Fabrication and Characterization of Low-Size and Compact AlN PMUT Arrays for Advanced Ultrasonic Applications

The piezoelectric micromachined ultrasonic transducers (PMUT) are based on the flexural movement of thin membranes coupled with thin piezoelectric film. These devices can find applications in several fields, like ultrasound imaging techniques for medical diagnostics and non-destructive and real-time structural monitoring. This work proposes the manufacturing, the computational study by Finite Element Method (FEM) and the characterization of Aluminum Nitride (AlN)-based PMUT arrays, suitable as ultrasound transmitters and receivers. In detail, two PMUT arrays, shaped as annular and square matrix, respectively, were developed by an optimized manufacturing process. It exploits a controlled Bosch process, used to release the membranes without using any stop layer to control the residual Si membrane thickness, resulting in great flexibility in tuning the PMUT operating frequency. The PMUT arrays were designed to operate at 2.5-3 MHz frequency. An annular configuration was deployed, featured by a common ground contact connecting all membranes’ bottom electrodes and separate contacts to drive the top electrodes of each ring. The modular structure allows easy customization depending on the application and operating scenario. AIN-based PMUT arrays were fully characterized as transmitters and receivers; in detail, time- and frequency-domain responses of a 33 square matrix and a 3-ring and 5-ring annular PMUT arrays were analyzed and discussed as a function of the number of membranes composing the probe’s rings and the amplitudes of driving pulse. Finally, fundamental mode bandwidth analysis is reported in relation of the driving voltage amplitude. Experimental tests have demonstrated that the designed annular PMUT arrays operate at a 3 MHz central frequency with a 1 MHz bandwidth.