Role of gas composition in weakened nonlinear standing wave excitation and improved plasma radial uniformity in very-high-frequency asymmetric capacitive Ar/CF4 discharges

Abstract

The higher harmonics generated by nonlinear sheath motion would enhance the standing wave effect, and thus lead to center-peaked plasma density profile in very-high-frequency (VHF) capacitive discharges. In this work, a nonlinear transmission line (NTL) model introduced in [Zhou et al. Plasma Sources Sci. Technol. 30 125017 (2021)] has been extended, with radial transport of various particles and nonlinear sheath motion into account, to investigate the effects of CF4 fraction α on the nonlinear standing wave excitation and plasma radial uniformity in VHF (60 MHz) capacitively coupled Ar/CF4 plasmas at 3 Pa. The results indicate that for pure Ar discharges (i.e. α=0%), the nonlinearly excited harmonics with short wavelength significantly enhance the electron power absorption at the radial center, resulting in a pronounced central-high plasma density profile. As α increases, the high-order harmonics are gradually damped due to the increase of resistance, as well as the longer wavelength caused by thicker sheath thickness. Thus, the radial profile of the electron absorbed power density shifts from center-peak to edge-high. Besides, at the radial center, the electron density and Ar+ ion density decrease with α, CF3+ ion density shows an increasing trend, while F- ion density initially rises and then decreases. Moreover, the density profiles of all the species become more uniform at higher α, due to the suppressed nonlinear standing wave excitation and the longer wavelength of the nonlinear harmonics.