Piezoelectric Micromachined Ultrasonic Transducers With Superior Frequency Control

It is a long-lasting issue that process variations, such as residual stress and poor dimension definition, deteriorate resonant frequency control of a piezoelectric micromachined ultrasonic transducer (PMUT). This letter presents a tapered cantilever cluster PMUT with tight frequency control. The PMUT could release residual stress via cantilever deformation and its front cavity could precisely define cantilever boundary. Theoretical analysis and test results show that the impact of residual stress on resonant frequency has been eliminated. Frequency precision and accuracy are evaluated across die, wafer and lot. Across wafer frequency uniformity and relative deviation of the proposed design are 0.8% and 1%, respectively, which are approximately an order of magnitude better than those of conventional design. The proposed PMUT demonstrates superior frequency control among reported PMUTs till date, meeting the 1% frequency control requirement for the first time. The measured transmitting and receiving sensitivities are 1.15 Pa/V/mm2 at a distance of 5 mm and 0.15 mV/Pa, respectively. The design is simple and cost effective to implement without stringent fabrication process requirements such as trimming or etch optimization. It is competitive in air-coupled applications where tight frequency control for frequency matching is a must, such as ultrasonic gas flowmeter and beam-forming array. [2023-0139]