Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Performance of LGADs and AC-LGADstowards 4D tracking
G. D’Amen1, W. Chen1, G. Giacomini1, L. Lavitola2,S. Ramshanker3, A. Tricoli11Brookhaven National Laboratory (US)2Universita’ degli studi Federico II (IT)3Oxford University (UK)
9 December 2019
CPAD INSTRUMENTATIONFRONTIER WORKSHOP 2019
University of Wisconsin-Madison
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
OutlineTime resolution - LGADI. Introduction to LGADs
II. LGAD response to 90Sr β−
III. Response to DT fast neutronsIV. Comparison with Geant4 simulationV. Response to 252Cf fast neutrons
Space & Time - AC-LGADVI. The AC-LGAD concept
VII. Characterization with IR laser and 90Sr
Conclusions and Future activities
LGAD wafer (BNL)
AC-LGAD matrix (BNL)2 / 21
Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Low Gain Avalanche DiodeIntroduction
Low Gain Avalanche Diode (LGAD): highlydoped layer of p-implant (Gain layer) near p-njunction creates a high electric field thataccelerates electrons enough to startmultiplication.
I Electric Field: ∼300 kV/cm in Gain LayerI Silicon-based technology with low,
adjustable gain (2 - 100)I Breakdown Voltage ∝ Gain parameters
(dose, energy)I High Signal/Noise ratioI Ability to achieve fast-timing O(20-30) ps
in high radiation environments
Efield
Questions to be answered:I MIPs detection capabilities already proven,
fast neutron response to be characterizedI How fast is the response to fast neutrons?I What are out limits of detectable neutron
energy?3 / 21
Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
LGAD structure
Wafer structure (W1836,W1837,W1840)
I 1×1 mm2 sensor sizeI 50 µm 28Si p epitaxial layer, 10B and 11B doped
(7×1013cm−3)I Different doping concentrations (3, 3.25 and 2.7×1013cm−3) and gain layer thickness
I 500 µm substrateI Aluminum thin layerI Silicon Oxide SiO2
I n++ layer, 31P dopedI Gain p+ layer, 11B doped
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
90Sr interactionsSignal waveforms
Waveforms from β− 90Sr signals
> W1836, W1837, W1840 show narrowpeaks with widths O(1 ns)
> Sensors Gain for β− compatible to that ofX-rays
> σj = 〈σnoise
(dVdt
)−1〉 ∼ 20 ps
Sensor Gain (X-Ray):W1836: ∼ 15W1837: ∼ 20W1840: ∼ 25
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Deuterium-Tritium neutron generator
BNL Thermo-Fisher MP 320 Neutron Generator (prototype)3T +2 D →4 He+ n(14.1 MeV ) (1)
Neutron energy spectrum very narrow σE = O(10−2 MeV) and isotropic, with estimated neutronproduction of 6×107 neutrons/sec, with a flux of 7×104 neutrons/(cm2 sec) at sensor position
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Fast Neutron interactionsSignal waveforms
Waveforms from neutron signals (Vtrig = 10mV)
> W1836, W1837, W1840 show narrowpeaks with widths O(1 ns)
> Sensor Gain for neutrons compatible to theone measured with X-rays
> σj = 〈σnoise
(dVdt
)−1〉 ∼ 20 ps
Sensor Gain (X-Ray):W1836: ∼ 15W1837: ∼ 20W1840: ∼ 25
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Fast Neutron interactionsDeposited Energy distributions
Energy deposited by the neutron interactioncomputed as integral of each signal:
Edep [eV ] =3.6 [eV ]
Gn Rfb qe
∫wf
Adt
Sensitive Range in deposited energy (∝ (Gn)),limited by trigger voltage and maximumsignal amplitude in oscilloscope window.
For a 10 mV trigger level and Gn = 15,sensitivity to neutron signals with depositedenergy as low as ∼ 30 keV.
