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Piezo to
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 Scientific
innovation from a multidisciplinary Piezo Institute
research collaboration has produced high-performance
thin film piezoelectric transducers for specialist
medical applications.
The new piezo thin films resonate with exactly the high
frequencies needed by ultrasound high resolution imaging
in dermatology and ophthalmology. The project is led by
LUSSI, a multidisciplinary research group at Francois
Rabelais University in Tours, with support from the
Josef Stefan Institute in Slovenia and piezo ceramics
manufacturer Ferroperm in Denmark.
The challenge
Standard 1-15MHz ultrasound scanners provide image
resolution around a millimetre, which is not good enough
for the tiny structures in the skin. Dermatology
applications typically need resolutions around 100
micrometres.
The quality of the resolution depends on the frequency
of the ultrasonic wave, which in turn is inversely
proportional to the thickness of the film.
So thinner films produce higher resolution, and the
resultant loss of signal power doesn’t have much impact
in skin imaging applications where the target is on or
near the surface.
The Piezo Institute team set out to make high frequency
transducers in bulk at a low cost. The traditional way
to make thin films is by machining bulk ceramics, but
this is costly, causes material defects and degrades the
properties of the material.
“Machining bulk ceramics into thin films with the high
frequencies required for high resolution medical imaging
is a process with high waste, low yield and poor
results,” says Prof Marc Lethiecq at LUSSI.
The group has developed several technologies to enable
deposition of precise thin films for exactly the right
frequency, including screenprinting for flat surfaces
and pad printing for curved substrates.
“With both techniques, we’ve been able to make films
with piezoelectric properties to rival the best bulk
ceramics on the market,” Lethiecq says.
Avoiding diffusion
The second challenge comes from the firing process. High
temperatures can cause diffusion of chemicals between
the substrate and thin film, which alters its
properties.
Additives in the ceramic powder become tiny gas bubbles
at high temperatures and help to produce a porous
substrate with the same chemical composition as the thin
film. The large amount of tiny gas bubbles gives the
substrate the acoustic properties to make it an element
of the transducer itself.
The addition of dopants reduces the required temperature
by 300ºC which in turn reduces the diffusion.
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