An Efficient Model of Nanoelectromechanical Systems Based Ultrasonic Sensor With Fringing Field Effects

In this work, modeling of parasitic capacitance due to fringing field and its influence on the performance of nano-electro-mechanical-system (NEMS) capacitive micromachined ultrasonic sensor (CMUT) is reported. The fringing effect was modeled with the help of empirical approach which is based on equivalent parallel plate capacitor employing circular membrane approximation. While accommodating the fringing field, the finiteness of the electrode diameter was taken care of. Two important performance parameters, electromechanical coupling coefficient and collapse voltage, were modeled. Influence of the fringing field from a finite dimensional electrode on various structural parameters was also investigated. Capacitance of a single cell, including the static deformation produced by polarizing voltage was evaluated numerically. As an important figure of merit, it was observed that the coupling factor is strongly dependent on the bias voltage, radius, membrane thickness, and also on the electrode separation. The factor approaches unity for higher bias (close to collapse) and large diameter thin membranes. Comparative study of percentage errors for direct and fringing model shows a significant improvement in the case of fringing approach, as expected. The error percentage for the evaluation of the collapse voltage shows a reduction of more than 10%.