List of Authors:

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List of Authors Maria Rita Coluccia, J. A.
Appel, G. Chiodini, D. C. Christian, S. W. Kwan,
G. Sellberg with Fermi National Accelerator
Laboratory L. Uplegger with INFN Milano (Italy)
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ABSTRACT
We present IV and CV curves for irradiated
prototype n/n/p silicon pixel sensors, intended
for use in the BTeV experiment at Fermilab. We
tested pixel sensors from different vendors and
with two pixel isolation techniques p-stop and
p-spray. Results are based on exposure with 200
MeV protons up to 6 x 1014 protons/cm2.
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• BTeV 1 is a new heavy quark experiment that
• will run at the TEVATRON collider at Fermi
• National Accelerator Laboratory.
• It is designed to cover the forward region of
  the
• proton-antiproton interaction point running at
  a
• luminosity of 2 x 1032 cm-2s-1.
• The experiment will employ a silicon pixel
  vertex
• detector to provide high precision space
  points for an
• on-line lowest-level trigger based on track
  impact
• parameters 2.

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THE VERTEX DETECTOR
ONE STATION
The baseline BTeV silicon pixel detector has
rectangular 50 µm x 400 µm pixel elements and
consists of a regular array of 30 stations of
planar pixel detectors distributed along the
interaction region.
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L-SHAPE HALF PLANE
Each station contains one plane with the narrow
pixel dimension vertical, and one with the narrow
dimension horizontal, and is split in order to
allow the sensors to be moved away from the beam
during acceleration and other unstable beam
conditions.
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HYBRID ASSEMBLY
The basic building block of the detector is a
hybrid assembly consisting of a sensor, a number
of readout chips and a flexible printed circuit
(a high-density interconnect HDI).
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SENSOR DESIGN
The BTeV pixel sensors have n/n/p configuration
and therefore it is necessary to provide explicit
electrical isolation between neighboring n
electrodes. We explore two techniques.
P-STOP ISOLATION TECHNIQUE
P-STOP isolation technique 3 in which a high
dose p-implant surrounds each n-type region.
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P-SPRAY ISOLATION TECHNIQUE
P-SPRAY isolation technique 4, developed by the
ATLAS collaboration, that consists of a medium
dose p-implant that is applied to the entire
n-side and is overcompensated by the high dose n
pixel implants. To increase radiation hardness
and the breakdown voltage before irradiation, a
grading of p-spray implantation (moderated
p-spray) 5 is used, that leads to a step in the
effective p-spray dose along the gap between two
n-implants.
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DEVICES TESTED

• P-STOP sensors
• From SINTEF (Norway).
• Low resistivity (1.0-1.5 KOcm) lt100gt silicon,
  270µm thick.
• Two arrays tested
• test-sized sensors 12 x 92 cells that, except
  for four, are all connected together.
• FPIX1-sized sensors 18 x 160 cells (same size as
  the readout chip FPIX1 6, no bias grid
  structure).
• P-SPRAY sensors
• From TESLA (Czech Republic) and CiS (Germany).
• High resistivity (2-5 KO cm) lt111gt silicon, 250
  µm thick.
• One array tested (FPIX1-sized sensors).
• Bias grid structure for biasing all the pixel
  simultaneously.

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P-STOP SENSOR PERFORMANCES
I-V CURVES BEFORE IRRADIATION
Fig. 1 Typical I-V curves for both devices
before irradiation. The test-sized sensors show
very good performance. The FPIX1-sized sensors
have lower breakdown voltage. This appears to be
related to the fact that it is difficult to bias
all of the pixel cells during wafer testing.
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I-V CURVES BEFORE IRRADIATION
Fig. 2 The breakdown voltage of the FPIX1-sized
sensors increases considerably after bump bonding
to a readout chip. We tested 5 sensors that were
bump bonded to a readout chip and 3 sensors, with
indium bumps deposited on the n side, that were
glued with conductive silver epoxy to a piece of
silicon in order to mimic the presence of the
readout chip.
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I-V CURVES AFTER IRRADIATION
Fig. 3 I-V curves for a test-sized p-stop sensor
irradiated up to 1.5 E14 p/cm-2 The current after
irradiation increases by a few orders of
magnitude. However, operating at lower
temperature can alleviate this problem.
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Leakage Current Temperature Dependence
Fig. 4 We repeated the measurements at various
temperatures (10 oC, 0 oC and -10 oC). As
expected, we observed that the current decreases
exponentially with temperature (Ileak ? T2 exp
(-E / 2kBT) 7). The figure shows the comparison
between data and the predicted dependence of the
leakage current vs temperature. There is good
agreement between the fit and the data.
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C-V CURVES AND DEPLETION VOLTAGE
Fig. 5 Depletion voltage as a function of proton
fluence. Note that for 1 year of BTeV running at
nominal luminosity the fluence will be 1 x 1014
p/ cm2
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C-V CURVES AND DEPLETION VOLTAGE
Fig. 6 Here a typical example of bulk capacitance
versus bias potential for a test-sized p-stop
irradiated up to 1.2 x 1014 p/cm2 . We use this
method to determine the depletion voltage.
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P-SPRAY SENSORS PERFORMANCE
I-V CURVES BEFORE AND AFTER IRRADIATION
Fig. 7 We irradiated two of these sensors, one up
to 8 ? 1013 p/cm2 and the other one up to 1.2 ?
1014 p/cm2. The figure shows the increase in the
leakage current due to the irradiation for the
most irradiated sensor.
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C-V CURVE AFTER IRRADIATION
Fig. 8 CV measurement for sensor irradiate to 1.2
? 1014 p/cm2 . Vdep 70 V. Before irradiation was
Vdep 60 V . Type inversion already occurred.
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CONCLUSIONS

• Two different pixel isolation techniques were
  studied
• P-stop isolation
• Most of the tested sensors meet specifications.
• Problem to measure the breakdown voltage before
  bump bonding to readout chip.
• P-spray isolation
• Results obtained are promising.
• It is possible to determine the breakdown voltage
  due to a bias grid.
• Need more irradiation studies.

We will study the performance of these sensors,
bump bonded to a readout chip, in a test beam.
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REFERENCES
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M. R. Coluccia, et al., Characterization of
prototype BTeV silicon pixel sensors
before and after irradiation," accepted for
publication in IEEE Trans. Nucl. Sci.,
Aug. 2002.