Striations in dual-frequency capacitively coupled CF₄ plasmas: the role of the high-frequency voltage amplitude

Abstract

Striations in dual-frequency (DF, 8/40 MHz) capacitively coupled CF4 plasmas have been investigated by phase resolved optical emission spectroscopy and via Particle-in-Cell/Monte Carlo collision simulations. The properties of striated structures of various plasma parameters in a DF discharge and the effect of the high-frequency voltage amplitude, ϕH, on the striated structures and charged species densities were studied at a gas pressure of 100 Pa. The measured spatiotemporal electronic excitation rates at different ϕH are in good agreement with the simulation results. It was found that the excitation/ionization patterns are modulated not only in space, but also in time by two frequencies. As ϕH increases, the width of a single ion density peak, dpeak, generally increases, leading to a decrease of the number of striations and finally to the disappearance of striations at higher ϕH. dpeak is believed to be determined by a local balance between the generation (via electron-impact ionization and dissociative attachment) and losses (primarily via recombination of the positive and negative ions, and detachment) of ions. We observed a hysteresis of the axial profiles of the measured plasma emission intensity and the simulated electron-impact excitation rate induced by increasing and decreasing ϕH semi-continuously. The dependence of dpeak on ϕH was found to play a key role in the appearance of the hysteresis.