Piezoelectric MEMS acoustic emission sensors

In this paper, piezoelectric MEMS (piezo-MEMS) sensors are designed and characterized for detecting elastic waves released by active flaws, known as acoustic emission (AE). The designed sensors operate in plate flexural mode or rigid body mode driven by the vibration of silicon diaphragm where piezoelectric layer is deposited on. The designed device contains two different frequency sensors tuned to 40 kHz and 200 kHz. The microstructural layers consist of doped silicon, aluminum nitride (AlN) and metal layer, which function as the bottom electrode, sensing layer and top electrode, respectively. The silicon layer also functions as a mass-spring system where a movable plate is connected with four partially clamped beams to the substrate. As the geometry sizes are restricted by the design rules of the manufacturing process, an additional mass is provided for the 40 kHz sensor through the substrate etching process. The sensors are numerically modeled using COMSOL Multiphysics software to obtain the frequency response and the dynamic response under excitation. Afterwards, the sensors are manufactured using Piezoelectric Multi-User MEMS Process (PiezoMUMPs) provided by MEMSCAP foundry. The performance of the designed sensors is demonstrated through electromechanical characterization experiments. A face-to-face response is obtained for each sensor, and the performance of the designed sensors is compared with the conventional piezoelectric AE sensors. The developed piezo-MEMS AE sensors have advantages of small foot-print, low cost and no need for bias voltage for operation.