Improving the thermal stability of nickel thin films on sapphire by a minor alloying addition of gold

Thin metal films deposited on ceramic substrates tend to undergo dewetting (agglomeration)at elevated temperatures even in the solid state. We characterized the dewetting of 40 nm thick Ni films on sapphire in the temperatures range of 700–900 °C and explored the effect of a minor (below 1 at.%)Au doping on the dewetting kinetics. The Ni film was doped with Au in three different ways: by a uniform alloying, by depositing a Ni-Au-Ni trilayer, and by depositing a Au overlayer on the Ni film. We found that Au additions decrease the dewetting rate of Ni films and increase their stability at high temperatures. We discuss this phenomenon in terms of increase of surface anisotropy of Ni caused by surface segregation of Au. The increased anisotropy reduces the extent of thermal grooving at the grain boundaries and retards the formation of dewetting holes. The strongest retardation of dewetting was observed in Ni films with Au overlayer. We discuss this observation in terms of the dominance of the surface diffusion path in the dewetting process.