Row-Column Capacitive Micromachined Ultrasonic Transducers for Medical Imaging

Abstract

Ultrasound imaging plays an important role in modern medical diagnosis. Recent progress in real-time 3-D ultrasound imaging can offer critical information such as the accurate estimation of organ, cyst, or tumour volumes. However, compared to conventional 2-D ultrasound imaging, the large amount of data and circuit complexity found in 3-D ultrasound imaging results in very expensive systems. Therefore, a simplification scheme for 3-D ultrasound imaging technology is needed for a more wide-spread use and to advance clinical development of volumetric ultrasound. Row-column addressing 2-D array is one particular simplification scheme that requires only N + N addressing lines to activate each element in an N × N array. As a result, the fabrication, circuit, and processing complexity dramatically decrease. Capacitive micromachined ultrasonic transducer (CMUT) technology was chosen to fabricate the array as it offers micro-precision fabrication and a wide bandwidth, which make it an attractive transducer technology. The objective of this thesis is to investigate and demonstrate the imaging potential of row-column CMUT arrays for RT3D imaging. First, the motivation, physics, and modelling of both CMUTs and row-column arrays are described, followed by the demonstration of a customized row-column CMUT pseudo-real-time 3-D imaging system. One particular limitation about row-column arrays discovered as part of this dissertation work is the limited field-of-view of the row-column arrays’ imaging performance. A curved row-column CMUT array was proposed to improve the field-of-view, and the resulting modelling of the acoustic field and simulated reconstructed image are presented. Furthermore, a new fabrication process was proposed to construct a curved row-column CMUT array. The resulting device was tested to demonstrate its flexibility to achieve the necessary curvature. Finally, a new wafer bonding process is introduced to tackle the next generation of RC-CMUT fabrication. Many of the new fabrication techniques reported in this work are useful for CMUT fabrication engineers. The analysis on row-column array also provides additional insights for 2-D array simplification research.