Multilayer Masking Technology for Fabricating Airborne CMUTs With Multi-Depth Fluidic Trenches

This paper presents a novel multilayer masking technology for fabricating airborne capacitive micromachined ultrasonic transducers (CMUTs) with multi-depth fluidic trenches, in which a plurality of masking layers for different etch depths are selectively patterned prior to the real etching to keep a planar wafer surface. The multi-depth fluidic trenches are utilized to effectively control the squeeze film within the CMUT gap and tune the fractional bandwidth (FBW) of CMUT. As such, the FBW can be changed over a wide range by only adjusting the trench height. With further lowering the gap height, the FBW can be extremely widened up to 168%. The receive sensitivity has also been significantly improved through the process of local oxidation of silicon to reduce the parasitic capacitance. The proposed multilayer masking technology enables the fabrication of airborne CMUTs with different FBWs and sensitivities on the same wafer and further combine these devices into arrays for high-resolution imaging and photoacoustic or thermoacoustic applications. These devices also provide a low minimum detectable pressure (MDP), as low as 1.37 mPa in the FBW of 13.5% for the
$8.5~\mu \text{m}$
deep fluidic trenches and 1.52 mPa in the 17.2% FBW for the
$16.2~\mu \text{m}$
trenches. Furthermore, the multilayer masking technology demonstrates the capability of building microelectromechanical systems (MEMS) with multi-depth micro/nanostructures. [2021-0090]