Recent development of finely
segmented AC-LGAD sensors
High Energy Physics Laboratory

Sayuka Kita
Tatsuki Ueda, Ikumi Goya, Koji Nakamura(KEK), Kazuhiko Hara

Motivation
Issuessolution

Future hadron collider →→ High luminosity

Tracking detector with 30ps timing resolution is one solution.

ex ) HL-LHC

multi-layer tracking detector
with timing resolution

No timing resolution

layer330 ps timingresolution

layer2
HL-LHC : 200 interactions in an event
FCC : 1500 interactions in an event

layer1

At high luminosity collider, inner tracker has problems
1.

Pile up2.

High occupancy3.

High radiationdifficult tracking
need radiation tolerance

improvereconstruction

hard to find rightcombination ofhits for tracks

Distinguish
1cm of speed
timing information helpsof lightto find correct hit
combinations for tracks.

Inner tracker detector for future hadron collider must have…
1. High spatial resolution : ~o(10) um
2. High timing resolution : 30ps
3. High radiation tolerance : 1 x 1016 neq/cm2

TCHoU Workshop, Photon & Particle Detectors Division2

Low-Gain Avalanche Diode detector
Normal LGAD detector

AC-LGAD signal readout model
Make electrode pitchfine for precise
spatial resolution

High electric field : Avalanche→ Precise timing resolution

signalcrosstalk

Signal size

AC-LGADimplant resistivity and coupling capacitancecharge𝒁𝑹𝒊𝒎𝒑chargereadout as 𝑸 =𝑸 created in𝒁𝑹𝒊𝒎𝒑 + 𝒁𝑪𝒄𝒑 𝟎 bulk
signalAC elec.

Uniform Gain layer : 100% fill factor
increase crosstalk ?

Finer pitch electrode: small signal and large crosstalk

TCHoU Workshop, Photon & Particle Detectors Division3

Goal and samples
p+ doping concentration

HPK several dose samples

2019 samples

Sensor for hadron collider…

2020 samples

C-3

C-2b

B-3

A-3

Lower breakdown
voltage (Radiation torelance)

C-2

B-2

A-2

D-a/bE-b

Larger n+ resistivity

C-1

B-1

DCA-LGAD

1

resistivity x 40
n+ doping concentration

Our samples✓ electrode type x 3 : pad, strip, pixel✓ dose type x 11 : A2~E-b

500umx500um
pad80um pitch strip

50um pitch pixel

E-b type (resistivityx40, coupling capacitance x 1.5)

➢ SN : Signal efficiency 99.98% @ noise rate 10-4
➢ Crosstalk : 82.68±0.06um
Can
thisdevice :assensor !!
➢ use
Spatialresolution
20.3±3.2um(low radiation condition)(binary readout, pitch/sqrt(12))➢ Timing resolution : ~37ps (C2 pad type, FNAL testbeam)
Can use for collider experiment if low radiation condition !!
(outer tracker of hadron collider or e+e- collider)

TCHoU Workshop, Photon & Particle Detectors Division

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Recent development
AC-LGAD

Topic1 : Timing resolution measurement at lab
Need to measure timing resolution in various condition• HV dependency
developed timing resolution• irradiated samples …
measurement method at lab

For inner tracker forfuture hadron collider

Topic2 : new idea for pixel sensor
To make pixel sensor
need signal size large

New type of LGAD for pixel sensor→ TCAD simulation

Topic3 : Radiation tolerance
To check radiation tolerance

Application for industry
and medical care

proton / gamma irradiation

Topic4 : measurement of polysilicon electrode LGAD sample
Can use LGAD sensor as photodetector ?
laser measurement of polysilicon electrode LGAD sample

TCHoU Workshop, Photon & Particle Detectors Division

5Recent development
AC-LGAD

Topic1 : Timing resolution measurement at lab
Need to measure timing resolution in various condition• HV dependency
developed timing resolution• irradiated samples …
measurement method at lab

