Fluid model simulations of a 13.56-MHz rf discharge: Time and space dependence of rates of electron impact excitation

Abstract

Results from fluid model simulations of a 13.56-MHz rf discharge qualitatively reproduce many of the experimentally observed features of time and space resolved electron impact excitation in several gases (oxygen, nitrogen, and silane). The shape of the excitation rate waveform, its direction of propagation, time of occurrence in the rf period, and the initial increase in intensity followed by attenuation are all observed experimentally as well as in the simulation. Secondary electrons play no direct role in the excitation waveform in the simulation, although secondary electron creation is included in the model. The excitation waveform is the result of the combination of electron motion, electric field profiles, and the electron energy balance in the discharge. Electron heating by the rf field peaks at the plasma-sheath boundary, resulting in a local rise in electron mean energy there. It is suggested that this electron heating mechanism is common to all high-frequency rf discharges in electropositive or weakly electronegative gases, and that this mechanism is responsible for increased rates of electron impact molecular dissociation around the plasma-sheath boundary. The qualitative agreement between simulation results and experimental measurements implies that the fluid model captures essential elements of rf discharge physics, and that fluid models are useful for the interpretation of discharge diagnostics.