A tethered front-plate electrode CMUT for broadband air-coupled ultrasound

Capacitive micromachined ultrasonic transducers (CMUTs) typically consist of a back-plate electrode on a substrate wafer, separated from a front-plate electrode by a small cavity. The electrode structures are usually sealed and when operating in air the majority of these CMUTs are highly resonant. However, for air-coupled applications, this sealing is not strictly necessary allowing other more open electrode structures to be explored. A CMUT structure specifically for air-coupled operation was investigated, consisting of a front-plate electrode that was tethered at only a few points around its periphery. The front-plate was also perforated to increase the overall squeeze-film damping and hence the bandwidth of the device. A series of CMUTs up to 800 μm by 800 μm square was manufactured in a standard CMOS process, using a sacrificial polyimide etch to leave a free-standing aluminum front-plate electrode 1.0 μm thick with a nominal electrode gap of 1.5 μm. A number of thin tethers along the device edges attached the front-plate electrode to the substrate, producing a CMUT structure that was completely open at the edges, with low front-plate stiffness and high squeeze-film damping. A one-dimensional analytical model was formulated to predict the response of the devices, and compared to the measured response of the manufactured CMUTs. The structures were not optimized, but initial results on the prototypes were promising. The devices had a pull-in voltage of only 5 V and a nominal capacitance of 70 pF. The devices were tested as transmitters and receivers over a 15 mm path in air, using a well-characterized broadband transducer as a standard transmitter or receiver. The tethered CMUTs had a center frequency of 400 kHz with a usable bandwidth of over 1 MHz in air, giving a Q-factor of less than 1. However, the devices were not very efficient, with an insertion loss of almost 70 dB and highly damped, as expected. The analytical model also gave reasonably good agreement with the experimental measurements.