Charge collection in irradiated pixel sensors V. Chiochia a, C.Amsler a, D.Bortoletto c, L.Cremaldi...

Charge collection in Charge collection in irradiated pixel sensorsirradiated pixel sensors

V. Chiochiaa, C.Amslera, D.Bortolettoc, L.Cremaldid, S.Cucciarellie, A.Dorokhova,b*, C.Hörmanna,b, M.Koneckie, D.Kotlinskib, K.Prokofieva,b, C.Regenfusa, T.Roheb, D.Sandersd, S.Sonc, T.Speera, D.Kimf, M.Swartzf

a Physik Institut der Universität Zürich-Irchel, 8057, Zürich, Switzerlandb Paul Scherrer Institut, 5232, Villingen PSI, Switzerland

c Purdue University, Task G, West Lafayette, IN 47907, USAd Department of Physics and Astronomy, Mississippi State University, MS 39762, USA

e Institut für Physik der Universität Basel, Basel, Switzerlandf Johns Hopkins University, Baltimore, MD, USA

* Now at: Institut de Recherches Subatomiques, F67037 Strasbourg, France

Beam test measurements and simulationBeam test measurements and simulation

V. Chiochia – Charge collection in irradiated pixel sensors10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005

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OutlineOutline

1.1. The CMS pixel detector and data reconstructionThe CMS pixel detector and data reconstruction

2.2. Analysis ingredients: beam test data and detector simulationAnalysis ingredients: beam test data and detector simulation

3.3. Physical modelling of radiation damage:Physical modelling of radiation damage:a)a) Models with a constant effective doping concentrationModels with a constant effective doping concentration

b)b) EVL models (V.Eremin, E.Verbitskaya, Z.Li)EVL models (V.Eremin, E.Verbitskaya, Z.Li)

c)c) Advanced double junction models (V.C., M.Swartz)Advanced double junction models (V.C., M.Swartz)

4.4. Conclusions Conclusions


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The CMS pixel detectorThe CMS pixel detector• 3-d tracking with 66 million channels • Barrel layers at radii = 4.3cm, 7.2cm and 11.0cm• Pixel cell size: 100x150 µm2

• Fluence 3(1)x1014 neq/cm2 year, inner layer for high(low) luminosity• Modules are unit cells of the system (1% of X0)• 704 barrel modules / 96 barrel half modules / 672 endcap modules • ~15k front end chips and ~1m2 of silicon


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The LHC radiation environmentThe LHC radiation environment

Fluence per year at full luminosity

• 4 cm layer 3x1014 n/cm2/yr• Fluence decreases quadratically

with the radius• Pixel detectors = 4-15 cm mostly

pion irradiation• Strip detectors = 20-110 cm

mostly neutron irradiation

ppinelastic = 80 mb

L = 1034 cm-2s-1

ppinelastic = 80 mb

L = 1034 cm-2s-1

What is the sensors response after few years of operation?


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Impact on reconstructionImpact on reconstruction

Sensor irradiation

Charge Carriers Trapping

Asymmetric pixel clusters

Example: Long clusters along

the z-coordinate at high

Variation of the electric field profile

Lorentz deflection

Example:Non-linear charge sharing

in the r- plane


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Prototype sensors for beam testsPrototype sensors for beam tests

• p-sprayp-spray design with biasing grid and punch design with biasing grid and punch through structures (CiS, Germany)through structures (CiS, Germany)

• 125x125 125x125 mm22 cell size cell size• 22x32 pixel matrix, 285 22x32 pixel matrix, 285 μμm thick <111> m thick <111>

DOFZ wafer, DOFZ wafer, nn-in--in-nn type type• Samples irradiated with 21 GeV protons at Samples irradiated with 21 GeV protons at

the CERN PS facilitythe CERN PS facility

• Fluences: Fluences: eqeq=(0.47,2.0,5.9)x10=(0.47,2.0,5.9)x101414 n neqeq/cm/cm22

• AnnealedAnnealed for three days at 30 for three days at 30º Cº C• Bump bonded at room temperature to non Bump bonded at room temperature to non

irradiated front-end chips with irradiated front-end chips with non zero-non zero-suppressed readoutsuppressed readout, stored at -20, stored at -20ºCºC

125 m2


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2004 Beam test setup2004 Beam test setup

CERN Prevessin siteH2 area

3T Helmoltz magnet

beam:150 GeV B field

pixel sensorsupport

Silicon strip beam telescope:50 μm readout pitch,~1 μm resolution

Cooling circuitT =-30 ºC or -10ºC


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Charge collection measurementCharge collection measurement

