DC plasma potential pattern in low-pressure RF discharge

Summary form only given. RF discharges are widely used in various technological processes, therefore studies of the properties of such a discharge are of considerable interest. If one knows the spatial distribution of the dc (time-averaged) plasma potential /spl psi//sub pl/ then one can clarify the mechanisms of many phenomena occurring in RF discharge. As is known /spl psi//sub pl/ increases steadily when one moves from the electrode to the boundary of the electrode layer and its value remains constant in the quasineutral plasma. This paper shows experimentally that one observes the minimum of the dc plasma potential in the central part of the RF discharge at gas pressures of p/spl ges/0.1 Torr. Experiments have been performed in air and argon at gas pressures between 0.005 and 2 Torr with RF voltage values U<1000 V, RF frequency f=13.56 MHz and spacings between plane electrodes L=10/spl divide/54 mm. A DC plasma potential has been measured with single cylindrical probes. At low gas pressures (p<0.1 Torr) and moderate spacings (L<1.5 cm) it remains constant or decreases slowly when one moves from the discharge center whereas it experiences fast decrease up to zero in electrode layers. At pressures p>0.1 Torr and sufficiently large L one observes a minimum in the axial distribution of the dc plasma potential in the central part of the discharge. This minimum appears when the RF discharge is going from the weak current form of burning to the strong current one.