Ge-on-Si single-photon avalanche diode detectors: design ...
Single-photon detectors in micro-electronics technology part...
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Single-photon detectorsin micro-electronics technology – part 2
Claudio PiemonteChief Scientist, Fondazione Bruno [email protected]
Claudio Piemonte, FBK 1
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Low-level light detectiontechnologies
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The problem: processing of extremely weak signals
Current signal:
1 pair/photon
Detector noise:
fluctuation of leakage
current
Electronics noise:
shot + thermal noise
Need of a detector with internal amplification
to reduce the impact of electronic noise!
Dominating
noise
Claudio Piemonte, FBK 3
Recap
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Solid-state photodetectorswith internal gain.
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Photodiode
Detectors with gain
Claudio Piemonte, FBK 5
n+
p+
p
-
+ Efield
multiplication
region
drift
region
p
n+
p+
p
-
+
p
APD
PDAPD
1 photo-carrierImpact ionization:
- linear mode: gain M
- Geiger mode: avalanche
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Turn-off
probability
GM-APD/SPAD: model
Claudio Piemonte, FBK 6
CD
RS
VBD
RQVBIASVD
SPAD
n+
p+
p
-
+
multiplication
region
drift
region
p
t
ID
t0
t2
t
IINT
~(VBIAS-VBD)/RQ
t1
~(VBIAS-VBD)/RQ
t0
Photon
detection:
Avalanche
triggering
probability
IINTID
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P10 – Turn-off probability
Claudio Piemonte, FBK 7
t
IINT
t1t0
t
IINT
t1t0
good quenching (high Rq)
bad quenching (medium Rq)
Fluctuation
(VBIAS-VBD)/RQ < 20uA
(VBIAS-VBD)/RQ
t
IINTno quenching (low Rq)
(VBIAS-VBD)/RQ
CD
RS
VBD
RQVBIASVD
SPAD
IINTID
fast, efficient!
RQ>~300kohm
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time
Current
2 photons1 photon
The SPAD response is not proportional to light
intensity (in flashes).
1 photon 2 photons
avalanche
propagates
all over the
diode
+-
+- +
-
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SiPM concept
Array of tiny
independent
SPADs can
provide a
proportional
information.
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SPADs are connected in
parallel.
Output analog signal is
proportional to # of photons.
Very simple technology, fully
custom, optimized.
Analog SiPM
Signal digitized at SPAD level.
Integrated Digital architecture
produces information on # of
photons and time stamp.
Deep sub-mm CMOS technology
required.
time
current
1pe
2pe
3pe
Digital SiPM
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SPADs/SiPMs: when, where?1960
Theory of Geiger-mode discharge in pn junctionsHaitz, McIntyre…
1970
Idea of SPAD exploitingG-M theory Various sources
1980
First SPADsPoliMIEG&G, Canada
Some papers
from 80-90s:
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SPAD/SiPMs: when, where?1995
Analog SiPMconceptRussian researchers
~2005
First SiPM deviceswith good performance
2000+
SPAD & SiPMdevelopmentsin custom and standard CMOSfew research centers
2010+
Growing industrial applications
Hamamatsu
FBK
SensL
Ketek
Philips Digital
U. Edinburgh
PoliMI
DELFT
Excelitas
PrincetonLightwave
grey = CMOS
red = custom Si
blue = custom InGaAs
Some of the main actors in SPADs/SiPMs:
ST
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…from SPAD to SiPM
New issues:
- SPAD fill factor limits the efficiency
- Interactions between SPADs
- Large area
SPAD1 SPAD3
active area
SPAD2
silicon
high Ef region
active area
high Ef region
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Analog SiPM
SEM picture of a 1x1mm2 SiPMmetal quenching resistor
active area
Simple technology:
• ~10 lithographic layers layers
• passive quenching
• «easily» customizible to application
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Analog SiPM
high-field region
(efficiency, noise)
guard struct.
(fill factor)
isolation
struct.
(fill factor,
cross-talk)
Silicon
drift region
(operation
voltage
stability)
silicon
properties
Technology can be optimized in
all aspects to get an optimal SPAD.
