CMS Pixel Simulation Vincenzo Chiochia Physik Institut der Universität Zürich-Irchel CH-8057...

CMS Pixel SimulationCMS Pixel Simulation

Vincenzo Chiochia

Physik Institut der Universität Zürich-Irchel CH-8057 Zürich (Switzerland)

PIXEL 2005 WorkshopChuzenji Lake, Nikko (Japan)

November 7-11, 2005

Vincenzo Chiochia

Physik Institut der Universität Zürich-Irchel CH-8057 Zürich (Switzerland)

PIXEL 2005 WorkshopChuzenji Lake, Nikko (Japan)

November 7-11, 2005

Quarter pixel: Electrostatic potential

V. Chiochia – CMS Pixel Simulation – Vertex 2005 - Chuzenji Lake, Nikko, Japan , November 7-11, 2005 2

OutlineOutline

1.1. The CMS pixel detectorThe CMS pixel detector

2.2. Radiation damage and impact on reconstructionRadiation damage and impact on reconstruction

3.3. Beam test data and detailed sensor simulationBeam test data and detailed sensor simulation

4.4. Physical modelling of radiation damage:Physical modelling of radiation damage:

a)a) Models with a constant space charge densityModels with a constant space charge density

b)b) Double-trap modelsDouble-trap models

5.5. Fluence and temperature dependence Fluence and temperature dependence


~1 m0.3 m

The CMS pixel detectorThe CMS pixel detector3-d tracking with about 66 million channels

Barrel layers at radii = 4.3cm, 7.2cm and 11.0cm

Pixel cell size = 100x150 µm2

704 barrel modules, 96 barrel half modules, 672 endcap modules

~15k front-end chips and ~1m2 of silicon


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 the first years of operation?


Radiation effectsRadiation effects

B field

chargewidth

chargewidth

after irradiation

r-plane r-zplane

no B fieldE

Irradiation modifies the electric field profile:

varying Lorentz deflection

Irradiation causes charge carrier trapping

chargecharge

after irradiation

E


Prototype sensors for beam testsPrototype sensors for beam tests

nn-in--in-nn type type withwith moderated p-spray moderated p-spray isolation, isolation, biasing grid and punch through structures biasing grid and punch through structures (producer: CiS, Germany)(producer: CiS, Germany)

285 285 μμm thick <111> DOFZ waferm thick <111> DOFZ wafer

125x125 125x125 mm22 cell size, 22x32 pixel matrix cell size, 22x32 pixel matrix

Samples irradiated with 21 GeV protons at Samples irradiated with 21 GeV protons at the CERN PS facilitythe CERN PS facility

Fluences: Fluences: eqeq=(0.5, 2.0, 5.9)x10=(0.5, 2.0, 5.9)x101414 nneqeq/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


Beam test setupBeam 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

Data collected at CERN in 2003-2004Data collected at CERN in 2003-2004


Charge collection measurementCharge collection measurement

Charge collection was measured using cluster profiles in a row of pixels illuminated by a 15º beam and no magnetic field

Charge collection was measured using cluster profiles in a row of pixels illuminated by a 15º beam and no magnetic field

½ year LHC low luminosity

eq = 5×1013 n/cm2

2 years LHC low luminosity

n+side p-side

2 years LHC high luminosity

AGEING

eq = 2×1014 n/cm2 eq = 6×1014 n/cm2


Detector simulationDetector simulation

Chargedeposit

Chargetransport

Trapping

Trapping times from

literature

Electronicresponse +

data formatting

ROC+ADC response

ROOTAnalysis

Double trapsmodels

(DESSIS)

3-D Electric field mesh

ISE TCAD 9.0ISE TCAD 9.0

PIXELAVPIXELAV

M.Swartz, M.Swartz, Nucl.Instr. MethNucl.Instr. Meth. A511, 88 (2003);. A511, 88 (2003);V.Chiochia, M.Swartz et al., V.Chiochia, M.Swartz et al., IEEE Trans.Nucl.SciIEEE Trans.Nucl.Sci. 52-4, p.1067 (2005).. 52-4, p.1067 (2005).


