Pixel sensors under heavy irradiation

1
Pixel sensors under heavy irradiation
A. Dorokhov a, b,, C. Amslera, D. Bortolettoc,
V. Chiochiaa, L. Cremaldid, S. Cucciarellie, 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, USAd 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
Vertex 2004 Villa Vigoni Menaggio Como 13
18 September 2004
Corresponding author, e-mail address
Andrei.Dorokhov_at_cern.ch Slides are available at
http//web.cern.ch/dorokhov/pixel/como/adorokhov_v
ertex04.ppt
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Contents

• Introduction pixel detectors
• Silicon sensor design for CMS
• Test setup, data reconstruction, measurement
  technique
• Lorentz angle measurements
• Signal and noise, charge collection efficiency
  measurements
• A new method to measure electric field in
  heavily irradiated pixel sensor
• Summary and outlook

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Introduction pixel detectors
Used to provide a precise (12um CMS Pixel)
position for track seeds, primary and secondary
vertex reconstruction, particles tagging
highly segmented (CMS pixel size 100x150mm (f,Z))
operate very close (4cm CMS Pixel) to the
particles interaction point at high luminosity at
LHC
large number of channels (60 millions CMS Pixel)
has to be readout very fast zero suppressed
(only amplitude above a threshold) readout
high irradiation dose 3x1014 Neq/cm2 per year
at full luminosity - change properties during the
operation time
need high signal-to-noise ratio, charge sharing
(due to the Lorentz drift in magnetic field)
proper design and study the detector properties
exposed to heavy irradiation
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Silicon sensor designs
p-stop
p-spray
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Test setup
X4,Y4 plane (25mm)
X3,Y3 plane (25mm)
X2,Y2 plane (25mm)
X1,Y plane (25mm)
Beam, Magnetic field
Trigger PIN diode (3x6 mm)
Pixel Array (22x32) of 125 mm x 125 mm one pixel
size

• beam telescope planes, trigger PIN diode and
  pixel installed between two Helmholtz coils,
  maximum magnetic field 3T
• beam telescope position resolution 1 mm
• pixel frame can be tilted, pixel is cooled by
  the Peltier cooler
• full analogue pixel amplitudes readout (no zero
  suppression)
• magnetic field parallel/perpendicular to the
  pions beam
• data collected in June03, September03 and
  August04 at CERN SPS H2 area

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Data reconstruction

• analog amplitudes correction (pedestals, common
  mode noise subtraction)
• event pre-selection using geometry and simple
  event topology
• reconstruction beam entry points
• alignment of the pixel with beam telescope

After the reconstruction pixel amplitudes (in ADC
counts) and beam entry points (in the pixel
system coordinates) predicted by the telescope
are available for the analysis.
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Pixel amplitude calibration/cross-calibration
By injecting of 22400 electrons with calibration
pulse to non-irradiated pixel readout chip and
comparing the resulting ADC counts with the
signal from pions the sample and hold delay time
can be measured.
Each pixel for each sample is calibrated with the
calibration pulse. The readout chip is linear up
to about 1 MIP signal.
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Measurement of signal-to-noise, charge collection
efficiency beam is perpendicular to the pixel
plane
Charge collected by single pixel in dependence of
particle entry point for non-irradiated and
irradiated sensors for both designs.

• charge in the centre of pixel decreases after
  irradiation by 30, what is design independent
  property of silicon
• the integrated charge depends on the design
  features

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Particles detection efficiency beam is
perpendicular to the pixel plane
From particle detection efficiency point, p-spray
design has the following advantages over p-stop

• better particle detection efficiency(98) than
  p-stop at threshold 5 sigma noise
• the inefficiency is less bias voltage dependant
• the inefficiency is not increasing very fast
  with threshold more safe

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Signal and noise of single pixel
p-spray design signal-to-noise ratio for
non-irradiated is 70, for the heavily irradiated
at 81014 neq/cm2 S/N 45
p-stop design signal-to-noise ratio for
non-irradiated is 30, for the heavily irradiated
40 the reason is larger charge spread between
pixels for non-irradiated device
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Measurement of Lorentz angle and charge cluster
profiles grazing angle technique

