Reactive-environment, hollow cathode sputtering: Basic characteristics and application to Al2O3, doped ZnO, and In2O3:Mo

Abstract

A method for thin-film deposition has been studied. The method is based on metal sputtering in a hollow cathode configuration with supply of a reactive gas in the vicinity of the substrate. The working gas and entrained sputtered atoms exit the cathode through an elongated slot. The reactive gas is thereby largely prevented from reaching the target. The basic operation of the cathode was studied using a Cu target and pulsed power excitation. These studies included the dependence of deposition rate on power, pressure, and flow rate, film thickness profiles, and film resistivity as a function of substrate conditions. Modeling was conducted to calculate the gas velocity distribution and pressure inside the cavity. Al2O3 films were prepared in a reactive environment of oxygen by sputtering an Al target. It was demonstrated that only a very small amount of oxygen passing through the cathode will oxidize (poison) the target, whereas large quantities of oxygen supplied externally to the cathode need not affect the target at all. A very stable discharge and ease of Al2O3 formation were realized in this latter mode. The method was applied to the preparation of transparent, conductive films of ZnO doped with either Al or B. High deposition rates were achieved, and, at appropriate oxygen flow rates, low film resistivities. High-mobility In2O3:Mo transparent conductors were also prepared, with resistivities as low as 1.9×10−4 Ω cm. Scaling relations for hollow cathodes, and deposition efficiency, and process comparisons between magnetron sputtering and linear, reactive-environment, hollow cathode sputtering are presented.