Novel Fabrication Scheme Using Lateral Stacking of Hybrid Piezo/Ferroelectric Thin-Films for MEMS Resonators

This paper introduces a novel fabrication framework for the lateral stacking of two distinct piezo/ferroelectric materials for the first time. The design integrates lead zirconate titanate (PZT), with its high piezoelectric coefficient, and aluminium nitride (AlN), known for its low permittivity, within a single circular piezoelectric micromachined ultrasonic transducer (PMUT). To validate the concept, the first resonant mode of the PMUT is used, with annular electrodes that individually cover the AlN and PZT regions. A significant improvement in the output magnitude of the electrical response is observed when AlN is used for sensing, and unpoled PZT is driven at varying AC voltages, compared to the reverse combination. Through this scheme, we are able to use the DC tuning capability of ferroelectric material while still benefiting from the low permittivity offered by AlN. The performance is already strong with unpoled PZT, and it can be further enhanced by poling the PZT. Experimental thin film characterization confirms that the intrinsic properties of both materials remain intact and are not compromised by the stacking process. The results demonstrate that this fabrication scheme is an effective method to explore the potential and harness the advantages of multiple piezo/ferro thin film materials at the same time on a single device.