Download Prospects of Using Silicon Photomultipliers for the

Prospects of Using Silicon Photomultipliers for the
Astroparticle Physics Experiments EUSO and
MAGIC
A. Nepomuk Otte, Boris Dolgoshein, Jürgen Hose, Sergei Klemin, Eckart Lorenz, Gerhard Lutz,
Razmick Mirzoyan, Elena Popova, Rainer H. Richter, Lothar W. J. Strüder and Masahiro Teshima
Abstract— We discuss the silicon photomultiplier (SiPM) as
a novel photon detector for the Major Atmospheric Gamma
Imaging Cerenkov Telescope (MAGIC) and the Extreme Universe
Space Observatory (EUSO). For these astroparticle experiments
we pursue two different developments of the SiPM. One development is pursued at MEPhI, where prototypes are available. In
the course of this paper we will describe some characteristics of
these devices. The second development, which is using the back
illumination principle is at present in its design phase.
Index Terms— high energy astroparticle experiments, silicon
photomultiplier (SiPM), avalanche photo diode (APD), air shower
experiments.
I. I NTRODUCTION
M
ANY existing and planned experiments in high energy
astroparticle physics rely on photon detectors with fast
time response, high photon detection efficiency and single
photon counting capabilities. In this context we investigate a
novel photon detector concept — the silicon photomultiplier
— for the experiments MAGIC and EUSO.
MAGIC is the worlds largest air Cherenkov telescope for
ground based γ-astronomy. It is located on the island La Palma
on top of the Roque de Los Muchachos at an altitude of
2200 m. The experiment detects very high energy gammas
(VHE-γ) from a few tens of GeV up to several TeV.
After entering the atmosphere, a high energy γ-ray initiates an electromagnetic shower in which many thousands of
electrons are produced. A certain fraction of them propagates
through the atmosphere faster than light and therefore emits
Cherenkov light. The sum of all Cherenkov photons result in a
light flash lasting a few nanoseconds which is detected by the
MAGIC telescope. The flux of Cherenkov photons arriving on
the ground is about 100 photons (300 nm-550 nm) per square
meter for a 1 TeV-γ and scales in first order linear with the
γ-energy. In order to detect such a small flux, large collection
areas in combination with efficient and fast photon detectors
are needed. Moreover, the photon detector has to operate in
A. Nepomuk Otte (e-mail: [email protected]), Jürgen Hose, Eckart
Lorenz, Gerhard Lutz, Razmick Mirzoyan, Rainer Richter and Masahiro
Teshima are with the Max-Planck-Institut für Physik (Werner-HeisenbergInstitut), München 80805, Germany
Boris Dolgoshein and Elena Popova are with the Moscow Engineering and
Physics Institute (MEPhI), Moscow 115409, Russia
Sergei Klemin is with ”Pulsar” Enterprise, Moscow, Russia
Lothar W. J. Strüder is with the Max-Planck-Institut für Extraterrestrische
Physik, Garching 85748, Germany
the presence of a large light background from the night sky
(stars, airglow, etc.).
The physics objective of the MAGIC experiment is the study
of the most violent processes known so far in our universe.
Objects of interest are potential VHE-γ sources like e.g. active
galactic nuclei, supernova remnants, gamma ray bursts and
pulsars; fundamental physics questions as extragalactic background, quantum gravity and dark matter are also investigated.
A detailed description of the MAGIC physics program can be
found in [1].
Orbiting the Earth at about 500 km, EUSO looks down
to the atmosphere and searches for air showers initiated by
extreme energetic particles (> 1019 eV). The flux of these
cosmic rays ( 1 particle per year per km2 ) asks for huge
detection volumes like the atmosphere which can easily be
viewed from space.
In an extended air shower, initiated by a cosmic ray,
nitrogen molecules are excited. When falling back into the
ground state, the molecules may emit fluorescence photons
of distinct energies. The most prominent emission lines are
in the wavelength region between 330 nm and 400 nm. This
fluorescence light is the signature of a cosmic ray which
EUSO is looking for. By recording the arrival time and
position of each fluorescence photon in the EUSO-detector,
the impact direction and energy of the primary cosmic ray
can be reconstructed. For an extended air shower initiated by
a cosmic ray with an energy of about 1020 eV, EUSO will
detect several hundred photons within a time window of about
100 µs.
