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Philips Digital Photon Counting
Press Backgrounder
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Philips' fully digital light detection technology
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Silicon photomultipliers (SiPMs) have recently gained a lot of interest as replacements for photomultiplier tubes. Like photomultiplier tubes, they are capable of measuring extremely low light levels, to the point of being able to detect single photons. However, compared to photomultiplier tubes, SiPMs offer the
'solid-state' advantages of lower operating voltages, ruggedness, smaller physical size, lighter weight and excellent immunity to magnetic fields.
Nevertheless, current SiPMs do have their limitations. Although they feature very high internal gain, they generate a relatively weak analog output signal that must be processed by a power-hungry readout ASIC (Application-Specific Integrated Circuit) in order to recover the photon count and the time of arrival of the first photon. The cost, size and power consumption of this ASIC makes the transition from using conventional photomultiplier tubes to SiPMs more difficult.
The innovative new all-digital SiPM technology developed by Philips eliminates the need for these external ASICs, opening up the viability of using a solid-state solution in many more applications.
The Philips approach
Although photon counting is by definition a digital task, conventional silicon photomultipliers combine the electrical pulses generated by multiple photon detections into a single analog output signal. As mentioned above, this signal has to be processed by expensive power-consuming electronics in order to recover the photon count.
By integrating low-power CMOS electronics into the silicon photomultiplier chip, the team at Philips has developed a digital silicon photomultiplier in which each photon detection is converted directly into an ultra high speed digital pulse that can be directly counted by on-chip counter circuitry. In contrast to conventional silicon photomultipliers, the Philips digital silicon photomultiplier is therefore an all-digital (digital-in/digital-out) device. As a result, it produces faster and more accurate photon counts with extremely well defined timing of the first photon detection, both of which are important factors in applications such as medical imaging scanners and high-energy nuclear particle detectors.
Moreover, these revolutionary new digital silicon
photomultipliers can be manufactured using a
conventional CMOS process technology.
Applications
The planar nature of Philips' digital silicon photomultipliers allows them to be closely coupled to suitable scintillator materials for the detection of nuclear particles and high-energy electromagnetic radiation. When struck by an incoming particle or high energy electromagnetic radiation, scintillator materials absorb the energy and re-emit it in the form of a weak flash of light, typically in the visible spectrum. This weak flash of light is then detected by the photomultiplier. Philips' digital silicon photomultipliers are therefore suited for use in particle physics experiments and a wide range of medical imaging equipment, for example in Positron Emission Tomography (PET) scanners.
PET is a molecular imaging technique that produces
three-dimensional images of functional processes in
the body, e.g. the uptake of glucose that fuels
metabolic activity. The PET system detects pairs of
gamma rays (high energy electromagnetic radiation)
originating from a radioactive tracer, a small
amount of which is injected into the patient prior
to the scan. To image metabolic activity, PET
typically uses a radioactive derivative of glucose
called fluorodeoxyglucose (FDG). This compound
mimics the behavior of glucose in the body and can
be detected by the PET system.
For so-called 'time-of-flight' PET scanners,
accurately determining the time at which the first
photon arrives at the detector is extremely
important. Philips' digital silicon photomultiplier
prototypes achieve a timing accuracy for the
detection of the first photon of around 190 ps
(full-width, half-maximum using a standard scintillator crystal (LYSO) at 511 keV for two
detectors in coincidence).
Other applications for Philips digital silicon
photomultipliers include fluorescence-based DNA
sequencing, protein/DNA microarray assays,
surveillance systems and night-vision systems. In
fact, almost any application that currently uses
photomultiplier tubes for the detection of low light
fluxes.
Technology details
Conventional silicon photomultipliers (SiPMs)
consist of a two-dimensional array of avalanche
photodiodes (APDs) each of which is connected in
series with its own polysilicon 'quenching'
resistor. All of these diode/resistor 'microcells'
are then connected in parallel and the entire
microcell array is reverse-biased to a voltage above
the diodes' normal breakdown voltage - typically in
the range 30V to 70V. Operating in this so-called
'Geiger mode', the diodes are ultra-sensitive to
single electron-hole pairs that result in individual
diodes experiencing avalanche breakdown. These
electron-hole pairs can be generated either by the
absorption of a photon (the desired signal), or by
thermal energy or electron tunneling (unwanted
background noise). The unwanted background noise
produced by thermally generated electron-hole pairs
and/or electron tunneling, together with false
counts due to defective microcells, are collectively
referred to as the SiPM's 'dark count'.
To eliminate a conventional SiPM's need for an
external digitizing ASIC, the digital silicon
photomultiplier developed by Philips equips each
individual avalanche photodiode with its own 1-bit
on-chip ADC (Analog to Digital Converter) in the
form of a CMOS inverter. Each microcell that
experiences avalanche breakdown therefore produces
its own digital output that is captured, along with
the digital outputs from all other triggered
microcells, by an on-chip counter. The Philips
digital SiPM therefore converts digital events
(photon detections) directly into a digital photon
count. As a result, it is capable of achieving
significantly better resolution than conventional
SiPMs.
To overcome the 'dark count' problem associated with
conventional SiPMs, each microcell in the Philips
digital SiPM is also equipped with an addressable
static memory cell that can be used to disable or
enable the microcell. Microcells that show high dark
count levels can therefore be prevented from
contributing false counts to the SiPM's output. This
facility allows the Philips' digital SiPM to achieve
better signal-to-noise ratios than conventional
devices. Because defective microcells in the array
can be disabled, it also helps to improve production
yield.
Additional circuitry is added to each microcell to
actively (rather than passively) 'quench' and
recharge the microcell after triggering. This active
quenching/recharging in the Philips device improves
the detector's recovery time as well as reducing its
power consumption. Detector modules constructed
using Philips' new digital SiPM technology typically
only require air cooling. Cooling below ambient
temperature is only required in applications that
require ultra-low dark count levels.
In contrast to conventional analog SiPMs, in which
parasitic capacitance and inductance degrade timing
performance, all microcells in the Philips' digital SiPM are connected via a low-skew balanced trigger
network to an on-chip time-to-digital converter. The
timing resolution of this converter is 20 ps,
thereby preserving the excellent intrinsic timing
performance of the Geiger-mode avalanche
photodiodes.
In implementing this new digital SiPM technology,
the challenge for Philips was to integrate the
relatively high voltage avalanche photodiodes, which
must be reverse-biased to around 30V, alongside
low-voltage CMOS logic on the same silicon chip,
while maintaining dark count and photon sensitivity
performance. Nevertheless, the company's
revolutionary new digital SiPMs can still be
manufactured using a standard high-volume CMOS
process technology.
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