Sequence of Mg segregation, grain growth, and interfacial MgO formation in Cu–Mg alloy films on SiO2 during vacuum annealing

Abstract

Cu-alloy films are being explored for integrated circuits, for creating low-resistivity interconnects with stabilized metal/dielectric interfaces via solute segregation, and for interfacial reactions. Here, we describe the pathways of microstructure evolution in supersaturated Cu– 5–12 at. % Mg films, and phase formation at the film/SiO2 interface during annealing. The as-deposited films consist primarily of a Cu–Mg solid solution with trace amounts of orthorhombic CuMg2. Upon annealing to 400 °C, Mg segregates to the surface and the Cu–Mg grains grow from an average size of 20 to 60 nm, resulting in a ∼25%–40% decrease in film resistivity. In the same temperature regime, CuMg2 phase dissolves and fcc Cu2Mg forms. Upon annealing to higher temperatures, Mg segregates to the film/silica interface, reduces SiO2, and forms fcc MgO on the silica side of the interface. The Si released by this interfacial reaction diffuses into the metal film resulting in a ∼40%–190% increase in resistivity, for films with 8–12 at. % Mg, respectively. These results are of relevance for understanding microstructure evolution in alloy films and exploring the use of Cu alloys as interconnects in integrated circuits.