CMOS-Integrated Array Transducers For High-Frequency Ultrasonic Imaging
One of the challenges in the fabrication of array transducers suitable for high-frequency ultrasonic imaging is providing electrical connections between the array elements and external systems such as pulser and beamforming circuits. Element connectivity is difficult due to the fine element pitch, as well as strict limitations on the physical size of the probe required for some high-frequency imaging applications such as intravascular ultrasound (IVUS) imaging. Previously, channel reduction has been demonstrated through single-element or partially-sampled array transducers that are mechanically translated during image acquisition. These systems offer ease of fabrication and connectivity due to their low channel count, however, the reliance on mechanical scanning is undesirable. This thesis presents the proof-of-concept of a new type of high-frequency array transducer that maintains the low system complexity associated with a reduced channel count, while at the same time offers the ability to electronically scan an active array pattern. A unique feature of this design is the presence of a custom scanning application-specific integrated circuit (ASIC), which is fully integrated into transducer stack. The ASIC reduces the number cables required by multiplexing a collection of array elements into a small number of channels. To allow for simplified methods of fabrication, two novel substrate bonding techniques are demonstrated that allow full integration of the scanning ASIC into the acoustic stack while not relying on the precise alignment of the two substrates. The integration schemes were demonstrated using a 68 element linear array that includes a custom scanning ASIC in the acoustic stack. The concept is extended to show the feasibility of an electronically-scanned, 50 MHz annular array, which would be suitable for catheter based IVUS imaging for the management of vascular disease. Feasibility has been shown through simulation and experiment, demonstrating that satisfactory transducer performance can be achieved when incorporating a combination of unique materials and structures into the integrated transducer.
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