PhD Project Summary
My PhD research focused on developing flexible Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) towards imaging applications. Targeting at the intersection of the piezo-MEMS and flexible ultrasound ideas, I developed a flexible PMUT towards imaging, addressing an unexplored area. The research employs an innovative, cost-effective fabrication approach without relying on expensive silicon-on-insulator (SOI) technology. Thin piezoelectric films (1-2 µm thick) were created using a custom PZT (lead zirconate titanate) solution and sol-gel process, showing promising properties for PMUT devices. Both rigid and flexible PMUT arrays were developed, incorporating polyimide membranes with novel fabrication techniques. These arrays were waterproofed using polyimide, epoxy, and parylene-C coatings, tested in air, water, and under bending conditions, and finally, integrated with imaging systems. Rigid-flex designs, offering higher sensitivity than fully flexible ones, successfully captured images of wire targets and tissue-like phantoms, showcasing their potential for low-frequency medical imaging. Moreover, this project demonstrates, for the first time, proof of concept of thin film PZT-based flexible PMUT arrays, suitable towards more versatile, efficient, and user-friendly imaging solutions, subject to further optimization.
Review Paper on Flexible MUTs
Advancements in micromachining techniques led to the development of micromachined ultrasound transducers (MUTs). On the other hand, emerging demands in healthcare, such as the need for continuous monitoring, have prompted the exploration of flexible alternatives to rigid bulk piezoelectric transducers, a well-established technology in medical imaging. Our review paper focuses on the convergence of these trends, offering a comprehensive examination of the state-of-the-art in flexible MUTs and their emergence in biomedical applications. It discusses the need for flexibility in ultrasound transducers, highlighting potential applications and explores the potential of flexible PMUTs and CMUTs in meeting the evolving demands of biomedical research.
The manuscript delves into various aspects, primarily focusing on fabrication techniques, while also addressing materials, performance specifications, and integration with electronics. It compares the performance of flexible PMUTs and CMUTs, including factors such as resonance frequency, sensitivity, and flexibility. The review concludes with an assessment of the challenges and opportunities in realizing efficient MUTs with high performance and flexibility, providing valuable insights for researchers, engineers, and healthcare professionals.
PZT Development
We report a reliable sol-gel process for crack-free PZT thin films up to 2 µm thickness, suitable for piezo-MEMS applications including PMUTs. The PZT solution development involved controlled stoichiometry with excess lead, tailored solvent molarity, PVP addition, and Pt seed layer tuning to achieve [111]-oriented, crack-free films. XRD and DBLI characterizations confirm the film’s suitability for proof-of-concept devices. While there's more room for refinement, this is a solid step towards democratizing PZT thin-film development for research and early-stage applications in piezo-MEMS such as PMUTs.
Awards, Scholarships and Grants
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Ultrasound Imaging with Rigid-Flex PMUTs
Using a custom-developed sol-gel PZT process, I fabricated rigid-flex PMUT arrays on conventional silicon wafers, completely avoiding the need for expensive SOI substrates. Using polyimide passive layer based membranes formed through backside etching enables simplified fabrication and packaging.
Two types of arrays were developed: a row-column (RC) array and a fully addressed matrix array. The RC array simplifies channel routing whereas the matrix array includes an innovative interconnect design that eliminates the need for through-silicon vias (TSVs), reducing complexity and improving yield. while the RC addressed array functionality is demonstrated by B-mode imaging, matrix captures near real-time 3D images. Together, these designs show the potential of low-cost, PMUT technology for wearable and point-of-care applications at low frequencies.
Awards, Scholarships and Grants
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Flexible PMUT Development Towards Ultrasound Imaging
Flexible piezoelectric micromachined ultrasound transducer (PMUT) arrays development is explained thoroughly in a reserach article accepted for publication in TUFFC! This work introduces a 30×12 row-column addressed flexible PMUT array on a polyimide substrate, leveraging a top-down fabrication process using a temporary carrier wafer with a PZT thin film. Here, the flexible PMUT assembly and characterizations take a significant step towards imaging application. The results show potential for wearable applications in shallow-depth regions. While there's more to optimize, I strongly believe this could lead to exciting new directions in wearable tech.