Surface segregation of Al of the bilayers of pure Cu and Cu-Al alloy films

The segregation of Al at the surface of pure Cu and Cu-5 atom % Al alloy th in film bilayers was investigated as a function of the Al content, anneal t emperature and time, and annealing ambient. Rutherford backscattenng spectr ometry 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 segregat ing on the surface of the samples annealed at temperatures less than or equ al 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 to 3 nm Al2O3. The formation of this oxide was self-limiting at temperatures l ess than the crystalline transformation temperature of Al2O3 and has struct ure similar to the surface oxide on pure Al, grown at similar temperatures. At higher temperatures (500 and 600 degreesC) the oxide apparently crystal lizes, allowing migration of Al and O-2, thus leading to continued growth o f the oxide. It is hypothesized that the residual oxygen (or other oxidizin g 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 homog enization of the Al content in the bilayer and oxide formation on the surfa ce. (C) 2001 The Electrochemical Society.