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| Stretchable micro-LED display, consisting of an
interconnected mesh of printed micro LEDs bonded to a rubber
substrate. Photo by D. Stevenson and C. Conway, Beckman Institute,
University of Illinois |
Applications for the arrays, which can be printed onto flat or
flexible substrates ranging from glass to plastic and rubber,
include general illumination, high-resolution home theater
displays, wearable health monitors, and biomedical imaging
devices.
"Our goal is to marry some of the advantages of inorganic LED
technology with the scalability, ease of processing and resolution
of organic LEDs," said John Rogers, the
Flory-Founder Chair Professor of Materials Science and
Engineering at the University of Illinois.
Rogers and collaborators at the U. of I., Northwestern
University, the Institute of High Performance Computing in
Singapore, and Tsinghua University in Beijing describe their work
in the Aug. 21 issue of the journal Science.
Compared to organic LEDs, inorganic LEDs are brighter, more
robust and longer-lived. Organic LEDs, however, are attractive
because they can be formed on flexible substrates, in dense,
interconnected arrays. The researchers' new technology combines
features of both.
"By printing large arrays of ultrathin, ultrasmall inorganic
LEDs and interconnecting them using thin-film processing, we can
create general lighting and high-resolution display systems that
otherwise could not be built with the conventional ways that
inorganic LEDs are made, manipulated and assembled," Rogers
said.
To overcome requirements on device size and thickness associated
with conventional wafer dicing, packaging and wire bonding methods,
the researchers developed epitaxial growth techniques for creating
LEDs with sizes up to 100 times smaller than usual. They also
developed printing processes for assembling these devices into
arrays on stiff, flexible and stretchable substrates.
As part of the growth process, a sacrificial layer of material
is embedded beneath the LEDs. When fabrication is complete, a wet
chemical etchent removes this layer, leaving the LEDs undercut from
the wafer, but still tethered at anchor points.
To create an array, a rubber stamp contacts the wafer surface at
selected points, lifts off the LEDs at those points, and transfers
them to the desired substrate.
"The stamping process provides a much faster alternative to the
standard robotic 'pick and place' process that manipulates
inorganic LEDs one at a time," Rogers said. "The new approach can
lift large numbers of small, thin LEDs from the wafer in one step,
and then print them onto a substrate in another step."
By shifting position and repeating the stamping process, LEDs
can be transferred to other locations on the same substrate. In
this fashion, large light panels and displays can be crafted from
small LEDs made in dense arrays on a single, comparatively small
wafer. And, because the LEDs can be placed far apart and still
provide sufficient light output, the panels and displays can be
nearly transparent. The thin device geometries allow the use of
thin-film processing methods, rather than wire bonding, for
interconnects.
In addition to solid-state lighting, instrument panels and
display systems, flexible and even stretchable sheets of printed
LEDs can be achieved, with potential use in the health-care
industry.
"Wrapping a stretchable sheet of tiny LEDs around the human body
offers interesting opportunities in biomedicine and biotechnology,"
Rogers said, "including applications in health monitoring,
diagnostics and imaging."
Rogers is affiliated with the Beckman Institute, the department
of mechanical science and engineering, the Frederick Seitz
Materials Research Laboratory, and the Micro and Nanotechnology
Laboratory.
Ford Motor Co., the National Science Foundation and the U. S.
Department of Energy funded the work.
SOURCE