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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, 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
Vertex 2004
Villa Vigoni
Menaggio – Como
13 – 18 September 2004
* Corresponding author, e-mail address: [email protected]
Slides are available at http://web.cern.ch/dorokhov/pixel/como/adorokhov_vertex04.ppt 1
Vertex 2004, Villa Vigoni Menaggio – Como, 13 – 18 September 2004
A. Dorokhov et al.
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…
operate very close (~4cm CMS Pixel) to
the particles interaction point at high
luminosity at LHC
highly segmented (CMS pixel size
100x150mm (f,Z))
high irradiation dose ~ 3x1014 Neq/cm2
per year at full luminosity - change
properties during the operation time
large number of channels (60 millions
CMS Pixel) has to be readout very fast –
zero suppressed (only amplitude above a
threshold) readout
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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A. Dorokhov et al.
Silicon sensor designs
p-stop
p-spray
p-stop ring
opening
p-spra y
bump pa ds
bia s grid
+
p -type impla nts
n-type silicon
n-type silicon
n+ -ntype
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A. Dorokhov et al.
Test setup
X1,Y plane X2,Y2 plane
(25mm)
(25mm)
X3,Y3 plane
(25mm)
X4,Y4
plane
(25mm)
Beam, Magnetic
field
Pixel Array (22x32) of
125 mm x 125 mm one
pixel size
Trigger PIN
diode (3x6 mm)
 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 June’03, September’03 and August’04 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.
S/H is here
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 nonirradiated 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 8*1014 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 )
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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 10*1014 neq/cm2 the Lorentz angle is
~8 degrees.
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Charge collection efficiency: beam enters the
pixel at a=150
p-spray again reveal better charge collection
for inclined tracks even after irradiation at
10*1014 neq/cm2
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Charge collection efficiency: beam enters the pixel at
a=150, bias dependency for 0.5*1014neq/cm2
As it was expected for not heavily
irradiated detector the depletion
voltage is very small.
For 0.5*1014neq/cm2 irradiation dose at
bias ~100V already above 95% charge is
collected.
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Charge cluster profiles: beam enters the pixel at
a=150, bias dependency
For about 1 year full luminosity at CMS Pixel
the irradiation dose is 2*1014neq/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
a=150, bias dependency
For about first 4 years of LHC at CMS Pixel
the irradiation dose is 6*1014neq/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 doesn’t start from one side of the silicon bulk, which means that
linear electric field approximation and, hence, constant effective dopant concentration
assumption doesn’t 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 Poisson’s 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,Vb=150V
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) = mH m(depth) B
tan(QL)(depth) determined from
Lorentz deflection
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Electric field measurement from known mobility
m = m0[1+(E/Ec) b]–1/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 nonuniform 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 a=150,
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(>98%)
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
(Vbias=100V) and 8.30 (4T) for the irradiated 1015 neq/cm-2
(Vbias=600V) 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
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