Future silicon trackers: 4D tracking, very high fluences, very small pixels - Nicolò Cartiglia INFN - Italy - Indico
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N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Future silicon trackers:
4D tracking, very high fluences, very small pixels
Nicolò Cartiglia
INFN - Italy
1
Outline
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
• A brief history of silicon trackers
• Requests for the next generation of silicon trackers
• 4D tracking:
- what is it
- is it possible?
• Sensors for extreme fluences
2
A brief history
The beginning of the Silicon detector era is set in the period
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
1978-1982, and the NA11/NA32 experiments are credited to
be the first one to have used a silicon tracker
Shortly after, successful tests of silicon strip detectors with VLSI
readouts were carried out in 1985.
During the 1990s, CDF and the LEP experiments were
instrumented with Silicon trackers, with the electronics at the
edges.
Here at DESY, we even manufactured a curved silicon
detector, to be placed near the proton beam.
The ZEUS experiment was also instrumented with the silicon
vertex detectors
3
Evolution up to LHC
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
This incredible evolution was made possible by the development of the “silicon”
industry and by the collaboration of our community with several silicon foundries
4
The LHC and HL-LHC era
Incredible development of manufacturing
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
capability
Very good understanding of the silicon
properties under irradiation: modelling of silicon
detectors and the effect of irradiation is well
modelled.
Taken from Doris Eckstein
Similar development in read-out capability
HL-LHC: the CMS-ATLAS upgrades are very large, however, they are in spirit similar to the present
LHC detectors. Higher radiation levels, more channels and much more performing electronics.
One novel request: need to measure the time of each track, to bundle correctly the tracks of
each vertex.
5
What’s next
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
There are many futures in Silicon trackers: some are clever redesigns of existing
systems, some requires much higher radiation tolerance, some extremely good
position resolution.
One of the most challenging design:
the Future Circular Collider tracker
Tracker requirements:
position: 7.5 - 9.5 μm
time resolution = 5 ps
Radiation levels: up to ~1E17 n/cm2
Note: there are many R&D directions in Silicon detectors.
This presentation is not a review but it is about a possible future.
6
Tracking particles in space and time at FCC
First question: Can we design a single detector that can concurrently measure
(a) time with
~ 10 ps precision
(b) position with
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
~ 10 micron precision
This is an extraordinary challenge in sensor design and ASICs
Second question: can we make silicon detectors able to work at fluences about 1E16 –
1E17 n/cm2?
A lot has been understood regarding the design of radiation hard silicon detectors, with
a key contribution from Hamburg, however, currently we don’t know how to do design a
sensor for extreme fluences, F = 1E16 – 1E17 n/cm2
7
First question: ~ 10 micron and 10 ps precision
Silicon sensors were never considered accurate timing devices
However, in the last 10 years there has been a very intense R&D
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
At present, silicon sensors are the ONLY detector able to provide excellent timing
capability (~ 30 ps) , good radiation hardness (fluence ~ 1E15 n/cm2), good pixelation
(10um – 1 mm), and large area coverage (many m2)
Important:
Sensors provide the current signals, read-out chips use them
Timing is the to combination of these two parts,
that succeed and fail together
8
The effect of timing information
The inclusion of track-timing in the event information has the capability of changing radically how we
design experiments.
Timing can be available at different levels of the event reconstruction, in increasing order of
complexity:
1) Timing in the event reconstruction è Timing layers (time, position)
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
• this is the easiest implementation, a layer ONLY for timing
2) Timing at each point along the track è 4D tracking (time, position)
• tracking-timing
Timing
3) Timing at each point along the track at high rate è 5D tracking (time, position, and rate)
• Very high rate represents an additional step in complication, very different read-out
chip and data output organization 9
Signal formation in silicon: induced current
The charge carriers motion induces Induced
variable charge on the read-out charge ++++
++++++
electrode.
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
The signal ends when the charges are
collected
Signal shape is determined by Ramo’s Theorem:
i ∝ qvE w
Drift velocity
Weighting field
10The sensors’ role: provide good signals
The goal of a sensor designer is to minimize the differences in the sensor’s output,
providing well defined, uniform current signals to the electronics.
