Shear Mode Polarity Inverted ScAlN Multilayer for Application to BAW Transformer in Rectifying Antenna

Abstract

Energy harvesting from surrounding environment is required for battery-less sensors. RF-DC conversion with a rectifying antenna (so-called rectenna) is generally used to obtain energy from ambient electromagnetic wave. However, RF-DC conversion efficiency of the diode significantly decreases for ambient electromagnetic wave due to their weak RF power. In this study, we report a new type of bulk acoustic wave (BAW) piezoelectric transformer based on c-axis zig-zag multilayers ScAlN film in order to amplify the RF signal in the rectenna. c-Axis approximately 45°-tilted film exhibit high k’ 152 without undesired longitudinal wave excitation. The theoretical voltage gain was simulated by Mason’s equivalent circuit model considering polarization inversions due to c-axis zig-zag structures. Voltage gain increase with k 2 Q value (= FOM) increasing or the number of layers n in the output layer increasing. S parameters of a BAW piezoelectric transformer with c-axis zig-zag 4 layers ScAlN film are measured by a network analyzer. As a result, voltage gain approaching + 20 dB with the 4 layers BAW piezoelectric transformer is obtained. This new type of polarization inverted FBAR transformer is attractive for a voltage transformer in the rectenna.