Fabrication and Electromechanical Modeling of a Flexural-Mode MEMS Piezoelectric Transformer in AlN

This paper presents the fabrication and electromechanical characterization of a novel AlN-based microelectromechanical systems (MEMS) flexural-mode piezoelectric transformer (PT) realized in a silicon-on-insulator bulk-micromachining process with segmented electrodes at the secondary side, which are series-connected in order to increase the output voltage. The goal of this work is to propose a MEMS-based alternative to inductors and magnetic transformers for power management in micro-power mm-scale electronic systems. The fabricated device is fully modeled by means of the Butterworth-Van Dyke (BVD) two-port network. The device is modeled analytically with the classic equations of a fully clamped-edge membrane and through finite-element method simulations. Characterization is performed through impedance measurements and an alternative empirical method suitable for MEMS devices is proposed for directly extracting its lumped parameters electromechanical circuit. Finally, the effect of the feed-through capacitance is fully analytically modeled, and this paper presents a variant of the BVD network of the PT with an inner BVD circuit, allowing an easier estimation of the effects of the complex zeros introduced by the feed-forward capacitance. The presented device achieves a measured maximum voltage gain of 58mV/V at ~36.3 kHz and maximum efficiency of ~75%.