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Silicon detectors eye-up photomultiplier markets

15 May 2006

Photomultiplier tubes are facing stiff competition from silicon-based low-light detectors. Jacqueline Hewett asks Joseph O'Keeffe, a founder of Irish firm SensL, why he believes that silicon devices have what it takes to displace incumbent technologies.

SensL of Ireland believes strongly that its silicon-based low-light detectors have the potential to displace alternatives such as photomultiplier tubes (PMTs). Backing up this confidence is the firm's ability to produce silicon photodiodes using CMOS-compatible processing that are optimized for Geiger mode operation.

A closer look at the technology reveals crucial advantages such as low operating voltages, low cost, no excess light damage and potential for integrated electronics. With a suite of products now available and more releases on the cards, SensL could certainly be a name to remember when it comes to visible low-light detection and photon counting. "Our core capability is making photodiodes that are stable in Geiger mode and all the associated silicon processing and optimal device geometries," Joseph O'Keeffe, SensL's chief operating officer and company founder, told OLE. "We can keep the dark count down and quantum efficiencies up for Geiger mode and photon counting."

Although it has been in operation for just over two years, the company's foundation is 15 years of research into low-light detection at the Tyndall National Institute, which is part of the University College Cork, Ireland.

A quick browse through the company's website and you will see that with a workforce of 18, SensL has turned academic success into commercially viable products, many of which it claims are world firsts.

This is largely thanks to the use of CMOS-compatible processing, which allows the firm to make arrays of devices. The approach also allows read-out circuits to be integrated onto the sensor and accessed through a USB interface.

The company also benefits from the Tyndall Institute's silicon fab. "As part of our set-up agreement, we have access to the [Tyndall] foundry," explained O'Keeffe. "This is extremely useful when you are trying to do quick and numerous iterations to fine tune different parameters. It gives us the ability to turn around wafer runs in a matter of weeks and customize structures to specific applications."

O'Keeffe explains that SensL is developing products under three different umbrellas, each of which is basically a different way of exploiting its core technology. The first platform is what SensL calls its intelligent photon counting module, the second is large-area high-gain avalanche photodiodes (APDs) and the third is a photon counting imager/array.

"In each case the building block is exactly the same," explained O'Keeffe. "The first platform uses one of our photodiodes, the second is an array of photodiodes connected together and the third is again an array but here each pixel/photodiode is individually addressable."

Photon counting

SensL's range of photon counting modules includes its PCDMini, which it brands as the world's smallest photon counting module and as being ideal for OEMs, and the PCMPlus, which it believes is the first USB photon counting system.

"A typical photon counting architecture would have a sensor, a counting card and software to analyse the data," said O'Keeffe. "But with our module, the counting is built-in. You can optimize parameters like bias voltage and integration time through software. Most applications have one parameter that is important and the user can go and optimize for this."

And it doesn't stop there. The company says that its PCSTime is the first integrated photon timing system. With a high timing resolution (time stamping down to 2.5 ns), this product is said to be ideal for applications such as time-correlated single photon counting and fluorescence decay.

Silicon photomultipliers

Such is the confidence in its silicon technology, SensL is branding its large-area high-gain APDs as the first direct replacement for analogue PMTs. As O'Keeffe explains, low-light detection can be performed with a PMT or an APD. While a PMT has a high, stable gain, its disadvantages include its bulky size, a high operating voltage in the region of 1000 V and its robustness. On the other hand, a silicon APD is small and uses a low operating voltage but its gain is limited to around 100.

"Our SPMMini product is what we call a silicon photomultiplier," said O'Keeffe. "It has all of the advantages of silicon combined with the 106 gain of the PMT. The operating voltage is less than 100 V so this is a great combination of what silicon can do whilst competing with the gain of the PMT."

SensL's silicon photomultiplier is essentially an array of around 1000 of its photon counting photodiodes connected in parallel. From a user's point of view, the product still has a single active area. "It's just that here the active area is made up of many photodiodes all packed together," said O'Keeffe.

"Other important points are that you cannot overexpose the SPMMini whereas a normal PMT will be damaged if you give it too much light," continued O'Keeffe. "The devices are also insensitive to magnetic field, which is extremely important for a number of applications such as nuclear medicine (PET and SPECT scans) and high-energy particle detection." SensL has high hopes for these two "radiation detection" and other security-based applications.

In addition, SensL is receiving interest from firms developing point-of-care instruments where the advantages of the silicon photomultiplier really begin to make a difference. SensL also expects to see adoption into biomedical fluorescence instrumentation in the short term as the radiation detection applications have a longer lead-time. That said, PMTs are used in an enormously wide range of applications and O'Keeffe is keen to stress that anywhere a PMT is used, its devices could be used instead.

The next release in this product platform will be an array of APDs. "You could have 1 × 32 or 1 × 64 for example," said O'Keeffe. "The technology allows you to have arrays of highly sensitive APDs, which you cannot do with PMTs. We are planning to release arrays in the coming months."

The rest of 2006 will certainly be busy for SensL as it also hopes to release a photon counting array (its third product platform) within the next six to nine months. "A big aspect of the photon counting array is that there is nothing like that out there - it is an enabling technology," explained O'Keeffe. "The first two platforms have a displacement aspect because you could take out a PMT or an APD and put our product in its place."

With several products on the market and more in the pipeline, it now appears to be make-or-break time for SensL. Will industry adopt its technology? Can SensL convince customers to replace PMTs with its silicon alternatives? These crucial questions resemble a "David and Goliath" battle but having got so far in such a short time, it's hard to bet against SensL making a success of its silicon know-how.

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