[2025] Modelling and fabrication of PDMS-based Capacitive Micromachined Ultrasound Transducers (CMUTs)

Flexible ultrasonic probes generally fall into three categories: (a) those using
organic piezoelectric films as transducers, (b) those embedding piezoelectric
ceramics within polymer substrates, and (c) those based on capacitive
micromachined ultrasonic transducers (CMUTs). Although organic piezoelectric
films (like polyvinylidene fluoride and its copolymers) offer considerable
flexibility, they are poorly suited for transmitter applications due to their low
electromechanical coupling coefficients, low dielectric constants, high dielectric
losses, and low Curie temperatures—which lead to processing challenges and
phase transformations at elevated temperatures that fully degrade their
piezoelectric properties. By contrast, piezoelectric ceramics achieve high
electromechanical performance and are relatively easy to process, and could be
developed into multi-element probes connected through island-bridge multi-
layered electrode structures[1], but most widely used piezoceramics like PZT
contains lead, which raises concerns about long-term biocompatibility.
CMUTs, typically fabricated on silicon wafers and filled with polydimethylsiloxane
(PDMS) in the trenches, gain some flexibility through this passive polymer filler,
but still face limitations when conforming to curved or irregular surfaces. Their
efficiency also tends to be lower than that of piezoelectric ceramics due to
inherent inhomogeneities and parasitic capacitances arising from their rigid
silicon-based structures. Consequently, most existing flexible CMUT probes only
conform well to “developable” surfaces (such as cylinders), not to more complex
“non-developable” geometries[2].
Nonetheless, PDMS-based CMUTs have the potential to enhance electro-acoustic
efficiency, because the soft elastomeric matrix allows membranes to deform
more readily under an applied bias, which can increase membrane displacement
and boost electromechanical coupling. For optimal performance, however,
ultrasonic probes must establish a seamless interface with irregular nonplanar
surfaces to minimize signal distortion. Current rigid probes frequently suffer from
air gaps at the interface, causing significant acoustic reflections and degrading
system performance. Seamless probe integration, coupled with beam-correction
algorithms, is therefore essential.
The aim of this project is to optimize critical design parameters—such as
membrane thickness and air-gap dimensions—in PDMS-based CMUTs, and microfabricate the resulting designs in cleanroom environment, followed by acoustic testing.
 

[1] H. Hu et al., “Stretchable ultrasonic transducer arrays for three-dimensional
imaging on complex surfaces,” 2018. [Online]. Available: https://www.science.org
[2] S. V. Joshi, S. Sadeghpour, N. Kuznetsova, C. Wang, and M. Kraft, “Flexible
micromachined ultrasound transducers (MUTs) for biomedical applications,” Dec.
01, 2025, Springer Nature. doi: 10.1038/s41378-024-00783-5.

Assignment

1st part: Literature review on Flexible CMUTs and COMSOL simulation to study
the design parameters for a PDMS-based CMUT on a complex surface.

2nd part: Microfabrication of Flexible CMUTs in cleanroom environment followed by acoustic validation.

Requirements

MSc students from Microelectronics, Biomedical Engineering, Applied Physics or
Mechanical Engineering. Prior knowledge on COMSOL is preferred.
Interested students should include their CV, the list of courses attended, and a
motivation letter, and send it to Tiago Costa (t.m.l.dacosta@tudelft.nl) and
Eshani Sarkar (E.Sarkar@tudelft.nl).

Contact

Last modified: 2025-02-28