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Generated energy spectrum
Distribution of energy deposited by DT neutroninteractions as simulated by Geant4 shows goodagreement with experimental data from W1836in the sensor sensitive range Edep = [30, 450] keV
Superimposing Edep distributions generated byneutrons with different energies can give us anestimate of minimum neutron energy sensitivity
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Neutron energy sensitivity
Extrapolation of sensitivity to various neutron energies based on 14.1 MeV data
W1836 sensitivity (according to 14.1 MeVdeposited E distribution) to 300- and 500- keV
neutrons
W1836 sensitivity (according to 14.1 MeVdeposited E distribution) to 20 MeV neutrons
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Californium 252 Decays
252Cf decay scheme:
- ∼ 96% Alpha decay
- ∼ 3% Spontaneous Fission (SF) (n, γ)
- < 1% rare decays
Energy spectrum (SF):
> Neutrons: Landau(µ = 2 MeV, σ = 0.5 MeV)
> Photons: Landau(µ = 400 keV, σ = 100 keV)
> α: either 6.076 MeV or 6.118 MeV, entirelyabsorbed
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
LGAD sensitivity to 252CfUnshielded sensor (Geant4 simulation)
Spontaneous Fission photon flux ∼ 8/3 neutronflux. Lead shielding should decrease γpopulation.
Lead shielding (2.5 cm) (Geant4 simulation)
• Edep < 80/90 keV Photon dominated
• Edep = 90 - 200 keV Photon/Neutronpopulation
• Edep > 200 keV Neutron dominated12 / 21
Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
LGAD sensitivity to 252Cf
Distribution of energy deposited by 252Cfneutron and photon interactions assimulated by Geant4 shows goodagreement with experimental data fromW1840 in the sensor sensitive range Edep
= [15, 140] keV (photon dominated)
Jitter from Cf signals ∼ 20 ps, compatibleto DT and MIPs.
Additional data covering mixed- andneutron- dominated regions are beingcollected as we speak.
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
4D detectors: AC-LGAD tests with IR laser and 90Sr
> The AC-LGAD concept> LGAD vs AC-LGAD comparison> Cross-Talk studies> Timing performance
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
AC-LGADconcept
LGAD limits:I Dead volume (local gain ∼ 1)
within the implanted region ofthe gain layer
I Pixels/strips (pitch ∼ 100 mm)with gain layer below the implanthave a Fill Factor «100%
I Good for timing, hardly for 4Dreconstruction
AC-LGAD goals:I ∼ 100% Fill Factor and fast timing information at a
per-pixel level achievedI Signal generated by drift of multiplied holes into the
substrate but AC-coupled through dielectricI Electrons collect at the resistive n+ and then slowly
flow to a ohmic contact at the edge.
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
AC-LGADSignal comparison with LGADs
I Sensor wire-bonded to 16 channel Trans-impedanceAmplifier board by FermiLab
I AC-LGAD: 3×3 pixel matrix, 200µm × 200µmAC-coupled pads bonded to TAs
I LGAD: same AC-LGAD device where the n++ isread-out by the TA (same bias conditions and gain)
I Comparison of pulse amplitudes of betas from 90Sr.I Essentially equal distribution (same gain) for LGAD
and AC-LGAD AmplitudesI Is this signal well spatially localized? Need to
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Cross-talkStrip Map
Cross-talk measured as ratio between signal amplitudepeaks in different strips
Crosstalkratio A2/A1 100%ratio A3/A1 13%ratio A4/A1 6%ratio A6/A1 4%
Response of a single strip asa function of shining positionof IR or red laser (TCTscan).
Border effect: n++ is a lowresistance path that couplesthe signals back to the stripunder measure.
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Cross-talkPixel Map
Cross-talk measured as ratio between signal amplitudepeaks in different pixels
Dose n+ 1/100 Dose n+ 1/10ratio A5/A1 7% 9%ratio A9/A1 11% 16%
Response of a single pixel asa function of shining positionof IR or red laser (TCTscan).
Border effect: n++ is a lowresistance path that couplesthe signals back to the pixelunder measure.
