Charge collection in irradiated pixel sensors

1
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
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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

3
The CMS pixel detector

• 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

4
The LHC radiation environment
sppinelastic 80 mb L 1034 cm-2s-1

• 4 cm layer F3x1014 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
5
Radiation effects
r-f plane
r-z plane
E
E
? B field
no B field
Irradiation modifies the electric field
profile varying Lorentz deflection
Irradiation causes charge carrier trapping
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Prototype sensors for beam tests

• n-in-n type with moderated p-spray isolation,
  biasing grid and punch through structures
  (producer CiS, Germany)
• 285 µm thick lt111gt 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)x1014 neq/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

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Beam test setup
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Charge collection measurement
Charge collection was measured using cluster
profiles in a row of pixels illuminated by a 15º
beam and no magnetic field
Feq 51013 n/cm2
½ year LHC low luminosity
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Detector simulation
Charge deposit
PIXELAV
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The classic picture
after type inversion

• 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

NeffND-NAlt0
Based on C-V measurements!
-
is all this really true?
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Models with constant Neff
F 6x1014 n/cm2
A model based on a type-inverted device with
constant space charge density across the bulk
does not describe the measured charge collection
profiles
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Two traps models
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)
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The double peak electric field
c) Space charge density
a) Current density
detector depth
detector depth
b) Carrier concentration
d) Electric field
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Fit results

• Data
• --- Simulation

F16x1014 n/cm2 NA/ND0.40 sh/se0.25
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Scaling to lower fluences
F30.5x1014 n/cm2 NA/ND0.75 sAh/sAe0.25 sDh/sDe
1.00

• 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)

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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

n-type
ND
space charge density
p-type
NA
sensor depth (mm)
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Lorentz deflection
Switching on the magnetic field
tan(?) 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
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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 hit reconstruction algorithms optimized
  for irradiated pixel sensors. Results expected
  soon!

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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),
  eprintphysics/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.,
  eprintphysics/0506228

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Backup slides
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EVL models
F16x1014 n/cm2
100 observed leakage current s1.5x10-15 cm2
30 observed leakage current s0.5x10-15 cm2
The EVL model based on double traps can produce
large tails but description of the data is still
unsatisfactory
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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
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E-field profiles
Constant space charge density
Double trap model
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Scaling to lower fluences (1)
Preserve linear scaling of Ge/h and of the
current with Feq
F22x1014 n/cm2 NA/ND0.68 sAh/sAe0.25 sDh/sDe1.
00
!
Not shown Linear scaling of trap densities does
not describe the data!
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Temperature dependence
F16x1014 n/cm2
T-25ºC
T-10ºC

• 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

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Lorentz deflection
Lorentz angle
Electric field
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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
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ISE TCAD simulation
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PIXELAV simulation
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SRH statistics
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SRH generation current
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Test beam setup
Magnetic field 3 T
or
?

• 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