For inner tracker for
future hadron collider

Topic2 : new idea for pixel sensor
To make pixel sensor
need signal size large

New type of LGAD for pixel sensor→ TCAD simulation

Topic3 : Radiation tolerance
To check radiation tolerance

Application for industry
and medical care

proton / gamma irradiation

Topic4 : measurement of polysilicon electrode LGAD sample
Can use LGAD sensor as photodetector ?
laser measurement of polysilicon electrode LGAD sample

TCHoU Workshop, Photon & Particle Detectors Division6

Pad sensor(500x500um)

Timing resolution measurement
Timing resolution : calculated time difference of two
stacked sensors

Setup

Cooling box(20 degC)

WaveRunner 8208HD
2GHz, 8ch, 10GS/s, 12bit

Designed and produced
metal frame

90Sr(β-ray)KEK16ch ampboad

sensordizitalize ampboad↑XY stage→ get closer distanceof two sensors→ align position
preciselyMPPC

β-ray

KEK 16ch ampboad

SensorModified ampboad→ improve SN

Conductive tapeTrigger : Scintillator with MPPC
TCHoU Workshop, Photon & Particle Detectors Division7

Pad sensor(500x500um)

Timing resolution measurement

resistivity
E-b : highC2 : low➢ Result➢ Analysis

Timing resolution with threshold
60mV(C2), 100mV(E-b)

fit arrival timedifference by gaussianex) time difference
ch0→ch1
calculate
timingresolution
from σTiming resolution [ns]definition of arrival time(pulse shape)

Eb
C2

C2 43.9±1.9ps(181V)
E-b 44.2±1.5ps(176V)High timing
resolution !!!

Time difference [ns]Pad1

FNAL testbeam : ~37ps

Timing resolution
𝝈 𝒕 = 𝝈 𝑻𝟏 − 𝑻𝟐 / 𝟐

Pad2

Evaluate time resolution for eachcombination then combined(cable route length difference considered)Best timing resolution(signal efficiency>99%)

bias voltage [V]✓ C2 and E-b have high timing resolution✓ method of measurement timing resolution
at lab.

TCHoU Workshop, Photon & Particle Detectors Division

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Recent development
AC-LGAD

Topic1 : Timing resolution measurement at lab
Need to measure timing resolution in various condition• HV dependency
developed timing resolution• irradiated samples …
measurement method at lab

For inner tracker forfuture hadron collider

Topic2 : new idea for pixel sensor
To make pixel sensor
need signal size large

New type of LGAD for pixel sensor→ TCAD simulation

Topic3 : Radiation tolerance
To check radiation tolerance

Application for industry
and medical care

proton / gamma irradiation

Topic4 : measurement of polysilicon electrode LGAD sample
Can use LGAD sensor as photodetector ?
laser measurement of polysilicon electrode LGAD sample

TCHoU Workshop, Photon & Particle Detectors Division9Pixel sensor challenge!
Toward fine pitch… → 50um pitch pixel sensor①

depletion region : deplete ①→②

E-b type (high resistivity, high coupling capacitance)
Event display

n++

n++

n+n++

n++

p++

②②p+

First observation of pixel signal !!
depleted region is spread into n+ layer
For pixelate…
Pulse height distribution

Too small signal toseparate signal
from noise

need improvement to make signal larger
Idea for improvement✓ To make larger Ccp→ thinner oxide thickness(next sample)✓ To make larger n+ resistivity→ limitation of Foundry process

𝑊𝑛 =2𝜖1𝑉𝑒𝑵𝑫 1 + 𝑵𝑫 𝐷𝑁𝐴

ε：permittivity
e：elementary charge
VD：Diffusion potential
NA：p+ doping concentration
ND：n+ + doping concentration

reduce n+ doping concentration (=large n+ resistivity)→ early breakdown due to reaching depletion layer tofront surface.