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
The prerequisite for this goal is the capability of simulating the physics of the particle-sensor
interaction.
Chip designers need to test their solutions on a realistic sets of current signals that reproduce
the full variability of the sensor’s output.
Good sensor simulation is necessary to achieve excellent time resolution
11Simulator Weightfield2
Available at:
http://personalpages.to.infn.it/~cartigli/Weightfield2/Main.html
It requires Root build from source, it is for Linux and Mac.
It will not replace TCAD, but it helps in understanding the sensors response
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
12Weightfield2 and friends
Weightfield2:
- It is completely open source
- It is fast
- It generates the signal from several sources (MIP, alpha, lasers..)
- Runs in batch mode writing output files
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
- It loads/save configurations
- It has basics electronics simulation
It crashes occasionally
Other simulators:
KDrtSim, https://indico.desy.de/indico/event/12934/session/3/contribution/26/material/slides/
TRACS
https://indico.desy.de/indico/event/12934/session/3/contribution/29/material/slides/
13The art of weighing field
Calculating the correct weighting field for a variety of situations is a very difficult task.
Most of the time we rely on simulator to do it.
Please note a series of papers that are approaching this problem analytically:
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
W. Riegler, “An application of extensions of the Ramo-Shockley theorem to signals in
silicon sensors” Nucl.Instrum.Meth. A940 (2019) 453-461 arXiv:1812.07570
Academic training at CERN: https://indico.cern.ch/event/843083/
Joern Schwandt, Robert Klanner, On the weighting field of irradiated silicon detectors,
https://arxiv.org/abs/1905.08533
14Silicon time-tagging detector
(a simplified view)
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Time is set when the signal crosses the comparator threshold
The timing capabilities are determined by the characteristics of the signal at
the output of the pre-Amplifier and by the TDC binning.
Strong interplay between sensor and electronics
15Time resolution
#
#
%&'() # # #
!" = + ∆'&/'01"'&/ + ∆(213) + 456
*+/*"
Subleading,
Sensor design ignored here
Usual “Jitter” term Amplitude variation:
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Here enters everything that is “Noise” and variation in the total charge
the steepness of the signal Shape distortion:
non homogeneous energy deposition
total current
electron current
hole current
total current
electron current
Need large dV/dt hole current
16Sensor geometry: how to minimize its contribution to !#"
Signal shape is determined by Ramo’s Theorem:
i ∝ qvE w
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Drift velocity
Weighting field
The key to good timing is the uniformity of signals:
Drift velocity and Weighting field need to be as uniform as possible
Basic rule: parallel plate geometry
17Larger dV/dt from thick detectors?
(Simplified model for pad detectors)
Thick detectors have higher number of charges:
d + -
+ -
Q tot ~ 75 q*d
+ -
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
However, the weighting field is 1/d, so each charge
+ -
contributes to the initial current as: D + -
1 + -
i ∝ qv + -
d
The initial current for a silicon detector does not depend
on how thick (d) the sensor is:
k k
i = Nq v = (75dq) v = 75kqv ~ 1− 2*10 A
−6
d d
Number of e/h = 75/micron
velocity
Weighting field
è Initial current = constant
18Summary “thin vs thick” detectors
(Simplified model for pad detectors)
Thin detector
d + -
+ -
i(t)
+ -
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
S Thick detector + -
D + -
+ -
+ -
dV S
~ ~ const
dt t r
Thick detectors have longer signals, not higher signals
We need to add do something about this problem…
19On the road, summary - I
The study of the signal in silicon sensors has highlighted a few crucial aspects:
- The signal in rather small, the initial current is a constant and dV/dt cannot be made
steeper using thinner/thicker sensors
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
- The fluctuation of ionization (Landau noise) are a physical limit to the time resolution
- The geometry of the sensor needs to be as much as possible similar to a “parallel plate”
capacitor
- The noise of the electronics is crucial in determining the jitter
20Gain in Silicon detectors
