Yield and Deformation in Biaxially Stressed Multilayer Metallic Thin Films

Multilayer thin films of CuNb, CuNi, and CuNbNi with 20 nm individual layer thicknesses were fabricated by magnetron sputtering on oxide coated silicon wafers. The mechanical properties of the films were measured using bulge testing and nanoindentation. Elastic and plastic properties were determined for freestanding films in both square and rectangular window geometries. A finite element model of the window was used to determine the yielding behavior to account for stress concentrations in the membranes. The initial yield in a pressurized membrane, on the order of 400–500 MPa, is approximately one-half of the flow stress inferred from nanoindentation hardness results between 1.9 GPa and 3.4 GPa, and is a result of microscale yielding prior to uniform deformation in these high strength systems.