Angular dependence of preferential sputtering and composition in aluminum-copper thin films

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 the film composition will vary as a function of the surface topography.