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Digital SiPM
Main Features:
• standard CMOS technology
• integration of «intelligence» in the sensor
• switch off noisy SPADs
• distributed TDCs for better timing
• smart validation systems
• …
• architecture is usually customized to the application
• SPAD sensor is sub-optimal
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Digital SiPM (for PET) - examplesPhilips DPC
Spadnet D-SiPMdesigned by FBK
produced by ST
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• STM SPAD-based TOF Proximity Sensor
From high-end applications to consumer
@Chipworks (http://ww2.chipworks.com/e/4202/6180-Time-of-Flight-Sensor-pdf/hwvfs/713665047)
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Claudio Piemonte, FBK 19
Main characteristics of a SiPM
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Parameters of a SiPM
1) Gain
Number of electrons produced per detected photon
2) Primary Noise (Dark Count Rate [Hz])
Cells firing spontaneously
3) Correlated Noise excess noise factor (ENF)
after-pulsing, optical cross-talk
4) Photo-detection efficiency (PDE)
5) Single Photon Time Resolution (SPTR)
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Description of each parameter with reference to an FBK technology:
NUV-HD
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Original technology
FBK SiPM technology roadmap
Electric fieldengineering
New cell border(trenches)
RGBNUV
RGB-HDNUV-HD
NUV-HD optimization
RGB-UHD
2005
2010
2012
2015
2012
VUV-HD NIR
Ongoing developments
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• Narrow dead border region → High Fill Factor
• Trenches between cells → Low Cross-Talk
• Make it simple: 9 lithographic steps
HD technology
(C. Piemonte et. al., (2016) IEEE T. Electr. Dev., 10.1109/TED.2016.2516641)
< 2 µm
High-field region
Poly
resistor
Metal
SPAD of SiPM
Trench
< 1 µm
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SPAD Pitch 15 µm 20 µm
Fill Factor (%) 55 66
SPAD/mm2 4444 2500
HD: layout features
25 µm 30 µm 35 µm 40 µm
73 77 81 83
1600 1111 816 625
High Dynamic Range High PDE
SPAD size active area
NUV
SiPM NUV-HD
SPAD pitch (mm)
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Gain = IMAX*tQ = (VBIAS-VBD)*tQ = (VBIAS-VBD)*CD__ ________ __ ____________
q RQ q q
t
i~(VBIAS-VBD)/RQ
exp(-t/RQ*CSPAD)
Gain of a SPAD/SiPM
Gain = number of electrons per photo-carrier generation
Uniformity and stability of VBD is crucial!
CD
RS
VBD
RQVBIASVD
DIODE
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26
SiPM Signal – NO amplifier
0.E+00
1.E-03
2.E-03
3.E-03
4.E-03
5.E-03
6.E-03
7.E-03
0.0E+00 1.0E-07 2.0E-07 3.0E-07 4.0E-07
Time (s)
Am
pli
tud
e (
V)
Thanks to the large gain it is possible to connect the SiPM
directly to the scope
VBIAS
SiPM
50W
Digital
Scope
SiPM: 1x1mm2
single SPAD fired
two SPADs fired
recovery
fast
component
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Gain of a NUV-HD SiPMT
ime
co
nsta
nt (n
s) 120
100
80
60
40
20
Ga
in (
mill
ion
s)
6
5
4
3
2
1
t
i
exp(-t/RQ*CSPAD)
t
i
Over-voltage (V)
Over-voltage (V)Claudio Piemonte, FBK 27
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SiPM photon counting capability
gain uniformity in SPAD and SPAD-to-SPAD
# of detected photons
T = 20 C
SiPM area 1x1mm2
Histogram of the signal area
at 4 illumination levels
Claudio Piemonte, FBK 28
SiPM illuminated
with weak pulses
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VBD Uniformity
6” Silicon wafer
1532 3x3mm2 SiPMs per wafer
SPAD pitch is 35mm
Each SiPM has 6295 SPADs
Reverse IV measurement on all
SiPMs on 3 sample wafers:
Breakdown voltage
Gain uniformity (intra- and inter- device) is mostly
affected by Vbd. What is the Vbd uniformity?
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VBD Uniformity
Distribution of VBD on
three wafers.
Max variation 200mV!
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VBD Temperature dependenceThe mean free path of the carriers in the high-field region
increases with decreasing temperature.
