Curvilinear capacitive micromachined ultrasonic transducer (CMUT) array fabricated using a reverse process

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.