Charge collection was studied with the cluster profiles in a row of pixels illuminated by a 15º beam and no magnetic field

Charge collection was studied with the cluster profiles in a row of pixels illuminated by a 15º beam and no magnetic field

½ year LHC low luminosity

2 years LHC low luminosity

n+side p-side

chargetrapping

chargetrapping

2 years LHC high luminosity

Temperature = -25 ºC and -10ºCeq = (0, 0.5, 2, 6)x1014 n/cm2


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Detector simulationDetector simulation

Chargedeposit

Chargetransport

Trapping

Trapping times from

literature

Electronicresponse +

data formatting

ROC+FED response

ROOTAnalysis

Double trapsmodels

(DESSIS)

3-D Electric field mesh

ISE TCAD 9.0ISE TCAD 9.0

PIXELAVPIXELAV


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The classic pictureThe classic picture

• After irradiation the sensor bulk becomes more acceptor-like

• The effective doping concentration is constant (and negative) across the sensor thickness

• The p-n junction moves to the pixel implants side

• Based on C-V measurements!

after type inversion

Neff<0

-


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Models with constant NModels with constant Neffeff

AA model based on a type-inverted device with constant Nmodel based on a type-inverted device with constant Neffeff

across the bulk does not describe the measured charge collection profilesacross the bulk does not describe the measured charge collection profiles

= 6x1014 n/cm2


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Two traps modelsTwo traps models

EConduction

EValence

Electrontraps

Holetraps

Eremin-Verbitskaya-Li Model (EVL)Eremin-Verbitskaya-Li Model (EVL)

NA and ND are fixed to TCT measurements

215Ah

Ae

Dh

De cm10σσσσ

1.12 eVdonor D

hDeD σ,σ ,NEV+0.48 eV

acceptor

Ah

AeA σ,σ ,NEC-0.525 eV

Given these parameters the charge carriers dynamics is governed by the Shockley-Read-Hall statistics


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The double peak electric fieldThe double peak electric field

n+p junction

np+ junction

-HV

a) Current density

b) Carrier concentration

c) Effective doping concentration

d) Electric field

dopantsAADD

dopantsAA

ADD

Deff

ρfNfNe

ρfNefNeρ

p-like

n-like


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EVL modelsEVL models

100% observedleakage current=1.5x10-15 cm2

30% observedleakage current=0.5x10-15 cm2

The EVLThe EVL model based on double traps can produce large tails model based on double traps can produce large tails but description of the data is still unsatisfactorybut description of the data is still unsatisfactory

1=6x1014 n/cm21=6x1014 n/cm2


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Advanced EVL modelsAdvanced EVL models

• The recipe: 1. Relax the assumption on the cross sections

2. Let the parameters (NA, ND, A/De, A/D

h) vary

3. Keep the traps energy levels (EA, ED) to the EVL values

• Constraints to the model:1. Charge collection profiles (at different Vbias and eq)2. Trapping rates 3. Generated leakage current

DDhheqhhh

AAeeeqeee

NσvΦβ1/τΓ

NσvΦβ1/τΓ

DDhheqhhh

AAeeeqeee

NσvΦβ1/τΓ

NσvΦβ1/τΓ

e/h from literature

eq known within 10%

e/h from literature

eq known within 10%

t

τ

1expQ(t)Q

e/hh0e,he,

t

τ

1expQ(t)Q

e/hh0e,he,


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Best fit: Best fit: 11=6x10=6x101414 n/cm n/cm22

Data--- Simulation

Data--- Simulation

1=6x1014 n/cm2

NA/ND=0.40h/e=0.25

1=6x1014 n/cm2

NA/ND=0.40h/e=0.25

profiles field E


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Temperature dependenceTemperature dependence

• Charge collection profiles depend on temperature

• T-dependent recombination in TCAD and T-dependent variables in PIXELAV (e/h, e/h, ve/h)

• The model can predict the variation of charge collection due to the temperature change

T=-10ºCT=-10ºC T=-25ºCT=-25ºC

1=6x1014 n/cm21=6x1014 n/cm2


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Scaling to lower fluences (1)Scaling to lower fluences (1)

2=2x1014 n/cm2

NA/ND=0.68

h/e=0.25

Dh/D

e=1.00

2=2x1014 n/cm2

NA/ND=0.68

h/e=0.25

Dh/D

e=1.00

)(Φδ)N(1R)(ΦNR)(ΦN

)(Φδ)N(1R)(ΦNR)(ΦN2

RR/ΦΦR ; )(ΦΓR)(ΦΓ

1DΓ1DD2D

1AΓ1AA2A

DA12Γ1e/hΓ2e/h

Preserve linear scalingof e/h and of the current

with eq

Not shown: Linear scaling of trap densities does not describe the data!!


