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 mu s and a decay time between 50 mu s and 80 mu s is derived from TPOES and probe measurements at 5 Pa. The maximum time-averaged electron density, (n) over bar(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 (n) over bar(e) is reached decreases to f approximate to 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 nu(loss) controls both the rise and decay times of n(e)(t). The nu(loss) is in the first order a function of the plasma system dimensions and geometry. The decay of T-e(t) depends on nu(loss) and the losses due to inelastic scattering.