Temporal evolution of electron energy distribution function and plasma parameters in the afterglow of drifting magnetron plasma

Abstract

The temporal behaviour of the electron energy distribution function (EEDF) and the plasma parameters such as electron density, electron temperature and plasma and floating potentials in a mid-frequency pulsed dc magnetron plasma are investigated using time-resolved probe measurements. A negative-voltage dc pulse with an average power of 160 W during the pulse-on period, a repetition frequency of 20 kHz and a duty cycle of 50% is applied to the cathode of a planar unbalanced magnetron discharge with a grounded substrate. The measured electron energy distribution is found to exhibit a bi-Maxwellian distribution, which can be resolved with the low-energy electron group and the high-energy tail part during the pulse-on period, and a Maxwellian distribution only with low-energy electrons as a consequence of initially rapid decay of the high-energy tail part during the pulse-off period. This characteristic evolution of the EEDF is reflected in the decay characteristics of the electron density and temperature in the afterglow. These parameters exhibit twofold decay represented by two characteristic decay times of an initial fast decay time τ1, and a subsequent slower decay time τ2 in the afterglow when approximated with a bi-exponential function. While the initial fast decay times are of the order of 1 µs (τT1 ∼ 0.99 µs and τN1 ∼ 1.5 µs), the slower decay times are of the order of a few tens of microseconds (τT2 ∼ 7 µs and τN2 ∼ 40 µs). The temporal evolution of the plasma parameters are qualitatively explained by considering the formation mechanism of the bi-Maxwellian electron distribution function and the electron transports of these electron groups in bulk plasma.