An AlN-Based Piezoelectric Ultrasonic Transducer Structure With Low Residual Stress

Residual stress has long been a critical issue in the development of MEMS, as its presence can substantially affect the performance and reliability of the device. This paper presents a high-performance piezoelectric micromachined ultrasonic transducer (PMUT) array with a stress-release (SR) structure. The PMUT array is a multilayer stacked structure that utilizes specific mechanical grooves etched into the device’s active sensing layers to release residual stress. The piezoelectric layer of the PMUT structure is made of aluminum nitride (AlN). The PMUT array is developed on a silicon-on-insulator (SOI) platform, with dimensions of 4 mm × 4 mm. The array consists of 13 × 13 sensitive units, with each unit designed using a biomimetic honeycomb structure. The basic dimensions of the PMUT arrays with SR and conventional (CV) structures are identical. On the outer side of the SR structure’s sensitive units, four mechanical grooves with a 120° opening angle and a width of 20 μm are etched. Electrical characteristics and transceiver performance tests were conducted on PMUTs with both structures. The results showed that the PMUTs with the SR structure exhibited higher mechanical quality factors and electromechanical coupling coefficients, with improvements of 37.6compared to the CV structure. Within the bandwidth range, the transceiver performance of the SR structure is 2 dB higher than that of the CV structure. These results validate the feasibility of the SR structure for achieving device stress matching, offering superior vibration and transceiver performance compared to the CV structure.