A Novel Vertical Dual-Gap CMOS-MEMS CMUT Enabling Large Fractional Bandwidth

This study introduces a vertical dual-gap capacitive micromachined ultrasonic transducer (CMUT) array featuring a large fractional bandwidth (FBW) achieved through the combined operation of In-Phase (IP) and Out-of-Phase (OOP) modes within a two-degree-of-freedom (2-DOF) structure. Leveraging complementary metal-oxide-semiconductor (CMOS) <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.18\ \mu \mathrm{m}$</tex> 1P6M technology and a five-step maskless post-fabrication process, the proposed design integrates a vertically stacked dual-gap structure, comprising a 400 nm titanium nitride composite (TiN-C) gap and a 125 nm metal-insulator-metal (MIM) gap. With comparable energy loss, the reception (Rx) measurements of the vertical dual-gap CMUT demonstrate a 1.58-fold improvement in FBW compared to the conventional single-gap CMUT, increasing the 6-dB FBW from 72.3% to 114%. The proposed FBW-enhancement mechanism could also be applied to other CMUT designs, thus improving imaging performance in the future.