Dynamics of ion-ion plasmas under radio frequency bias

A time-dependent one-dimensional fluid model was developed to study the dyn amics of a positive ion-negative ion (ion-ion) plasma under the influence o f a rf bias voltage. The full ion momentum and continuity equations were co upled to the Poisson equation for the electrostatic field. Special emphasis was placed on the effect of applied bias frequency. Due to the lower tempe rature and greater mass of negative ions compared to electrons, the sheath structure in ion-ion plasmas differs significantly from that of conventiona l electron-ion plasmas, and shows profound structure changes as the bias fr equency is varied. For low bias frequencies (100 kHz), the charge distribut ion in the sheath is monotonic (switching from positive to negative) during each half cycle. For intermediate frequencies (10 MHz), when the bias peri od approaches the ion transit time through the sheath, double layers form w ith both positive and negative charges coexisting in the sheath. For high f requencies (60 MHz), beyond the plasma frequency, plasma waves are launched from the sheath edge, and the sheath consists of multiple peaks of positiv e and negative charge (multiple double layers). For a relatively large rang e of bias frequencies (up to the plasma frequency), each electrode is bomba rded alternately by high energy positive and negative ions during a rf bias cycle. For bias frequencies greater than the plasma frequency, however, th e electrode is bombarded simultaneously by low energy positive and negative ions with ion energies approaching the thermal value. The ion energy was f ound to increase with the applied bias potential. Also, at relatively high pressures (20 mTorr), the ion energy at low frequencies (100 kHz) is limite d by collisions. The peak ion energy may then be increased by using an inte rmediate bias frequency (10 MHz). At lower pressures, however, the effect o f collisions is mitigated while the effect of ion transit time becomes sign ificant as the bias frequency increases. In this case, a low bias frequency (100 s of kHz) is favorable for extracting high energy ions from the plasm a. (C) 2001 American Institute of Physics.