Zig-zag ScAlN 12-layer for shear mode BAW transformer application in rectenna.

Wireless power transfer (WPT) with rectennas is important for IoT sensor applications. Miniature GHz voltage transformers are more attractive than a large-size charge pump to operate the rectifiers efficiently. In this study, GHz bulk acoustic wave (BAW) piezoelectric transformers based on c-axis zig-zag polarization-inverted ScAlN thin films are proposed. The capacitive impedance (Z) of n-layer c-axis zig-zag multilayers resonator is n-times larger than that of single-layer resonator. Therefore, shear mode operation in the c-axis zig-zag structure acts as a piezoelectric transformer. To demonstrate this principle, c-axis zig-zag ScAlN multilayers are grown using glancing angle sputtering deposition (GLAD). 12-layer c-axis 40-50 degrees zig-zag structure is clearly observed by SEM and XRD pole figure analysis. The open-circuit voltage gain approaching +15 dB in 600 MHz range in the HBAR type transformer based on polarization-inverted 12-layer thin film is observed. The experimental results and the theoretical predictions computed by the Mason's equivalent circuit model considering the effect of the polarization-inverted structure are in good agreement.