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Timing Resolution
I AC-LAGDs and LGADs show similar response(waveforms)→ expected ∼ same timingperformance
I Using beta signals from a 90Sr source on AC-LGADslead to estimated σjitter ∼20 ps
I NEXT: Measuring timing resolution in coincidenceswith a trigger sensor, using 3D-printed Beta Scopesetup ready with ∼ 180 MBq 90Sr source
I Developed a setup such that our probe station canoperate both at room temperature and at -30◦Cwhich will be used for pre/post irradiation IV andCV scans
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Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Conclusions
LGADs can be used to detect neutrons in the 100s keV - MeV (and beyond?)energy range in high flux conditions for applications where fast time (∼20 - 30ps) measurements are needed
Fast timing for fast neutrons ensured by jitter measurement of O(20) ps
Good agreement between data and G4 simulation; extrapolations from Geant4simulations shows potential sensitivity to neutrons with energies <100 keV
By changing a few photolithographic masks and tuning process flow parameters,AC-LGADs have been fabricated as well
Precision space resolution (50-100 µm) available with AC-LGAD technology
Cross-talk and time resolution tested with mips and TCT, leading to positiveresults 20 / 21
Performance of LGADs and AC-LGADs towards 4D trackingIntroduction Data Geant4 Simulation Californium AC-LGADs Conclusions
Additional info/links
I G. Giacomini, W. Chen, F. Lanni, and A. Tricoli, Development of a technologyfor the fabrication of Low-Gain Avalanche Diodes at BNL
I G. Giacomini, W. Chen, G. D’Amen, A. Tricoli, Fabrication and performanceof AC-coupled LGADs
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Performance of LGADs and AC-LGADs towards 4D tracking
Backup
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Performance of LGADs and AC-LGADs towards 4D tracking
Motivation
Low-Gain Avalanche Diodes (LGAD) are gathering interest in thePhysics community thanks to fast-timing and radiation-hardness:
I HEP: ATLAS (HGTD) and CMS (MTD) timing detectors atthe HL-LHC
I NASA: neutron flux studiesI Medical Imaging: PET scansI Quantum information, Nuclear and forward physics,
etc...
MIPs detection capabilities already proven, investigating theresponse to neutrons in the O(MeV) region (fast neutrons)
Wafer of LGADs produced at BNL
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Performance of LGADs and AC-LGADs towards 4D tracking
Sensor gain computationSignals max amplitude Max amplitude scaled by Gain (normalized)
Distributions of maximum signal amplitude (left) aredivided by the sensor gain Gn (right), as obtainedfrom X-ray measurements.
• Sensor Gain:W1836: ∼ 15W1837: ∼ 20W1840: ∼ 25
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Performance of LGADs and AC-LGADs towards 4D tracking
Slew Rate
Average signal NoiseI W1836: (0.39±0.54) mVI W1837: (0.10±0.43) mVI W1840: (0.19±0.5) mVI W1849: (-0.11±0.42) mV
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Performance of LGADs and AC-LGADs towards 4D tracking
Sensitive range
Full width at half maximum (normalized)
Sensitive region limited by trigger voltage(10 mV for W1836, W1837, W1840, 3.5 mVfor W1849) and maximum signal amplitudein oscilloscope window.
Energy distributions constrained in regionbetween:
Ith =√2π Vth
〈FWHM〉2.355
with V minth = trigger level and V max
th = maxwindow amplitude
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Performance of LGADs and AC-LGADs towards 4D tracking
Signal waveforms
Waveforms acquired withTektronix MSO64 mixed-signalsoscilloscope;
W1836, W1837, W1840 (50 µm)show narrow peaks with widthsO(1 ns), while W1849 (300 µm)produces longer (∼ 8 times)signals.
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Performance of LGADs and AC-LGADs towards 4D tracking
Sensor gain computationSignals max amplitude Max amplitude scaled by Gain (normalized)
Distributions of maximum signal amplitude (left) aredivided by the sensor gain Gn (right), as obtainedfrom X-ray measurements.
• 50 µm Gain:W1836: ∼ 15W1837: ∼ 20W1840: ∼ 25
• 300 µm Gain:
W1849: ∼ 10
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Performance of LGADs and AC-LGADs towards 4D tracking
Jitter measurementJitter is an important component of the timeresolution of the sensor and is computed as ratiobetween the noise (∼0.5 mV for all the sensors)and slew rate (dV/dt):
σj = 〈σnoise
(dV
dt
)−1
〉
Sensor Gain Jitter [ps]W1836: ∼15 14.8 ± 3.6
W1837: ∼20 17.5 ± 4.3
W1840: ∼25 21.3 ± 4.3
W1849: ∼10 222.4 ± 42.7
Slew rate (normalized)
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Performance of LGADs and AC-LGADs towards 4D tracking
Deposited Energy distributions300 µm sensor comparison
W1849 (300µm) has been compared to the 50µm sensors:
