1. Effect of microcell size on timing performance of
silicon photomultipliers for ToF-PET imaging
Ashwin A. Wagadarikar, Sergei Dolinsky and Fabrizio Guerrieri
Abstract– The timing resolution of a Silicon Photomultiplier
(SiPM)-based scintillator detector depends on its ability to detect
optical photons in the first ns after a scintillation event and is
thus directly proportional to its photon detection efficiency
(PDE). As a result, an SiPM with a larger microcell size, and thus
larger fill factor and PDE, is expected to demonstrate better
timing resolution. However, coincidence timing resolution
measurements we performed with Hamamatsu 3x3 mm2
SiPMs
with 25, 50 and 100 um microcells showed that the timing
resolution of larger microcell devices are adversely impacted by
larger dark count rates. A simple high pass filter can be used to
reduce the impact of the dark counts on the baseline of an SiPM
output and thus improve the timing resolution to reflect what is
expected from a device with larger fill factor and PDE.
I. INTRODUCTION
ILICON photomultipliers (SiPMs) are a promising
alternative to photomultiplier tubes (PMTs) for scintillator-
based nuclear medical imaging. An SiPM is an array of single
photon avalanche photodiodes (microcells) connected in
parallel. Each microcell operates in Geiger mode, where they
are biased above the breakdown voltage. In response to
incident light, each microcell generates an avalanche of
current that is quenched by a resistor connected in series with
the microcell. A single microcell only provides logical on/off
information in response to incident light, while the combined
output of all the microcells in an SiPM is the sum of all the
activated microcells, thus providing an output proportional to
the scintillation light flux [1].
While retaining key properties of PMTs such as high gain
(105
to 106
) and short pulse duration, SiPMs have other
attractive features including compact size, lower operating
voltages (<100V), higher photon detection efficiency (PDE),
and insensitivity to magnetic fields [2]. Thus, several research
and commercial groups have investigated their applicability in
revolutionary SiPM-based positron emission tomography
(PET) and combined PET / magnetic resonance (MR) systems.
Due to their high gain and response times comparable to
PMTs, SiPMs also offer good timing resolution, a key
property that can be exploited in time of flight PET (ToF-
Manuscript received July 31, 2011.
A. A. Wagadarikar is with GE Global Research, Niskayuna, NY 12309
USA (telephone: 518-387-5156, e-mail: wagadari@ge.com).
S. Dolinsky is with GE Global Research, Niskayuna, NY 12309 USA
(telephone: 518-387-5400, e-mail: dolinsky@research.ge.com).
F. Guerrieri was with GE Global Research, Niskayuna, NY 12309 USA.
He is now with Volterra Semiconductor Corp., Fremont, CA 94538 USA
(telephone: 510-743-1629, e-mail: fabrizio@volterra.com).
PET) to better localize the annihilation events in the scanner’s
field of view [3]. The timing resolution, , is dominated
by the ratio between and / :
, (1)
where , is the combined electronic and detector noise,
and / is the slope of the signal fed into the timing
pickoff circuit. A major source of detector noise is dark
counts, which are avalanches induced by thermally generated
free carriers within individual microcells. They typically
increase with higher operating voltages due to the larger
electric field [4].
Since an SiPM’s output signal is a summation of the pulses
from the individual microcells after they are triggered by
incident photons, the signal slope is proportional to the gain of
the microcells and the number of microcells that are triggered,
which depends on the PDE of the microcells. The PDE is
described as
. . Pr , (2)
where QE is the quantum efficiency, FF is the fill factor of the
device and Pr(Avalanche) is the probability of avalanche
being triggered in a microcell, which increases with higher
SiPM bias voltage [5].
A key SiPM design feature that affects its behavior is the
microcell size. For a given SiPM area, a larger microcell
implies a larger fill factor and thus larger PDE based on (2) as
well as a larger gain due to the larger capacitance of the SiPM.
Since the timing resolution is expected to improve with larger
PDE, one would expect to achieve better timing resolution
from devices with a larger microcell size. However, the
increase in microcell size is typically accompanied by a larger
number of dark counts and secondary avalanches, as well as a
longer pulse duration due to the larger capacitance of the
device. These effects increase the noise on the baseline of the
SiPM signal and thus adversely impact the timing resolution.
In addition, a reduction in the total number of microcells
within the area of the SiPM implies a reduction in the dynamic
range of the device [2]. Thus, the choice of microcell size can
lead to a number of design tradeoffs.
