The Bohm criterion for rf discharges.

The well-known de Bohm criterion is extended to rf discharges. Both low- (ω[sub rf « ω[sub pi]]) and high- (ω[sub pi] « ω[sub rf]) frequency regimes are considered. For low frequencies, the dc Bohm criterion holds. This criterion states that the initial energy of the ions entering the sheath must exceed a limit in order to obtain a stable sheath. For high frequencies, a modified limit is derived, which is somewhat lower than that of the de Bohm criterion. The resulting ion current density in a high-frequency sheath is only a few percent lower than that for the dc case. [ABSTRACT FROM AUTHOR]