Time-resolved investigations of pulsed microwave excited plasmas

Pulsed microwave excited (2.45 GHz) argon plasmas generated by a slot antenna type plasma source are investigated by various diagnostic tools. Through the combined use of time-resolved planar optical emission spectroscopy (TPOES), microwave interferometry (MWI) and Langmuir probes the temporal behaviour of the electron density, n e ( t ), and effective electron temperature, T e ( t ), for the pulse frequency range of 0.2-20 kHz are measured. Additionally, from TPOES maps of Ar * and Ar + , the qualitative spatially and time-resolved electron temperature distribution is derived. The n e ( t ) and T e ( t ) rise and decay times are almost constant throughout the examined frequency range. A n e ( t ) rise time of 1 ms and a decay time of 0.6 ms is derived from probe and MWI data at 5 Pa. A T e ( t ) rise time between 5 and 10 µs and a decay time between 50 µs and 80 µs is derived from TPOES and probe measurements at 5 Pa. The maximum time-averaged electron density, e , at 5 Pa is obtained at a pulse frequency f of 200 Hz. With increasing pressure and power the pulse frequency f at which a maximum of e is reached decreases to f 50 Hz. The temporal n e ( t ) and T e ( t ) behaviour for the investigated pressure range is described by a simple set of equations based on the `Global Model' of pulsed plasmas. It can be concluded that the electron loss rate loss controls both the rise and decay times of n e ( t ). The loss is in the first order a function of the plasma system dimensions and geometry. The decay of T e ( t ) depends on loss and the losses due to inelastic scattering.