The classic pictureThe classic picture

after type inversion

Neff=ND-NA<0

- Based on C-V measurements!Based on C-V measurements!

After irradiation the sensor bulk becomes more acceptor-like

The space charge density is constant and negative across the sensor thickness

The p-n junction moves to the pixel implants side

Sensors may be operated in “partial depletion”

…is all this really true?


Models with constant NModels with constant Neffeff

AA model based on a type-inverted device with model based on a type-inverted device with constant space charge densityconstant space charge density across the bulk across the bulk does not describedoes not describe the measured charge collection profiles the measured charge collection profiles

= 6x1014 n/cm2 = 6x1014 n/cm2


Two traps modelsTwo traps models

EConduction

EValence

Electrontraps

Holetraps

1.12 eVdonor D

hDeD σ,σ ,NEV+0.48 eV

acceptor

Ah

AeA σ,σ ,NEC-0.525 eV

Model parameters(Shockley-Read-Hall statistics):

EA/D = trap energy level fixedNA/D = trap densities extracted from fite/h = trapping cross sections extracted from fit


a) Current density

b) Carrier concentration

c) Space charge density

d) Electric field

The double peak electric fieldThe double peak electric field

dopantsAADD

dopantsAD A

ADDeff

ρfNfNe

ρfNefNeρ

dopantsAADD

dopantsAD A

ADDeff

ρfNfNe

ρfNefNeρ

n+p junction

np+ junction

-HV

p-like

n-like

There areP-N junctions at bothsides of the detector

detector depth detector depth


Model constraintsModel constraints

The two-trap model is constrained by:

1. Comparison with the measured charge collection profiles

2. Signal trapping rates varied within uncertainties

DDhheqhhh

AAeeeqeee

NσvΦβ1/τΓ

NσvΦβ1/τΓ

DDhheqhhh

AAeeeqeee

NσvΦβ1/τΓ

NσvΦβ1/τΓ

t

τ

1expQ(t)Q

e/hh0e,he,

t

τ

1expQ(t)Q

e/hh0e,he,

3. Measured dark current

ADj

kTEi

jhh

kTEi

jee

iDje

jheh

jj enpvennv

nnpNvvI

,//

2

)()(

)(

ADjkTE

ijhh

kTEi

jee

iDje

jheh

jj enpvennv

nnpNvvI

,//

2

)()(

)(

Typical fit iteration: (8-12h TCAD) + (8-16h PIXELAV)xVTypical fit iteration: (8-12h TCAD) + (8-16h PIXELAV)xVbias bias + ROOT analysis+ ROOT analysis


Fit results Fit results

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

Electric field andspace charge density

profiles

Electric field andspace charge density

profiles


Scaling to lower fluencesScaling to lower fluences

Near the ‘type-invesion’ point: the double peak structure is still visible in the data!Profiles are not described by thermodynamically ionized acceptors aloneAt 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


Space charge densitySpace charge density

Space charge density uniform before irradiation

Current conservation and non uniform carrier velocities produce a non linear space charge density

after irradiation

The electric field peak at the p+ backplane increases with irradiation

p-type

n-type

sensor depth (m)

V.Chiochia, M.Swartz,et al., physics/0506228V.Chiochia, M.Swartz,et al., physics/0506228


Temperature dependenceTemperature dependence

Comparison with data collected at lower temperature T=-25º C.

Use temperature dependent variables:– recombination in TCAD– variables in PIXELAV (, D,

)

The double-trap model is predictive!

