The effect of electrode area ratio on low-frequency glow discharges

In many radio frequency plasma applications, use of unequal area electrodes leads to a dc self-bias voltage that is correlated with sputtering, etching, deposition, and surface damage rates. Using the techniques of laser-induced fluorescence (LIF) and Stark-mixed LIF, we measure (1) ion densities in N2 discharges and (2) electric fields in BCl3 and 5% BCl3/Ar discharges as a function of electrode area ratio (from 1:1 to 7.4:1). We find that the discharge current density is constricted and nonuniform. As a consequence, the dependence of sheath field strengths on electrode area ratio is weaker than expected, ion bombardment of the larger electrode is nonuniform, and large radial electric fields periodically pull ions into the small electrode sheath. These observations are consistent with ballistic (or beam) electron models for low-frequency glow discharges.