Oxidation of an aluminum magnetron sputtering target in Ar/O2 mixtures

Abstract

The oxidation kinetics during sputtering of an Al target in a planar magnetron have been studied by accurate measurement of the total pressure produced by argon/oxygen mixtures. When a 5×8‐in. target is rf sputtered at 500 W with a fixed argon flow, the pressure is constant at 4.6 mTorr until the oxygen flow rate is increased to 2.1 cm³/min. At this critical flow, the total pressure increases to 7.85 mTorr in 100 min and the time dependence is explained by a parabolic oxidation rate. The equilibrium oxide thickness is about 100 nm and has been measured by determining the time necessary to cause a sudden increase from 180 to 290 V in self‐bias voltage. When the target was oxidized, Al₂O₃ films were deposited at 3 nm/min, whereas Al was deposited in an argon discharge at 70 nm/min. The ratio in sputtering rates of Al and Al₂O is a factor of 2 greater than predicted from calculated yield values. Before target oxidation, oxygen added to the system is gettered by the film until, at values just below the critical value, Auger analysis shows the films have a composition AlO_x where 1<x<1.5.