Download Planar silicon SPADs with 200-μm diameter and 35-ps

Planar Silicon SPADs with 200 µm diameter and 35 ps
photon timing resolution
a
Massimo Ghioni * a, b, Angelo Gulinatti a, Piera Maccagnani c , Ivan Rech a and Sergio Cova a, b
Politecnico di Milano, Dipartimento di Elettronica e Informazione, Piazza Leonardo da Vinci 32 20133 Milano, Italy
b
MPD Micro-Photon-Devices, via Stradivari 4 – 39100 Bolzano, Italy
c
IMM-CNR sezione di Bologna, Via Piero Gobetti, 101 – 40129 Bologna, Italy
ABSTRACT
Time-correlated single photon counting (TCSPC) is exploited in emerging scientific applications in life sciences, such as
single molecule spectroscopy, DNA sequencing, fluorescent lifetime imaging. Detectors with wide active area (diameter
> 100 µm) are desirable for attaining good photon collection efficiency without requiring complex and time-consuming
optical alignment and focusing procedures. Fiber pigtailing of the detector, often employed for having a more flexible
optical system, is also obtained more simply and with greater coupling efficiency for wide-area detectors. TCSPC,
however, demands to detectors also high photon-timing resolution besides low noise and high quantum efficiency.
Particularly tight requirements are set for single-molecule fluorescence analysis, where components with lifetimes of tens
of picoseconds are often met. Small photon timing jitter and wide area are considered conflicting requirements for the
detector.
We developed an improved planar silicon technology for overcoming the problem and providing a solid-state alternative
to MCP-PMTs in demanding TCSPC applications. We fabricated Single Photon Avalanche Diodes (SPADs) with 200
µm active area diameter and fairly low dark counting rate (DCR). At moderately low temperature (-25 °C with Peltier
cooler) the typical DCR is 1500 c/s and it is not difficult to select devices with less than 1000 c/s. The photon detection
efficiency peaks at 48% around 530 nm and stays above 30% over all the visible range. A photon timing resolution of 35
ps FWHM (full width at half maximum) is obtained by using our patented pulse pick-up for processing the avalanche
current.
Keywords:
single photon avalanche diodes, SPAD, time-correlated single photon counting, TCSPC, photon timing
1. INTRODUCTION
Time-correlated single-photon counting (TCSPC) techniques were developed relying on photomultiplier tubes (PMTs),
that is, vacuum tube detectors with high internal gain. High performance PMTs have been produced industrially with
sophisticated technologies since the 1960s; amongst their advantages, the most valuable one is the wide sensitive area (∼
cm2), which in some cases greatly simplifies the design of the optical system. PMTs also attain remarkable performance
at high counting rate and offer picosecond timing resolution with Micro Channel Plate (MCP) models. However, they
suffer from low quantum efficiencies (QE) in the visible range: the QE of conventional bialkali and multialkali
photocathodes reaches 20-25% between 400 and 500 nm [1].
In more recent years, Single-Photon Avalanche Diodes [2, 3] (SPADs) have emerged as a solid state alternative to PMTs.
Besides the well known advantages of solid state versus vacuum tube devices (small size, ruggedness, low power
dissipation, low supply voltage, high reliability, etc.), SPADs provide inherently higher quantum efficiency, particularly
in the red and near infrared spectral regions.
The smaller active area of SPAD devices is often considered a disadvantage compared to PMTs. However, this is not
true in a variety of TCSPC techniques relying on confocal or near field microscopy such as Fluorescence Lifetime
Imaging (FLIM), combined FRET-FLIM and time-resolved FCS [4]. In these applications, the spot size at the
*
[email protected], phone +39-02-23996093, fax: +39-02-2367604
Advanced Photon Counting Techniques, edited by Wolfgang Becker, Proc. of SPIE
Vol. 6372, 63720R, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.685834
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microscope image plane can easily be made underfill the SPAD active area, provided that the diameter is sufficiently
large (≥ 100 µm). In confocal microscopy, the SPAD device can also be used as confocal pinhole, thus simplifying the
whole optical system and increasing the miniaturization potentiality. Furthermore, fiber pigtailing of the detector, often
employed for having a more flexible optical system, is also obtained more simply and with greater coupling efficiency
for SPAD detectors with large area.
