Micromachined Integrated Transducers for Ultrasound Imaging

Søren Elmin Diederichsen

Research output: Book/ReportPh.D. thesis

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Medical ultrasound imaging is a widely used real-time technique for non-invasive diagnostics. A key part of an ultrasound scanner system is the transducer probe, which enables the transmission and reception of ultrasound waves. Conventional ultrasonic transducers are based on bulk-machined piezoelectric materials, such as lead zirconate titanate (PZT).
In this thesis, an emerging transducer technology is investigated, namely the capacitive micromachined ultrasonic transducer (CMUT). The CMUT relies on an electrostatic transduction mechanism and provides attractive properties such as high design flexibility and large frequency bandwidth. The CMUT has been investigated since the early 1990s
and the technology is now matured to the stage of commercialization.
The overall purpose of this PhD thesis is to develop CMUT-based ultrasound probes for medical imaging. In particular, the focus is to design, fabricate and characterize 1-D linear CMUT arrays. The CMUT arrays are mounted in prototype probes with integrated electronics. The CMUT probes presented in this thesis are designed with specifications similar to commercially available piezoelectric transducer probes from the company BK Medical ApS (Herlev, Denmark). This allows for a direct comparison between the two transducer technologies.
A theoretical background is provided, including models for the static behavior and dynamic behavior of a CMUT. Finite element analysis and experimental studies serve as a foundation for optimizing the CMUT operating conditions and CMUT design parameters for medical imaging purposes. 1-D linear CMUT arrays are fabricated using a direct fusion bonding technique in combination with different schemes for realizing the CMUT cells: Reactive ion etching, one local oxidation of silicon process, or two local oxidation of silicon processes.
Seven different CMUT prototype probe generations, based on five different CMUT array generations, are presented in this thesis. Selected CMUT prototype probes and their corresponding piezoelectric reference probes are characterized and the results compared. The CMUT probes are fully functional with commercial BK Medical ultrasound scanner systems, including bk3000 and bk5000 scanners.
Selected 1-D linear CMUT probes developed during this project are used for ultrasound imaging of tissue mimicking phantoms as well as in vivo imaging of the carotid artery. The CMUT probes generally support the same widely used imaging modes as their piezoelectric equivalents. This PhD project has confirmed that the CMUT technology is a viable alternative for medical ultrasound imaging.
Original languageEnglish
PublisherDTU Health Technology
Number of pages273
Publication statusPublished - 2019


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