The role of interface diffusion in solid state dewetting of thin films: The nano-marker experiment

We proposed using an ultrathin-film marker to study the accretion/depletion of material at the interface between the thin nickel (Ni) films and sapphire substrates. The accretion/depletion of material at the film-substrate interface is closely related to the divergence of the self-diffusion flux of Ni along the interface. Thus, our method allowed determining the role played by interface self-diffusion of Ni along the interface during solid state dewetting of the Ni film. An ultrathin Si film sandwiched in the Ni–Si–Ni trilayer served as a marker, and its position after different heat treatments has being tracked by acquiring the composition maps of cross-sectioned samples. We observed the increase of the Si marker height relative to the substrate, indicating the accretion of Ni atoms at the Ni-sapphire interface due to the interface self-diffusion of Ni. This is the first direct experimental proof of interface self-diffusion during the solid state dewetting of thin metal films deposited on inert ceramic substrates. However, saturation of the upward drift of the marker with increasing annealing time, as well as its small amplitude relative to the film thickness, indicate that the interface self-diffusion is mainly active during initial stages of dewetting associated with grain boundary grooving. The edge retraction and holes expansion at the later stages of the process are controlled by the surface self-diffusion of Ni.