Unfortunately, large active area, low dark counting rate, high quantum detection efficiency and small photon timing jitter
are conflicting requirements for a SPAD device.
Single-photon counting modules based on SPAD diodes with thick depleted region (20-25µm) and large active area
(180µm diameter) are commercially available from PerkinElmer Optoelectronics [5]: they have very good photon
detection efficiency (PDE) and low dark counting rate (DCR), but they have a moderate photon timing resolution of
typically 350ps full-width at half maximum (FWHM). Such a performance is unsuitable for tissue autofluorescence
measurements [6] or protein conformational dynamic analysis [7], where fluorescence components as short as 100 ps or
less are often met.
SPAD devices with thin depleted region (1-2µm), fabricated in a standard CMOS silicon technology can attain time
resolution down to less than 50ps FWHM at room temperature [8, 9]. This performance is comparable with that of the
best MCP-PMTs. Standard CMOS technology makes it possible to monolithically integrate the SPAD device and its
associate quenching circuit, taking great advantage of reduced size and low parasitic capacitances. Unfortunately, CMOS
technologies available in industrial silicon foundries are not optimized for SPAD device fabrication, resulting in reduced
photon detection efficiency and relatively high dark counting rate. So far, no CMOS-based SPAD devices with active
area diameter > 50 µm have been reported.
To overcome these limitations and provide a solid-state alternative to MCP-PMTs in demanding TCSPC applications we
have developed an improved planar silicon process. We fabricated SPAD devices with 200 µm active area diameter and
fairly low dark counting rate. At moderately low temperature (-25 °C with Peltier cooler) the typical DCR is 1500 c/s and
it is not difficult to select devices with less than 1000 c/s. The photon detection efficiency peaks at 48% around 530 nm
and stays above 30% over all the visible range. A photon timing resolution of 35 ps FWHM is obtained by using our
patented pulse pick-up for processing the avalanche current.
2. DEVICE STRUCTURE
Figure 1 shows a schematic cross section of the SPAD structure. A double-epitaxy structure is used in order to
reduce the diffusion effects that adversely affect the time response of the detector [10]. The active n+p junction is built in
the upper low-doped p-epilayer. The buried p+ epilayer provides a low-resistance path to the side ohmic contact. A
boron implantation in the central part of the n+p junction defines the high electric field region, that is, the active area of
the detector. Deep, highly doped n+ regions connected to the bulk n-silicon guarantee electrical isolation between
adjacent SPADs. Devices with active area diameter of 50, 100 and 200 µm were fabricated on 4” wafers at IMM-CNR
by using the process described in ref. [10]. The efforts for improving the device technology were mainly directed to:
- exploit specific gettering processes for removing metal impurities from the detector active volume, therefore
reducing DCR and afterpulsing effects;
- reduce the electric field strength within the p-n junction depletion region in order to attenuate the tunnel-assisted
generation of carriers, thus making it possible to reduce more effectively the DCR by cooling the detector.
Thanks to this approach we were able to avoid the steep increase of the dark counting rate with the detector active area,
observed in the previous generations of SPAD devices [11].
hv
Anode
sinker
>/
5O-200pnr
a
n++ shallow
n-I--I-
isolation
///
7
Cathode
-
p- epilayer
5n
p -- honed layer
n -- soksnrane
Fig. 1 Cross section of the planar SPAD structure.
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3. DEVICE CHARACTERIZATION
A large number of devices fabricated with the improved technology were fully characterized in order to achieve
statistically significant data.
3.1. Breakdown voltage
We first evaluated the uniformity of the breakdown voltage, VBD, of SPAD devices, since this is usually a good
marker of process quality. Figure 2 shows the distribution of the breakdown voltages measured at room temperature over
half of a wafer (about 500 devices with active area diameter of 200µm). The average VBD is 36.3 V, with a remarkably
low standard deviation of 0.27 V. Similar results have been obtained for the 50 and 100µm devices, thus confirming the
good uniformity of the fabrication process.
100
90
80
Device count
70
60
50
40
30
20
10
0
35,6
35,8
36,0
36,2
36,4
36,6
36,9
Breakdown Voltage (V)
Fig. 2 Distribution of breakdown voltages of SPAD devices having an active area diameter of 200 µm, measured at room
temperature over half of a wafer.