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Scaling to lower fluences (2)Scaling to lower fluences (2)

• Near the ‘type-invesion’ point: the double peak structure is still visible in the data!

• Profiles are not described by thermodynamically ionized acceptors alone

• At these low bias voltages the drift times are comparable to the preamp shaping time (simulation may be not reliable)

3=0.5x1014 n/cm2

NA/ND=0.75

h/e=0.25

Dh/D

e=1.00

3=0.5x1014 n/cm2

NA/ND=0.75

h/e=0.25

Dh/D

e=1.00

field E


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Scaling summaryScaling summary• Donors concentration increases faster than acceptors

• NA/ND increases for decreasing fluences

• Electric field peak at the p+ backplane increases with irradiation


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Lorentz angle vs depthLorentz angle vs depth

• Lorentz angle and electric field extracted from the test beam measurements

• The Lorentz angle is not constant across the sensor thickness

Lorentz angle Electric field


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Conclusions (1)Conclusions (1)

1. A simulation based on a constant effective doping (or “type inverted”) across the sensor bulk does not describe the measured charge collection profiles

2. A effective model based on two defect levels can be tuned to describe the observed charge collection profiles

3. Trapping of the leakage current produces an electric field profile with two maxima at the detector implants. Is it time to leave the classical notion of ‘partial depletion’?

4. The model can: • account for the expected leakage current and, within the uncertainties,

for free carriers trapping

• predict the temperature dependence of charge collection


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Conclusions (2)Conclusions (2)

5. In reality the chemistry of Si defects is more complicated and there are several trap states

6. The levels in this model have no physical reality and have to be considered as an ‘effective sum’ of multiple charged states

7. The simulation is a very nice tool for predicting the behavior of our pixel sensors during the operation in CMS. The hit reconstruction algorithms need to be fine tuned to cope with radiation effects


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ReferencesReferences

• V. Eremin, E. Verbitskaya, and Z. Li, “V. Eremin, E. Verbitskaya, and Z. Li, “The origin of double peak electric field The origin of double peak electric field distribution in heavily irradiated silicon detectorsdistribution in heavily irradiated silicon detectors”, ”, Nucl. Instr. Meth.Nucl. Instr. Meth. A476A476, pp. 556-, pp. 556-564 (2002)564 (2002)

• M.Swartz, “M.Swartz, “CMS Pixel simulationsCMS Pixel simulations”, ”, Nucl.Instr.MethNucl.Instr.Meth. A511, 88 (2003). A511, 88 (2003)

• V.Chiochia, M.Swartz et al., “V.Chiochia, M.Swartz et al., “Simulation of Heavily Irradiated Silicon Pixel Sensors Simulation of Heavily Irradiated Silicon Pixel Sensors and Comparison with Test Beam Measurementsand Comparison with Test Beam Measurements”, accepted for publication on ”, accepted for publication on IEEE Trans.Nucl.SciIEEE Trans.Nucl.Sci., eprint:physics/0411143., eprint:physics/0411143

• A.Dorokhov et al., A.Dorokhov et al.,

• ISE TCAD 9.0: http://www.synopsys.com/products/acmgr/ise/dessis_ds.html


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Backup slidesBackup slides


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CMS pixel sensor designCMS pixel sensor design

n+

p+

n- bulk

Si3N4

passivation

Si3N4

Alnitride

Indium-Bump

p+pspray

Alnitride + LTO

GoldGoldNickel

Titanium

punch-throughbiasing

Bump-bondcontact

n+/Alopening

cross section

metal line

Vendor: CiS, Erfurt - www.cismst.de


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Test beam setupTest beam setup

Magnetic field = 3 Tor

pixel sensorpixel sensor

• Four modules of silicon strip detectors

• Beam telescope resolution ~ 1 m

• Sensors enclosed in a water cooled box (down to -30ºC)

• No zero suppression, unirradiated readout chip

• Setup placed in a 3T Helmoltz magnet

PIN diode trigger


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ISE TCAD simulationISE TCAD simulation


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PIXELAV simulation PIXELAV simulation


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SRH statisticsSRH statistics


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SRH generation currentSRH generation current


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Lorentz angle vs biasLorentz angle vs bias

• ‘Effective’ Lorentz angle as function of the bias voltage

• Strong dependence with the bias voltage (electric field)

• Weak dependence on irradiation

• This is a simplified picture!!

Magnetic field = 4 T

Charge collection in irradiated pixel sensors V. Chiochia a, C.Amsler a, D.Bortoletto c, L.Cremaldi...

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