Our goal was to understand how the interplay between these
competing characteristics could affect the timing resolution of
an SiPM based PET detector. We report results of an
experimental investigation into the timing performance of
SiPMs with varying microcell size from Hamamatsu. Timing
S

2. resolution measurements were performed with SiPM devices
at different operating voltages and variable threshold of the
leading edge discriminator. The results of these measurements
can be interpreted to determine the optimal microcell size for
an SiPM-based ToF-PET system.
II. EXPERIMENTAL SETUP
The timing performance of 3x3 mm2
SiPM devices from
Hamamatsu Photonics with 25 um (S10362-33-025C), 50 um
(S10362-33-050C) and 100 um (S10362-33-100C) microcells
was investigated. All timing measurements were made using
coincidence events from a Na-22 button source. A 3x3x10
mm3
LYSO crystal was wrapped with Teflon and coupled to
the SiPM under test using optical grease (BC-630, Sain-
Gobain). The SiPM signal was readout using a preamplifier
board with THS4303 transimpedance amplifiers. A 13x13 mm
LaBr3 scintillator coupled to a H6533 PMT was used as a
reference detector and its timing jitter was 120 ps. The signals
from the SiPM and reference detectors were sent for timing
and energy measurements to NIM and CAMAC modules, as
shown in the figure 1 below. For each device, the timing
resolution was measured over a range of operating voltages
and at each operating voltage the threshold on the leading
edge discriminator (LED) for the SSPM was varied until the
best timing resolution was achieved at that particular operating
voltage. All the timing resolution measurements reported in
this paper have been corrected for the walk effect on the
leading edge discriminator and re-calculated to be between 2
LYSO-SSPM detectors.
Fig. 1. Timing resolution measurement setup between LYSO/SiPM and a
LaBr3/PMT reference detector. An ADC was used to digitize the energy
outputs from the CAEN shaping amplifier and the timing output from the
time-to-amplitude converter.
III. PRELIMINARY TIMING MEASUREMENTS
After conducting preliminary timing measurements for the 25,
50 and 100 um devices over several operating voltages, we
found that the optimal timing resolution achieved with the 25
um device was no lower than 300 ps. This can be attributed to
the significantly smaller fill factor, and thus lower PDE, of the
25 um device (31%) as compared to that of the 50 um device
(62%) and 100 um device (79%). More interestingly, the best
timing achieved with the 50 and 100 um devices were similar
(~250 ps), and corroborates previously observed results [6].
IV. DEPENDENCE OF TIMING RESOLUTION ON MICROCELL SIZE
AND OVERVOLTAGE
To better understand why the timing resolution of the 100 um
device did not outperform the 50 um device despite its larger
fill factor, we compared the performance of the devices on the
basis of their overvoltage, i.e. the operating voltage above the
respective breakdown voltages of each device. The breakdown
voltage for each device was measured at 250
C by illuminating
the device with a weak laser to trigger a few of the microcells.
By recording the amplitude of the first few photoelectron
pulses as a function of bias voltage, it was possible to
backtrack what the breakdown voltage for each device was.
As the figure below shows, the breakdown voltage for the 50
um device was 67.6V. The figure inset is a screen capture
from an oscilloscope operating in persistence mode that shows
the discrete photoelectron (SPE) pulses being detected by the
SiPM. The breakdown voltage for the 100 um device was
similarly measured to be 69.8V.
Fig. 2. Determining the breakdown voltage of the 50 um SiPM by tracking the
amplitude of discrete photoelectron pulses as a function of the device bias
voltage.
As shown in figure 3 below, the timing resolution of each
device was measured over a range of overvoltages and
allowed a number of key observations to be made. First,
assuming that 300 ps is an acceptable timing resolution, the 50
um device has a wider useful operating range of ~1V, while
the operating range of the 100 um device is limited to ~0.5V.
Second, as previously reported, the best timing resolution
achieved with the 100 um is no better than the 50 um device.
Finally, the timing resolution of each device first increases
with increasing overvoltage due to the rising gain and PDE,
but there is a threshold overvoltage for each device beyond
which the timing resolution degrades rapidly.

3. Fig. 3. Timing resolution of the 50 um and 100 um devices as a function of
device overvoltage.
If the timing resolution depended solely on the PDE and
gain of the SiPM, then it would be expected to continue
improving with larger overvoltages, as indicated by the dashed
lines in figure 3. However, the measured degradation in timing
at the higher overvoltages suggested that the timing
performance of both devices was being affected by the noise
on the SiPM baseline due to the larger dark counts and after-
pulses that are typically observed at higher device bias
voltages.