Scientific objectives of the EUSO-mission are amongst
others to study the high energy ( 1019 eV) part of the cosmic
ray spectrum and trace back the origin of these high energy
cosmic rays. For a more detailed discussion of the EUSOexperiment, the interested reader is pointed to [2] and [3].
Both experiments now use (MAGIC) or plan to use (EUSO)
conventional photomultiplier tubes (PMTs) as photon detectors. From the physics point of view this is a far from optimal
choice, as the photon detection efficiency (PDE) of availabe
PMTs is limited to about 20%. A significant increase in signal
to noise ratio (SNR) could be obtained, if a detector with a
much larger PDE were be available comprising the high gain,
speed and single photon counting capabilities of PMTs.
For the experiments this means a lower energy threshold
and improved energy resolution at higher energies. For EUSO
the threshold must be as low as possible so results can be
sensibly compared to those produced by experiments like
AGASA, HiRes and Auger. The motivation for MAGIC is
to close the so far unobserved energy region between 30 GeV
down to some GeV. Summarizing one can conclude that better
photon detectors will produce better physics results in both
experiments.
For several years mainly Russian groups have developed
the SiPM concept, e.g. [4]–[6]. In the following we will
introduce the idea behind the so called silicon photomultiplier
(SiPM). We then focus on the development realized at the
Moscow Engineering and Physics Institute (MEPhI) and Pulsar
Enterprise before we report about a new development in
the semiconductor laboratory (HLL) attached to the MaxPlanck-Institut für Physik and Max-Planck-Institut für Extraterrestrische Physik. In that new development we want to
combine for the first time the back illumination principle with
the SiPM concept.
II. T HE S ILICON P HOTOMULTIPLIER C ONCEPT
A SiPM is composed of an array of small avalanche
photodiodes combined to form a macroscopic unit (typically
500 to 4000 diodes per mm2 ). In the following we call such
an avalanche photodiode a cell and the macroscopic unit
SiPM. Each APD cell is independent from the rest of the
unit and operates in limited Geiger mode, i.e. a few volts
above breakdown voltage (see Fig. 1). In this mode of operation a single photoelectron can initiate a diverging avalanche
confined to the APD cell (the photoelectron ”triggers” the
cell). The photoelectron can easily be detected, because in the
breakdown a large and fast current pulse is generated. After
the cell has been triggered, the breakdown is quenched by
limiting the current through the cell. In the SiPM this is done
by polysilicon resistors attached to the cells (s. Fig. 1).
Si* Resistor
+
n
Vbias
p+
Al-conductor
SiO2
Guardring n
-
Substrate p+
Fig. 1. Cross section through the topology of one SiPM APD cell. Photons
are incident from the top (from [7])
The fast and large output signal of single Geiger APDs is
very attractive for highly efficient photoelectron detection purposes and has been studied by several authors [8]–[11]. On the
other hand, one is by no means able to attain information about
the number of photoelectrons that initiated the breakdown, as
the Geiger-APD output signal charge is fixed. This problem
is circumvented in the SiPM, as the sensor is divided into
many small cells which are connected to the same readout
line (marked as Al-conductor in the sketch). The main area of
applications is in average light fluxes 1 photon/cell/recovery
time. In this way the number of fired cells is in first order
proportional to the photon flux, thus compensating for the
missing dynamic range of a single Geiger mode APD. In
reality the process is more complex because of the recovery
time of the cells and the influence of dark rate.
The main advantages of SiPMs are:
• standardized output pulses for single photoelectrons
5
6
• large intrinsic gain 10 –10
• no need for sophisticated preamplifiers
• fast rise–time pulses (about 1 ns)
• very low operating voltages 20–100 V
• very low time jitter for single photoelectrons (about
100 ps)
• no sensitivity to magnetic fields
• very low sensitivity to pickup, EMI
• no aging
• no damage when exposed to large photon fluxes
• compactness
2
• low power consumption (about 10 µW/mm )
• expected low costs because of production process
• high radiation hardness
A trade-off of available SiPM prototypes is a relative high
dark rate (0.2–2) MHz/mm2 at room temperature. The dark
rate originates from thermally generated electrons in the active
volume and tunnelling assisted charge carrier generation in the
avalanche region. The former contribution can in principle be
reduced by active cooling and the later one by a careful design
of the avalanche region.
