Wave propagation and plasma uniformity in a electron cyclotron resonance plasma etch reactor

Wave propagation and absorption have been studied experimentally in an electron cyclotron resonance (ECR) plasma etch reactor operating at 2.45 GHz, with various microwave couplers. A coaxial rf probe was used to measure the wave amplitude along the reactor axis, and radial profiles of the plasma ion saturation current were determined by a Langmuir probe. A single mode whistler wave was detected, and wave absorption was found to occur at magnetic fields corresponding to an ECR frequency which is Doppler-shifted above the cold resonance. The radial uniformity of the plasma is strongly determined by the radial power distribution of the microwave coupler at lower magnetic fields where absorption occurs near the microwave window. Plasma radial transport also influences the resulting profile, especially for modes which are hollow in the center, such as the TM01. Under conditions where the waves propagate from the window without absorption for several wavelengths, the waves refract towards regions of lower density where subsequent ionization tends to correct non-uniform plasma profiles. A model is presented and yields predictions which are in good agreement with the data. The model allows calculation of wave trajectories by ray-tracing techniques and includes simple plasma transport and power deposition.