6 V
5 V
33mV/C
25mV/C
DOI: 10.1109/TED.2016.2641586Claudio Piemonte, FBK 31
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Dark count rate
Thermal generation
(SRH model)
Tunneling generation
Main source of
carrier generation
in silicon at room T in
standard photodiodes
TAT is an important additional
contribution in devices relying
on impact ionization
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DCR measurement
(C. Piemonte et. al., (2012) IEEE NSS/MIC Conf. Rec., N1-206)
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DCR measurement
Inter-arrival time histogram
Scatter plot
DCR
Scatter plot
of amplitude vs
inter-times
DCRDCR event
distribution
with Poisson fit
Claudio Piemonte, FBK 34
T=70K
(example to separate
well the components)
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DCR / mm2 vs. Temperature
Standard field
Low-field
> 7 orders of
magnitude !
NUV-HD 4x4mm2 size 25mm SPAD pitch (25600 SPADs)
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-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
-1.0E-08 1.0E-08 3.0E-08 5.0E-08 7.0E-08
Time (s)
Vo
lta
ge
(V
)
Collection of primary Events with after-pulse
The amplitude of the after-pulse
increases as the cell recovers to
its opertional condition
After-pulsing Probability
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Carriers may be trapped during an avalanche,
released and may triggering another avalanche.
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AP measurement
Inter-arrival time histogram
Scatter plot
AP
Scatter plot
of amplitude vs
inter-times
AP
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AP in NUV-HD
Low-fieldStandard field
AP increases with decreasing temperature because the
release becomes slower..
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3x10-5 photons with energy higher than 1.14eV emitted per carrier
crossing the junction.
[A. Lacaita et al., IEEE TED, vol. 40, n. 3, 1993]
cell1 cell2
Optical cross-talk
Photons are emitted during an avalanche which may
trigger another avalanche in another SPAD
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substrate
activelayer
-+
external cross-talk
avalanche
--+
delayed cross-talk+
scintillator
-+direct cross-talk
high-field region
Optical cross-talk
Claudio Piemonte, FBK 40
0.E+00
1.E-03
2.E-03
3.E-03
4.E-03
5.E-03
6.E-03
7.E-03
0.0E+00 1.0E-07 2.0E-07 3.0E-07 4.0E-07
Time (s)
Am
pli
tud
e (
V)
single SPAD fired
two SPAD fired,
likely CT
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CT measurement
Inter-arrival time histogram
Scatter plot
Di-CT
De-CT
Amplitude histogram
De-CT
Claudio Piemonte, FBK 41
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CT in NUV-HD
Direct cross-talk Delayed events
25-30-35mm SPAD size 25-30-35mm SPAD size
Claudio Piemonte, FBK 42
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Photo-detection efficiency
2016 JINST 11 P11010
Claudio Piemonte, FBK 43
SPAD
Fill Factor
Quantum
Efficiency
Triggering
Probability
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QE
It depends on:
- Anti-reflective coating
- active layer thickness
- junction depth
no detection detection
Claudio Piemonte, FBK 44
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NUV-HD: QEQE Measured on a photodiode produced with SiPMs
+
Simulation of the anti-reflective coating
simulation of the anti-reflective coating
measured QE
Claudio Piemonte, FBK 45
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NUV-HD: QE*Pt
SPAD size is defined by
metal opening which is
within the high-field region
cSPAD (100% Fill Factor)
Claudio Piemonte, FBK 46
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SPAD Pitch 15 µm 20 µm
Fill Factor (%) 55 66
SPAD/mm2 4444 2500
NUV-HD: PDE
25 µm 30 µm 35 µm 40 µm
73 77 81 83
1600 1111 816 625
35mm cell pitch
Claudio Piemonte, FBK 47
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Statistical Fluctuations in the current growth:
1. Photo-conversion depth
2. Vertical Build-up at the very beginning of the avalanche
3. Lateral Propagation
t=0 pair generation
0<t>t1 drift to the high-field region
t>t1 avalanche multiplication
single carrier
current level
Intrinsic Time jitter
Claudio Piemonte, FBK 48
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Single-photon time resolution (SPTR)
Claudio Piemonte, FBK 49
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• 2-ps width pulsed
laser (425nm)
Measurement setup for SPTR
•Set-up time
resolution ~ 10ps FWHM
Claudio Piemonte, FBK 50
Laser
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F. Acerbi, et al IEEE TNS vol. 61, n. 5, 2014 , pp. 2678 - 2686
• SPAD (50x50um2)
• 1x1mm2 SiPM of 50x50um2
• 3x3mm2 SiPM of 50x50um2
Example of SPTR
Claudio Piemonte, FBK 51
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Noise contribution to SPTR
Signal shape
𝜎𝒕𝜎𝑎𝑓𝑡ℎ′
Is the electronic noise the origin of SPTR deterioration
moving to large areas?
