Influence of substrate conditions on the temporal behaviour of plasma parameters in a pulsed dc magnetron discharge

Using time-resolved optical emission spectroscopy and Langmuir probing, the effect of substrate bias (and the absence of the substrate) on the energetics and concentrations of the plasma species at different phases of the pulse have been investigated in a bi-polar unbalanced pulsed dc magnetron. The discharge was operated in a range of frequencies, 25–100 kHz, and duty cycles, 60–90%, at a constant Ar pressure of 0.66 Pa. In the presence of an electrically grounded substrate at the transition from discharge on to off (when the plasma potential is raised to values over +150 V relative to ground), we have detected a short-lived burst in optical emissions from transitions in Ar and Ti (of duration 200 ns) in the plasma bulk. We also detect an associated elevation in the effective electron temperature measured with the Langmuir probe (Teff> twice that during the rest of the cycle). This phenomenon, we believe, is due to the liberation and increased confinement of electrons emanating from the substrate due to local ion bombardment. With an electrically floating substrate there is a much weaker associated optical flash, and none in the absence of the substrate. The Langmuir probe results also show that in the on phases of the pulse, a general increase in effective electron temperature and density is observed with decreasing frequency and duty cycle, i.e. increased reverse times. The implications of the observed transient bulk heating of electrons on the pulsed sputter deposition process are briefly discussed