Competition between tensile and compressive stress creation during constrained thin film island coalescence

Various analytical models have been proposed to predict the tensile stress created when discrete islands contact during a Volmer-Weber thin film growth. Past efforts to experimentally validate these models have been hindered by the stochastic nucleation of islands, which results in coalescence over a large distribution of times and length scales. To avoid this we systematically varied island geometries using electrodeposition of Ni islands on lithographically patterned conductive substrates (Au film on Si), which allowed for independent control of island size and growth rate. Using this technique, we previously demonstrated that most of the coalescence stress occurred after the initial contact of the neighboring islands, reaching a steady state when the film surface became nearly planar. In this work, we expand on these initial results to examine the kinetics of the coalescence process and to systematically evaluate the stress transition from discrete islands to a planar film. The steady state stress in planar films increased with growth rate, but asymptotically approached a limiting value for higher growth rates that depended on the island size. We attribute this to the competition between the kinetically limited compressive stress generation and tensile coalescence stress processes. The interaction of these mechanisms is consistent with both the observed transient stress evolution during the initial stages of island coalescence and the steady state stress evolution later in the process. The instantaneous stress at both the initial contact and at longer times decreased with increasing island size, as predicted in the literature. However, the existing models predict significantly larger grain size effects than those observed in these experiments. (c) 2007 American Institute of Physics.