Thin film material parameters derived from full field nanometric displacement measurements in non-uniform MEMS geometries

MEMS-scale polycrystalline silicon 2 μm thin film specimens fabricated via the Multi User MEMS Processes (MUMPs) have been employed to obtain the non-uniform nanometric displacement fields in the vicinity of prefabricated circular and elliptical micron-sized perforations. For the hole diameter-to-specimen width ratios considered in this work, and for all practical purposes, the displacement solution for a hole in an infinite plate is applicable. This method requires the ability to reliably and repeatably acquire nanometer level displacements on freestanding thin films. These tensile tests were conducted by a custom microtensile tester with the aid of Atomic Force Microscopy (AFM) and a special gripper that makes use of electrostatically assisted UV adhesion to handle and load miniature MEMS specimens. This non-conventional procedure for material parameter determination relies on Digital Image Correlation (DIC) to compare two AFM images, one before and one after specimen loading, and thus compute the nanometer level displacement fields (<50 nm global displacements) in 15×15-μm² (or smaller) areas, with better than 2-3 nm resolution in displacements. By posing and solving a non-linear least squares inverse problem where, for known applied loads and measured displacement fields in an infinite plate with either a circular or an elliptical hole, it is possible to recover the elastic modulus (E). The main advantage of this approach is the full utilization of high-resolution displacement measurements over a specific specimen area, using only measurements acquired at one load level. Further statistical measurements of material properties may be obtained at varying load levels.