Need some idea to make signal larger
for pixel sensor…

TCHoU Workshop, Photon & Particle Detectors Division10New idea and simulation model➢ ACLGAD➢ TCAD model
MIP particle

buried gain
layer in bulk

50um
➢ buried gain layer LGADn+MIP simulation
MIP injected center electrode of fiveburied gain layer LGAD : suppress early breakdown due toreach depletion layer to surface of the sensor ??

Make sure✓ if it works as LGAD sensor✓ signal size change with n+ doping
concentration ?

TCHoU Workshop, Photon & Particle Detectors Division11

Signal size and crosstalk simulations
1

Signal size and crosstalk change with n+ resistivity ?

2

3

4

5

Pulse height ratio of leading ch

Signal size multiplication→ change n+ doping concentration(1e17 ㎝-3～1e19 ㎝-3 ）Signal size
make largen+ resistivity multiplicationch1 ch2ch4 ch5crosstalksuppressed

Simulated result of ACLGAD(same condition)
Signal size
multiplication

Crosstalk sizech1 ch2
ch4 ch5n+ resistivity multiplication

buried gain layer LGAD
✓ It works as LGAD sensor✓ signal size and crosstalk feature is similar to AC-LGAD

n+ resistivity multiplicationn+ resistivity multiplicationn+ resistivity make large→ signal size large, crosstalk suppressed

TCHoU Workshop, Photon & Particle Detectors Division

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Recent development
AC-LGAD

Topic1 : Timing resolution measurement at lab
Need to measure timing resolution in various condition• HV dependency
developed timing resolution• irradiated samples …
measurement method at lab

For inner tracker for
future hadron collider

Topic2 : new idea for pixel sensor
To make pixel sensor
need signal size large

New type of LGAD for pixel sensor→ TCAD simulation

Topic3 : Radiation tolerance
To check radiation tolerance

Application for industry
and medical care

proton / gamma irradiation

Topic4 : measurement of polysilicon electrode LGAD sample
Can use LGAD sensor as photodetector ?
laser measurement of polysilicon electrode LGAD sample

TCHoU Workshop, Photon & Particle Detectors Division

13Proton irradiation sample
Signal size vs bias voltage (B-3 type)Radiation damage✓ NIEL : Bulk damage✓ TID : Surface damage✓ acceptor removal (decrease p+ doping conc)

Irradiation at CYRIC (Tohoku University)

80GeV𝒑 Beam Line+Signal size remain after irradiation

non-irad(-20℃ correction)

1e14(-20℃)

The amount of increase ingain voltage is the sameeven if the p+ doping
concentration is changed.
Sample Slot

𝒅𝑽𝑮
= 𝟗𝟑. 𝟓𝟕 ± 𝟎. 𝟎𝟑 𝐕Τ 𝟏𝟎𝟏𝟒 𝐧𝐞𝐪 Τ𝐜𝐦𝟐𝒅𝝓
700 V → 0.67 × 1015 neq Τcm2

1𝑒15 neq Τcm2 : need >1000V

Pad sensor(B-1, B-2, B-3)
less5e14(-20℃)

need to find a way to reduce gain
voltage increase by radiation
damage (acceptor removal)

muchp+ doping
concentration

TCHoU Workshop, Photon & Particle Detectors Division

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Strip sensor (E-b)

Gamma irradiation sample
Radiation damage

Crosstalk✓ NIEL : Bulk damage✓ TID : Surface damage✓ acceptor removal (decrease p+ doping conc)

TID damage(positive trapping charge)

non-irrad
𝜏 = 1. 20strip

Irradiation at facility in Takasakisensor

Signal size
After irradiateddecrease

60Co gamma ray

Increased
apparent n+
resistance

1MGy
increase withradiation level𝜏 = 0.29strip

irradiated level✓ 1MGy✓ 100kGy

TID sample

TCHoU Workshop, Photon & Particle Detectors Division

Signal size : large
Crosstalk : small
15

Recent development
AC-LGAD

Topic1 : Timing resolution measurement at lab
Need to measure timing resolution in various condition• HV dependency
developed timing resolution• irradiated samples …
measurement method at lab

