Zero-Bending Piezoelectric Micromachined Ultrasonic Transducer (pMUT) With Enhanced Transmitting Performance

A piezoelectric micromachined ultrasonic transducer (pMUT) has enabled numerous exciting ultrasonic applications. However, residual stress and initial buckling may worsen the transmitting sensitivity of a pMUT, and also limit its application and commercialization. In this paper, we report a new innovative pMUT with a perfectly flat membrane, i.e., zero-bending membrane. Leveraging on the stress-free AlN thin film, framelike top electrode layout, and integrated vacuum cavity, the initial deflection of suspended membrane is significantly suppressed to only 0.005%. The transmitting sensitivity of the zero-bending pMUT is measured as 123 nm/V at a resonant frequency of 2.21 MHz, which is 450% higher than that of the reference pMUT with slightly non-zero initial deflection. Compared with the simulation results, the measured data of zero-bending pMUT achieve 94.5% of its ideal transmitting sensitivity. It is solid evidence that our approach is an effective and reliable way to overcome the residual stress and the initial buckling issue.