Design and test of a monolithic ultrasound-image-guided HIFU device using annular CMUT rings

This paper describes the design, fabrication, and characterization of a CMUT-based therapeutic ultrasound chip with built-in ultrasound imager for real-time monitoring of the object being operated on. Multiple concentric high-power (inner) CMUT rings and an annular imager CMUT array (outmost) comprising of 48 or 64 elements are integrated on a silicon substrate measuring 2mm × 2mm for simultaneous ultrasonic ablation/stimulation and imaging. The polysilicon membrane thickness and gap height of the high-power CMUT devices are 1.3μm and 0.35 μm, respectively, while these of the imager CMUT integrated on the same substrate are 1.0μm and 0.18μm, respectively. The thicker membrane and higher gap of the high-power CMUTs make them capable of delivering highpressure ultrasound, while the thin membrane and lower gap of imager CMUTs improves the receiver sensitivity in pulse-echo imaging. In order to maximize the overall membrane displacement of the high-power CMUT ring in transmission mode, the ring is designed as a one-chamber swimming-ring structure instead of being divided into multiple sub-chambers, like imager arrays. The high-power CMUT rings were successfully used for heating the liver tissue of a pig, creating a 2.5°C temperature increase after 6 minutes of ultrasound irradiation. Preliminary B-mode imaging using the CMUT imager element on this imager-guided therapeutic chip was also demonstrated.