An automated interferometric system for bulge and blister test measurements of micromachined membranes

摘要

An experimental set-up designed for the mechanical characterization of small-size membranes by the bulge test and blister test techniques is described. The differential pressure (0-10 bars) is applied to the membrane with a motorized microsyringe pump filled with water to minimize stored elastic energy in the system. An interferometric microscope equipped with a quasi monochromatic Na discharge lamp, a CCD camera and an apertured photomultiplier is used to get simultaneously full field interferograms of the membrane deformed shape and a point measurement of the membrane central height variation. Phase extraction by FFT and unwrapping of the photomultiplier output signal, and processing of some pixels corresponding to the substrate in the set of interferograms images allows to get, with an accuracy in the 3-30 nm range, the true membrane maximum deflection corrected from substrate bending, vertical drift and tilting. 2D or 3D profiles of the membrane deformed shape can as well be obtained with a similar accuracy and a spatial resolution of 3 μm. The good performances of the system are illustrated from measurements on micromachined Si_3N_4 and Mo membranes on silicon.