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Charge collection in irradiated pixel sensors. Beam test measurements and simulation.

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charge collection in irradiated pixel sensors

Charge collection in irradiated pixel sensors

Beam test measurements and simulation

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, Switzerlandc Purdue University, Task G, West Lafayette, IN 47907, USA

d Department of Physics and Astronomy, Mississippi State University, MS 39762, USAe Institut für Physik der Universität Basel, Basel, Switzerlandf Johns Hopkins University, Baltimore, MD, USA

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

outline
Outline
  • The CMS pixel detector
  • Radiation damage and pixel hit reconstruction
  • Analysis ingredients: beam test data and sensor simulation
  • Physical modelling of radiation damage:
    • Models with a constant space charge density
    • Double-trap models
  • Conclusions

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

the cms pixel detector
The CMS pixel detector

~1 m

0.3 m

  • 3-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

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

the lhc radiation environment
The LHC radiation environment
  • 4 cm layer F=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

sppinelastic = 80 mb

L = 1034 cm-2s-1

What is the sensors response after

few years of operation?

Fluence per year at full luminosity

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

radiation effects
Radiation effects

charge

charge

width

charge

width

charge

after irradiation

after irradiation

r-f plane

r-zplane

E

E

 B field

no B field

Irradiation modifies

the electric field profile:

varying Lorentz deflection

Irradiation causes

charge carrier trapping

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

prototype sensors for beam tests
Prototype sensors for beam tests

125 mm2

  • n-in-n typewith moderated p-spray isolation, biasing grid and punch through structures (producer: CiS, Germany)
  • 285 μm thick <111> DOFZ wafer
  • 125x125 mm2 cell size, 22x32 pixel matrix
  • Samples irradiated with 21 GeV protons at the CERN PS facility
  • Fluences: Feq=(0.5, 2.0, 5.9)x1014neq/cm2
  • Annealed for three days at +30º C
  • Bump bonded at room temperature to non irradiated front-end chips with non zero-suppressed readout, stored at -20ºC

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

beam test setup
Beam test setup

Silicon strip beam telescope:

50 μm readout pitch,~1 μm resolution

CERN Prevessin site

H2 area

beam:

150 GeV p

B field

pixel sensor

support

Cooling circuit

T =-30 ºC or -10ºC

3T Helmoltz magnet

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

slide8

Charge collection measurement

Feq = 2×1014 n/cm2

Feq = 6×1014 n/cm2

n+side

p-side

2 years LHC low luminosity

2 years LHC high luminosity

AGEING

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

Feq = 5×1013 n/cm2

½ year LHC low luminosity

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

detector simulation
Detector simulation

Charge

transport

ROOT

Analysis

Trapping

ISE TCAD 9.0

Double traps

models

(DESSIS)

3-D Electric

field mesh

Trapping times

from

literature

ROC+ADC

response

Electronic

response +

data formatting

Charge

deposit

PIXELAV

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

the classic picture
The classic picture
  • 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”

after type inversion

Neff=ND-NA<0

Based on C-V measurements!

-

…is all this really true?

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

models with constant n eff
Models with constant Neff

F = 6x1014 n/cm2

Amodel based on a type-inverted device with constant space charge density

across the bulk does not describe the measured charge collection profiles

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

two traps models
Two traps models

acceptor

EC-0.525 eV

donor

EV+0.48 eV

EConduction

Electron

traps

1.12 eV

Hole

traps

EValence

EA/D= trap energy level fixed

NA/D = trap densities extracted from fit

se/h = trapping cross sections extracted from fit

Model parameters

(Shockley-Read-Hall statistics):

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

the double peak electric field
The double peak electric field

n+p

junction

np+

junction

-HV

PN junctions at both

sides of the detector

p-like

n-like

c) Space charge density

a) Current density

detector depth

detector depth

b) Carrier concentration

d) Electric field

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

fit results
Fit results

Electric field and

space charge density

profiles

  • Data

--- Simulation

F1=6x1014 n/cm2

NA/ND=0.40

sh/se=0.25

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

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

F3=0.5x1014 n/cm2

NA/ND=0.75

sAh/sAe=0.25

sDh/sDe=1.00

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

scaling summary
Scaling summary
  • Donors concentration increases faster than acceptors
  • NA/ND increases for decreasing fluences
  • Electric field peak at the p+ backplane increases with irradiation

n-type

ND

space

charge density

p-type

NA

sensor depth (mm)

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

lorentz deflection
Lorentz deflection

LHC startup

2 years LHC low luminosity

2 years LHC high luminosity

Switching on the magnetic field

tan(L) linear in the carrier mobility (E):

The Lorentz angle can vary a factor of 3 after heavy irradiation:

This introduces strong non-linearity in charge sharing

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

conclusions and plans
Conclusions 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 constant.
  • 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 hitreconstruction algorithms optimized for irradiated pixel sensors. Results expected soon!

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

references
References
  • PIXELAV simulation:
    • M.Swartz, “CMS Pixel simulations”, Nucl.Instr.Meth. A511, 88 (2003)
  • Double-trap model:
    • V.Chiochia, M.Swartz et al., “Simulation of Heavily Irradiated Silicon Pixel Sensors and Comparison with Test Beam Measurements”, IEEE Trans.Nucl.Sci. 52-4, p.1067 (2005), eprint:physics/0411143
    • V. Eremin, E. Verbitskaya, and Z. Li, “The origin of double peak electric field distribution in heavily irradiated silicon detectors”, Nucl. Instr. Meth. A476, pp. 556-564 (2002)
  • Model fluence dependence:
    • V.Chiochia, M.Swartz et al., “A double junction model of irradiated pixel sensors for LHC”, submitted to Nucl. Instr. Meth., eprint:physics/0506228 (2005)

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

backup slides

Backup slides

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

evl models
EVL models

F1=6x1014 n/cm2

100% observed

leakage current

s=1.5x10-15 cm2

30% observed

leakage current

s=0.5x10-15 cm2

The EVLmodel based on double traps can produce large tails

but description of the data is still unsatisfactory

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

advanced evl models
Advanced EVL models
  • The recipe:
    • Relax the assumption on the cross sections
    • Let the parameters (NA, ND, sA/De, sA/Dh) vary
    • Keep the traps energy levels (EA, ED) to the EVL values
  • Constraints to the model:
    • Charge collection profiles (at different Vbias and Feq)
    • Trapping rates
    • Generated leakage current

be/h from literature

Feq known within 10%

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

e field profiles
E-field profiles

Constant

space charge

density

Double trap

model

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

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

Preserve linear scaling

of Ge/h and of the current

with Feq

F2=2x1014 n/cm2

NA/ND=0.68

sAh/sAe=0.25

sDh/sDe=1.00

!

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

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

temperature dependence
Temperature dependence

F1=6x1014 n/cm2

  • Charge collection profiles depend on temperature
  • T-dependent recombination in TCAD and T-dependent variables in PIXELAV (me/h, Ge/h, ve/h)
  • The model can predict the variation of charge collection due to the temperature change

T=-25ºC

T=-10ºC

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

lorentz deflection1
Lorentz deflection

Lorentz angle

Electric field

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

lorentz angle vs bias
Lorentz 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

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

ise tcad simulation
ISE TCAD simulation

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

pixelav simulation
PIXELAV simulation

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

srh statistics
SRH statistics

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

srh generation current
SRH generation current

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005

test beam setup
Test beam setup

pixel sensor

Magnetic field = 3 T

  • 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

or

PIN diode trigger

V. Chiochia – Charge collection in irradiated pixel sensors – TIME 05 - Zürich, October 3 -7, 2005