High Fill Factor Array of Piezoelectric Micromachined Ultrasonic Transducers with Large Quality Factor

In this paper, we present a novel piezoelectric micromachined ultrasonic transducer (PMUT) array with a high fill factor and a large quality factor (Q). With the reusable design of etching holes, the pitch between PMUTs is effectively minimized, improving the fill factor of the array. With a newly developed surface micromachining process, the fabricated PMUT array only needs few etching holes to release the structure, and all sealed etching holes are removed from the resonance region, achieving a large Q. The demonstrated process facilitates low-cost volume manufacturing. As a result, a 50 × 50 PMUT array containing 48-μm-diameter PMUTs with a 52 μm pitch achieves a high fill factor of 67%. For an individual PMUT, a large Q of 351 is achieved at a center frequency of 12.62 MHz, the peak displacement is 3.25 nm/V, and the effective electromechanical coupling coefficient is 1.6%. The achieved high fill factor and large Q render the proposed PMUT array promising in applications such as flow sensing, chemical detection, and energy transmission.