Development an AC-LGAD sensor with fine time andspatial resolutions
Koji Nakamrua, Sayuka Kita^A^, Tatsuki Ueda^A^, Kazuhiko Hara^A^
KEK, U.Tsukuba^A^
29nd Mar, 2021

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1Motivation
Higgs discovery and measurement by LHC experiment
New era!

• “Vacuum”– “Vacuum” is nothing? Filled by Higgs boson?
– How Higgs boson/field condensed to the “Vacuum”?
– Need to determine/observe the shape of Higgs Potential.
→Observe/measure “Higgs self coupling”.
interferenceHHHtHH• “Dark Matter/Energy”– We only know 4%. What’s the others?
– Beyond the Standard Model?
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Next generation of Collider experiment• Need “Higher Luminosity” and/or “Higher Energy”

Inner Tracking system– High Luminosity LHC (HL-LHC)• 20 times more data (~3000-4000fb-1) at 14TeV• Plan : Start at 2027

– High Energy LHC (HE-LHC)• Use Super Conducting Magnet with Higher Magnetic field(16T)• 28TeV collider in the same tunnel as LHC.

Very high density tracks

140 pileup @ HL-LHC
1500 pileup @ FCC-hh– Future Circular Collider (FCC-hh)– International Linear Collider (ILC)• 250GeV e+ e- collider in Japan

Only way to solve this so far…

finer pixel pitch• Use Super Conducting Magnet with Higher Magnetic field(16T)• 100TeV collider with 100km tunnel at CERN.

Current detector(ATLAS IBL)

250um

50um

New detector(Pixel @HL-LHC)
φ50um
50um

Z
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Future Semi-conductor Tracking Detectors• Further finer pitch pixel detector → Limited by front end Electronics (min : 50x50um2)– In addition to spatial resolution, Timing resolution helps!
→New generation of Tracking detector should have timing information for all hits!

• Tentative Requirement– 30ps timing resolution– ~o(10)um spatial resolution (Pixel type).
– (hadron collider) ~o(1016)neq/cm2 radiation tolerance
Particle identification

4D tracking！
Detector Hit

Mass spectrum for new particle

Tracking

β=1

β = 0.95

150ps difference at R=1m
Solve pileup hits in an event
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K+ π+ separation
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β measurement to obtain mass
e.g. Mass measurementfor Long lived chargeno4Low gain Avalanche Diode (LGAD)• Low gain Avalanche Diode (LGAD)– General n+-in-p type sensor with p+ gain layer under n+ implant to make higher
Electric Field → Good timing resolution.
– 30ps timing resolution achieved already.
beam– Next development• Finer electrode separation for spatial resolution• Radiation tolerance

Measure time difference

Signal drivers : Gain Holesn+ - eee--h--e- eeee--h--eh-hh-hehh++
pe-hhhhehhee- h hehp+

Electric field

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5Detector with both spatial and timing resolution• First prototype with 80um pitch strip (DC-LGAD)→Only 20% of active area has gain
• Common gain layer with AC-coupled readout (AC-LGAD) → Uniform gain expected!
DC-LGAD
n++

Aln+

n+

p++

p++

--+- e ne+- eee-ehh
hhe hh nee--e- h- e- h- he++p hhhhhe hh p++
e-eee--

SiO2

n+

n+

p++

p++

hn++

Low fill factor

hhp+

Al

AC-LGADDC elecn++

SiO2

Poly-Sin++n++

AC elec-eee- - +e- ee-e-- - - eehn-hhhhhe hh
e ehh ehhhh ph++he-n++eee-hhh100% fill factor expected
But need to check Cross-talkp+ doping concentration– Cross talk expected in the n+ implant → Increase resistivity of n+ implantParameter space for doping concentration

p+29nd Mar, 2021

Lower Operation Voltage
Radiation tolerance

DCLGAD
Smaller cross talkn+ doping concentration
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First AC-LGAD by HPK
Pad type sensor (4x 500umx500um) Pixel type sensor (14x14 50umx50um)
Bias ring

Bias ring
Pixel pitch

DC ring

50µm

500µm

DC ring

50µm

20µm

40µm

Wire bondpadGNDed DC ring via Poly-si→ To remove charge in n+42µm

Poly-si 30µm
resistor

Poly-si
resistor

Strip type sensor (16x 80um pitch)

Strip pitch

45µm

80µm

n+ and p+ doping concentration

Poly-siresistor

N+ doping concentration / resistivity

p+ doping

Bias ringDC ring

Varied Al size (AC coupling capacitance)
Pixel : 42/38/34/30 um width/length
Strip : 45/40/35/30 um widthC(Ax10 resistivity)
3 (high)P+ dopingconcentration

B(Ax3.3 resistivity)

A (~DC-LGAD)

B-3

A-3
A-2

2 (mid)C-2

B-21 (low)

C-1

B-1

40µm

n+ doping
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7Measurement setup and signal observation• Lab setup– Designed high speed amplifier board.
– Signal recorded by CAEN DT5742 digitizer– 90Sr β lay source– Triggered by Scintillator (MPPC readout)

KEK 16 ch Discrete Amp.

