Micromachined capacitive ultrasonic immersion transducer for medical imaging

Piezoceramics have been the dominant transducer technology in ultrasound medical imaging for several decades. Recent progress in surface micromachined capacitive ultrasonic immersion transducers makes them an alternative transducer technology, especially in highly integrated two-dimensional arrays. This paper demonstrates that the surface micromachined capacitive ultrasonic immersion transducer performs at a level competitive enough to challenge the established piezoelectric transducers. Single element transducers and a variety of array transducers are fabricated with CMOS compatible micromachining technology. The transducers are observed to operate from 2 MHz to 15 MHz in immersion operation. Better than 100 dB dynamic range is evident around 4.5 MHz for a single device with only 6 dB of unknown return loss. Similar performance is observed in a pulse echo experiment. The transducer's beam pattern indicates that the device behaves as a uniform piston transducer. Theoretical analysis shows that it is feasible to build an ideal immersion transducer with 100% of bandwidth and 3 dB insertion loss in wide frequency ranges. The study in this paper concludes that micromachined ultrasonic transducers are an attractive alternative to piezoelectric transducers in ultrasound medical imaging.