Plasma-density measurements by microwave interferometry and Langmuir probes in an rf discharge

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Abstract
Improvements for two established methods for plasma-density measurements have been developed: (a) microwave interferometry and (b) single Langmuir-probe technique. The new microwave interferometer design allows us to avoid a rigid waveguide as the reference line and therefore results in a high flexibility for the use at plasma chambers with different geometries. The system is operated at 35 GHz and has a bandwidth of 10 MHz. The cylindrical single Langmuir-probe system has been equipped with a new rf-suppression circuit. This allows the application of probes of nearly arbitrary geometrical dimensions in rf-driven plasmas. The circuit is simply mounted outside the vacuum chamber between the probe feedthrough and the dc circuit. The performance of these two developments has been demonstrated and comparative measurements of charge carrier densities in a conventional rf-driven plasma show good agreement between the two methods.