Coupled ion-ion oscillation and periodic emission pattern in electronegative capacitively coupled radio-frequency plasmas

Spatiotemporal dynamics in a capacitively coupled electronegative plasma discharge (Ar/O2 mixture) generated using a combination of 13.56 MHz sinusoidal voltage and a low frequency of a few hundred kHz tailored voltage is examined both experimentally and numerically. A periodic electron impact excitation pattern with a frequency of a few MHz has been observed by phase-resolved optical emission spectroscopy (PROES). Their formation is linked to the coupled oscillation between the positive and negative ions. The onset of this unique oscillation is set off by the electron motion triggered at the edge of the low frequency (LF) waveform, followed by the separation between positive/negative ions, and then maintained by the long-lasting ion-ion coupled motion. The established modulation of the electric field and the electron density by the ion motion generate periodic patterns in the excitation of metastable species. It is also shown in our particle in cell/Monte Carlo collision (PIC/MCC) simulations that the frequency of this ion-ion oscillation is determined only by the ion plasma frequency and its amplitude is determined by the peak-to-peak voltage of the LF and plasma electronegativity, independent of the LF waveform and frequency. The emission patterns from the experiments with different waveforms and frequencies of the LF voltage show a good agreement with the simulation results.