Experimental and numerical investigation of the plasma characteristics and mode transition in dual-frequency capacitively coupled argon plasmas: effects of low-frequency source and gas pressure

Capacitively coupled rf plasmas biased by a low-frequency (LF) source in the medium frequency range (0.3−3 MHz) have recently received growing attention due to high ion energy and narrow ion angular distribution, which are beneficial for the deep reactive ion etching. In this work, we report on a combined experimental-numerical investigation of the effects of the LF parameters (i.e. frequency and voltage) and gas pressure on the plasma characteristics and mode transition in dual-frequency (DF) capacitively coupled argon plasmas. The LF source is varied from 40 kHz to 2.72 MHz while the high-frequency (HF) source is fixed at 27.2 MHz. A hairpin probe is used to measure the plasma density while a phase-resolved optical emission spectroscopy technique is used to measure the spatio-temporal distributions of the electron-impact excitation rate; meanwhile, a 1d3v PIC/MCC simulation is employed to reproduce the experimental data. Results indicate that i) the modulation effect of LF source on the HF oscillation becomes enhanced (weakened) with increasing the LF voltage (HF voltage) and ii) the discharge might experience an α-to-γ mode transition with increasing the LF voltage/gas pressure or decreasing the LF frequency.