Fabrication and Characterization of a Wideband Low-Frequency CMUT Array for Air-Coupled Imaging

The capacitive micro-machined ultrasonic transducer (CMUT) has a good acoustic matching with air. In this paper, a 16-element CMUT array for airborne application is designed and fabricated by SOI wafer bonding process. All the CMUT cells have the same geometric parameters with circular and sealed membrane, which vibrate uniformly for good transmission efficiency. And the thin membrane with small mass is proposed to broaden the bandwidth of CMUT. The sensor array operates at a 215.6 kHz resonance frequency with a large -3dB fractional bandwidth of 15.7% and a good consistency. The selection of the driving parameters (i.e. the DC bias voltage and AC excitation signal) has been analyzed comprehensively, which can be used to optimize the driving strategy of the CMUT for better transmission and reception performance. The CMUT array proposed in this paper has been used to demonstrate a two-dimension non-contact air-coupled imaging by through-transmission mode and pitch-catch mode. The images of two kinds of objects have been reconstructed to exhibit the capability of the CMUT array in air-ultrasonic imaging, suggesting broad applications including 3D imaging, gesture recognition, and other human-machine interactions in air.