Indeed, there was a strong correlation between the
increasing dark current due to the larger dark counts measured
from each device and the degradation in the timing resolution,
as seen in the figure below. Beyond a threshold dark current of
~30 uA, the timing resolution of both devices began
degrading.
Fig. 4. Timing degradation of each device is well correlated with increasing
dark current as a function of increasing overvoltage.
V. EFFECT OF DARK COUNTS ON TIMING RESOLUTION
To quantify the effect of increasing dark current on timing
performance at higher overvoltages, the timing resolution of
each device was measured at 3 different overvoltages while
inducing dark counts from an external LED source. As the
figure below shows, even at the overvoltage that gave us the
best timing resolution for the 50 um device, turning up the
intensity of the LED could lead to a timing degradation of
almost 100 ps (from 250 ps to 350 ps). Similar degradation in
timing was also observed from the 100 um device. These
results clearly demonstrated the detrimental effect of the
baseline noise (due to dark counts) on the SiPM timing
resolution.
Fig. 5. At a fixed overvoltage, the SiPM timing resolution can be significantly
degraded by externally induced dark counts.
VI. REDUCING THE EFFECT OF DARK COUNTS ON TIMING
RESOLUTION
As we described earlier, at higher overvoltages, SiPM pulses
due to dark counts and after-pulsing start piling up, causing
the baseline of the SiPM signal to fluctuate. These fluctuations
are superimposed on the LYSO signal of interest, thus
degrading the timing resolution. By shortening the tail of the
SiPM single photoelectron response using a CR shaping
circuit, it is possible to reduce the effect of fluctuations in the
baseline [8]. The CR shaping circuit acts as a high pass filter
by preserving the fast rising edge of the LYSO pulse, while
reducing the low frequency components of the noise. We
introduced a CR high pass filter (time constant = 10 ns, cutoff
frequency = 16 MHz) in the front-end electronics to reduce the
detrimental effect of dark counts. The “filtered” curves in the
figure below indicate measurements of the timing resolution
from the 50 um and 100 um devices using the modified front-
end electronics.
Fig. 6. Measured improvement in SiPM timing resolution at higher
overvoltages from each device after the introduction of the 16 MHz high pass
filter.
As seen from the figure above, with the introduction of the
high pass filter, it was possible to operate both the 50 and 100
um devices at higher overvoltages without adversely affecting

4. the timing resolution. More importantly, the best timing
resolution of the 100 um device was almost 25 ps better than
the 50 um device.
VII. TIMING RESOLUTION DEPENDENCE ON PDE
To verify that the improved timing performance of the 100
um device was due to its larger larger fill factor, we measured
the PDE of each device relative to the other using the method
that was previously described by Otte et al. [8]. Each device
was illuminated by 10,000 low-intensity laser light pulses and
the number of events in which no photoelectrons are detected
was measured. Then, by assuming a Poisson distribution for
the number of detected photoelectrons per light pulse, the
relative PDE, which represents the mean number of
photoelectrons detected per pulse could be estimated using the
formula:
, (3)
where N(0) is the number of events where no photoelectrons
are detected.
The figure below shows the timing resolution and the
relative PDE for the 50 um and 100 um devices.
Fig. 7. With the use of the high pass filter in the front-end electronics, the 100
um device is able to achieve a better timing resolution than the 50 um device,
which reflects its larger PDE due to its larger fill factor.
As seen from the figure above, the best timing resolution
measured with the 50 um device was achieved at the
overvoltage where the PDE of the device had plateaued. The
best timing resolution measured with the 100 um device was
at an overvoltage where the PDE was beginning to plateau but
still increasing. Most importantly, the PDE of the 100 um
device was higher than that of the 50 um device at all the
overvoltages, particularly where it achieved its best timing
resolution.
VIII. OPTIMAL MICROCELL SIZE FOR TOF-PET
The results presented demonstrate that with the use of
proper front-end electronics to reduce the detrimental effect of
dark counts, it is indeed possible to achieve better timing
resolution with 100 um SiPMs than 50 um SiPMs, thus
reflecting their larger fill factor and PDE.
However, with the best timing resolution of the 50 um
device being only about 25 ps worse than that of the 100 um
device, it can be operated over a wider range of overvoltages.
This makes the 50 um device far less sensitive to uncontrolled
changes of the device’s breakdown voltage due to thermal
drifts. Moreover, a larger number of microcells per square mm
allows better dynamic range with the device. Finally, the
device delivers very good timing resolution (~250 ps) even
without the use of the high pass filter, thus allowing an
additional degree of freedom for the design of the ToF-PET
system. Thus, the 50 um microcell size is still the preferred
choice for timing applications.
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