The computer industry is nearing a crisis: microchips get smaller and
faster but they struggle to transfer data at sufficient speeds.
Electrons flowing through standard chip connections are just too slow.
Now EU-funded researchers have shown how chips with built-in lasers
which use multiple wavelengths of light could in the future transmit
data at terabit speeds.
are great for transmitting information. Every time you use the Internet
or make a telephone call data, in the form of light pulses or photons,
travels hundreds of kilometers through the optical fiber networks that
crisscross the continent.
the insides of computers still stick to old fashioned electronics.
Microprocessors do their calculations with electrons, and they transfer
data within and between chips using electrons too.
is fast approaching a crunch point,’ explains Dries Van Thourhout from
the Department on Information Technology at Ghent University, an
associated lab of imec, in Belgium.’Up to now we have been trying to
increase the speed of transistors, but that performance has stopped
increasing now, it is just a question of packing more into a smaller
space. But the biggest hindrance to performance is the speed of the
connections between chips and devices. We call it the ‘interconnectivity
a sweet factory which makes thousands of sweets per second, but the
plant can only bag the sweets and dispatch them to the shops at a rate
of a few hundred per second. Unless you slow down production you will
end up with sweets piling up, rolling over the floor and clogging the
powerful microprocessors in computers today use vast quantities of data
and perform millions of calculations per second. You need to transfer
this data around your computer (or your mobile phone for that matter).
But the connections can’t keep up, they simply can not shift electrons
fast enough. The only way to cope is to slow down data production.
is where light comes in: you can use lasers to send photons down
silicon “wires” (light at infrared wavelengths travels remarkably well
through silicon, says Mr Van Thourhout) instead of electrons. But the
speed of light is not why optical interconnects are better. The real
trick is that light can be ‘multiplexed’; basically you can send photons
of different wavelengths through your interconnect at the same time.
Use three wavelengths and you effectively triple the speed of data
this in mind the ‘Wavelength division multiplexed photonic layer on
CMOS’ ( Wadimos) project set out to develop a demonstration chip with
multiplexing optical interconnects. The chip was based on technology
developed in a predecessor project (PICMOS) which created the first ever
microchip with integrated microlaser light sources, thanks to a unique
bonding “glue” developed by the PICMOS partners.
PICMOS project was a great success. We showed that optical
interconnects could be manufactured and that they would work,” says Mr
Van Thourhout. “But it is one thing to make and demonstrate something in
the lab. You won’t get chips like these into the mainstream or solve
that interconnectivity bottleneck unless you can manufacture them at the
industrial scale, making millions of them. PICMOS demonstrated the
principle of optical interconnects. Wadimos is proving that multiplexing
is possible and that the chips can be made in a standard CMOS
largest chip manufacturer STMicroelectronics has worked in
collaboration with universities and research institutions from France
and Italy and a Dutch SME which specializes in lithography (etching) for
electronic components. Together these partners have extended the
results of PICMOS and adapted them to more commercial manufacturing
of the biggest challenges was to replace the gold connections on the
microlasers in the PICMOS prototype. “You can’t have gold in a chip
fabrication plant,” explains Mr Van Thourhout. “Gold is a contaminant,
so partner CEA-LETI developed a process that would mean the integrated
lasers mounted on the chips could be connected using metals commonly
used in chip manufacturing such as aluminum, titanium and titanium
project partner imec has also worked to optimize the passive router
structures in silicon and investigated the feasibility for their
industrial production. Other project partners have contributed their
expertise: the Lyon Institute of Nanotechnology (INL) in France
demonstrated a new type of “microsource” for which you can control the
output wavelength. INL also worked with STMicroelectronics to develop a
way to simulate the optical network on a chip.
the University of Trento, Italy, designed and demonstrated a new type
of silicon router which could be used to ‘switch’ photons down
particular optical pathways.
these developments together, the Wadimos team has produced a network of
eight fully interconnected silicon blocks. The researchers have
demonstrated successful multiplexing across these connections and the
feasibility of optical filtering to direct and control the passage of
photons through the silicon interconnects and their subsequent
is still plenty of research to do, however, especially to keep the
lasers working in the high temperature environment of a chip’s surface.
Mr Van Thourhout says that they will need to find new materials that can
cope with the heat.
we are very hopeful that this approach will prove very successful in
the long term,” he asserts. “We are taking an exploratory approach.”
explains that other research groups, especially those in the US, have
developed optical interconnects that use an ‘off chip’ laser source; the
laser beam is split and redirected for each interconnect.
chips are more advanced and will soon be used in supercomputers,” says
Mr Van Thourhout, “and may eventually trickle down to mainstream
computing, but in the long run it will be more efficient to have chips
with integrated laser sources.”
expect the Wadimos interconnects to allow computer processing power to
continue to increase and overcome the data transmission bottleneck. Our
goal is to make optical interconnects a standard technology that will
support the development of yet more powerful, smaller microprocessors
capable of transferring data at rates of 100 terabits per second.”
Wadimos project received EUR 2.3 million (of a total EUR 3.2 million
project budget) in research funding under the EU’s Seventh Framework
Programme (FP7), ICT (Next-Generation Nanoelectronics Components and
Electronics Integration) program.