• the beam enters the pixel at a small angle a
• the charge drifts in the direction of the
  Lorentz angle QL
• the charge collected by the pixels is deflected
  by the angle b

tan(QL) tan(b )/sin(a )
Along particle track , mm
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Lorentz angle measurements grazing angle
technique, averaged over depth Lorentz angle
Averaged Lorentz angle depends mainly on the bias
voltage. Since heavily irradiated sensors have to
operate at large bias, the Lorentz angle
decreases. At 4T magnetic field, after 101014
neq/cm2 the Lorentz angle is 8 degrees.
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Charge collection efficiency beam enters the
pixel at a150
p-spray again reveal better charge collection for
inclined tracks even after irradiation at 101014
neq/cm2
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Charge collection efficiency beam enters the
pixel at a150, bias dependency for
0.51014neq/cm2
As it was expected for not heavily irradiated
detector the depletion voltage is very small.
For 0.51014neq/cm2 irradiation dose at bias
100V already above 95 charge is collected.
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Charge cluster profiles beam enters the pixel at
a150, bias dependency
For about 1 year full luminosity at CMS Pixel
the irradiation dose is 21014neq/cm2 the bias
voltage has to be increased up to 300V. It
should be noted, that charge is collected even at
low bias voltage from both sides of the detector.
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Charge cluster profiles beam enters the pixel at
a150, bias dependency
For about first 4 years of LHC at CMS Pixel the
irradiation dose is 61014neq/cm2 . Again charge
is collected even at low bias voltage from both
sides of the detector the assumption that
depletion starts from pixel side only is not
valid.
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Heavily irradiated detectors a method of
electric field measurement
Motivation as it was seen from profiles of
charge cluster for the heavily irradiated
sensors, the depletion doesnt start from one
side of the silicon bulk, which means that linear
electric field approximation and, hence, constant
effective dopant concentration assumption doesnt
correctly describe the physics in the heavily
irradiated silicon detector.
There are models predicting non-linear electric
filed in heavily irradiated silicon the main
idea that the thermally generated electrons and
holes, while drift towards the electrodes, are
trapped by the trapping centers (induced by the
irradiation damage) in the silicon bulk and form
non-uniform space-charge region. The
concentration of traps can be measured with
standard Deep Level Transient Spectroscopy (DLTS)
or Thermally Stimulated Current (TSC) technique.
The electric field than can be calculated from
Poissons equation.
Another direct method of electric field
measurement in heavily irradiated silicon pixel
sensors is proposed. It is based on the precise
measurement of the Lorentz deflection in the
sensor bulk. The Lorentz deflection is related to
the mobility and the electric field can be
calculated, using mobility dependence on electric
field formula.
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Lorentz angle in dependence on the depth
Charge asymmetry, p-spray,Vb150V
The Lorentz deflection in the silicon bulk can be
calculated from asymmetry distribution.
Asymmetry fitted with Erf function in each
vertical slice. From each fitted slice one can
determine position where the asymmetry is equal
to zero.
The position where asymmetry is equal to zero
gives the Lorentz deflection in magnetic field in
dependence on the depth
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Electric field measurement Mobility extracted
from Lorentz deflection
tan(QL)(depth) determined from Lorentz deflection
tan(QL)(depth) mH m(depth) B
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Electric field measurement from known mobility
m m01(E/Ec) b1/b
Where Ec , b - for electrons and holes are known
functions of temperature, one of the standard
parametrization used.
Region with geometrical distortion
Region with geometrical distortion
One can see that for heavily irradiated sensors
the electric field is very non-uniform and has
U-like shape that means charge collection
starts from both sides
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From measured electric field to simulation of
induced charge
Very simple simulation to validate measurement of
electric filed flat drift field, but magnitude
is taken from measurements. Using measured
trapping constants and Ramo theorem one can find
induced charge. No diffusion, no delta-rays, no
energy deposition spread (Landau) are taken into
account. The effective potential is calculated,
assuming no gaps between pixels.
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From measured electric field to simulation of
induced charge
The results of the simulation beam enters the
pixel at a150, electrons and holes contribution
separately. Important to see, that the charge
cluster is mainly formed by the electrons.
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From measured electric field to simulation of
induced charge
The charge cluster simulated at zero magnetic
field gives very good agreement with the measured
data all the features of the cluster shape are
very similar to the measured ones.
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From measured electric field to simulation of
induced charge
The simulation using measured electric filed
gives also good agreement with the measured data
in the presence of magnetic field therefore
with the measured electric field it is possible
to describe correctly the physical processes in
the pixel sensor.
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Summary and outlook

• The sensors reveal sufficient particle detection
  efficiency(gt98) even after exposed to high
  irradiation doses (1015 neq/cm-2) and at high
  bias voltage (up to 600V)
• The Lorentz angle is up to 260 (4T) for
  non-irradiated (Vbias100V) and 8.30 (4T) for
  the irradiated 1015 neq/cm-2 (Vbias600V) devices
  
• Signal-to-noise ratio decreases from 70 to 40
  after irradiation, charge collection reaches 60
  
• A method of electric field measurement in heavily
  irradiated silicon pixel sensor is proposed and
  validated with simple simulation