Enhancement of Transmitting Sensitivity of Piezoelectric Micromachined Ultrasonic Transducers by Electrode Design

This article investigates the dependence of transmitting sensitivity on the top electrode design of piezoelectric micromachined ultrasonic transducers (PMUTs). Two typical top electrodes, namely inner electrode (IE) and outer electrode (OE), are designed and fabricated. The measured transmitting velocities of the fabricated PMUTs at resonance under a drive voltage of 5
$\text{V}_{\mathrm {p-p}}$
(peak-to-peak) are 15.36 mm/s for the IE design and 20.67 mm/s for the OE design with a circular diaphragm and 16.62 mm/s for the IE design and 22.18 mm/s for the OE design with a hexagonal diaphragm. The OE design demonstrates a transmitting velocity improvement of 34.57% for the circular diaphragm and 33.45% for the hexagonal diaphragm. The improvement is due to the fact that the OE design shows higher quality factor (
${Q}$
) than the IE counterpart. Moreover, the resonant frequency of the OE design is higher than that of the IE design, which results in a larger acoustic pressure output and hence higher transmitting sensitivity. This work highlights an effective and simple approach for PMUTs to achieve high transmitting sensitivity, which is an important parameter in the applications that require large sound pressures, such as fingerprint imaging, gesture recognition, and ranging.