3.2 Photon Detection Efficiency
Figure 3 shows the photon detection efficiency (PDE) of a 200µm SPAD device as a function of wavelength, measured
at different excess bias voltages, Vex. At Vex = 5V the PDE has a peak of 48% at 530nm and it is about 15% at 820nm
wavelength. These figures are consistent with the total thickness of the epitaxial layer (about 5µm). Similar results have
been obtained with 50 and 100µm SPAD devices. The PDE is quite uniform all over the wafer: small fluctuations were
observed (std deviation ∼ 5% of the average value), mainly due to the thickness fluctuations of the top oxide layer.
Photon Detection Efficiency %
70
Excess bias voltage
60
10 V
7V
5V
50
40
30
20
10
0
400
500
600
700
Wavelength (nm)
800
900
1000
Fig. 3 Photon detection efficiency of a 200µm SPAD device as a function of wavelength. Measurements were performed at different
excess bias voltages
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Optimal excess bias voltage depends on the actual experimental conditions. In photon counting, the highest Vexc should
be used if the measurement is background or photon noise limited. Conversely, if the detector noise is dominant, a
compromise between photon detection efficiency and dark counting rate must be found to achieve the best S/N ratio. A
Vex ranging from 4 to 5V was verified to be the best compromise for the SPAD devices reported here.
3.3 Dark counting rate
Thermal generation effects produce current pulses even in absence of illumination, and the poissonian fluctuation of
these dark counts represents the internal noise source of SPAD devices. Dark counts include primary and secondary
pulses. Primary dark pulses are due to carriers thermally generated within the SPAD junction. Afterpulsing effects
produce secondary dark pulses, and may strongly enhance the total dark counting rate [2].
Dark counting rate (DCR) was measured by operating the SPAD devices at 5V excess bias voltage. A relatively long
hold-off time of a few hundred nanoseconds was enforced by using an active quenching circuit (AQC), in order to reduce
afterpulsing effects to a negligible level [2].
Figure 4 shows the DCR as a function of temperature for SPAD devices having different active area diameters. DCR
decreases almost exponentially with temperature: at a relatively low temperature of –25°C (easily obtainable with a
thermoelectric cooler) the typical DCR is 5, 50 and 1500c/s for the 50, 100 and 200 µm SPADs.
100000
Counts (c/s)
10000
1000
200 µm
100
100 µm
10
50 µm
1
0.1
-50
-40
-30
-20
-10
0
10
20
Temperature (°C)
Fig. 4 Dark counting rate as a function of temperature for SPAD devices having different active area diameters.
The advantage of an engineered electric field profile is clearly visible in Figure 5, where the DCR of two 50 µm SPAD
devices made by using different processes are plotted. The DCR of the SPAD device with “standard” electric field [12]
can be accurately fitted by taking into account band-to-band tunneling (dotted line) and Shockley-Read-Hall generation
(dashed line): the weights of these two contributions become equivalent at a temperature of about –5 °C (corner
temperature). Since the dependence of tunneling on temperature is relatively weak, there is no advantage in cooling the
device well below the corner temperature. Conversely, for the SPAD device with “engineered” electric field there is no
evidence of the onset of tunneling down to -50°C: cooling can be therefore exploited for a strong reduction of the DCR.
DCR uniformity was assessed by testing about 2000 SPAD devices all over the wafer at room temperature. The percent
point function (i.e. the inverse of the cumulative distribution function) of the DCR is shown in Figure 6 for SPAD
devices having different active area diameter.
It must be noted that the distribution curves for the 100 and 200 µm devices are similar, whereas the DCR distribution
for the 50 µm shows a relatively flat region (about 35% of the 50 µm SPAD devices show a DCR between 1 and 2 kc/s)
followed by a steep increase. Furthermore, the dark counting rates of the best 50 and 100 µm devices do scale linearly
with the active area, whereas the DCR of the 200 µm devices does not. A hint for explaining these results is provided by
the Arrhenius plot of the DCR (Figure 7). Data shown in Fig. 7 were obtained from SPAD devices belonging to the first
10% of the distributions of Fig. 6. The Arrhenius plots highlight a substantial dependence of the activation energy on the
device active area. We argue that there are at least two types of defects acting as generation centers within the active
volume of SPAD devices. Defects of the first type (type A) have a low density, so that the probability of having no such
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defects in a small area device is significant. According to the slope of the Arrhenius plot for the 200 µm device these
defects have an energy level close to the midgap, being therefore highly efficient generation centers.
