The effect of radio frequency plasma processing reactor circuitry on plasma characteristics

Abstract

Past experiments have demonstrated that details of the external electrical circuitry can strongly influence the performance of radio frequency (rf) plasma processing reactors. Seemingly minor changes in the circuit, such as changing cable lengths, can lead to significantly different plasma characteristics. To investigate these couplings, a plasma equipment model has been developed which consists of a linked reactor simulation and a circuit model. In this hierarchy the results of the reactor simulation are periodically used in the circuit model to construct a simple representation of the plasma consisting of sheaths and resistors which are connected to the external circuit. Voltages (dc, fundamental and harmonics) and currents at all electrodes and reactor surfaces are computed, and are then employed as boundary conditions for the plasma reactor simulation. The models were used to investigate the effects of operating conditions, reactor geometry and stray coupling on the electrical characteristics of asymmetric capacitively coupled rf discharges. It was found that nonlinear sheaths lead to voltages and currents that have significant amplitudes at higher harmonics. As a consequence, external circuits that may appear identical to the plasma at the fundamental frequency may produce different plasma characteristics. Since plasmas are generally nonlinear and the combined plasma and circuit impedance is usually reactive, it was found that the voltage (or power) at the supply is not simply related to the voltages on electrode surfaces which generate the plasma.