Experimental and kinetic simulation study of electron power absorption mode transitions in capacitive radiofrequency discharges in neon

Abstract

The spatio-temporal ionization and excitation dynamics in low-pressure radiofrequency (RF) discharges operated in neon are studied and a detailed comparison of experimental and kinetic simulation results is provided for a wide parameter regime. Phase resolved optical emission spectroscopy (PROES) measurements and 1d3v particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations are performed in a geometrically symmetric capacitively coupled plasma (CCP) reactor at driving frequencies ranging from 3.39 MHz to 13.56 MHz, pressures between 60 Pa and 500 Pa, at a peak-to-peak voltage of 330 V. We examine the applicability of PROES (which provides information about the spatio-temporal distribution of the electron-impact excitation dynamics from the ground state into the Ne 2p1 state) to probe the discharge operation mode in neon (which is determined by the spatio-temporal distribution of the ionization dynamics). We find that the spatio-temporal excitation rates measured by PROES are in a good agreement with the excitation rates obtained from the PIC/MCC simulations, for all the discharge conditions studied here. However, the ionization dynamics is found to be significantly different from the excitation dynamics under most of the discharge conditions studied here, especially at higher values of the driving frequency and lower values of the pressure, when energetic heavy particle induced secondary electrons (γ-electrons) are more likely to ionize than to excite. PROES does not probe the discharge operation mode under these conditions. At a fixed frequency and peak-to-peak voltage, the spatio-temporal distribution of the ionization rate obtained from PIC/MCC simulations shows a transition of the discharge operation mode from the α-mode to the γ-mode by increasing the pressure. However, PROES fails to show this transition. While in the spatio-temporal distribution of the excitation rate obtained from the PROES measurements and the PIC/MCC simulations the α-peak (the intensity maximum at the bulk side of the expanding sheath edge) is dominant and a γ-peak (a maximum near the edge of the fully expanded sheath) becomes visible only at high values of the pressure or at the lowest frequency of 3.39 MHz, a γ-peak is visible in the ionization rate for all operation conditions, and it dominates the ionization in the vast majority of the cases investigated.