Comparative characterization of bimorph and unimorph AlN piezoelectric micro-machined ultrasonic transducers

A comparative study of bimorph and unimorph AlN piezoelectric micro-machined ultrasonic transducer (pMUT) is reported for the first time. The bimorph is a suspended square diaphragm, comprised of two active AlN layers sandwiched by three Mo layers. This bimorphic structure (Mo/AlN/Mo/AlN/Mo stack) enables higher dynamic amplitude as the stack serves both actuation and structural functions. A unimorph counterpart is obtained by subjecting a bimorph device through FIB post process, which deactivates the actuation function of top Mo/AlN layers while preserving the device mechanical integrity. It allows same-device, high-fidelity comparative study between AlN bimorph and unimorph. Besides, the Mo layers are appropriately patterned for optimized signal configuration to maximize the pMUT performance. The reported bimorph pMUT has dynamic deflection as high as 1155.7 nm/V (diaphragm central displacement), a 77% increase compared to the unimorph. It allows low voltage actuation and high acoustic power generation for various ultrasonic applications.