Electron power absorption in capacitively coupled neon-oxygen plasmas: a comparison of experimental and computational results
Phase Resolved Optical Emission Spectroscopy (PROES) measurements combined with 1d3v Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations are used to study the electron power absorption and excitation/ionization dynamics in capacitively coupled plasmas (CCPs) in mixtures of neon and oxygen gases. The study is performed for a geometrically symmetric CCP reactor with a gap length of 2.5 cm at a driving frequency of 10~MHz and a peak-to-peak voltage of 350 V. The pressure of the gas mixture is varied between 15 Pa and 500 Pa, while the neon/oxygen concentration is tuned between 10% and 90%. For all discharge conditions, the spatio-temporal distribution of the electron-impact excitation rate from the Ne ground state into the Ne $\rm{2p^53p_0}$ state measured by PROES and obtained from PIC/MCC simulations show good qualitative agreement. Based on the emission/excitation patterns, multiple operation regimes are identified. Localized bright emission features at the bulk boundaries, caused by local maxima in the electronegativity are found at high pressures and high O$_2$ concentrations. The relative contributions of the ambipolar and the Ohmic electron power absorption are found to vary strongly with the discharge parameters: the Ohmic power absorption is enhanced by both the high collisionality at high pressures and the high electronegativity at low pressures. In the wide parameter regime covered in this study, the PROES measurements are found to accurately represent the ionization dynamics, i.e., the discharge operation mode. This work represents also a successful experimental validation of the discharge model developed for neon-oxygen CCPs.