Gain in silicon detectors is commonly achieved in several types of sensors. It’s based
on the avalanche mechanism that starts in high electric fields: V ~ 300 kV/cm
α l
Charge multiplication G=e
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
æ be ,h ö
Gain: a e,h (E ) = a e,h (¥ )* expçç - ÷
÷
è E ø
a = strong E dependance
DV ~ 300 kV/cm
a ~ 0.7/µm for electrons,
a ~ 0.1/um for holes
- - - -
- - -
Concurrent multiplication of electrons and + + + +
holes generate very high gain + - -
- - - -
Silicon devices with gain: - + + +
+ + + -
• APD: gain 50-500 + - -
• SiPM: gain ~ 104 - +
21Standard vs Low Gain Avalanche Diodes
Gain layer
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
zoom Drift area with gain
0.5 – 2 um long
Gain implant
E field Traditional silicon detector Low Gain Avalanche Diode E field
The LGAD sensors, proposed and manufactured for the first time by CNM
(National Center for Micro-electronics, Barcelona):
High field obtained by adding an extra doping layer
E ~ 300 kV/cm, closed to breakdown voltage
LGAD optimized for timing applications are often called Ultra Fast Silicon Detector (UFSD)
22How gain shapes the signal
Gain electron:
- +
- + absorbed immediately
Initial electron, holes + - Gain holes:
long drift home
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Electrons multiply and produce
additional electrons and holes.
• Gain electrons have almost no effect
• Gain holes dominate the signal
è No holes multiplications
23Interplay of gain and detector thickness
The rate of particles produced by the gain does not depend on d
(assuming saturated velocity vsat)
Particles per micron
Gain layer Gain Layer
+
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Gain
- Gain +
dN Gain ∝ 75(vsat dt)G
è Constant rate of production
However the initial value of the gain current depends on d
(via the weighing field)
k
digain ∝ dN Gain qvsat ( ) è Gain current ~ 1/d
d
A given amount of new carriers has much more effect on thin detectors
24Gain current vs Initial current
(Real life is a bit more complicated, but the conclusions are the same)
è Go thin!!
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
300 micron:
~ 2-3 improvement
with gain = 20
Significant improvements in time resolution require thin detectors
25Gain and Signal current
dV G
∝
dt d
i Max ∝ Gain
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
i(t) medium
thick
thin
t1 t2 t3 t
The rise time depends only on
the sensor thickness ~ 1/d
26UFSD time resolution summary
The UFSD advances via a series of productions.
For each thickness, the goal is to obtain the intrinsic time resolution
Achieved:
• 20 ps for 35 micron
• 30 ps for 50 micron
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Resolution without gain
UFSD1
UFSD2, 3
27Gain, amplitude, and position in the Landau
The higher tail of the Landau distribution is
populated by events with very high ionization.
These events contain a strong secondary
ionization component, such as that caused by
delta rays.
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
The shape of the events in these bins varies
a lot, so they have worse time resolution
signals generated by 120GeV/c pions
crossing a 50 micron thick UFSD
28Consider the holes’ drift velocity
The amplitude depends on:
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
• Holes’ drift velocity (Bias) The combination of gain
• Gain and bias determines dV
& &
A ()*+$ ()*+$
!&"##$% = ~
,-/,# 0/#%*+$
trise
The rise time depends
on the electrons’ drift
i ∝ qvE w
velocity
530V
Vholes never saturates,
Bias: 150V
so higher the voltage,
50 micron
better dV/dt is
29The effect of the “never saturating” holes’ drift velocity
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
40 30-35 ps
At lower bias, higher gain
35-40 ps
35 is needed to achieve a
resolution of 30-35 ps
40-45 ps
30
45-50 ps
25 50-60 ps
Gain
20 Expon. (HPK 50C -20C)
HPK 3.2: too doped,
very poor time Expon. (HPK 3.1 -30C)
15 resolution
Expon. (FBK UFSF3 W5 -30C)
10
Expon. (HPK 3.2 20C)
5 Expon. (HPK 50D -20C)
Expon. (FBK UFSF2 W6 -20C)
0
0 100 200 300 400 500 600 700 800
Expon. (FBK UFSF2 W8 -20C)
Bias [V]
Time resolution for new UFSD FBK & HPK sensors in the bias-gain plane
30LGAD producers
Currently there are 6 companies that either have produced, or are about to produce LGADs
FBK, Italy
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
CNM, Spain
Hamamatsu, Japan
Up coming:
Brookhaven National Lab, USA
NDL China
Micron, England
Maybe more
31Electronics: What is the best pre-amp choice?
