Application of Resonance Microwave Probes as Electron Density Sensors

Abstract

A concept of measuring local electron density in weakly magnetized plasma based on quarter wave resonator was introduced by Stenzel in 1976. In the recent years, the technique has attracted noticeable interest for diagnosing industrial plasmas. The resonance probes, also popularly known as hairpin probe (HP) because of its characteristic shape, have been successfully applied in many industrial plasma systems including deposition plasmas and dual radio-frequency operated confined CCP discharge. The principle is based on supporting a standing wave that corresponds to the plasma permittivity in the near field region around the resonator. The above probing technique has the advantage over microwave interferometer because they are capable of providing local measurement of electron density. One important application of resonance microwave probe is its capability of providing spontaneous response to the change in dielectric medium around the probe head. This unique property can therefore be employed as a sensor for monitoring the plasma during long-pulse operation in industrial plasma tools. In this paper we describe the underlying principle behind the resonance hairpin prove and finally introduce the concept of split-ring-resonator as a possible substitute for plasma surface diagnostics. The split-ring-resonator (SRR) is unique in the sense that it can be mounted on the wall of the plasma chamber therefore it is less perturbing to the plasma. Additionally the diagnostics can provide useful information about the state of plasma adjacent to the substrate during dielectric etching and plasma deposition treatments. A comparative study of SEE with the conventional hairpin probe applied in a magnetized linear plasma device is presented and its prospective role as a diagnostic sensor is discussed