Effect of Torsional Beam Length on Acoustic Functionalities of Bio-Inspired Piezoelectric MEMS Directional Microphone

In MEMS, the vibrations of torsionally supported diaphragms largely depend on the torsional beam specifications. An amiss selection of the beam parameters can lead to significant imperfection in the microphone functionalities, such as modal frequency, sensitivity, directionality, signal-to-noise ratio (SNR), and input-referred noise floor. Here, we present two piezoelectric MEMS directional microphones inspired by the ear anatomy of fly Ormia ochracea, with a prime focus to identify the effect of torsional beam dimensions on their acoustic functionalities. The dimensions of both diaphragms and the width & thickness of the torsional beams are identical for both microphones, the only difference between these two microphones lies in the torsional beam length. The microphones are fabricated by a commercially available micromachining process; PiezoMUMPs. To convert the mechanical vibrations into the electronic signal, a unique piezoelectric sensing scheme is developed comprising of aluminium nitride (AlN) and D33 transducer mode. The performances of both microphones are carried out by the theoretical analysis and simulation and further verified with the experimental measurements. It is found from all analyses that the microphone having a shorter length of the torsional beam is better in terms of sensitivity, noise floor, and SNR regarding its low propagation delay.