PC_Advisor_Issue_264_July_2017

News: Analysis
Researchers build a microprocessor
from flexible materials
It’s a one-bit microprocessor with four instructions, but it could
open the way to more flexible electronics, writes Peter Sayer
R
esearchers have built a primitive
microprocessor out of a twodimensional
material similar
to graphene, the flexible conductive
wonder material that some believe will
revolutionise the design and manufacture
of batteries, sensors and chips.
With only 115 transistors, their processor
isn’t going to top any benchmark rankings,
but it’s “a first step towards the development
of microprocessors based on 2D
semiconductors,” the researchers at Vienna
University of Technology said in a paper
published in the journal Nature.
Two-dimensional materials have the
benefit of flexibility, meaning that they
can be incorporated more easily into
wearable devices or connected sensors, and
potentially making them less breakable:
Picture a smartphone that bends rather
than breaks if you drop it.
Today’s semiconductors and screens
are already pretty thin, but they still rely on
the three-dimensional physical properties
of the materials they’re made from in order
to function. Bend a silicon wafer and it will
crack. But 2D materials like graphene or
the transition-metal dichalcogenide (TMD)
used by the Vienna researchers, are truly
two-dimensional, made with crystals just
one layer of atoms or molecules thick,
allowing them to flex.
TMDs are compounds composed of a
transition metal such as molybdenum or
tungsten and a chalcogen (typically sulphur,
selenium or tellurium, although oxygen
is also a chalcogen). Like graphene, they
form into layers, but unlike graphene which
conducts electricity like a metal, they are
semiconductors, which is great news for
flexible chip designers.
Stefan Wachter, Dmitry Polyushkin
and Thomas Mueller of the Institute of
Photonics, working with Ole Bethge of
the Institute of Solid State Electronics
in Vienna, decided to use molybdenum
disulfide to build their microprocessor.
They deposited two molecule-thick
layers of it on a silicon substrate, etched
with their circuit design and separated by a
layer of aluminium oxide.
“The substrate fulfils no other function
than acting as a carrier medium and could
thus be replaced by glass or any other
material, including flexible substrates,”
they wrote.
Recent Intel microprocessors act on data
in 64-bit ‘words’, can understand hundreds
or even thousands of different instructions,
depending on how you count them, and
contain hundreds of millions of transistors.
In contrast, the microprocessor built by
the researchers is only capable of acting
on data one bit at a time, using a set of
just four instructions (NOP, LDA, AND and
OR), and the circuit features used to build
it are of the order of two micrometers
across, 100 times larger than those found
in the latest Intel and ARM processors. With
more work, though, the microprocessor’s
complexity could be increased and its
size reduced, the researchers said. They
deliberately chose an overly large feature
size for their manufacturing process to
reduce the effects of holes, cracks and
contamination in the molybdenum disulfide
film and to make it easier to inspect the
results with an optical microscope.
“We do not see any roadblocks that
could prevent the scaling of our 1-bit
design to multi-bit data,” they said, and
only the challenge of lowering contact
resistance stands in the way of submicrometer
manufacturing.
That’s not to say it will be easy: although
the manufacturing yield for subunits
was high, with around 80 percent of the
arithmetic-logic units fully functional, their
non-fault tolerant design meant only a few
percent of finished devices worked properly.
Commercial microprocessor
manufacturers deal with yield problems
by making their chip designs modular,
and testing them at a variety of speeds.
Processors that work at higher speed fetch
higher prices, while faulty subcomponents
can be permanently disabled and the
resulting chips, otherwise fully functional,
sold as lower-specification models.
It’s taken 46 years for Intel to get
from the 4004, a four-bit central
processor with 46 instructions, to the
latest incarnation of the x86 architecture,
Kaby Lake: with all that the industry
has learned about micromanufacturing
since then, progress with flexible
semiconductors may be a little faster. J
14 www.pcadvisor.co.uk/news July 2017