Surface Segregation of Al of the Bilayers of Pure Cu and Cu-Al Alloy Films

Abstract

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 backscattering 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 ≤400°C increases to about 14 × 1015 atoms/cm2. 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 ∼3 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°C) the oxide apparently crystallizes, allowing migration of Al and O2, 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. © 2001 The Electrochemical Society. [DOI: 10.1149/1.1387239] All rights reserved.