The segregation of Al at the surface of pure Cu and Cu-5 atom % Al alloy thin film bilayers was investigated as a function of the Al content, anneal temperature and time, and annealing ambient. Rutherford backscattenng spectrometry was used to follow Al movement through the thickness of the film. Al appears to segregate on the surface as an oxide. The amount of Al segregating on the surface of the samples annealed at temperatures less than or equal to 400 degreesC increases to about 14 x 10(15) atoms/cm(2). At the same time, a small amount of Al is retained in the overlying Cu through which Al is diffusing to the surface. The Al at surface is equivalent to similar to3 nm Al2O3. The formation of this oxide was self-limiting at temperatures less than the crystalline transformation temperature of Al2O3 and has structure similar to the surface oxide on pure Al, grown at similar temperatures. At higher temperatures (500 and 600 degreesC) the oxide apparently crystallizes, allowing migration of Al and O-2, thus leading to continued growth of the oxide. It is hypothesized that the residual oxygen (or other oxidizing species like water) reacts with Al arriving on the surface, providing the thermodynamically favored driving force to migrate Al from the alloy film through the overlying pure Cu film to the surface leading to both the homogenization of the Al content in the bilayer and oxide formation on the surface.