CMS Pixel Simulation

1
CMS Pixel Simulation
Quarter pixel Electrostatic potential
Vincenzo Chiochia Physik Institut der
Universität Zürich-Irchel CH-8057 Zürich
(Switzerland) PIXEL 2005 Workshop Chuzenji Lake,
Nikko (Japan) November 7-11, 2005
2
Outline

• The CMS pixel detector
• Radiation damage and impact on reconstruction
• Beam test data and detailed sensor simulation
• Physical modelling of radiation damage
• Models with a constant space charge density
• Double-trap models
• Fluence and temperature dependence

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 the first
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
6
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

7
Beam test setup
Data collected at CERN in 2003-2004
8
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
9
Detector simulation
Charge deposit
PIXELAV
M.Swartz, Nucl.Instr. Meth. A511, 88
(2003) V.Chiochia, M.Swartz et al., IEEE
Trans.Nucl.Sci. 52-4, p.1067 (2005).
10
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?
11
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
12
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)
13
The double peak electric field
c) Space charge density
a) Current density
detector depth
detector depth
b) Carrier concentration
d) Electric field
14
Model constraints

• The two-trap model is constrained by
• Comparison with the measured charge collection
  profiles
• Signal trapping rates varied within uncertainties

Typical fit iteration (8-12h TCAD) (8-16h
PIXELAV)xVbias ROOT analysis
15
Fit results

• Data
• --- Simulation

F16x1014 n/cm2 NA/ND0.40 sh/se0.25
16
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)

17
Space 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

n-type
p-type
sensor depth (mm)
V.Chiochia, M.Swartz,et al., physics/0506228
18
Temperature dependence
F22x1014 n/cm2 T-25º C

• Comparison with data collected at lower
  temperature T-25º C.
• Use temperature dependent variables
• recombination in TCAD
• variables in PIXELAV (m, D, G)
• The double-trap model is predictive!

F16x1014 n/cm2 T-25º C
V.Chiochia, M.Swartz,et al., physics/0510040
19
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
20
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 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.

21
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 and temperature dependence
• V.Chiochia, M.Swartz et al., A double junction
  model of irradiated pixel sensors for LHC,
  submitted to Nucl. Instr. Meth.,
  eprintphysics/0506228
• V.Chiochia, M.Swartz et al., Observation,
  modeling, and temperature dependence of
  doubly-peaked electric fields in silicon pixel
  sensors, submitted to Nucl. Instr. Meth.,
  eprintphysics/0510040

22
Backup slides
23
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
24
E-field profiles
Constant space charge density
Double trap model
25
Scaling to lower fluences
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!
26
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

27
Lorentz deflection
Lorentz angle
Electric field
28
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
29
ISE TCAD simulation
30
PIXELAV simulation
31
SRH statistics
32
SRH generation current
33
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