Hillocks formation in the Cr-doped Ni thin films: growth mechanisms and the nano-marker experiment

Stress relaxation in thin metal films often results in formation of hillocks-high grains protruding from the rest of the film. Their formation is discussed in terms of surface diffusion and accumulation of the film material, as well as in terms of material accretion at the film-substrate interface and grain boundary sliding. We employed an ultrathin film of Cr oxide embedded in the middle of a thin nickel (Ni) film deposited on sapphire substrate as a marker tracking the material flow during film annealing. We found that during annealing at the temperature of 700 degrees C the marker under the growing hillocks remains largely immobile, indicating the negligible role played by grain boundary sliding in hillock formation, and the lack of material accretion below the hillock and along the film-substrate interface. We suggest that the hillocks grew by lateral diffusion of Ni, either along the grain boundaries or on the film surface covered by Cr oxide, and material accumulation on the top of the hillock by surface diffusion. We discuss the hillock nucleation and growth in terms of defects in the layer of Cr oxide formed on the surface of the Ni film.