Energy deposition
Current Amplifier • Fast slew rate
in a 50 mm sensor
• Higher noise
• Sensitive to Landau
bumps
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
WF2 simulation
Current signal in a
50 mm sensor
Charge Sensitive Amplifier
WF2 simulation
• Slower slew rate
• Quieter
• Integration helps the
signal smoothing
32Electronics
To fully exploit UFSDs, dedicated
electronics needs to be designed.
The signal from UFSDs is different
from that of traditional sensors
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
WF2 simulation
No Gain,
No
Gain
300 micron
300 micron
50 micron
Much easier life!
Oscilloscope
Simulated Weightfield2
Pads with no gain Pads with gain
Charges generated uniquely by Current due to gain holes creates a longer
the incident particle 2 sensors and higher signal 33Consideration on the read-out chip
• The design and production of a large (2x2 cm2, 500 channels) ASIC for timing is a
very complex operation
• Analog pre-amplifier
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
• Power consumption,
• Clock distribution,
• TDC design
• It requires years (3-4 year is our present timescale), a lot of manpower and money
• It might not work
• Many groups working on this problem, exploring different technologies (CMOS 130
nm, 65 nm, 28 nm, SiGe, monolithic etc)
34Pixel termination and position resolution
As in every n-in-p sensor, the pads need to be isolated
Unwanted consequence: late signals
n++
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Particles hitting a pad create charges
underneath the multiplication layer: Gain implant p-stop + -
+ -
no delay between the passage of + -
p bulk
the particle and the start of + -
multiplication
~50 um
n++
Particles hitting between pads
p-stop + -
create charges far from the Gain implant
+ -
multiplication layers: + - 10 um = 100 ps
p bulk + -
they generate late signals
35A solution and a problem
n++
Solution: add a deep n implant to p-stop + -
Gain implant
collect the charges in the interpap + -
+ - 10 um = 100 ps
p bulk
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
+ -
No gain area
Deep implants collect the charge carriers preventing their
multiplications:
• signals that are “out of time” are not multiplied
• Setting a threshold high enough allows to be blind to the
interpad signals
We solved the “isolation” and the “late signals” problems, however we have created a
“no gain” area of ~ 30-40 micron: impossible to make small pixels!
36Position resolution: trenches
Trenches (the same technique used in SiPM):
- No pstop,
- No JTE è no extra electrode bending the field lines
Current version R&D goal
No gain area
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
JTE + p-stop design Trench design
FBK run @ RD50
37On the road, summary - II
Key points to achieve excellent position and timing performances.
1. Thin sensors to maximize the slew rate (dV/dt)
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
2. Parallel plate – like geometries (pixels..) for most uniform weighting field
3. High electric field to maximize the drift velocity
4. Small size to keep the capacitance low
5. Small volumes to keep the leakage current low (shot noise)
6. Use trench isolation
7. Know someone that can design a read-out chip for you
38Irradiation effects – the LGAD point of view
Irradiation causes 3 main effects:
• Decrease of charge collection efficiency due to trapping
• Doping creation/removal
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
• Increased leakage current, shot noise
LGAD are particularly sensitive to the doping creation/removal, as it
changes the electric field and therefore the gain.
39Sensitivity to doping changes
The amount of doping in the gain implant strongly
Nicolo Cartiglia, INFN, Torino – Tracking in 4D
affect the gain value
+3% doping doubles the collected charge
-4% halves the collected charge
The bias can be adjusted to keep the charge
constant as the doping in the GL changes.
