ELECTRICAL-CONDUCTIVITY IN HIGH-FREQUENCY PLASMAS

The electrical conductivity is an important parameter in understanding the mechanism by which power is coupled to a radio-frequency (rf) dis charge plasma, as well as in determining the external electrical chara cteristics of the discharge. We present the results of computations of the resistive and reactive components of the collisional impedance of an argon plasma at 13.56 MHz. The plasma conductivity is computed fro m the two-term solution to the Boltzmann equation, and includes the ve locity dependence of the electron collision frequency, as well as non- Maxwellian electron energy distribution functions. We compare these re sults with those obtained from the widely used classical expression fo r plasma impedance, in which the electron collision frequency is compu ted either in the de or high frequency limit. Our results show that ne ither of the classical limiting expressions are adequate for discharge pressures in the range of few mTorr to a few Torr, which includes the region of operation for many rf discharges used in many applications of plasma technology. Further, the classical formula assumes that in t he high-frequency limit the plasma reactance is due entirely to electr on inertia. We demonstrate that the plasma reactance may be strongly i nfluenced, and in some cases dominated, by electron collisions. Result s are presented in graphical form, which are useful in evaluating the importance of these effects on the interpretation of experimental resu lts and the modeling of rf discharges. (C) 1996 American Institute of Physics.