Novel Plasma Monitoring by Surface Wave Probe

This paper reviews a novel and simple technique for measuring electron density and temperature using a surface wave probe (SW probe). This probe is also called plasma absorption probe as it is based on absorption of surface waves by plasma. The SW probe enables us to measure the local absolute electron density even when the probe surface is soiled with processing plasmas. The technique relies on absorption of surface waves resonantly excited around the probe head at critical frequencies which mainly depend on the electron density. The probe consists of a small antenna connected with a coaxial cable and is enclosed in a tube of dielectric constant εd inserted in a plasma of electron plasma frequency ωp. A network analyzer feeds a rf signal to the antenna and displays the frequency dependence of the power absorption. The absorption is observed at frequencies slightly above the surface wave resonance frequency, ωp/ (1+ εd), which allows us to determine the electron density. Use of a pair of SW probes of different sizes enables measurements of both electron temperature and electron density.