CMUTs with dual electrode structure for improved transmit and receive performance

We report on the use of a dual electrode structure to improve the performance of capacitive micromachined ultrasonic transducers (CMUTs.). In the dual electrode structure, separate transmit and receive electrodes are embedded either in the same CMUT membrane or in the dielectric substrate beneath the same membrane. This configuration has several advantages. First, the maximum pressure amplitude during transmit mode can be significantly increased by locating the transmit electrodes near the edges of the membrane. In this case, leveraged bending increases the maximum displacement without collapsing the membrane. Second, since the DC bias of the electrodes can be controlled independently, the device capacitance can be increased during the receive time by biasing both receive and transmit electrodes. Additionally, separate electronics can be used for transmit and receive without the need for a switch. We have fabricated a 0.3×0.65 mm² CMUT element consisting of 20 μm wide, 100 μm long and 0.9 μm thick rectangular silicon nitride membranes. The transmit electrodes are 4 μm wide and located at the edges of the membrane, whereas the 8 μm wide receive electrode is at the center. With this non-optimized structure, we measured a 6.8 dB increase in maximum output pressure at 9 MHz with side electrode excitation. We also demonstrated the possibility of using dual electrodes for simultaneous excitation and detection of several CMUT membrane vibration modes.