Besides the dark rate another intrinsic feature of the SiPM
has to be taken into account. Photons generated in the
avalanche propagate within the SiPM and eventually are
absorbed in the active volume of a neighboring cell. This
gives rise to ”optical” crosstalk. The efficiency of the photon
generation is about 3 · 10−5 per avalanche charge carrier [12].
Therefore, a straightforward measure to reduce this kind of
crosstalk is to lower the gain as much as tolerable in the
final application. Another approach is to absorb the ”crosstalk”
photons between neighboring pixels. Both methods can have
an impact on the PDE. Because the probability of a Geiger
breakdown is dependent on the overvoltage above breakdown
voltage and therefore on the gain of a given device and grooves
between pixels for the absorbtion of crosstalk photons might
introduce additional dead area.
III. S I PM P ROTOTYPES BY MEP H I AND P ULSAR
We have studied prototype SiPMs produced by MEPhI and
Pulsar enterprise to test the general use of this photon detector
in experiments like MAGIC or EUSO. In this prototype, 576
APD cells are integrated on a total sensor area of (1 × 1) mm2 .
A single cell has an active area of (20 × 20) µm2 . This is about
four times smaller than the total cell size.
A summary of the most important characteristics of the
tested SiPM is given in Tab. I. In the following we will discuss
some of the features in more detail.
TABLE I
S PECIFICATIONS OF THE S I PM PROTOTYPE BY MEP H I AND P ULSAR
ENTERPRISE UNDER INVESTIGATION .
parameter
value
Sensor area
Nr. of individual Geiger cells
active area of the sensor
peak PDE (at 540 nm)
bias voltage
breakdown voltage
signal charge (gain)
APD cell recovery time
typ. noise rate at room temperature
(1 × 1) mm2
576
25%
20% averaged over SiPM [5]
52V-60V
50V
104 − 5 · 106
1 µs
106 counts/mm2 /s
Fig. 2. Gain dependence on the bias voltage for the SiPM operating at room
temperature
In Fig. 2 the gain dependence of the SiPM as a function of
the the applied bias voltage is plotted. It would be more correct
to speak of signal charge rather than gain as the output signal
of a single cell is independent of the generated photoelectrons.
But as long as the probability is small that more than one
photoelectron is generated in a cell — which is the case
for applications of SiPMs — signal charge and gain has the
same meaning. From the graph one can deduce the linear
dependence of the gain on the applied voltage. This is due
to the intrinsic structure of a single cell. In the process of a
Geiger breakdown, the intrinsic cell capacitance and parasitic
capacitance of the quenching resistor is discharged. Due to the
linear relationship between charge ∆Q and change of potential
on a capacitance C, the charge that is externally flowing is
therefore linearly dependent on the applied voltage UBias
∆Q = C · (UBias − Ucell@breakdown ) ∝ UBias .
(1)
From the slope of the gain vs. bias one can deduce the total
capacitance that is contributing to the output signal. Considering the above given equation we estimate a capacitance of
about 40 fF per cell.
After a cell has fired, it needs a certain time to recover to its
original state. The recovery time constant is given by the value
of the quenching resistor and a capacitance. The capacitance in
one part comes from the pn–junction and in another part from
the parasitic capacitance parallel to the quenching resistor.
For the measurement of the recovery time we have used a
fast pulsed diode laser. The laser intensity was set to a level
somewhat below at which a further increase did not show an
increase in signal amplitude of the SiPM. We then reduced the
time between subsequent laser pulses stepwise and recorded
the resulting signal amplitude. From the measurement we
derive a recovery time of 1 µs, i.e. the change in time needed
between subsequent laser pulses that results in a change of
63% in a signal amplitude. The recovery time has an impact
on the PDE of the SiPM as a certain amount of cells will
always be ”dead” due to dark noise or background light. Thus
the active area of the SiPM is reduced.
The largest limitation in PDE of the tested device is due to
the currently small fraction of the cell area which is sensitive
to photons (25%). This explains the peak 20% PDE of this
device at a wavelength of 540 nm [5], i.e. the active area alone
has a PDE of ∼ 80%.