2=2x1014 n/cm2

T=-25º C

2=2x1014 n/cm2

T=-25º C

1=6x1014 n/cm2

T=-25º C

1=6x1014 n/cm2

T=-25º C

V.Chiochia, M.Swartz,et al., physics/0510040V.Chiochia, M.Swartz,et al., physics/0510040


Lorentz deflectionLorentz deflection

BHL BEr sin)(tan

tan() linear in the carrier mobility (E):

LHC startup 2 years LHC low luminosity 2 years LHC high luminosity

Switching on the magnetic field

The Lorentz angle can vary a factor of 3 after heavy irradiation:This introduces strong non-linearity in charge sharing


Conclusions and plansConclusions and plans

After heavy irradiation trapping of the leakage current produces electric field profiles with two maxima at the detector implants. The space charge density across the sensor is not uniform.

What is the meaning of Vdep, depletion depth and type inversion? Measurements reflecting the electric field profile (e.g. TCT, CCE, long clusters etc.) are preferable to C-V characterization to understand radiation damage in running detectors

A physical model based on two defect levels can describe the charge collection profiles measured with irradiated pixel sensors in the whole range of irradiation fluences relevant to LHC operation

Our model is an effective theory: e.g. in reality there are several trap levels in the silicon band gap after irradiation. However, it is suited for applications related to silicon detector operation at LHC.

We are currently using the PIXELAV simulation to develop hit reconstruction algorithms optimized for irradiated pixel sensors.


ReferencesReferences

PIXELAV simulation:PIXELAV simulation:– M.Swartz, “M.Swartz, “CMS Pixel simulationsCMS Pixel simulations”, ”, Nucl.Instr.MethNucl.Instr.Meth. A511, 88 (2003). A511, 88 (2003)

Double-trap model:Double-trap model:– V.Chiochia, M.Swartz et al., “V.Chiochia, M.Swartz et al., “Simulation of Heavily Irradiated Silicon Pixel Sensors and Simulation of Heavily Irradiated Silicon Pixel Sensors and

Comparison with Test Beam MeasurementsComparison with Test Beam Measurements”, ”, IEEE Trans.Nucl.SciIEEE Trans.Nucl.Sci. 52-4, p.1067 (2005), . 52-4, p.1067 (2005), eprint:physics/0411143eprint:physics/0411143

– V. Eremin, E. Verbitskaya, and Z. Li, “V. Eremin, E. Verbitskaya, and Z. Li, “The origin of double peak electric field distribution The origin of double peak electric field distribution in heavily irradiated silicon detectorsin heavily irradiated silicon detectors”, ”, Nucl. Instr. Meth.Nucl. Instr. Meth. A476, pp. 556-564 (2002) A476, pp. 556-564 (2002)

Model fluence and temperature dependence:Model fluence and temperature dependence:– V.Chiochia, M.Swartz et al., “V.Chiochia, M.Swartz et al., “A double junction model of irradiated pixel sensors for A double junction model of irradiated pixel sensors for

LHCLHC”, submitted to ”, submitted to Nucl. Instr. Meth.,Nucl. Instr. Meth., eprint:physics/0506228 eprint:physics/0506228– V.Chiochia, M.Swartz et al., “V.Chiochia, M.Swartz et al., “Observation, modeling, and temperature dependence of Observation, modeling, and temperature dependence of

doubly-peaked electric fields in silicon pixel sensorsdoubly-peaked electric fields in silicon pixel sensors”, submitted to ”, submitted to Nucl. Instr. Meth.,Nucl. Instr. Meth., eprint:physics/0510040eprint:physics/0510040

22

Backup slidesBackup slides


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


E-field profilesE-field profiles

Constantspace chargedensity

Double trapmodel


Scaling to lower fluencesScaling to lower fluences

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!!


Temperature dependenceTemperature dependence

Charge collection profiles depend on temperatureT-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


Lorentz deflectionLorentz deflectionLorentz angle Electric field


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


ISE TCAD simulationISE TCAD simulation


PIXELAV simulation PIXELAV simulation


SRH statisticsSRH statistics


SRH generation currentSRH generation current

ADj

kTEi

jhh

kTEi

jee

iDje

jheh

jj enpvennv

nnpNvvI

,//

2

)()(

)(


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

CMS Pixel Simulation Vincenzo Chiochia Physik Institut der Universität Zürich-Irchel CH-8057...

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