Transient RF self-bias in electropositive and electronegative plasmas

The transient self-biasing of surfaces has been modelled to extend the utility of an isolated probe technique. The biasing is effected by the arrival of electrons drawn from the adjacent plasma but proceeds at a rate determined by the positive ion flux. Electron temperature and ion flux can be extracted from the initial stages of transient biasing. The model has been used to interpret data from a helicon plasma in argon. Shorter transients occur within the period of applied radio frequency (RF). Sheath reversal occurs during the initial stages of RF bias when the RF amplitude exceeds the normal DC floating potential. Very large RF bias signals, even after the primary transient phase, can reverse the sign of potential across the space charge sheath briefly during the cycle. The onset of this stage is mass dependent and may arise in hydrogen when the RF amplitude is only 47 times the electron temperature. The development of self-bias is also modelled for an electronegative plasma. Here, sheath reversal sets in at lower RF amplitude and the self-bias takes longer to establish than in equivalent electropositive plasmas. The model has been applied to data from a helicon plasma in sulphur hexafluoride, leading to a quantification of its electronegativity.