Recent innovations in microwave probes for reactive plasma diagnostics

Conventional Langmuir probe cannot be used in most reactive plasmas for materials processing, owing to the insulating layers that are deposited on the probe surface. To address this issue, a novel variety of microwave probes has been recently developed for the purpose of monitoring local electron density, which is a key parameter for plasma control. The probe diagnostic must be stable, compact, easy, exhibiting negligible disturbance to a plasma and materials processing. This article presents a review of the diagnostic principle, characteristics and applications of representative probes such as the plasma oscillation probe, surface wave probe (plasma absorption probe), multi-pole resonance probe, hairpin probe and curling probe. The merits and demerits of each probe are presented from a reactive plasma diagnostic point of view. Innovations in the applications of microwave probes to optical emission spectrometry and the monitoring of wall deposits are also presented. (C) 2019 The Japan Society of Applied Physics