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.
Caronti, A., Coppa, A., Savoia, A.S., Longo, C., Gatta, P., Mauti, B., et al. (2008). Curvilinear capacitive micromachined ultrasonic transducer (CMUT) array fabricated using a reverse process. In Ultrasonics Symposium, 2008. IUS 2008. IEEE (pp.2092-2095) [10.1109/ULTSYM.2008.0517].