Electron power absorption mode transitions in low-pressure capacitively coupled plasmas in gas mixtures*

The electron power absorption and excitation dynamics are studied by Phase Resolved Optical Emission Spectroscopy (PROES) measurements combined with Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations in low-pressure capacitively coupled plasmas (CCPs) operated in gas mixtures. Results of the PROES measurements performed in a geometrically symmetric CCP reactor show good qualitative agreement with the PIC/MCC simulation results in a wide parameter range in Ne-O₂ and Ar-O₂ mixtures. Various emission and excitation patterns develop at different pressures, driving frequencies and mixing ratios of the two gases. The mechanisms behind the formation of these patterns are revealed based on the Boltzmann term analysis. The transitions between different discharge operation modes and electron power absorption modes are examined. Significant changes in the excitation patterns are generally considered to be predictive of transitions of the dominant electron power absorption mode and discharge operation mode. The results show that it is not straightforward to infer the power absorption mode transitions based on the spatio-temporal distribution of the excitation rate alone as the same electron power absorption mechanisms could be associated with excitation patterns of significantly different characteristics, which indicate discharge operation in different modes.