For inner tracker forfuture hadron collider

Topic2 : new idea for pixel sensor
To make pixel sensor
need signal size large

New type of LGAD for pixel sensor→ TCAD simulation

Topic3 : Radiation tolerance
To check radiation tolerance

Application for industryand medical care

proton / gamma irradiation

Topic4 : measurement of polysilicon electrode LGAD sample
Can use LGAD sensor as photodetector ?
laser measurement of polysilicon electrode LGAD sample

TCHoU Workshop, Photon & Particle Detectors Division

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Application to LGAD photodetector
LGAD photodetector : Al electrode → polysilicon electrode

Transmittance measurement @ red laser
visible lightvisible light

Transmittance(red laser)
76.16±0.15 %on poly elec.

Timing resolution measurement
time resolution
frame

60

Entries
Mean xMean y58
on poly-Si electrode56

on slitStd Dev x
Std Dev y0000054

time resolution [ps]time resolution [ps]time resolution

120

Entries
Mean xMean yon poly-Si electrodeon slit

110

5250

frame

130

Std Dev x
Std Dev y

Red 73.1±0.3ps

on slit

10048

Timing resolution
Red > IROn elec. > slit

90

464400000

IR 44.5±0.2ps

804240140 142 144 146 148 150 152 154 156 158 160 162
voltage [V]70140 142 144 146 148 150 152 154 156 158 160 162
voltage [V]

TCHoU Workshop, Photon & Particle Detectors Division

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Strip sensor(80um pitch)

Signal attenuation by electrodes
Signal readout model (polysilicon elec. version)

Testbeam @ ELPH
Efficiency

efficiency
frame

0.5

Entries
Mean xMean y0.45

Std Dev x
Std Dev y

0.4
0.35

wirebond padregion

0.3
0.25
0.2
0.15
0.1
0.05
00

ZRstrip increases in proportion to thedistance from the wirebond pad

0.4
0.6
0.811.2
1.4
Distance from the middle of wire-bond pad𝒙𝑦 = 𝐴𝑒𝑥𝑝 − + 𝐵
𝝉𝟎. 𝟐𝟑 ± 𝟎. 𝟎𝟓 𝒎𝒎

Signal size : poly-Si elec. < Al elec.

✓ Signal cannot separate from noise.
✓ Reason : Large resistance of polySi ??
𝒁𝑹𝒊𝒎𝒑
𝑸=𝑸
𝒁𝑹𝒊𝒎𝒑 + 𝒁𝑪𝒄𝒑 + 𝒁𝑹𝒔𝒕𝒓𝒊𝒑 𝟎0.2

Signal was attenuated due to the resistanceof poly-Si elec. itself→ make sample with reduced polysilicon resistance

TCHoU Workshop, Photon & Particle Detectors Division

1800000Conclusion
LGAD detector : precise timing and position resolution
Topic1 : Method of timing resolution
measurement at lab

Topic3 : Radiation tolerance
Proton irradiation
Signal size : not reduced
Gain voltage : large

Timing resolution measurement
C2 43.9±1.9ps (181V)
E-b 44.2±1.5ps (176V)
Established method of timing resolution
measurement at lab

Gamma irradiate
Signal size : larger
crosstalk : smaller

Important step for developing
Topic2 : new idea for pixel sensor

need improvement
for hadron collider

Topic4 : measurement of polysiliconelectrode LGAD sample

buried gain layer LGAD
TCAD simulation → confirm to work as a sensor

produce prototype sample

Measurement poly-Si elec. sample
Transmittance : 76.16±0.15 % (red laser)
Timing resolution : 44.5±0.2ps (IR laser)

Serious study for the applicationis performed

TCHoU Workshop, Photon & Particle Detectors Division19