Monolithic CMUT-on-CMOS Integration for Intravascular Ultrasound Applications

• Published in: IEEE transactions on ultrasonics, ferroelectrics, and frequency control ferroelectrics, and frequency control, 2011-12, Vol.58 (12), p.2659-2667
• Main Authors: Zahorian, Jaime, Hochman, Michael, Xu, Toby, Satir, Sarp, Gurun, Gokce, Karaman, Mustafa, Degertekin, F. Levent
• Format: Article
• Language: English
• Subjects: Acoustics; Biological and medical sciences; Cardiovascular system; Electronics - instrumentation; Equipment Design; Equipment Failure Analysis; Exact sciences and technology; Fabrication; Fundamental areas of phenomenology (including applications); Image Enhancement - instrumentation; Imaging; Investigative techniques, diagnostic techniques (general aspects); Medical sciences; Metals; Micro-Electrical-Mechanical Systems - instrumentation; Miniaturization; Physics; Reproducibility of Results; Semiconductors; Sensitivity and Specificity; Silicon wafers; Surface topography; Surface treatment; Systems Integration; Transducers; Transduction; acoustical devices for the generation and reproduction of sound; Ultrasonic investigative techniques; Ultrasonic transducers; Ultrasonography, Interventional - instrumentation; Wires
• ISSN: 0885-3010; 1525-8955; 0885-3010
• DOI: 10.1109/TUFFC.2011.2128

One of the most important promises of capacitive micromachined ultrasonic transducer (CMUT) technology is integration with electronics. This approach is required to minimize the parasitic capacitances in the receive mode, especially in catheter-based volumetric imaging arrays, for which the elements must be small. Furthermore, optimization of the available silicon area and minimized number of connections occurs when the CMUTs are fabricated directly above the associated electronics. Here, we describe successful fabrication and performance evaluation of CMUT arrays for intravascular imaging on custom-designed CMOS receiver electronics from a commercial IC foundry. The CMUT-on-CMOS process starts with surface isolation and mechanical planarization of the CMOS electronics to reduce topography. The rest of the CMUT fabrication is achieved by modifying a low-temperature micromachining process through the addition of a single mask and developing a dry etching step to produce sloped sidewalls for simple and reliable CMUT-to-CMOS interconnection. This CMUT-to-CMOS interconnect method reduced the parasitic capacitance by a factor of 200 when compared with a standard wire-bonding method. Characterization experiments indicate that the CMUT-on-CMOS elements are uniform in frequency response and are similar to CMUTs simultaneously fabricated on standard silicon wafers without electronics integration. Ex- periments on a 1.6-mm-diameter dual-ring CMUT array with a center frequency of 15 MHz show that both the CMUTs and the integrated CMOS electronics are fully functional. The SNR measurements indicate that the performance is adequate for imaging chronic total occlusions located 1 cm from the CMUT array.