| Resonant ultrasound spectroscopy (RUS) is a recent method used by scientists in condensed matter physics to determine elastic constants of materials. In this dissertation, we introduce RUS into the field of MicroElectroMechanical Systems (MEMS), using RUS to characterize silicon-nitride membrane/silicon structures. The general theory of RUS is the comparison of the measured and calculated natural frequencies of solids. Ritz method is conventionally used for calculation but it is only used for the samples with simple and symmetric shapes, such as cubes and rectangular parallelepipeds. In MEMS, the shapes of microstructures are complicated and they usually consist of multiple materials. So finite element method (FEM) is used for the calculation of natural frequencies of solids in RUS applications. Then, RUS measurement, combined with FEM for calculation, is used to characterize silicon-nitride membrane/silicon structures. A linear model is developed to set up the relationships between resonance frequencies and mechanical properties and dimensions of silicon-nitride membrane/silicon structures. This allows the use of RUS to make a fast die-level characterization of silicon-nitride membrane/silicon structures, for which not only mechanical properties but also dimensions are determined. Sensitivity of using RUS to characterize silicon-nitride membrane/structures with different ratios of the membrane length to the substrate length is further studied and the preliminary result of using RUS to characterize the multilayer thin-film membrane/silicon structures is also presented. The final part of the dissertation is to study using bulk modes to excite the standing flexural plate wave, which can be regarded as the extension of RUS study. The normal modes of silicon-nitride-membrane/silicon structures are the coupled modes, for which bulk modes of the silicon substrate are coupled to the membrane modes of the suspending membrane part. This leads to a new method to excite flexural plate waves on the membrane. The piezoelectric plates (e.g. PZT4, Lead-Zirconate-Titanate-Oxide) can be bonded to the substrate and when an AC signal is applied on piezoelectric plates, the acoustic energy can be coupled to the membrane and make the membrane vibrate. Measurements by the use of laser interferometer verified this new method. |
