Electric potential screening on metal targets submitted to reactive sputtering

A very thin oxide layer is formed on top of metal surfaces that are submitted to reactive magnetron sputtering in an oxygen atmosphere. Having a few atomic monolayers thickness (1–5 nm), this oxide top layer shows properties of an electric insulator that retards the flux of incident ions. Here, the authors show that this layer can be modeled as a parallel combination of capacitance and resistance. The basic sputtering processes on the oxide layer have been mimicked by means of particle beam experiments in an ultra-high-vacuum reactor. Hence, quantified beams of argon ions and oxygen molecules have been sent to aluminum, chromium, titanium, and tantalum targets. The formation and characteristics of the oxide top layer have been monitored in situ by means of an electrostatic collector and quartz crystal microbalance. The charge build-up at the oxide layer interfaces generates a screening potential of the order of 1–10 V, which shows linear correlation with the total current through the target. The secondary electron yields of the oxides show the expected behavior with ion energies (500–1500 eV), thereby showing that this parameter is not significantly distorted by the screening potential. Charging kinetics of the oxide layer is investigated by means of time-resolved current measurements during bombardment with square-wave modulated ion fluxes. Finally, the dependence of secondary electron emission with surface oxidation state and surface charging issues in pulsed plasmas are studied within the context of the Berg's model.