Time-Resolved Investigations of the Capacitive to Inductive Transition in a Microwave-Driven Plasma Source

Measurement of resonances requires data acquisition at different frequencies. Tracing the evolution of a resonance, therefore, requires a long time for each step. Reproducible events allow us to record data in the time domain at different fixed frequencies and then to rebuild the resonance shapes at different times. Using this method, the ignition process and the transition from electrostatic to electromagnetic coupling (E–H) have been investigated for plasma formation in different gases (He, Ar, N₂, and O₂) and pressures (20–2000 Pa). The microwave source used offers a miniature model of an inductively coupled plasma (ICP) inside a quartz tube and has a relatively narrow resonance in the range of 2.4–2.5 GHz with or without plasma. After a short time with only capacitive coupling, at low pressures, there is a coexistence of two resonances, indicating that capacitive and inductive coupling exist. At high pressures, the ignition time is much longer, and a common hybrid resonance appears. Helium and argon show an increase in time over tens of microseconds of the resonance frequency corresponding to inductive coupling, which means, in our global model, a very slow increase of the electron density. Nitrogen and oxygen show, on the contrary, a relatively long initial phase of capacitive coupling and then a stable electron density with inductive coupling. Moreover, oxygen at high pressures shows a plateau, initially indicating an attachment of electrons to oxygen atoms (O $^{-}$ ) and after hundreds of microseconds followed by the formation of positive oxygen ions.