Ultra-narrow gap CMUT cell structure for highly sensitive photoacoustic imaging

A highly sensitive capacitive micromachined ultrasound transducer (CMUT) cell with an ultra-narrow gap was investigated. The electromechanical coupling coefficient (k
_T
) as a figure of merit of sensitivity of ultrasound transducers was 0.67 in a 30-MHz CMUT cell with a 30-nm height vacuum gap and in 100-nm thick dielectric layers, and the coefficient was superior to that of piezoelectric transducers. By combining the highly sensitive CMUT cell array of a 2-mm diameter with a customized receiving circuit, the receive sensitivity was 116 μV/Pa, and the noise-equivalent pressure was 25 Pa. We found a photoacoustic signal from water with the prototyped CMUT receiver.