/Effect of element properties randomness on the pMUT array acoustic performance

Effect of element properties randomness on the pMUT array acoustic performance

Leuven | More than two weeks ago

Harness process uncertainty for ultrasonic devices with better certainty

Piezoelectric ultrasonic transducers consist of a piezoelectric membrane sandwiched between two electrodes. When AC voltage is biased on the electrodes, the piezoelectric material will be strained due to the intrinsic electro-mechanical coupling mechanisms of the piezoelectric material. The piezoelectric membrane will be set into vibration and acoustic waves will be generated in the medium in contact. Inversely, when acoustic waves were impinged on the membrane, the piezoelectric layer will be deformed. Dielectric charge will be induced, and AC voltage can be detected on the electrodes. The piezoelectric device can thus be used as both an ultrasonic transmitter and a sensor.

Incorporating piezoelectric materials with modern cleanroom microfabrication techniques has resulted in a new device called pMUT (piezoelectric miciromachined ultrasonic transducers). These miniaturized ultrasonic devices allow for a much smaller form factor and ease of integrating big arrays of ultrasonic transducers with electronic circuits. pMUTs have shown great prospects in applications such as medical imaging, ultrasonic haptic device, acoustofluidic devices and so on.

The ultrasound imaging quality relies much on the beamforming performance of the transducer arrays. Beamforming is the operation of producing an image out of the received ultrasound echo signals from each element of an array. In beamforming different elements will be driven with AC signal of different magnitude and phase such that complex ultrasound fields can be generated and received. In the existing pMUT designs the beamforming algorithm is usually based on the assumption that all the elements within the array have identical electromechanical properties. This assumption is not valid, however, since the microfabrication technique based on multilayer deposition processes always introduce some fluctuation to different elements in both the geometrical and material aspects. The process fluctuation results in different elements showing different resonances and displacement and these cause deterioration of the beamforming properties of the array compared to the designed ones.

In many natural materials, the randomness among arrayed structures turns out to be a useful factor which nature can make use of to improve the material performance. The moth scaled wing, for example, composed scales of varied aspect ratios [2]. These makes the scaled wing an acoustic absorber showing very wide bandwidth. This thesis aims at exploring the effect of element property fluctuation caused by the microfabrication processing on the whole pMUT array beamforming performance. The vibrational properties of some pMUT array as a whole and the composing elements will be characterized. Both numerical and theoretical models will be built to understand the effect of the randomness distribution of the element properties on the array beamforming properties. Lessons learnt from the modelling and from some natural materials will be used to guide the new pMUT array design. In the new design, systemic randomness factors will be purposely introduced into the pMUT array so that the array will show boosted performances for example a wide bandwidth.

Depending on the students' interest, the balance between simulations and experimental work will be steered. Generally, this is a perfect topic for students eager to understand modern microfabrication techniques and get hands-on experience in characterizing MEMS devices. The work is estimated to be: 40% simulations/calculations (software such as COMSOL, Matlab will be used), 40% characterization on some fabricated pMUTs arrays, 20% on designing new pMUT arrays with randomness design features. The design will be fab-out if improved bandwidth performance could be predicted by the knowledge learnt.

For further information or for application, please contact Dr. Zhiyuan Shen (zhiyuan.shen@imec.be) or Dr. Veronique Rochus (veronique.rochus@imec.be).






[1] Hang Gao, Pieter Gijsenbergh, et. al, Design of polymer-based PMUT array for multi-frequency ultrasound imaging, IEEE Ultrasonics Symposium, 2019.


[2] Thomas R. Neil *, Zhiyuan Shen* (*co-first author), et. al, Moth wings are acoustic metamaterials, PNAS, 2020.


Type of project: Thesis

Duration: 6-12 months

Required degree: Master of Engineering Technology, Master of Science, Master of Engineering Science, Master of Bioengineering

Required background: Electromechanical engineering, Biomedical engineering, Electrotechnics/Electrical Engineering, Mechanical Engineering, Nanoscience & Nanotechnology, Physics

Supervising scientist(s): For further information or for application, please contact: Zhiyuan Shen (Zhiyuan.Shen@imec.be) and Veronique Rochus (Veronique.Rochus@imec.be)

Only for self-supporting students.