Surface recombination in Pyrex in oxygen-containing plasmas

Surfaces interact with either active discharges or their afterglow in most plasma processes, via heterogeneous surface kinetics. These processes can affect both plasma and surface properties. In particular, in oxygen-containing discharges, the adsorption and recombination of atomic oxygen on reactor surfaces determine the gas composition, the availability of O for important volume reactions (e.g.: CO₂ + O → CO + O₂; CO + O + M → CO₂ + M) and eventually the flux of reactive oxygen species (ROS) towards target surfaces. The loss frequencies of O atoms have been measured in the positive column of O₂ and CO₂ glow discharges in a Pyrex tube (borosilicate glass), for several pressures, currents and wall temperatures. However, the surface mechanisms determining the recombination of O are not fully known yet. In this work the LoKI global model [1] is employed to self-consistently simulate the volume kinetics in the plasma and the kinetics of plasma species interacting with the surface. The conditions of the experiments are addressed in the simulations, including the different types of current modulation employed to measure the loss frequencies in discharge and post-discharge. The simulation results are directly compared with experimental measurements, describing the experimental dependence of the atomic oxygen loss frequencies on pressure, current, gas temperature, wall temperature and modulation type. Moreover, the newly developed model allows to identify and understand the most important O recombination mechanisms for each condition.

[1] Dias et al., Plasma Sources Sci. Technol. 32 (2023) 084003