I Compatible shape in the sensitive rangeafter gain correction
I Higher detection efficiency (×54 timesvolume)
I Different minimum threshold of sensitiverange:Emin
dep =∼ 30keV (50µm) vs ∼ 200keV(300µm)
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Performance of LGADs and AC-LGADs towards 4D tracking
Characterization of neutron processes
I Neutron Elastic interactionsignificant for 14 MeV neutroninteractions with depositedenergy up to ∼ 1.85 MeV
I Neutron Inelastic interactiondominant contribution for highdeposited energies
I In the range Edep = [30, 450] keVminimal contributions fromphotons and electronselectromagnetic processes(ionization, Compton effect,photoelectric effect) and decays
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Performance of LGADs and AC-LGADs towards 4D tracking
Scan of neutron energy sensitivity
Distributions of deposited energyfor neutrons with:
I K = 10/100 keV(top-left)
I K = 200/300 keV(top-right)
I K = 500/700 keV(bottom-left)
I K = 1 MeV(bottom-right)
for Trigger threshold 10 mV andGn = 15, expected sensitivity to300 keV neutrons
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Performance of LGADs and AC-LGADs towards 4D tracking
LGAD structure
Wafer structure (W1836,W1837,W1840)
I 1×1 mm2 sensor sizeI 50 µm 28Si p epitaxial layer, 10B and 11B
doped (7×1013cm−3)I Different doping concentrations (3, 3.25 and
2.7 ×1013cm−3) and gain layer thickness
I 500 µm substrateI Aluminum thin layer, thickness 0.5 µmI Silicon Oxide SiO2, thickness 0.3 - 0.5 µmI n++ layer, 31P doped, thickness 0.5 µmI Gain p+ layer, 11B doped, thickness 0.5 µm
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Performance of LGADs and AC-LGADs towards 4D tracking
Geant4 simulationIntroduction
Sensor response modelled with Geant4 10.4MonteCarlo simulation software
Simulation parameters:I QGSP_BIC_HP physics list used for
high precision simulation of neutrons ≤ 20MeV
I 10 million 14.1 MeV neutrons generatedeach simulation run with randomized initialdirection
I 1.6 mm of 27Aluminum interposed betweenneutron generator and sensor, to simulatethe Deuterium-Tritium generator casing
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Performance of LGADs and AC-LGADs towards 4D tracking
AC-LGAD characterizationIV-curve
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Performance of LGADs and AC-LGADs towards 4D tracking
AC-LGADFabrication at BNL
Process:I Process starts from a Std (DC-) LGAD PadI Change METAL (Aluminum) and thus ContactsI n++ runs at the periphery only; replaced by resistive
n+ in the active area with 10/100 less doseI Thin insulator (100 nm SiN ) over the n+
Std-LGAD Pad:
AC-LGAD Pixels:
AC-LGAD Strips:
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Performance of LGADs and AC-LGADs towards 4D tracking
Near future plans
• Lower trigger threshold from10 mV to 2 mV (×4 averagenoise); expected sensitivity toEn < 100 keV:
• Edep th @10mVW1836: ∼30 keVW1837: ∼20 keVW1840: ∼22 keV
• Edep th @2mVW1836: ∼6 keVW1837: ∼4 keVW1840: ∼4 keV
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Performance of LGADs and AC-LGADs towards 4D tracking
Limits of LGADs
Lateral dimensions of Gain layer must be much larger than thickness of substrate, to createuniform multiplication.Dead volume (local gain ∼ 1) extends within the implanted region of the gain layer:
I Pixels/strips (pitch ∼ 100 mm) with gain layer below the implant have a Fill Factor «100%(Voltage dependent)
I Large pads (∼ 1 mm) are preferred (e.g. ATLAS HGTD or CMS MTD)I Good for timing, hardly for 4D reconstructionI Various possible ways to overcome this issue with different geometries
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Performance of LGADs and AC-LGADs towards 4D tracking
AC-LGADconcept
Main differences w/r to LGADs:
1. One large low-doped high-ρ n+
implant running overall the activearea, instead of a high-dopedlow-ρ n++
2. Thin insulator over the n+, wherefine-pitch electrodes are placed,patterned over the insulator
Expected Results:I ∼ 100% Fill Factor and fast timing information at a
per-pixel level achievedI Signal generated by drift of multiplied holes into the
substrate but AC-coupled through dielectricI Electrons collect at the resistive n+ and then slowly
flow to a ohmic contact at the edge.
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Performance of LGADs and AC-LGADs towards 4D tracking
Fast Neutron interactionsJitter measurement
Jitter is an important component of the timeresolution of the sensor; computed as ratiobetween the noise (∼0.5 mV for all the sensors)and slew rate (dV/dt):
σj = 〈σnoise
(dV
dt
)−1
〉 ∼ 20 ps
Slew rate (normalized)
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Performance of LGADs and AC-LGADs towards 4D tracking
AC-LGADSignal Sr90
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