Pad Sensor

ch1ch2ch0 ch3BOX
LV
HV

90Sr

Collimator
Sensor
Amp. board

Scinti.

digitizer
Trigger
CAEN DT5742
Desktop Digitizer
5GS/s 10bit
12bit ADC 2V full range

Scintillator /29nd Mar, 2021

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ch1ch0ch3ch2
Pulse height

Positive overshootfor highest signal only

Typical Event :Particle through ch2
Detected cross talk in ch0,1,3First time : AC-LGAD signal
observed with small crosstalk8Equivalent circuit for Signal readout• Crosstalk and Readout charge
Assuming Zcbulk>>ZCcp…

𝑄=𝑍𝑅𝑖𝑚𝑝

𝑍𝑅𝑖𝑚𝑝 + 𝑍𝐶𝑐𝑝

𝑄0

Equivalent Circuit

Ccp Q Ccp
Rimp

Rimp

Ccp

Q0 QxtalkRimp

n+p++

RimpCbulk– Generated charge Q0– Readout charge Q
To have larger signalLarger Ccp
Larger Rimp

Pad type

Strip type
500um

45um

9880um500um
SignalMPV = 62 mVx3

MPV = 21mV

JPS : 12aT3-7 Sayuka Kita (Tsukuba)
See more detail in Last THoU member meeting in 30th Nov 2020

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9Radiation Effect in LGAD sensor•The same as general n+-in-p sensor

AlAC-LGAD
DC elec– Bulk damage (NIEL) : Lattice defect.
– Surface damage(TID) : Positive charge @ SiO2-Sin++

SiO2

Poly-Sin++n++

AC elec-eee- - +e- ee-e-- - - eehn-hhhhhe hh
e ehh ehhhh ph++he-n++ee-• In addition to this ”Accepter Removal”

e-hhh

p+– p+ (Boron) accepter change to doner levelp+ doping profile measured by bulk capacitance

What happened if p+ dope reduced by accepter removal?

Reduce p+ doping concentration
Current condition

non-irrad
non-irrad
non-irrad

Higher Gain (operation) Voltage

Improvement
After radiation damage
1.5e15neq/cm2

Lower p+ doping
Operation Voltage > Max voltage(~700V)End of life

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10IV performance after irradiation• Irradiated sensors at CYRIC (Tohoku university) with 70MeV Proton.
• Operation/Gain voltage get higher by irradiation (almost linearly)– Current sensor does not work after 1x1015neq/cm2 fluence or more.
Max. operation V
Non-irrad

Signal Size

1 x 1014 neq/cm2

5 x 1014 neq/cm2

Fluence

Signal MPV

Non-irrad35±2 mV

1x1014neq/cm2 39±1 mV
5x1014neq/cm2 30±2 mV
60V
400V

5x1014neq/cm2 may haveslight decrease of signal(But keep at least ~75%)

JPS 12aT3-8 Tatsuki Ueda (Tsukuba)
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11Test beam in Feb 2021 @ Fermilab
Fermilab Test Beam Facility (FTBF)
120GeV proton beam
Strip Detector based Telescope : ~15um pointing resolution

Beam Telescope

Beam Telescope

Cooling Box

Amp board
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Readout by Ocilloscope
LeCroyWR8208HD scope
12bit, 10GSa/s, 2GHz
8 channel

Timing reference Detector
PHOTEK
MCP photomultipliers (PMT140)
450ps FWHM with 5e3 Gain
~5ps timing resolution(SPEC: Multi-photon jitter below 10 ps)

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12Time resolution measurement @ testbeam• Used PHOTEK : MCP PMT140 as a timing reference detector– Including 5ps PMT140 time resolution (<1% effect)
Very fresh results : Obtained 30-40ps time resolution for a couple of types of sensors

C-2 : 180V
σ = 37ps

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B-2 : 230V
σ = 33ps

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13Efficiency and signal sharing @ testbeam

Beam position X• Efficiency measurement for the top left pad.

