THE BOHM CRITERION FOR RF DISCHARGES

The well-known dc Bohm criterion is extended to rf discharges. Both low- (omega-rf << omega-pi) and high-(omega-pi << omega-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 dc Bohm criterion. The resulting ion current density in a high-frequency sheath is only a few percent lower than that for the dc case.