Grain growth and complex stress evolution during Volmer-Weber growth of polycrystalline thin films

Volmer-Weber growth of polycrystalline thin films involves poorly understood kinetic processes that occur far from equilibrium and lead to complex co-evolution of the surface, microstructure and intrinsic stress of the films. Here we present a comprehensive study consisting of in situ stress measurements, microstructure characterization and analytical modeling for various metallic films that grow by the Volmer-Weber mechanism. We find that, under conditions of intermediate atomic mobility, stress evolution after coalescence involves a turnaround from a compressive to a tensile stress state. The thickness at which the stress turnaround is observed increases as the substrate temperature increases or the deposition rate decreases. We show that this phenomenon is associated with two competing mechanisms: grain growth during deposition (at homologous temperatures as low as 0.17) and adatom attachment to surface sites at grain boundaries. Grain growth during film deposition not only causes a tensile component of the intrinsic stress, but also leads to changes in the magnitude of the compressive stresses, from being independent of, to scaling with, the inverse of the grain size. Analyses of these phenomena lead to insights into stress evolution under conditions of low and high atomic mobility, as well as intermediate mobility, and are general for stress evolution during Volmer-Weber growth of polycrystalline films.