Optical fingerprint scanners suffer from limited depth of penetration and are particularly sensitive to the surface conditions of the skin. Fingerprint scanners based on ultrasounds offer the possibility to explore the surface and the underlying tissues of the finger and to detect blood flow, leading to enhanced robustness and reliability in biometric applications. Capacitive Micromachined Ultrasonic Transducers (cMUTs) have shown to have great potential for use in medical imaging applications. The ease of fabricating broadband high-frequency ultrasound transducers makes the cMUT technology a good candidate for ultrasound based biometrics. This paper presents the design, fabrication and characterization of a cMUT linear array probe optimized for near-field ultrasound imaging. Ultrasound images of fingerprints are obtained using a customized 3D ultrasound scanning system

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This paper presents the manufacturing of a flexible 192-elements curvilinear CMUT array developed using our patented reverse fabrication process. In this process the silicon nitride CMUT membranes are surface micromachined over a silicon substrate such that the non-radiating surface of the transducer corresponds to the last layer deposited, while the radiating membranes are uncovered by completely etching the silicon substrate. Before the removal of the silicon substrate a flexible backing material is poured and cured on the non-radiating surface, and after the removal another layer of flexible suited material is applied to cover the membranes. At the end of the process the large 24-mm by 6-mm CMUT die with a total thickness of just 6.5 mum, embedded in the flexible backing and coating layers, was bent to a radius of curvature of less than 10 mm in an azimuth direction, showing great flexibility. The functionality of the device wire-bonded to a flex printed circuit and mounted on a curved backing with a radius of curvature of 25 mm was demonstrated by means of electrical impedance tests and pulse-echo measurements in water. The tested elements showed a pulse-echo center frequency of about 11.0 MHz with a fractional bandwidth of at least 100%, with no apparent performance degradation resulting from curving the array

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