Enhancement of Transmitting Sensitivity for Piezoelectric Micromachined Ultrasonic Transducers via Back-cavity Package Design

In this letter, the influence of back-cavity package design on the transmitting sensitivity of piezoelectric micromachined ultrasonic transducers (PMUTs) with open back cavities is investigated. The reported PMUTs are fabricated based on a thin-film piezoelectric on silicon-on-insulator (SOI) platform with backside cavities in substrate to define the vibration diaphragms. An adjustable package with back-cavity depth control and reflective interface selection is proposed. The transmitting sensitivity of the PMUTs is largely influenced by the package design, due to the constructive or destructive interferences between the reflective acoustic waves in the back cavity and the originally transmitted forward acoustic waves. Experimental results illustrate that the measured vibration amplitude of the diaphragm in the PMUTs demonstrates sinusoidal-shaped variations as the back-cavity depth continuously increases under a reflective interface, such as aluminum alloy and cardboard. With a proper back-cavity package design, the reported PMUTs demonstrate a 1.3-fold enhancement of vibration amplitude compared with those without the interferences of reflective waves. This work highlights an effective approach for enhancing the transmitting sensitivity of PMUTs.