Fully integrated 2D CMUT ring arrays for endoscopic ultrasound

Abstract

Ultrasound has become more prevalent as a medical tool for real-time volumetric imaging. Sparse 2D CMUT ring arrays generate high resolution volumetric images with fewer elements than a fully populated 2D array, allow for integration with electronics in an endoscope form factor, and provide a central lumen for simultaneous use of additional diagnostic and therapeutic tools, such as HIFU or photoacoustic fibers, without increasing the overall package size. We have previously fabricated QuadRing capacitive micromachined ultrasound transducers (CMUTs). Each of the four independent, concentric rings in the array contains 128 elements and operates at a different center frequency. In this work, we use one of the four concentric rings at 4MHz for a fully integrated endoscope assembly. Custom charge-amplifier ASICs were used in these assemblies rather than transimpedance amplifiers, reducing the noise figure of the system. The CMUT arrays are flip chip bonded to a custom 8-leg flexible PCB (flex) that provides electrical connections between the CMUT array, ASICs, and Verasonics imaging system. The flip chip bonded assembly is integrated with a custom 3D printed tip that encases and mechanically supports the assembly, and provides a convenient built-in reference for the passivation layer. Additionally, one flex version flips the CMUT bias, grounding the top electrode without additional circuitry by level-shifting the IC supplies. This feature is particularly desirable for clinical applications, as it shields the patient from the CMUT bias voltage. This new 128-element forward-facing CMUT endoscope has been used for real-time 3D imaging on the bench and expands the toolkit beyond previous work in several ways: high quality images are obtained using a relatively sparse array; new ASICs have shown improvements in SNR; and the array size has enabled use of new 3D-printed, highly customizable assembly tools. We have validated operation with a grounded CMUT top electrode, a critical step towards clinical use. Furthermore, a large lumen increases the breadth of tools that can be used in conjunction with the imaging array.