Claudio Piemonte, FBK 52
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SPTR: ultimate limit?
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 6,
NOVEMBER/DECEMBER 2014
Claudio Piemonte, FBK 53
We designed a small
SPAD with masked
edges.
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Metal ring all-around active area
Better signal extraction
more uniform signal shape
SPTR: analysis at a single SPAD level
uncovered edge
uncovered edge
covered edge
uncovered edge
Claudio Piemonte, FBK 54
20 ps FWHM!!!
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Main numbers in one table
ParameterNUV-HD technology 1mm2 40mm SPAD
DCR (20C) <100 kHz
Di-CT <20%
De-CT <5%
AP (20C) <5%
PDE 60% (400nm)
SPTR 80 ps FWHM
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PMT vs SiPM for NUV light
compactness ↓ ↑
robustness ↓ ↑
sensitivity to magnetic fields
↓ ↑
low operating voltage ↓ ↑
PDE ↑ ↑
DCR ↑ ↓
dynamic range ↑ ↓
SPTR ↑ ↑
cost ↑ ~↑
market competition ↓ ↑Claudio Piemonte, FBK 56
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Hot topics in SIPM/SPAD
• High Dynamic Range
• NIR sensitivity
FBK is working on those aspects.
UHD technology is an example.
Claudio Piemonte, FBK 57
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UHD Technology
Reduction of all the feature sizes
Circular active area (smallest cells)• No corners (with lower field)
• Hexagonal cells arranged in
honeycomb configuration
D = 0.5 D = 0.75
D = 1 D = 1.25
L < 1um
Claudio Piemonte, FBK 58
New design
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RGB-UHD
12.5mm
10mm
7.5mm
HD technology: small cells
L
RGB
RGB-HD
20mm
25mm
15mm
30mm
12mm
cell pitch (mm) cells/mm2
12 7000
15 4500
20 2500
25 1600
30 1100
cell pitch (mm) cells/mm2
7.5 20530
10 11550
12 7400
RGB-HD
RGB-UHD
Claudio Piemonte, FBK 59
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Comparison to other technologies
40 um cell
RGBSiPMs
25 um cell
RGB-HDSiPMs
<7.5 um cell
RGB-UHDSiPMs
Claudio Piemonte, FBK 60
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…new design is not enough…
F.Acerbi et al., presented at IEEE NSS, Strasbourg 2016Claudio Piemonte, FBK 61
Border region plays an important role for small cells.
We have to develop a new guard ring (NGR) especially
for the smallest SPAD (5um)
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SEM images
Microcells separated by
trenches (picture taken during
processing)
Finished SiPM
(10 um)
Claudio Piemonte, FBK 62
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Oscilloscope waveforms
7.5 um cell 10 um cell
The 5um SPAD does not work with the standard guard
ring while it works with NGR!!
Claudio Piemonte, FBK 63
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RGB-UHD: PDE
• Optimization of the technology
• Development of even smaller cell size
5 mm SPAD pitch
46200 cells / mm2
Measurement at CERN
by Y. Musienko
Claudio Piemonte, FBK 64
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RGB-UHD: Signal
Claudio Piemonte, FBK 65
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66
Thanks to all collaborators
Fabio Acerbi
Giacomo Borghi
Gabriele Faes
Nicola Furlan
Alberto Gola
Marco Marcante
Stefano Merzi
Claudio Piemonte
Giovanni Paternoster
Antonino Picciotto
Veronica Regazzoni
Nicola Zorzi