10000
1000
Counts (c/s)
SPAD with "standard" electric field
100
10
SPAD with "engineered" electric field
1
0.1
-80
-60
-40
-20
0
20
Temperature (°C)
Fig. 5 Dark counting rate vs. temperature for 50 µm SPAD devices with “standard” and “engineered” electric field profile.
NDL: limit to DCR (c/s)
1000000
200 µm
100000
100 µm
10000
50 µm
1000
100
0
20
40
60
80
100
% of devices with DCR<NDL
Fig 6. Percentage of SPAD devices (horizontal scale) found within a given limit of the individual dark counting rate (vertical
scale) measured at room temperature.
The exact energy level of type A defects cannot be directly extracted from data of Fig. 7, since the activation energy is
strongly influenced by the electric field [13]. A rough estimation shows that the electric field might account for a
reduction of the activation energy of about 0.1 eV. Possible candidates for type A defects are extended defects such as
dislocations or precipitates, which might also be decorated by metal impurities [14]. It can be easily verified that an
average defect density of ∼ 4 108 cm-3 leads to a 40% probability of no defect occurrence in a 50 µm device, whereas the
probability is 3% for a 100 µm device and practically negligible for a 200 µm device. This observation might justify the
relatively flat region of the distribution curve in Fig. 6 for the 50 µm devices. Defects of the second type (type B) have a
greater density, so that they may occur with high probability also in smaller devices. However, according to the slope of
the Arrhenius plot for the 50 µm device these defects have an energy level displaced by about Eg/4 from the midgap,
being therefore less efficient generation centers. Point defects due to transition metal contaminants are strong candidates
for type B defects [15]. In general, good devices have no type A defects and their DCR decreases very quickly by
lowering the temperature (Ea=0.7 eV). Devices with dominant type A defects show a DCR that decreases with a smaller
slope (Ea=0.44 eV), whereas devices with a mix of non-dominant type A and type B defects show a DCR that decreases
with intermediate slopes (for instance, Ea=0.6 eV for the 100 µm device shown in Fig. 7). To support this interpretation,
we selected a subset of 50 µm and 100 µm devices with similar defect populations by choosing devices with the same
Arrhenius plot slope. As expected, at a given temperature the DCR of the selected devices scale exactly with the active
area. A further confirmation is given by data shown in Fig. 6, where it can be easily observed that the DCR of 100 and
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200 µm devices belonging to the 40-80% range of the distributions (where type A defects are presumably dominant)
scales exactly by a factor 4. Studies aimed at elucidating the nature of defects and their impact on SPAD performance are
still at an early stage. Further theoretical and experimental work is under way in our laboratory at Politecnico di Milano
to fully clarify this point.
1
Ea=0.441 eV
Counts/T 2 (s -1 K -2 )
0.1
Ea=0.603 eV
200 µm
0.01
Ea=0.7 eV
0.001
100 µm
0.0001
50 µm
0.00001
3.4
3.6
3.8
4
4.2
4.4
-1
1000/T (K )
2
Fig 7. T corrected Arrhenius plot of the DCR of SPAD devices with different active area diameters. Tested devices belong to
the first 10% of the distributions shown in Fig. 6.
3.4 Afterpulsing
Afterpulsing effects produce secondary dark pulses, and may strongly enhance the total dark counting rate. During the
avalanche some carriers are captured by deep levels in the depletion layer and then released with a statistically
fluctuating delay. The emission probability per unit time has a characteristic value for each type of level involved, whose
reciprocal is the time constant (trap lifetime) of the exponential depopulation transient for that level type. Afterpulsing
effects can be evaluated by using the time-correlated carrier counting (TCCC) technique described in ref. [16]. TCCC
essentially consists in: a) filling the deep levels with a pulsed stimulus; b) measuring the time interval from the filling
pulse to the detection of a released carrier; and c) repeating the procedure for collecting a histogram of the carrier
emissions versus time. Deep levels can be populated by current or light pulses. We adopted an avalanche current pulse,
which has the advantage of filling only the traps involved in the normal device operation. Note that only electrons
released in the p-side and holes released in the n-side cross the high-field region, thus being able to trigger the avalanche.
