Flexible transducer arrays with through-wafer electrical interconnects based on trench refilling with PDMS

This paper reports on a method to fabricate flexible onedimensional (1D) and two-dimensional (2D) micromachined transducer arrays that are electrically connected to flip-chip bond pads on the back side of the array. In our case, the transducers are capacitive micromachined ultrasonic transducers (CMUT) intended for medical ultrasound imaging. For ultrasound imaging, flexible arrays conform to the body part being imaged. Flexible arrays are also desired for certain catheter and fixed-focus array geometries. Electrical connection to bond pads on the back side of the array is provided for flip-chip bonding to an integrated circuit or flexible PCB. The arrays are made flexible by etching through-wafer trenches and filling the trenches with polydimethylsiloxane (PDMS). The flexibility of the substrate is demonstrated by wrapping it around a needle tip with a radius of 650 μm (French catheter size of 4).