Ion energetics in collisionless sheaths of rf process plasmas

Abstract

Ion energy distribution functions in collisionless radio-frequency (rf) sheaths are discussed from the viewpoint of kinetic theory. Effects of rf fields on ion density and velocity profiles in plasma sheaths are also derived, based on ion fluid equations. It is shown that the ponderomotive force due to rf modulation of the magnitude of the sheath electric field exerts a retarding effect on the ion motion that counteracts the dc-bias field when the ratio of the ion transit frequency ωtr to rf modulation frequency ω is small but finite. Consequently, the time-averaged ion density is higher and the time-averaged ion fluid velocity is lower in rf sheaths by the order of (ωtr/ω)2 than those in corresponding dc sheaths. The influence of an oscillating plasma/sheath boundary on the ion energetics is also considered. Under suitable conditions, this induces rapid "quasiperiodic" variations in the ion energy distribution as the rf ω is increased. © 1992 American Institute of Physics.