An experimental and computational study on the ignition process of a pulse modulated dual-RF capacitively coupled plasma operated at various low-frequency voltage amplitudes

Abstract

The ignition process of a pulse modulated capacitively coupled argon discharge driven simultaneously by two different radio frequency voltages (12.5 MHz (high frequency, HF) and 2.5 MHz (low frequwncy, LF)) is investigated by multifold experimental diagnostics and particle in cell / Monte Carlo collision (PIC/MCC) simulations. In particular, (i) the effects of the low frequency voltage amplitude measured at the end of the pulse-on period, V L,end, on the spatiotemporal distribution of the electron impact excitation rate determined by phase resolved optical emission spectroscopy, and (ii) the electrical parameters acquired by analyzing the measured waveforms of the plasma current and voltage, are studied. Computed spatiotemporal distributions of the electron impact excitation rate and electrical parameters show a good qualitative agreement with the experimental results. Generally, the HF and LF electrical parameters (amplitudes and relative phase of the voltage and the current) change with time in a similar manner during the ignition process for each V L,end. However, various scenarios of the breakdown mechanism are found as a function of V L,end. At low values of V L,end, the “RF-avalanche” mode dominates the electron multiplication process. By increasing V L,end, the ionization caused by the volume electrons is suppressed and the electron loss at the electrodes is enhanced, leading to a delayed ignition. At higher values of V L,end, the avalanche ionization is significantly enhanced by ion-induced secondary electron emission at the electrodes.