European researchers have created a CMOS (semiconductor)
camera capable of filming individual photons one million times a second. The
breakthrough will impact on all the most advanced areas of science and makes Europe the world leader in the technology.
The scientists wanted to create the fastest, highest
resolution CMOS (semiconductor) video camera, but to do that they needed to
choose an ultra-fast photo detector. They also needed to choose between two
competing timing mechanisms or stopwatches, Time to Analogue Convertors (TAC)
and Time to Digital Convertors (TDC).
A 32 x 32 Single Photon Avalanche Diode (SPAD) array fabricated in 0.8-micrometer CMOS technology (2004)
The timing mechanism is important. It can tell, to within a
few tens of picoseconds, when the photon arrives at the detector. It creates
stunning resolution in time.
As they studied TAC and TDC, the researchers realized that
both methods had peculiarities that provided advantages in different
applications. Instead of eliminating one technology, they characterized two. It
was a Eureka
moment for the Megaframe project
behind the research.
“We got really excited, and we are going to explore this
further in the second phase of our project,” explains Edoardo Charbon, formerly
with EPFL and now at TU Delft, coordinator of Megaframe.
It is an indication of the drive and enthusiasm of the team.
The dust has hardly settled on the worldwide breakthroughs achieved by the
first phase of the project, and already Megaframe is looking forward to the
next breakthrough.
Imaging technology has advanced rapidly over the last few
years. But the demands of science have advanced even more rapidly. New
scientific fields like proteomics – studying the different proteins that form
the human body – pose a problem because they require cameras that are capable
of recording data at photon-resolution, and extremely fast.
That problem is now essentially solved thanks to the work of
the Megaframe project, which developed a CMOS video camera that can capture
1024 individual photons at one million frames a second. It can record, to
within 100 picoseconds, when the photon arrived at each detector. It is extremely
fast.
Big challenges
There were enormous challenges on the way to achieving their
goal. The first challenge was to miniaturise the photo detector they had
selected, the Single-Photon Avalanche Diode (SPAD).
“We started the project with the redesign of a SPAD, at the
time available only in 0.35 micrometer CMOS technology, a technology for which
we had pioneered SPAD design,” notes Charbon. “We soon realised that if we
wanted to fit a stopwatch in each pixel we needed to go further down in feature
size. We needed to go to technologies that are much more compact, so we chose a
0.13 micrometer CMOS technology.”
The stopwatches are essential to record when the photon
arrives, SPADs are necessary to detect the light, and both need to be present
in each pixel. And the consortium wanted to do a 32 by 32 array of pixels,
making a total of 1024 individual SPADs and stopwatches!
“We spent a good year and a half optimising the design of
SPADs in 0.13 micrometer CMOS technology. So the challenges were mostly to
reduce noise, increase sensitivity, and to miniaturise the device.”
Then they had to decide which stopwatch they needed to use,
the TDCs or TACs mentioned earlier, which led to one of the most exciting of
the project’s many breakthroughs.
Megaframe phase I simply sought to establish that a
photon-resolution, high-speed CMOS camera was possible at all. Both the TDCs
and the TACs met the design targets, but instead of eliminating one, they
discovered that they could potentially use the two in different ways.
It was a research breakthrough because the team expanded the
potential usefulness of the underlying technology. “In the second phase of the
project, we decided to focus on TDCs, because they were easier to implement,”
stresses Charbon. But the work on both stopwatches is useful for further study
in later phases.
The upshot is that their camera can time the arrival of a
photon to within 100 picoseconds, and they can do it up to 1 million times a
second. “And every microsecond we can count up to 64 photons,” declares Charbon.
Pinhead watches
"That was a big, big challenge. In integrated circuit
design, achieving a certain performance is usually possible, but achieving that
performance in a large number of miniaturised components, working
simultaneously, with a supply current optimised in a way that it does not
disturb the other components, that is very hard.
“It is a bit like making one incredibly accurate watch. You
can do it; you can make it run with very high precision. But building 1024
miniaturised watches, working with the same power supply and the same
precision, on a space about the size of the head of a pin? That is the real
challenge.”
Of course, all these electronics in 32 by 32 array produce a
staggering amount of data, so the partners needed to develop new and improved
ways of moving the data from the pixels into storage. In the end, they developed
a high-speed, highly parallel readout capable of achieving 10Gb/s.
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Detail of the CMOS 32 x 32 Single Photon Avalanche Detector Array (2004)
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“There were two more very important challenges. One was the
optical challenge. If you have a pixel that is 50 microns-by-50 microns, with a
lot of functionality in it, the area that is sensitive to photons can only be a
small proportion of this pixel. So you can imagine that only maybe 1% of the
pixel can be used to detect light,” Charbon explains.
The consortium included a partner specialising in
microlenses, which are able to concentrate the light to the sensitive element.
It was a complex task, but the high-precision lenses work amazingly well to
lead the photons onto the photosensitive areas that are just 10 microns in size.
Another challenge was to make the camera compatible with
existing microscopes, essential if the technology is to gain acceptance in, for
instance, advanced biomedical research labs.
Enormous potential
The technology passed validation and the Megaframe camera
is, in itself, an enormous breakthrough in imaging science and worthy of a
celebration on its own. It has validated European expertise, established the
Continent’s leadership in the domain, and proven an excellent training ground
for future engineers and scientists. What’s more, it will have enormous
ancillary benefits for allied fields like lithography, integrated circuit
design and a host of others.
But by far the most exciting result from Megaframe’s work
will be the science that it will enable in dozens of unrelated fields,
everything from molecular biology and medicine, to automotive and aeronautic
testing and development, entertainment and 3D television, and many others.
The legacy of the Megaframe project will not be that it
established European leadership, but that it will foster innovation in so many
other areas.
The Megaframe project received funding from the FET-Open
scheme for future and emerging technologies in the EU’s Sixth Framework
Programme for research.
Source:
ICT Results
A 32 x 32 Single Photon Avalanche Diode (SPAD) array
fabricated in 0.8um CMOS technology (2004)
Detail of the CMOS
32 x 32 Single Photon Avalanche Detector Array (2004)