Moreover, during the avalanche, minority carriers are present only where impact ionizations occur. It follows that TCCC
with avalanche filling stimuli is inherently sensitive only to minority-carrier traps in the high-field region.
TCCC measurements were performed at room temperature. The dead time of the AQC circuit was set at 80ns. Figure 8
shows the probability density in time for the occurrence of an afterpulse after an initial avalanche pulse for a 200µm
SPAD device operated at 5V excess bias voltage. It is worth noting that the probability of having an afterpulse quickly
decays, being negligible after about 1 µs from the initial avalanche pulse. The total afterpulsing probability is 2%, which
is a typical value for these SPAD devices.
3.5 Time resolution
Time resolution is determined by the precision with which the arrival instant of the incident photon on the photodetector
is identified. We recently demonstrated [17] that remarkable timing performance is achievable with large-area single
photon avalanche diodes (SPADs), provided that the avalanche current is sensed at very low level (at a hundred µA
level) when the multiplication process is still confined within a small area around the photon absorption point. In order to
perform a true low-level discrimination of the avalanche current it is mandatory to preserve the shape of the first part of
the leading edge by avoiding any filtering action. To this end, we designed and patented [18] a special current pick-up
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-3
-1
Afterpulsing Probability Density (ns )
1.E-03
10
-4
1.E-04
10
1.E-05
10 -5
-6
1.E-06
10
1.E-07
10 -7
0
500
1000
1500
2000
Time (ns)
Fig. 8 Afterpulsing probability density for a 200 µm SPAD device, measured at room temperature.
circuit that can be added to any of the quenching circuit configurations described in the technical and scientific literature.
Time resolution measurements were performed in a conventional time-correlated single photon counting setup [4] by
using an ultrafast laser diode (Antel MPL-820 laser module) emitting 10ps FWHM optical pulses at 820nm wavelength.
Thanks to the current pick-up circuit we achieved time resolutions in the range between 30 and 35 ps FWHM at room
temperature, independent of the SPAD active area diameter.
Fig. 9 shows the time resolution curve of a 200 µm SPAD detector. The curve shows a prompt peak with a full-width at
half maximum of 35 ps. The laser light was coupled to a single-mode fiber and the fiber tip was deliberately placed
~30cm away from the SPAD active area in order to obtain a rather uniform light distribution over the detector. As
expected, we didn’t notice any variation of the FWHM by focusing the light into a small spot at the center or by scanning
the spot over the active area. It must be noted that the curve shows a clean exponential diffusion tail with a time constant
of 240ps. This tail can be easily deconvolved from experimental data when fluorescent decay events with very short
lifetimes are under investigation.
100000
Counts (c/s)
10000
FWHM = 35 ps
1000
100
10
1
11.5
12.0
12.5
13.0
13.5
14.0
14.5
Time (ns)
Fig. 9 Time resolution curve of a 200 µm SPAD detector. The curve shows a prompt peak with a full-width at half maximum of
35ps and a clean exponential diffusion tail with a time constant of 240ps.
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4. CONCLUSIONS
We reported a planar silicon technology that allows the fabrication of large-area single photon avalanche diodes. We
fabricated Single Photon Avalanche Diodes (SPADs) with 200 µm active area diameter and fairly low DCR. At
moderately low temperature (-25 °C with Peltier cooler) the typical DCR is 1500 c/s and it is not difficult to select
devices with less than 1000 c/s. Measurements performed at different temperatures on a large number of detectors made
it possible to gain a deeper insight into the thermal generation processes responsible for the DCR.
A fairly low total afterpulsing probability of 2% was measured with a dead-time of 80 ns. The photon detection
efficiency peaks at 48% around 530 nm and stays above 30% over all the visible range. By using our patented pulse
pick-up circuit for processing the avalanche current, a photon timing resolution of 35 ps FWHM was obtained at room
temperature, not dependent on the size of the SPAD active area.
These unprecedented characteristics make these SPAD devices a real alternative to MCP-PMTs in demanding TCSPC
applications.
5. ACKNOWLEDGEMENTS
This work was supported by European Commission, Sixth Framework Programme, Information Society Technologies
(NANOSPAD project) and the Italian Ministry of University and Research (MIUR-FIRB project n. RBNE01SLRJ;
MIUR-PRIN project n. 2005027857).
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