Plastic MEMS promise pervasive ultrasound imaging

September 13, 2018 // By Julien Happich
With the aim to make ultrasound transducers more affordable for the medical market, researchers from the University of British Columbia have designed capacitive micromachined ultrasound transducers (CMUTs) out of polymer instead of traditional silicon-based designs.

Detailing their findings in a paper titled "Fabrication and testing of polymer-based capacitive micromachined ultrasound transducers for medical imaging" published in the Microsystems & Nanoengineering journal, the researchers explain how they were able to use inexpensive materials, such as the SU-8 photopolymer and Omnicoat, to create thin yet very sensitive arrays of ultrasound transducers whose imaging capabilities where on part with silicon-based designs.

A 64-element polyCMUT linear array mounted on PCB.
b) Six CMUT elements separated by a pitch of 550μm.
c) CMUT cells showing the interconnected top
electrodes and cavities underneath.

In just six steps and for an estimated unit cost under 100 dollars, they manufactured micrometre-thin biocompatible parylene-sealed polymer CMUTs (polyCMUTs). Their device was built as a linear array of 64 CMUT elements, each containing 4×75 CMUT cells (the actual plastic MEMS), with a pitch of 550μm between elements. The linear array allowed the researchers to experiment with beamforming techniques for accurate ultrasound imaging.

Key to making these polyCMUTs was the encapsulation of the metal electrode inside the membrane, instead of being on top, allowing for a thin stack and low operational voltages comparable to traditional CMUTs fabricated in polysilicon or silicon nitride. In their paper, the researchers report that the polyCMUTs could be pre-biased so as to be operated even as passive devices during reception and with low excitation voltages (a 12 VAC signal superimposed on a 10 VDC signal) during ultrasound transmission.


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