Measuring local mechanical properties of membranes applying coherent light projection moiré interferometry

The bulge test is a versatile and reliable way to determine mechanical properties of thin films. It can be applied to obtain constitutive equations of the material at different ranges of deformation including time effects. It provides a biaxial state of stresses that is the prevailing condition in thin film operate. In this paper the bulge test information is retrieved with interferometric nano-moiré. A special set up was designed and built to pressurize a membrane in the platina of a conventional metrological microscope. The utilized field of view is 326 × 326 microns and the spatial resolution is 318 nanometers, the depth information is within 10 nanometers. An aluminum foil was cemented to a plate that has a circular aperture of 10 mm. The foil was inflated to 3.5 psi at 0.5 psi intervals and images were recorded at each pressure level. Local properties of the deformed surface were compared with the results of the membrane theory by determining from the experimentally measured values the surface trend. The comparison shows that the membrane takes the parabolic trend with maximum observed deviation of 43 nm. At the testing pressure of 3.5 psi the calculated radius of curvature from the surface trend is 96.8 mm, while the theoretical radius of curvature according to the geometry and material properties of aluminum foil is 96.3 mm.