Electric fields in a microwave-cavity electron-cyclotron-resonant plasma source

A disk-shaped electron cyclotron resonant (ECR) plasma is generated inside a 2.45 GHz, cylindrical microwave cavity. The microwave electric field pattern within the cavity is measured using a microcoaxial probe while a plasma is ignited. TE211, TE311, and hybrid electromagnetic modes are identified. The radial component of the electric field adjacent to the cavity wall varies in strength from 7 to 46 kV/m. Double Langmuir probe density measurements show little variation in the plasma ion density among the various cavity modes, but discharge stability is clearly dependent on the alignment of cavity electric field patterns with the ECR magnetic field. Emission spectroscopy is used to measure the spatial distribution of ions in the ECR zones (where Langmuir probe measurements are not practical) and the distribution of fluorine radicals in SF6/Ar discharges. Peak ion densities in the ECR region are estimated at 1012 cm−3 in Ar at 9×10−4 Torr with 250 W of microwave input power.