Potential oscillations of electronegative plasmas have been experiment
ally studied in a strongly asymmetric rf (13.56 MHz) discharge. Oscill
ation amplitudes in the plasma were found to increase suddenly from 10
to 60V with increasing concentration of SF6 gas in He gas, while grad
ually from 10 to 40V in CF4 mixed plasma. The electron energy distribu
tion functions (EEDFs) measurement predicted an electron beam travelli
ng from the plasma towards the rf electrode because of the potential d
ouble layer in the sheath picked up by an emissive probe. A clear diff
erence in potential formation and EEDFs profiles in comparison to thos
e in electropositive plasmas has been detected. A more marked gas conc
entration dependence was obtained in a SF6 gas than in a CF4 one due t
o the more active production of negative ions in a SF6 plasma.