Effective secondary electron yields for different surface materials in capacitively coupled plasmas

Abstract Processes at the plasma boundaries, including the electrodes, can significantly influence plasma properties, among them the plasma density, the flux-energy distribution of various particle species, etc. The emission of secondary electrons, in particular, can lead to ionization avalanches, which strongly increase the plasma density and change the discharge operation mode as a function of the operating conditions. Using reliable values to characterize the efficiency of such processes is indispensable for accurate numerical modeling. There is, however, a lack of such data for surface coefficients for arbitrary combinations of the plasma species and electrode materials and surface conditions. In this work, we investigate the α- to γ-mode mode transition induced by changes of the operating conditions (voltage, pressure) in capacitively coupled argon plasmas for different electrode surface materials (copper, nickel, gold, aluminum, and stainless steel) and target the determination of the effective in-situ secondary electron emission coefficient, γ ∗ . The first is accomplished by phase-resolved optical emission spectroscopy applied to measure the spatio-temporal distribution of the electron-impact excitation rate from the ground state into a high-threshold-energy level of a tracer gas (neon). The studies are conducted for pressures between 50 Pa - 200 Pa and voltage amplitudes ranging from 150 V - 350 V at a driving frequency of 13.56 MHz. A mode transition from the α- to the γ-mode is shown to take place at different pressures for different materials. The combination of these measurements with particle-in-cell / Monte Carlo collision simulations employing a range of γ ∗ values allows the determination of the effective 'in-situ' electron yield for the given set of operating conditions. The simulations also shed light on the contributions of the various species, argon ions, metastable atoms, and vacuum-ultraviolet photons to electron emission from the electrodes. The findings suggest that for precise modeling individual secondary electron yields specific to different electrode surface materials should be used and multiple species should be included in the models that describe secondary electron emission at the electrodes.