Plasma absorption probe for measuring electron density in an environment soiled with processing plasmas

In this paper, we describe a novel and simple technique for measuring electron density using. a plasma absorption probe (PAP). PAP enables us to measure the local absolute electron density even when the probe surface is soiled with processing plasmas. The technique relies on the resonant absorption of surface waves (SWs) elicited in a "cavity" at the probe head. The PAP consists of a small antenna connected with a coaxial cable and is enclosed in a tube (dielectric constant epsilon) inserted in a plasma (electron plasma frequency omega(p)). A network analyzer feeds a rf signal to the antenna and displays the frequency dependence of the power absorption. A series of resonant absorptions are observed at frequencies slightly above the SW resonance frequency omega(sw) = omega(p)/(1 + epsilon)(1/2), which allows us to determine the electron density. The measured electron densities are in good agreement with the data obtained by the plasma oscillation method.