Building CMUTs for imaging applications from top to bottom

This paper focuses on an innovative fabrication process of a capacitive micromachined ultrasonic transducer (CMUT) array for medical imaging. Our approach consists in inverting the function of each layer to build the CMUT capacitive cell starting from the membrane, made of silicon nitride, up to the bottom electrode, which is supported by an appropriate acoustic backing to absorb any ultrasound energy transmitted towards the backside of the array and to prevent the ringing of the pulse-echo signal. The fabricated devices exhibited a large bandwidth (>100%) and an increased sensitivity, in respect to the classical CMUT devices. Using the fabricated devices as linear probes, echographic images are obtained.