40Acceptor removal
Unfortunate fact: irradiation de-activate p-
doping removing Boron from the reticle
! ∅ = ! $ ∗ &'(∅
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Boron
Radiation creates interstitial defects that
inactivate the Boron: Si_i + B_s è Si_s + B_i
B_i might interact with Oxigen, creating a
donor state
Two possible solutions: 1) use Gallium, 2) Add Carbon
Gallium
From literature, Gallium has a lower probability
of becoming interstitial
Carbon
Carbon competes with Boron and Gallium in
reacting with Oxigen
41LGAD radiation hardness improvement
Defect Engineering of the gain implant
• Carbon co-implantation mitigates the gain loss after irradiation
• Replacing Boron by Gallium did not improve the radiation hardness
Modification of the gain implant profile
• Narrower Boron doping profiles with high concentration peak (Low Thermal Diffusion)
are less prone to be inactivated
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Boron + Carbon
Boron profile
Doping Concenration (a.u).
Gallium + Carbon
Boron (Low Diff)
Boron (High Diff)
Gallium
Acceptor removal is no understood from a microscopic point of view
42N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Time resolution
43First timing layer: CMS Endcap Timing Layer
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
7 m2 of sensors
on each side
First detector for
~ 16000 sensors: precision timing
• 2x4 cm2 --- small sensors in Silicon
• Thickness of active area: 40-50 microns
• Pad size: 1.3 x 1.3 mm2 (512 pads)
44Second question: silicon at fluences about 1E16 – 1E17 n/cm2?
Very large volume with fluence in the range 1E16-1E17 n/cm2
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
45Silicon at fluences about 1E16 – 1E17 n/cm2?
Irradiation causes 3 main effects:
• Decrease of charge collection efficiency due to trapping
• Doping creation/removal
• Increased leakage current, shot noise
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Irradiation models developed in the fluence range 1E14 – 1E15 n/cm2 predict
standard silicon detectors (~ 200 um thick) almost impossible to operate
è Mission impossible
(New) VFD > VBias
200V
500V
750V
Fully depleted at low Vbias Fully depleted at high Vbias Partially depleted at very high Vbias
46A new hope: saturation of displacement damage
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Overlapping clusters
At high fluence the damage might saturate since clusters of damage start overlapping
472D calculations of superposition
• What is the probability of a particle to hit a square of 1 Å2 that has not been hit before?
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Probability of hitting an empty square of area 1 Å2
At 1E16 n/cm2 only 30% of particles will hit an “empty square”
Note: Silicon lattice has a cube of 5 Å; every cell has already been hit at 1E15.
Damage on damaged Silicon probably has different consequences.
48Evidence of Saturation
Marko Mikuž at AIDA2020 Topical Workshop on Future
There is a consensus building that:
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
of Tracking, Oxford, April 2nd, 2019
• Low fluence extrapolations do not work at all
• Go out and measure to get anything working at extreme fluences
Charge trapping
Saturation is a key aspect of the R&D in the next few years, we should learn how to take
advantage of this effect
The bottom line is: Silicon detectors irradiated at fluences 1E16 – 1E17 n/cm2
do not behave as expected, they behave better
49Use thin sensors
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
What does it happen to a 25-micron sensor after a fluence = 5E16 n/cm2?
• Trapping is almost absent
• It can still be depleted
• Leakage current is low (small volume)
However: Charge deposited ~ 0.25 fC
è Need a gain of at least ~ 5 in order to provide enough charge
50Sensors evolution with fluence
Use thin (25 um) LGAD sensors: as the irradiation deactivate the gain
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
layer, increase bias to obtain gain in the bulk
(F~1E16 n/cm2) ) VFD < VBias
- ----- 500V
Gain: Gain:
+++++
- ---
+++++
- Gain layer - Gain layer
+++
- -
+++++
- Bulk
+
Trenches
+++++
n-in-p n-in-p
Below 5E15 n/cm2
è Use LGAD design to obtain a gain of ~ 5 without breakdown
è Vbias controls gain
Above 5E15 n/cm2
è is the gain still there?
è Is the mobility decreasing to a point where no gain is possible?
è Damaged bulk acts as a quenching resistor?
è No holes multiplications?