Below 450 nm the efficiency of the SiPM drops rapidly. This
can be explained by the intrinsic structure of the cells. As can
be seen from Fig. 1, the avalanche structure is implanted on a
p-doped substrate (hereafter named n-on-p-structure). The pnjunction is reverse biased. Therefore only electrons that are
generated in the depleted p-doped bulk (white in the drawing)
below the structure drift into the avalanche region. UV-photons
are absorbed within a short distance (< 100 nm) compared to
the extension of the high field structure. Because of this the
electrons generated by UV-photon absorbtion will not drift
into the high field region. The hole that is also generated
and which is drifting into the avalanche region has a much
lower probability to initiate the avalanche breakdown. A high
recombination rate close to the surface reduces further the UV
sensitivity
It is planned to invert the structure from n-on-p to pon-n in the next prototypes in order to enhance the UVsensitivity . Then the photoelectrons generated after a UVphoton absorbtion close to the surface drift into the high field
region as the electric field has changed its sign.
Only inverting the structure will not improve the PDE much
beyond 20% because of the dead area between cells. That
is why we also plan to increase the overall cell sizes to
(100 × 100) µm2 . In doing so we want to increase the active
area to 60%. If this development is successful we think about
applying optical micro structures like micro lenses or micro
Winston cones to the SiPM. This will further increase the
effective active area of this type of SiPM.
The ”optical” crosstalk in this SiPM is 40%, if the sensor
operates at a gain of 2 · 106 . This 40% is the fraction in
which more than 1 cell fires at the same time while recording
dark noise. After lowering the gain to 5 · 105 , the crosstalk is
reduced to 4%, a value which is acceptable for both EUSO
and MAGIC.
In future developments it is planned to introduce grooves
between the cells of the SiPM. Photons that are generated in
an avalanche and propagate into a neighboring cell will then
be partially inhibited from doing so as they are either absorbed
or scattered away at the grooves.
Power consumption of the SiPM is almost negligible and
dominated by dark rate. Operating the SiPM at room temper-
ature at a bias voltage of 55 V, corresponding to a gain of
106 , the dark rate is about 1 MHz. This results in an average
power dissipation P per mm2 of
P = 55 V · q · 106 · 1 MHz ≈ 10 µW/mm2 ,
photoelectron driftpath
drift rings p+
depleted bulk
50µm…450µm
deep n
where q denotes the elementary charge. The power consumption due to the photon detectors in EUSO would then be about
50 W if equipped with this kind of SiPM. On the other hand,
the SiPMs will need moderate cooling if used in EUSO, thus
increasing the overall power consumption.
avalanche region
quenching resistor
output line
100µm
Fig. 4. Blowup of one cell of the back illuminated SiPM principle. Please
note that the drawing is not to scale.
photon
photoelectron path
photon
shallow p+
avalanche region
depleted bulk
50µm...450µm
Si
output
well defined drift field between the two surfaces the backside
is biased to -500 V. The avalanche region consists of a deep pimplantation which is enhanced in the middle by an additional
low doped p-implantation. This ensures a homogenous electric
field smoothly falling off to the sides (s. Fig. 5).
Fig. 3. Principle of the back illuminated SiPM. The box marks one cell of
which the blowup is shown in Fig. 4
IV. S I PM WITH BACK ILLUMINATION
The design of the SiPM as introduced in the previous
section is conceptionally limited in PDE due to the dead space
between adjacent cells and shadowing from aluminium contact
lines. In the MPI Semiconductor Laboratory in Munich we
pursue a different approach in which we want to combine the
SiPM principle with the back illumination principle that is
already used with great success, e.g. with CCDs.
The basic idea of the back illuminated SiPM is sketched
in Fig. 3. Photons are incident on the detector side opposite
to the avalanche regions (hereafter named backside) and drift
into the avalanche region. As in the front side illuminated
SiPM the sensor itself is divided into quasi independent cells.
In Fig. 4 we sketch the cross section of such a cell. The cell
can be divided into a completely depleted drift volume and
the avalanche region, which is much smaller than the rest
of the cell. On both sides of the drift volume, contacts at
different potentials shape the electric field in such a way that
photoelectrons drift into the avalanche region. Upon entering
the avalanche region, a photoelectron will initiate a Geiger
breakdown as in the front illuminated SiPM.
