Prints of light
Electronic printing process could lead to transparent solar panels that are thin
enough to be used as tint on windows. Siobhan Wagner reports
A TECHNIQUE that allows electronic
circuits to be printed on a wide array of
surfaces could lead to new products
such as transparent solar panels that are
thin enough to be used as tint on
windows.
The technology from Semprius, a
University of Illinois spin-out, uses a
rubber stamp-like method to bond
circuits to substrates.
With this new approach, circuits are
first formed on a semiconductor wafer
using conventional wafer processing
techniques.
Then a special wet etching chemical
process slices chips off the surface of the
host wafer. The chips can range in size
from 10 to 100 times thinner than the
wafer depending on the application.
‘We’re unaware of anyone who has
sliced them in this way for the purpose of
electronics,’ said one of the developers,
Prof John Rogers of the University of
Illinois.
A silicone rubber stamp picks
up the chips and bonds them to a
substrate coated with a thin polymer
adhesive, which is used in conventional
electronics.
But Rogers said there are ways to do
the printing without an adhesive layer.
‘The silicone rubber has very low surface
energy, so when it makes contact with
the chiplet there is some adhesion there
but it is very weak,’ he said. The intrinsic
differences in the material properties
between the rubber and the final device
substrate, he said, allow transfers to
occur without an adhesive layer.
The simple stamping method can
transfer tens of thousands of individual
silicon ribbons or device components,
such as transistors and diodes, to a range
of conventional or unusual substrates.
Rogers said a conventional substrate
such as glass could be used for LCD
displays. The process would allow
transistors to be placed directly on to an
LCD back panel, which could increase
pixel response times for the display and
eliminate motion blur.
He said one of the unconventional
substrates his group is researching is
The team is still
perfecting the
printing process
that could one day
lead to electronic
circuitry providing
solar-tinted
windows
rubber. ‘We have ways of integrating
silicon devices on pieces of rubber, like
a rubber band, so they can be stretched
and formed in ways that enable you to
integrate electronics within the body,’
he said. ‘We’re looking at a variety of
biomedical applications.’
The choice of substrate is almost
limitless because the fabrication of the
silicon wafer is completely separated
from the final device substrate.
This means that high-temperature
processing, high resolution and high
registration lithography patterning can
be used, as well as cleaning processes
using strong acids and bases and other
chemicals.
Rogers said that his team is still
perfecting the printing process.
‘The two things we are concerned
about are the yield and the
registration,’ he said. ‘The yield is
defined by the fraction of the chips
picked up on the stamp and printed on
the substrate. The registration is
defined by how well the chiplets are
positioned relative to one another in
their printed state compared to the
state they were fabricated on the
source wafer.’
Rogers said with the tools they are
the EnGIneeR 27 OCTOBER– 9 NOVEMBER 2008 37
using, yields are in the order of
99.9 per cent, but that figure needs to be
improved for applications such as
displays. ‘We are very encouraged
because there are many ways we can
improve it,’ he said.
The placement accuracy is
currently about one micron. This, said
Rogers, is good enough for all the
applications including displays. ‘Yet we
still need improved yield.’
Semprius has a number of working
prototype displays it developed with a
team of industrial partners that cannot
yet be named. ‘They will go public in a
couple of months,’ said Rogers.
The company has also developed a
number of working prototypes for solar
modules that are designed to reduce the
cost per watt. The modules look similar
to standard solar panels, but Rogers’
team at Illinois is exploring new ideas
for the solar industry, such as
transparent panels that are thin
enough to be used as tint on windows.
‘Semprius is interested in these
ideas, but in the short term the most
immediate opportunity is creating a
low-cost replacement for the solar
panels people are already installing,’
said Rogers.
ELECTRONICS
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