Advancing sensitivity and performance of Capacitive Micromachined Ultrasound Transducers (CMUTs) for medical imaging

Nekoeian, Dayhim (2025) Advancing sensitivity and performance of Capacitive Micromachined Ultrasound Transducers (CMUTs) for medical imaging. MSc(R) thesis, University of Glasgow.

Abstract

Ultrasound transducer technology has significantly contributed to advancements in medical imaging, continuously improving resolution, efficiency, and integration. Capacitive Micromachined Ultrasound Transducers (CMUTs) use electrostatic transduction to generate and detect sound waves, promise broader bandwidth, smaller size, and better integration with electronics than piezoelectric transducers, which improves Transduction efficiency and frequency response for next-generation imaging such as X-radiation (X-rays), Magnetic Resonance Imaging (MRIs), or standard photography.

This thesis focuses on improving CMUT sensitivity and performance for medical imaging through innovative design strategies, analytical modelling and optimized fabrication processes. The literature review covers ultrasound imaging principles, CMUT operation, and their use in diagnostics, therapy, biosensing, and airborne systems, while a comparison with PMUTs emphasizes CMUTs’ improved electromechanical coupling and frequency response, supporting their role in advanced imaging. A fabrication process compatible with Complementary Metal Oxide Semiconductor (CMOS) is developed to support the integration of CMUTs with advanced electronic circuits, enabling the creation of compact and efficient imaging devices.

The research takes an experimental approach to improve a low-temperature sacrificial release method, enabling precise membrane formation with reduced stress. Various microfabrication techniques, including photolithography with photomask design using COMSOL, thin-film deposition, and etching, are refined to develop a scalable and reproducible fabrication process. To enhance CMUT sensitivity, the study uses analytical modelling with Hooke’s Law and EulerBernoulli Beam Theory to analyse axial and bending stiffness in straight beams, then compares them to meander beams to assess their effect on CMUT sensitivity.

Experimental validation through capacitance-frequency (C-F) and capacitance-voltage (C-V) measurements confirms reliability, showing the Silicon Oxide (SiO₂) sacrificial layer remained stable without causing capacitance loss. Additionally, the measured permittivity of the sputtered SiO₂ closely aligns with values reported in previous studies, demonstrating consistency in material properties and fabrication precision.

In summary, this research has established a CMOS-compatible fabrication process and refines the dry etching release method to reduce stiction, laying the groundwork for advancing CMUT technology with improved reliability and adaptability for medical imaging. Future work aims to improve CMUT fabrication by adjusting material choices and enhancing the sacrificial release process.

• Item Type: Thesis (MSc(R))
• Qualification Level: Masters
• Additional Information: Supported by funding from the EU BRAINSTORM Project.
• Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Funder's Name: EU BRAINSTORM project
• Supervisor's Name: Cochran, Professor Sandy and Heidari, Professor Hadi
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85257
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 27 Jun 2025 08:53
• Last Modified: 27 Jun 2025 08:53
• Thesis DOI: 10.5525/gla.thesis.85257
• URI: https://theses.gla.ac.uk/id/eprint/85257