase. When the LF voltage becomes negative again, species production and excitation remain suppressed for some time before returning to the pre-positive-pulse conditions. This reduction in plasma production is linked to the spike in electron current to the powered electrode during the positive LF voltage period, which depletes the electrons in the plasma bulk and adjacent to the grounded electrode and also raises the mid-chamber plasma potential. Plasma production suppression after the LF positive negative voltage transition lasts longer at higher LF voltage and lower high frequency voltage due to lower plasma density.