Measurement of residual stresses in films of unknown elastic modulus

In expressions of film stress, the term containing the film elastic modulus is negligibly small (with respect to other terms) when the ratio of substrate‐to‐film thickness is large, thus allowing the calculation of stress in films of unknown modulus. However, the consequent large radii of curvature require large‐field‐of‐view interferometers. A Michelson interferometer capable of producing interferograms from 25‐mm‐diam samples was constructed using standard large precision optics. The large field of view compared to the usual 1‐mm diameter allows determination of very large radii of curvature.