Indentation testing of axisymmetric freestanding nanofilms using a MEMS load cell

We have developed a new scientific tool for the mechanical characterization of axisymmetric freestanding nano-thickness films. The tool consists of a calibrated MEMS-based load cell with a spherical loading tip, and a positioning system capable of sub-nm accuracy. The loading tip is positioned over the center of a free-standing film using a two-axis positioning stage. A Michelson Interferometer is positioned below the film to record the displacement field of the membrane, in situ, as the load cell deflects the membrane from above. As the MEMS load cell is lowered into the axisymmetric freestanding thin film, concentric fringes emanate from the center of the thin film sample. This tool allows us to record the applied load and membrane displacement simultaneously. To date, load cells with stiffnesses as low as 2 nN/μm have been fabricated and calibrated. We have validated the operation of the apparatus using gold films 1080 μm in diameter and 170 nm thick.