• Pedersen, Stine Løvholt Grue

Abstract

Medical ultrasound is a widely used imaging modality, which is considered harmless and cost-effective compared to other imaging modalities like X-rays and CT scans. The ultrasound system relies on a transducer responsible for emitting and receiving ultrasonic waves. Conventional scanner systems utilize a piezo ceramic transducer, typically composed of lead zirconate titanate (PZT) materials.<br/>This thesis investigates an alternative transducer technology, called Capacitive Micromachined Ultrasound Transducers (CMUT), which is fabricated using conventional semiconductor methods. CMUT-based transducers offer various advantages compared to conventional PZT probes, such as temperature stability, greater design flexibility, and broader bandwidth.<br/>The primary objective of this Ph.D. project was to design, develop, and fabricate a linear CMUT-based array for super-resolution ultrasound imaging in real-time of erythrocytes, with the ultimate goal of detecting cancer and diabetes at an earlier stage.<br/>Additionally, the project aimed to design and develop an alternative transducer implementation method utilizing backside contacting, allowing for the integration of a CMUT-based array into a laparoscope. Related process optimization was conducted for processes essential to achieve successful backside contacting and was implemented on a 2D CMUT-based array.<br/>A theoretical framework was provided to enhance the understanding of the CMUT behavior, including the derivation of classical expressions for essential CMUT parameters. Finite element analysis was employed to extend the classical theory to a wider range of plate geometries and clamping conditions.<br/>A 15MHz transducer with an element pitch of λ/2 was fabricated and electrically characterized using a novel wafer-level characterization method. The method employed a step-wise selection approach, exclusively focusing on the most promising arrays to save time compared to a comprehensive characterization of all arrays. Four 16-element arrays were assembled and subsequently acoustically characterized, demonstrating successful performance with bandwidths exceeding 100% during transmission and ranging from 85% to 120% based on pulse-echo measurements.

Topics

• impedance spectroscopy
• theory
• semiconductor
• ultrasonic
• ceramic
• finite element analysis
• computed tomography scan