Microstructure-controlled interdiffusion of Cu/Co/Au thin films investigated by three-dimensional atom probe

The dependence of thin-film microstructure on the curvature of the substrate is investigated by field ion microscopy (FIM) and transmission electron microscopy (TEM). For this purpose, Cu–Au multilayers were deposited on W single crystals, which are prepared both as curved (radius of curvature ∼50 nm) and as planar substrates. 3D atom probe analysis shows that the curved substrates lead to heterogeneous reaction patterns by grain boundary reaction beside planar reaction patterns. This result is explained by local variations of microstructure. Grain boundary reaction is dependent on the atomic structure of the grain boundary and can lead to the early formation of intermetallics. At temperatures too low for significant volume diffusion, interdiffusion between enriched grain boundaries and neighbored grains is observed under the formation of ordered compounds. The elastic strain, which is caused by a considerable atomic size mismatch, is found to hinder interdiffusion and to cause alloying under grain boundary motion (DIGM). The introduction of a Co diffusion barrier between Au and Cu slows down interdiffusion, resulting in higher reaction temperatures. Thus, it is possible to observe that the grain boundary reaction in the Au layer disappears at elevated temperatures, at which volume diffusion of Cu in Au gets possible, whereas in the Cu layer the grain boundary reaction is still observed.