Device simulations with the TOSCA program were performed to show the functionality of the back illuminated
SiPM. We investigated a cylinder symmetric shaped cell with
a diameter of 100 µm and a thickness of 50 µm and 450 µm
respectively as shown in Fig. 4. First test structures and
prototypes will be fabricated using 450 µm thick silicon. If
necessary, further developments will be done using 50 µm
thick silicon detector material. The drift field is shaped by
three rings on the front side, including the deep p-implantation
which is also part of the avalanche region. The applied voltages
to these rings starting with the innermost is -50 V, -55 V and
-60 V, respectively. For the full depletion of the bulk and a
*10 3 V/cm
720.00
560.00
400.00
240.00
0.00
0.16
Distance from
the frontside
80.00
0.32
0.02 *10
0.48
*10 -3 cm
-2 cm
0.06
0.10
0.14
0.18
Distance from center
of the Avalanche cell
Fig. 5. Absolute electric field distribution of the Avalanche region. The
values on the lower axes give the coordinates in cm, and the corresponding
electric field is given on the vertical axis in V/cm.
We simulated the position dependent charge collection efficiencies of this device by subdividing the avalanche region into
six boxes of equal volumes and generated charges at different
positions in the drift volume.
For these simulations the avalanche mechanism was
switched off. From the results we can deduce that the charge
is nicely collected in the middle of the avalanche region.
From the difference in drifttimes we estimate a time jitter
below 1 ns.
In Fig. 6 we simulated an avalanche breakdown in one cell.
The quenching resistor was 1 MΩ with a parallel parasitic
capacitance of 10 fF. The current pulse width is 100 ps with
an amplitude of 4 mA.
current [mA]
0.5
ACKNOWLEDGMENT
0.0
We would like to thank P. Liebig and S. Rodriguez for
carefully reading this manuscript.
R EFERENCES
-0.5
-1.0
-1.5
-2.0
0.0
0.4
0.6
0.8
time [ns]
1.0
1.2
1.4
Fig. 6. Current pulse of a simulated avalanche breakdown. The value of the
quenching resistor was 1 MΩ with a parallel parasitic capacitance of 10 fF.
V. S UMMARY AND C ONCLUSION
We have investigated the SiPM as a photon detector for the
experiments MAGIC and EUSO. The SiPM shows features
very similar to those of conventional photomultiplier tubes
(PMTs). Moreover, it has many advantages compared to
PMTs, e.g. it can be exposed to a strong light source without
degrading the SiPMs properties, insensitivity to magnetic
fields or pickup, etc. .
The tested prototypes from MEPhI can be used as a photon
detector in MAGIC and EUSO if further developments will
succeed in enhancing the PDE beyond 50% in the blue wavelength region and in increasing the sensor size to (4 × 4) mm2
for EUSO and at least (10 × 10) mm2 for MAGIC. The focal
plane of both experiments is in the order of a few square meter.
A coverage of these surfaces with existing one square millimeter SiPMs is therefore not feasible. The PDE enhancement is
pursued by increasing the cell size and changing the structure
to p-on-n in future SiPMs. “Optical” crosstalk is in principle
no problem if the SiPM operates at a gain of 105 . Additional
efforts in decreasing the “optical” crosstalk will be made by
introducing grooves between cells to absorb crosstalk photons.
The SiPMs under investigation have a fairly high dark noise
rate. As both experiments operate in the noisy environment of
the night sky, only moderate cooling down to -50◦ C is needed
to bring the dark noise down to an acceptable level.
The front side illuminated SiPM is limited in PDE, as some
areas will always be shadowed or otherwise insensitive to
photons. For this reason we develop a new kind of SiPM at the
MPI semiconductor laboratory that uses the back illumination
principle. We have performed simulations to find a structure
in which photons enter the detector from the backside and the
photoelectrons drift into an avalanche region which is much
smaller than the rest of the cell. The simulation results are very
promising and meet the timing requirements needed in both
experiments. We will now do test implantations to verify our
technology simulation and in parallel develop test structures
to pinpoint important parameters of the device like crosstalk,
dark rate and breakdown behavior.
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