Correlated mode analysis of a microwave driven ICP source

Microwave and optical measurements are correlated to identify the mode evolution in a miniature, microwave driven, inductively coupled plasma (ICP) source. The very compact design of the source is derived from previous work (Porteanu et al 2013 Plasma Sources Sci. Technol. 22 035016). Microwave spectroscopy of the system resonances during the simultaneous microwave excitation of the plasma ('Hot-S-Parameter' spectroscopy) is a novel method to determine the electron density and to identify the type of coupling mode. The method corresponds directly to the kind of numerical simulations employed. The purpose of this analysis is finally to find the minimum power necessary to drive the source into the ICP mode. The efficiency of microwave energy transfer to the plasma is also discussed. Nitrogen at pressures between 50 and 1000 Pa and a gas flow of 150 sccm is used as test plasma, for which the electron density is determined. Analysis of the microwave resonance frequency shows that the electron density exceeds 1019 m−3 at 50 Pa for 11 W and at 1000 Pa for 26 W absorbed power. 3D theoretical analysis of this source confirms that at this electron density an ICP mode is present.