Using Langmuir probes to measure the plasma decay rates in pulsed RF magnetron discharges

Time-resolved measurements of the electron temperature T., plasma density n, floating V-f and plasma potentials V-p. have been made at different positions along the centre line in a pulsed RF magnetron plasma (frequency 50 Hz, 50% duty cycle). In the afterglow, T-e and n were found to decay with two characteristic times. These are in the ranges tau (1t) similar to 25-50 mus, tau (2t) similar to 120-130 mus, and tau (1n) similar to 35-330 mus, tau (2n) similar to 100-485 mus for T-e and n, respectively. The density decay times increase with pressure over the chosen range (0.27-2.13 Pa), while the temperature decay times are generally insensitive to pressure. The rate of particle loss is found to increase at positions closer to the cathode. A simple one-dimensional diffusion model indicates that during the 'off' times, the rate of density decay is governed by the ion flux across the floating sheath formed at the cathode. This sheath itself decays due to the falling electron temperature.