The angular dependence of preferential sputtering and composition in aluminum–copper thin films

Abstract

The copper concentration in aluminum–copper alloys can be altered by ion bombardment during film deposition. We have measured the sputtering yields of aluminum and copper in Al–Cu alloys as a function of the Cu concentration (5–13 at. %) and the angle of ion incidence (0–40° from normal). During deposition, the films were partially resputtered by 500-eV Ar+ ion bombardment from a Kaufman ion source. We found that the Cu sputtering yield decreases by up to a factor of 10 in the alloy, relative to elemental Cu. The Al sputtering yield remains close to the elemental value. The net effect is a strong preferential sputtering of Al relative to Cu, which enhances the Cu concentration in an ion bombarded film. The Al/Cu sputtering yield ratio for normal incidence ion bombardment ranges from 3 to 5 as a function of Cu concentration. This ratio decreases with increasing angle of incidence to as low as 2 for 40° incident ions. However, since a higher fraction of the film is resputtered from a sloping surface, a higher Cu concentration is found on a sloping surface relative to a flat surface. These results show that in multicomponent film deposition under ion bombardment, the film composition will vary as a function of the surface topography. We will also show how the level of argon left trapped in the films varies inversely with respect to the ion flux.