Analysis of charge effects in high frequency CMUTs

This paper presents a theoretical analysis of charge effects in silicon nitride membranes in Capacitive Micromachined Ultrasonic Transducers (CMUTs) working at 30 MHz in air. The analysis was motivated by observations of drift in the resonance frequency to the CMUTs during electrical and heterodyne interferometric measurements while keeping the DC voltage level constant over time. After applying a DC voltage of -40 V for 3 hours, a drift of 0.6 MHz is observed. Analysis showed that this corresponded to a transfer of 15 per cent of the electrode charge into the silicon nitride membrane. While adjusting the voltage to keep the coupling factor constant equal to 0.5 and comparing a charge ratio of 0 and 1, the electrical Q-factor is increased by 8.5 % while the mechanical Q-factor is decreased by 7.8 % for a cavity height of 100 nm. The mechanical resonance frequency is reduced by 6.9 %. However, these changes are moderate and do not impact the response of the CMUT significantly in wide band transducer operation.