CMUT-on-CMOS for forward-looking IVUS: Improved fabrication and real-time imaging

The capability to monolithically integrate CMUTs with underlying front-end electronics is promising for forward-looking (FL) imaging catheters with improved SNR and smaller size. We previously demonstrated feasibility of CMUT-on-CMOS arrays for FL imaging and obtained pulse-echo results from individual elements. Here we describe recent improvements in the fabrication process and initial results from a test setup capable of real-time image data collection using CMUT-on-CMOS arrays. Dual-ring CMUT arrays were fabricated on silicon wafers with 0.35 μm CMOS front-end electronics processed at a commercial foundry. The critical changes made in the fabrication process involved in-house polishing followed by a chemical stripping of the aluminum oxide slurry. We also added 0.2 μm of silicon nitride before CMUT to CMOS interconnect via etching. We made these modifications to improve surface quality, alleviating wirebonding stiction issues. The real-time imaging test setup uses an FPGA to control Tx/Rx element selection and data collection functions. The Tx electronics are capable of generating high voltage, broadband, bipolar pulses up to 100V in amplitude. The 4 Rx channels coming out of the CMUT-on-CMOS chip are simultaneously digitized using a 14 bit 250 MS/s digitizer. 12 MHz dual-ring CMUT-on-CMOS arrays were used for real-time imaging of various targets. The results show that these arrays, coupled with an FPGA controlled data acquisition system, can produce true volumetric images in front of the array in real time.