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, Switzerland
b Paul Scherrer Institut, 5232, Villingen PSI, Switzerland
c Purdue University, Task G, West Lafayette, IN 47907, USA
d Department of Physics and Astronomy, Mississippi State University, MS 39762, USA
e Institut für Physik der Universität Basel, Basel, Switzerland
f Johns Hopkins University, Baltimore, MD, USA
* Now at: Institut de Recherches Subatomiques, F67037 Strasbourg, France
Outline
1. The CMS pixel detector and data reconstruction
2. Analysis ingredients: beam test data and detector simulation
3. Physical modelling of radiation damage:
a) Models with a constant effective doping concentration
b) EVL models (V.Eremin, E.Verbitskaya, Z.Li)
c) Advanced double junction models (V.C., M.Swartz)
4. Conclusions
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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The CMS pixel detector
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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
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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The LHC radiation environment
sppinelastic = 80 mb
L = 1034 cm-2s-1
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•
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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
What is the sensors response after
few years of operation?
Fluence per year at full luminosity
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Impact on reconstruction
Sensor irradiation
Charge Carriers Trapping
Variation of the
electric field profile
Asymmetric pixel clusters
Lorentz deflection
Example:
Long clusters along
the z-coordinate at high h
Example:
Non-linear charge sharing
in the r-f plane
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Prototype sensors for beam tests
125 mm2
• p-spray design with biasing grid and punch
through structures (CiS, Germany)
• 125x125 mm2 cell size
• 22x32 pixel matrix, 285 μm thick <111>
DOFZ wafer, n-in-n type
• Samples irradiated with 21 GeV protons at
the CERN PS facility
• Fluences: Feq=(0.47,2.0,5.9)x1014 neq/cm2
• Annealed for three days at 30º C
• Bump bonded at room temperature to non
irradiated front-end chips with non zerosuppressed readout, stored at -20ºC
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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2004 Beam test setup
CERN Prevessin site
H2 area
beam:
150 GeV p
Silicon strip beam telescope:
50 μm readout pitch,~1 μm resolution
B field
pixel sensor
support
3T Helmoltz magnet
Cooling circuit
T =-30 ºC or -10ºC
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Charge collection measurement
Charge collection was
studied with the cluster
profiles in a row of pixels
illuminated by a 15º beam
and no magnetic field
Temperature = -25 ºC and -10ºC
Feq = (0, 0.5, 2, 6)x1014 n/cm2
n+side
½ year LHC
low luminosity
charge
trapping
p-side
2 years LHC
low luminosity
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
2 years LHC
high luminosity
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Detector simulation
ISE TCAD 9.0
Double traps
models
(DESSIS)
3-D Electric
field mesh
Charge
deposit
Charge
transport
PIXELAV
Trapping
Trapping times
from
literature
Electronic
response +
data formatting
ROOT
Analysis
ROC+FED
response
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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The classic picture
after type inversion
Neff<0
-
• 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!
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Models with constant Neff
F = 6x1014 n/cm2
A model based on a type-inverted device with constant Neff
across the bulk does not describe the measured charge collection profiles
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Two traps models
EConduction
Electron
traps
acceptor
1.12 eV
N A , σ eA , σ Ah
EC-0.525 eV
EV+0.48 eV
donor
Hole
traps
N D , σ eD , σ Dh
EValence
Given these parameters the charge carriers dynamics
is governed by the Shockley-Read-Hall statistics
Eremin-Verbitskaya-Li Model (EVL)
σ eD = σ Dh = σ eA = σ Ah = 10 15 cm 2
NA and ND are fixed to TCT measurements
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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The double peak electric field
ρ eff = e N D f D  e N A f A  ρ dopants
D
a) Current density
c) Effective doping
concentration
A
 eN D f D  N A f A   ρ dopants
n+p
junction
np+
junction
-HV
p-like
b) Carrier concentration
d) Electric field
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
n-like
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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 EVL model based on double traps can produce large tails
but description of the data is still unsatisfactory
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Advanced EVL models
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The recipe:
1.
2.
3.
•
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:
1.
2.
3.
Charge collection profiles (at different Vbias and Feq)
Trapping rates
Generated leakage current
 1
Qe,h (t) = Q0e,h exp  
 τ e/h

t 

Γ e = 1/τ e = β e Φ eq  v e σ eA N A
Γ h = 1/τ h = β h Φ eq  v h σ Dh N D
be/h from literature
Feq known within 10%
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Best fit: F1=6x1014 n/cm2

E field profiles
 Data
--- Simulation
F1=6x1014 n/cm2
NA/ND=0.40
sh/se=0.25
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Temperature dependence
F1=6x1014 n/cm2
T=-10ºC
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T=-25ºC
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
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Scaling to lower fluences (1)
Preserve linear scaling
of Ge/h and of the current
with Feq
Γ e/h (Φ 2 ) = R Γ Γ e/h (Φ1 ) ;
R Γ = Φ 2 /Φ1 =
RA  RD
2
N A (Φ 2 ) = R A N A (Φ1 ) = R Γ (1  δ)N A (Φ1 )
N D (Φ 2 ) = R D N D (Φ1 ) = R Γ (1  δ)N D (Φ1 )
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
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Scaling to lower fluences (2)
F3=0.5x1014 n/cm2
NA/ND=0.75
sAh/sAe=0.25
sDh/sDe=1.00

E field
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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)
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Scaling summary
• Donors concentration increases faster than acceptors
• NA/ND increases for decreasing fluences
• Electric field peak at the p+ backplane increases with irradiation
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Lorentz angle vs depth
Lorentz angle
Electric field
• Lorentz angle and electric field extracted from the test beam
measurements
• The Lorentz angle is not constant across the sensor thickness
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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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
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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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
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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References
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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. 556564 (2002)
M.Swartz, “CMS Pixel simulations”, Nucl.Instr.Meth. A511, 88 (2003)
V.Chiochia, M.Swartz et al., “Simulation of Heavily Irradiated Silicon Pixel Sensors
and Comparison with Test Beam Measurements”, accepted for publication on
IEEE Trans.Nucl.Sci., eprint:physics/0411143
A.Dorokhov et al.,
ISE TCAD 9.0: http://www.synopsys.com/products/acmgr/ise/dessis_ds.html
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Backup slides
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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CMS pixel sensor design
Gold
Indium-Bump
Nickel
Titanium
Si3N4
cross section
punch-through
biasing
nitride
pspray
Al
n+
p+
metal line
n- bulk
n+/Al
opening
Bump-bond
contact
p+
nitride + LTO
Si3N4
passivation
Al
Vendor: CiS, Erfurt - www.cismst.de
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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Test beam setup
pixel sensor
Magnetic field = 3 T
or 
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
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PIN diode trigger
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Four modules of silicon strip
detectors
Beam telescope resolution ~ 1 mm
Sensors enclosed in a water cooled
box (down to -30ºC)
No zero suppression, unirradiated
readout chip
Setup placed in a 3T Helmoltz
magnet
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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ISE TCAD simulation
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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PIXELAV simulation
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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SRH statistics
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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SRH generation current
V. Chiochia – Charge collection in irradiated pixel sensors
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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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
10th European Symposium on Semiconductor Detectors - Wildbad Kreuth, June 12 -16, 2005
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