Engineers at the University of Washington have
added an LED array into a flexible contact lens, ultimately aiming
at in-eye head-up displays.
"This is a very small step toward that goal, but I think it's
extremely promising," said researcher Dr Babak Parviz.
Medical sensing is another possibility. "There is a large area
outside of the transparent part of the eye that we can use for
placing instrumentation," he added.
Transparent PCB
The lens, which is also a two layer transparent PCB, is built on
a 100µm polyethylene terephthalate (PET) substrate patterned with
two layers of fine metal tracks separated by an insulating
layer.
320nm diameter flat-bottomed circular self-assembly targets for
LEDs are created by forming apertures in the insulating layer.
These disc-shaped holes expose tracks of the lowest metal layer
- which will eventually be the LED bottom contact.
Side contacts for the LEDs are formed during the deposition of
the second metal layer which drapes down the sides of the apertures
where appropriate.
A further insulating layer, with its own lithography step to
make circular holes which match the original insulator, completes
the self-assembly targets and covers the top metal.
Thermal processing during these steps is kept below the 70°C
distortion temperature of PET. Finally the proto-lenses are cut
from the substrate and exposed contacts 'tinned' with a low-melting
point indium alloy.
Custom fabricated AlGaAs devices
The 320µm diameter disc-shaped LEDs are custom fabricated
AlGaAs
devices tuned to emit red at 689nm.
These were built by epitaxy with both anode and cathode contacts
on top, etched circular, then released by etching through a
sacrificial layer in the epitaxial stack.
Shape-based attraction, said Washington, causes the diodes to
self-assemble into their targets, the right way up, when the
lens/PCBs is washed in a suspension of the diodes.
Low temperature re-flow soldered the diodes in place, and a
polymethyl methacrylate (PMMA) top coat sealed the assembly and
made it bio-compatible, claimed Washington.
Pressing into a mould at 240°C converts the assembled lens blank
into a lens.
For a head-up display, images have to be focussed at infinity,
can this be done?
"Constructing a focused image on the surface of cornea is
definitely challenging, nobody has tried it or demonstrated it
before," Parviz told Electronics Weekly. "We are looking into two
ways of doing this: integrated a micro lens with each pixel, or
using small ultra-low power lasers for each pixel."
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