Out-of-plane stresses arising from grain interactions in textured thin films

Face-centered-cubic thin films often have mixed (1 1 1)/(1 0 0) fiber texture. These orientations can have very different in-plane stiffnesses, leading to the possibility of significant stress inhomogeneities. Previous X-ray studies appeared to confirm this, reporting much higher stresses in (1 1 1)- than (1 0 0)-oriented grains. In those studies, the stress in the film normal direction was assumed to be zero everywhere, but Poisson effects suggest that out-of-plane stresses may be significant. Here, an X-ray data analysis that allows for out-of-plane stresses is presented and applied to X-ray data taken from a Cu film. The in-plane stress is shown to be homogeneous, and significant out-of-plane stresses arise. This analysis is shown to be more accurate and more consistent with the microstructure than previous methods. Consideration of inhomogeneous triaxial stress states is seen to be critical to understanding mechanical behavior of films with mixed fiber texture. Models for yielding and texture development are discussed.