Mechanical characterization of thin films via constant strain rate membrane deflection experiments

Abstract

The mechanical behavior of freestanding metal thin films (gold, aluminum, and nickel-molybdenum-tungsten alloy) is characterized using micro-tensile testing and membrane deflection experiment (MDE). Micro-tensile and membrane samples are simultaneously fabricated on a single wafer to avoid microstructural differences. A constant strain rate MDE methodology is developed to directly compare each test method. Finite element analysis is performed to investigate potential sources of error in MDE. Specifically, the effects of in-plane and out-of-plane misalignment, tip blunting, sample dimension, and substrate deformation are studied, and physical insights gained from the analysis were applied to the experiments. By comparing with the micro-tensile tests, it is experimentally demonstrated that the stress-strain curve of freestanding metal thin films can be successfully measured via constant strain rate MDE. In particular, yield stress shows a good agreement with that obtained from micro-tensile tests, demonstrating the usefulness of applying the constant strain rate methodology in MDE testing. Furthermore, MDE enables residual stress measurements of the deposited films. Constant strain rate MDE developed in this study can serve as a useful technique for high-throughput measurement of in-plane mechanical properties of metal thin films.