Effect of humid environment on electromechanical performance of piezoelectric micromachined ultrasonic transducers (PMUTs)

Abstract

In recent years, piezoelectric micromechanical ultrasonic transducers (PMUTs) have received widespread attention. The deployment of PMUTs in humid environment exposes them to changes in relative humidity, which effects their electromechanical performance. This paper investigates the effect of humid environments (30 %−80 %) on the electromechanical performance of uncoated PMUTs from the perspective of electromechanical conversion model and equivalent circuit. Two types of transducers with different top layer materials were experimented to compare the effects of moisture absorption on the electromechanical performances of the transducers. The top layer material of Device A is SiO2 film, and the top layer material of Device B is Si3N4 film. The experimental results show that with the rise of relative humidity, the frequency of Device A decreases by −0.47 %, while the frequency of Device B increases by 0.13 %, and the phase resonance peaks of both increases by 0.441° and 0.285°, respectively. Equivalent circuit fitting was conducted on the impedance characteristics of Device A and Device B, and the fitting results were discussed based on acoustic impedance and moisture absorption theory. The electromechanical performances of the two devices show a different variation trend, with the potential reason being that Device A is mainly affected by the combined effects of moisture absorption leading to a decrease in Young’s modulus of the membrane and a decrease in the acoustic impedance of the humid air, while Device B is mainly affected by the acoustic impedance.