51Putting things together
Future trackers need to provide:
- Accurate timing
- Excellent pixellation
- Be very rad-hard
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
A very simplified design:
• Very inner layer:
Ø Position, thin sensors, with small gain
• Medium – far away layers:
Ø timing layers
Ø Position, thick sensors
Note: Limited number of timing layer: probably, they require too much power
52One sensor does not fit all
Silicon sensors for tracking come in many shapes, fitting very different needs:
• Spatial precision: from a few microns to mm (pixels, strips)
• Area: from mm2 up to hundred of square meter
• Radiation damage: from nothing to >1E16 neq/cm2 (3D, thin planar, thick planar)
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Likewise, silicon sensors for time-tracking are being developed to fit different needs with
respect of time and space precision.
Depending on the amount of optimization, several resolution ranges can be identified
- Excellent time precision ~ 10-20 ps per plane è small area, lot’s of power
- Very good precision ~ 30-50 ps per plane è large area, medium power
- Good time precision ~ 50-100 ps per plane è relaxed
53Summary and outlook
• Building the next generation of trackers is a formidable challenge
• The requirements become more demanding (5 ps, 5 um, few ~100m2)
• At FCC, the fluence will be unprecedented
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
• New development in Silicon detectors suggest some (all) of these demands can be met
• The key is a strong R&D activity over the next decade
There are no real alternatives: hopefully, as it happened in the past, Silicon detectors will
be the enabling technology to new discoveries
54Acknowledgments
We kindly acknowledge the following funding agencies, collaborations:
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Ø INFN - Gruppo V
Ø Horizon 2020, grant UFSD669529
Ø Horizon 2020, grant no. 654168 (AIDA-2020)
Ø U.S. Department of Energy grant number DE-SC0010107
Ø Dipartimenti di Eccellenza, Univ. of Torino (ex L. 232/2016, art. 1, cc. 314, 337)
55N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Bonus
56Signal shape for equal gain at different biases
Equal rise time:
saturated electron drift velocity The electrons’ drift velocity
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
saturates at about ~ 150V.
low bias The holes’ drift velocity never
saturates. For equal gain:
medium bias Higher bias è higher dV/dt
high bias
Bias = 120V, gain = 9 Bias = 170V, gain = 9 Bias = 500V, gain = 9
For equal gain, better resolution at higher voltage
57Effect of acceptor removal
! ∅ = ! $ ∗ &'(∅
Acceptor removal,
Gain layer deactivation
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
To some extent, the gain layer disappearance might be compensated
by increasing the bias voltage
58Acceptor removal data
! ∅ = ! $ ∗ &'(∅
Acceptor removal coefficient
Puzzle: the removal of acceptor depends on their density
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
è the removal is slower for higher densities
59Time walk corrections
Constant Fraction Time-over-Threshold
On paper both seem
V V
feasible, in practice
ToT is much easier to (a) (b)
10%
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
implement Vth
t1 t2
t t
Amplitude
Constant Fraction Time-over-Threshold
My favorite: ToA and Amplitude
V V
è The tail of the signal is prone
to large changes due to charge (a) (b)
10% (C)
trapping Vth
ToA t1 t2
t t
What is the influence of the sensor on the level of the CFD or of Vth?
60What is the signal of one e/h pair?
(Simplified model for pad detectors)
Let’s consider one single electron-hole pair.
The integral of the current is equal to the electric charge, q:
∫ [i el
(t)+i h (t)]dt = q
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
However the shape of the signal depends on the thickness d:
thinner detectors have higher slew rate
Thin detector d + -
i(t)
Thick detector
D + -
t
è One e/h pair generates higher 1
i ∝ qv
current in thin detectors d Weighting field
61RD50 production of trenched LGAD
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
Common RD50 run with FBK: preproduction run demonstrated the proof of
principle: nice isolation and nice gain
62Electric fields in Silicon sensors
Gain happens when the Efield is near the critical values, 300 kV/cm
3 methods to increase Efield:
1. Doping in the bulk
2. Doping in the gain layer
3. Bias
N. Cartiglia, INFN. Terascale meeting - 27-Nov-2019
• The “low gain avalanche diode” offers the most stable situation
• Gain due to interplay between gain layer and bias
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