We have used both conventional cantilever and new micro-machined microelectromechanical devices to make high-sensitivity measurements of the bending forces exerted on substrates during nucleation, pre-coalescence growth, coalescence and post-coalescence growth of polycrystalline films have been made. Measurements of stress evolution as a function of substrate temperature and deposition rate have been made at all stages of growth. Measurements of stress evolution during interruptions of growth have also been made at different stages of film formation and thickening, and for interruptions of growth that have been carried out under different steady-state conditions.
We find, as others have in other systems, that Cu films are in an apparently compressive state prior to, and in the early stages of, coalescence, and evolve into a tensile state during coalescence. These films evolve back into a compressive state during post-coalescence thickening. We also find, as have others in other systems, that growth interruptions during post-coalescence thickening lead to reversible tensile relaxations. We argue that the reversible stress evolution that occurs during growth interrupts is associated with differences in the shapes of the grain surfaces during growth and during growth interruptions. This argument is supported by simulations of the morphological evolution that occurs during growth and during growth interruptions, and by analyses of the effects of shape evolution on forces exerted by the deposit on the substrate. These models are consistent with a wide range of data, both in prediction of the magnitudes of the stress changes, and in predictions of the kinetics of stress evolution.