Surface-energy-driven intermixing and its effect on the measurement of interface stress

We show that the technique of extracting interface stresses in multilayers using a force balance between layer stresses and substrate forces is extremely sensitive to small amounts of intermixing. Intermixing of as little as one half of a monolayer can produce apparent interface stresses of -2 N/m unless changes in the unstrained lattice parameter are accounted for. Diffusion and intermixing driven by large surface energy differences has been observed in several systems with large positive heats of mixing and limited bulk solid solubility, and is consistent with recently reported lattice parameter measurements in Ag/Ni multilayers. We also present in situ stress measurements in the Ag/Ni system that are consistent with this surface-energy-driven intermixing. We conclude that the reported imbalance between substrate and layer stress forces in Ag/Ni is probably not due to negative interface stresses but is more consistent with the effects of surface-energy-driven intermixing. (C) 2000 American Institute of Physics. [S0021-8979(00)09505-0].