– Close to 100% efficiency @ 90mV threshold– ~40mV crosstalk observed. (consistent to lab meas.)
Need more study for the reason of flat crosstalk. (inter elec. Cap?)
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Beam position X

Mean amplitude [mV]Beam position X

Efficiency, 20mV threshold
Beam position Y

Efficiency, 45mV threshold
Beam position Y

Beam position Y

Efficiency, 90mV threshold

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Parameter space for doping concentration
C-3

B-3

Existing Samples
New Samples (26th Mar)

A-3

Coupling capacitanceRadiation tolerance

Nominal x 1

Nominal x 1.5

Ax10

C-2

C-2-b

Ax20

D-a

D-b

C-2b
C-2

B-2

D-a/b

DCLGAD

Smaller cross talk
Larger signal

E-b

C-1A-2B-1n+resistivity

p+ doping concentration

New samples (4 types of sensors)

Ax40

E-b

A-1

Thinner SiO2

Ccpn+

x 1/2
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x 1/2

x 1/10n+ doping concentration
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Rimp

Rimpp++Cbulk
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Summary and plan
HPK Fabrication
JPY2019

JPY2020

Nov. : First AC-LGAD design

Oct. : New parameters

improve

New parameterization
Reception

JPY2022

Former TCAD simulation

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Readout
Amplifire

Measurement of Pad and Strip sensor
Understanding Signal and crosstalkfeatureSamples
Time resolution
Radiation tolerance CYRIC irrad.

improve

Large scale ACLGAD (20mm x 20mm)

Pixel sensor & rad hardness
CYRIC irrad.

Testbeam

Testbeam

Sensor Production
JPY2023

US Collaborators
BNL AC-LGAD
SampleComparison

Rec. in Mar

Rec. in Mar
JPY2021

KEK/Tsukuba measurement

Solder Bump bonding(SnAg bump bonding developedby ATLAS experiment)
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Feb. : Testbeam

Si-Ge
BipolarASIC
Si-Ge
BipolarASIC

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backup29nd Mar, 2021

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Photo29nd Mar, 2021

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Leakage current vs Bias voltage

High p+ doping

High p+ doping
High p+ doping

C-2 type :– Lower operational voltage– Smaller crosstalk
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Variation of p+ and n+ doping concentration
N+ doping concentration / resistivity

p+ doping

Break down (gain) voltage get lower• Higher P+ dope
• Lower N+ dope
→ Radiation trelanceC(Ax10 resistivity)
3 (high)P+ dopingconcentration

B(Ax3.3 resistivity)

A (~DC-LGAD)

B-3

A-3
A-2

2 (mid)C-2

B-2

1 (low)

C-1

B-1

n+ doping
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Pulse Height and Bias Voltage dependence
Pedestal distribution is evaluated from off timing region
On timeExample : 190V Bias Voltage
Off time

PAD : C-2 typeOn time event

Operation region
Large gain increase→ Similar to leak current

Off time event

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Noise increasea bit later

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20How to reduce “Accepter Removal” effect?
• Study by FBK LGAD sensors.

1. Lower diffusion of Boron doping profile
2. Adding carbon (or Gallium) to p+ layer
Not simple changes… (HPK)

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21Position dependent mean amplitude

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22Radiation Tolerance• Radiation Tolerance

将来ハドロンコライダー(FCC-hh)

Timing resolution

Current : 0.6-2 x 1015neq/cm2
Goal : 1x1016neq/cm22x1015neq/cm26x1014neq/cm2
30ps

Idea for improvement?
Thinner active thickness and optimized p+ doping

Fluence
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23Radiation environment• Expected radiation level for 4000fb-1– Non Ionizing Energy Loss (NIEL):• 3rd layer: 2.8x1015 neq /cm2 1st layer : 2.6x1016neq/cm2– Total Ionizing Dose (TID) :• 3rd layer : 1.6MGy 1st layer : 19.8MGy

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Could replace detector
at the middle of runs.

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Accepter removal

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新型LGAD検出器(AC-LGAD)以前の検出器• 電極の細密化が課題– 以前の検出器は80umピッチのストリップ型で有感領域が20um程度– 各ストリップごとで増幅層が独立(大きくすると電場が不安定)– AC-LGAD : 一つの増幅層で、AC電極を配置→n+のドープ量を減らして抵抗値を高くしてクロストークを減らす。
➔n+とp+のドープ量で増幅率が決まるので最適化が必要
AC-LGADの検出器

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TCHoU symposium27時間分解能の測定• 二枚の同種のセンサーを上下に配置– 時間差分布の標準偏差: σ(T1-T2)= (σ1)2+(σ2)2– 同種のセンサーなので: σt = σ(T1-T2)/ 𝟐
Pulse Height Distribution

Timing resolution with certain threshold

Time difference of pad 1 and 2

閾値0.06V

Time resolution
48.9±0.7 ps

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