Time-resolved Langmuir probe measurements at the substrate position in a pulsed mid-frequency DC magnetron plasma

Abstract

The time evolution of the electron density N e, effective electron temperature T eff, floating V f and plasma potential V p has been measured using a Langmuir probe in a mid-frequency bipolar pulsed DC magnetron system. The measurements were made at the position of the substrate for pulse frequencies of 50 and 100 kHz, and a duty cycle of 80%. Non-reactive sputtering of an Al target in argon at a pressure of 0.27 Pa was performed. On the application of the duty power (‘pulse on’), a burst of ‘hot’ or ‘beam-like electrons’ are detected with T eff≈20 and 8 eV for pulse frequencies of 50 and 100 kHz, respectively. Approximately 1 μs later, secondary energetic, but cooler groups are seen. During the rest of the duty cycle, T eff remains roughly constant, however, above the value obtained in the DC equivalent conditions, (there is a 25% increase in time-averaged value of T eff at 50 kHz and 33% at 100 kHz). During the positive voltage reversal (‘pulse off’), T eff decays slowly with a characteristic cooling time of approximately 20 μs. However, in this phase, N e falls by approximately an order of magnitude with a decay time constant of approximately 1 μs. As well as electron heating, measurements show pulsing the discharge increases the time averaged electron density, (21% at 50 kHz and 18% at 100 kHz). The presence of energetic electrons detected at the probe is discussed in terms of their acceleration in the rapidly expanding cathode sheath and subsequent transport both along and across the magnetic field. A simple model for the observed net electron heating due to pulsing of the cathode sheath is proposed and the effect of denser and more energetic plasmas on the flux and energy flux of impinging ions at the substrate is also discussed.