Agglomeration and long-range edge retraction for Au/Ni bilayer films during thermal annealing

Abstract The solid state dewetting of Au/Ni bilayer films from SiO2/Si substrates exhibits both homogeneous and localized dewetting of Ni and long-edge retraction for Au under isothermal annealing condition. The top Au layer retracted up to 1 mm from the edge of the substrate wafer to reduce the energetically unfavored Au/Ni interface. In contrast, Ni dewetted and agglomerated locally due to its limited diffusivity compared to Au. Film morphology and local chemical composition varied significantly across hundreds of micrometers along the direction normal to the retracting edge. The accumulation of Au from long-range edge retraction resulted into the separation of Au and Ni, which are affected by alloying between the two elements. The experimental observations suggest that alloying combined with unequal self-diffusion coefficient for the metal components control the bilayer dewetting process.