The validity of Bohm’s sheath criterion in rf discharges

Abstract

In the limit of a small Debye length λD the plasma boundary layer is split into a thin space-charge sheath and a quasineutral presheath. In dc discharges, the well-known Bohm criterion expresses a necessary condition for the formation of a stationary sheath. With the increasing importance of rf discharges there has arisen some confusion on the validity of the sheath condition for rf sheaths. It is shown that Bohm’s sheath criterion holds without modification in rf discharges with arbitrary frequency ratio ω0 /ωpi (ion plasma frequency).