An investigation of plasticity in MEMS materials

We have developed a membrane deflection experiment particularly suitable for the investigation of sub-micron thin films that directly measures actual load and film stretch. The experiment consists of loading a fixed-fixed membrane with a line load that is applied to the middle of the span with a nano-indenter column. A Mirau microscope-interferometer is conveniently aligned with the nano-indenter to directly measure strains. This is accomplished through a specially manufactured wafer containing a window to expose the bottom surface of the membrane. The sample stage incorporates the interferometer to allow continuous monitoring of the membrane deflection during both loading and unloading. As the nanoindenter engages and deflects the sample downward, fringes are formed due to the motion of the bottom surface of the membrane and are acquired through the use of a CCD camera. Digital monochromatic images are obtained and stored at periodic intervals of time to map the strain field. Through this method, loads and strains are measured directly and independently without the need for mathematical assumptions to obtain the parameters describing material response. Additionally, no restrictions on the material behavior are imposed in the derivation of the model. In fact, inelastic mechanisms including strain gradient plasticity effects can be characterized by this technique.