Electron density measurements on an inductively coupled plasma with a one-port microwave interferometer

Abstract

A one-port microwave interferometer has been employed to measure the electron density in a conventional reactor for plasma-enhanced chemical vapour deposition (PECVD). As opposed to standard two-port configurations, which require two accessible viewports in the reactor, this setup uses only one port and is therefore readily coupled to standard industrial vacuum equipment. The interferometer works at 45.75 GHz and separates forwarded and reflected waves. A microwave mirror is positioned at the reactor wall opposite the viewing port. The capability of the method is demonstrated on the basis of exemplary measurements in an inductively coupled plasma (ICP, 13.56 MHz) suitable for PECVD. Line-integrated electron densities are measured for different conditions (Ar, p = 0.1–1 Pa, Prf = 30–1000 W). Langmuir probe measurements provide radial profiles of the electron density and are used as a comparative reference. These profiles are fitted with Bessel functions and serve as the basis for the interpretation of the interferometric data in terms of peak density, where the amplitude of these generic profiles is used as the free parameter. The densities obtained are in good agreement with the Langmuir probe results. Maximum values 1 × 1018 m−3 are obtained in the centre of the vessel at an axial distance of 0.24 m from the antenna window of the ICP source.