Radiation tolerant sensors for the ATLAS pixel detector

Nuclear Instruments and Methods in Physics Research A 466 (2001) 327–334
Radiation tolerant sensors for the ATLAS pixel detector $
Abstract
R. Wunstorf*
Universit .at Dortmund, Lehrstuhl f .ur Experimentelle Physik IV, D-44227 Dortmund, Germany
For the ATLAS Pixel Collaboration
The pixel detector in the ATLAS experiment at the LHC, Geneva, is an important detector component for high
resolution tracking and vertex identification. For this demanding task the hybrid pixel detector with silicon sensors has
to work in a very harsh radiation environment with up to 3.5 10 14 n eq/cm 2 per year. On the basis ofthe known
radiation effects a dual-track strategy was followed for the development of radiation tolerant silicon pixel sensors. The
ATLAS pixel collaboration successfully developed the radiation hard sensor design which meets the challenging
requirements for the ATLAS pixel detector. In parallel, the hardening of the silicon itself was followed within the ROSE
collaboration, which developed the radiation tolerant DOFZ-silicon with oxygen enrichment by diffusion. Taking all
the results together the radiation tolerant silicon sensors have been designed, produced and showed excellent
performance before and after irradiation. # 2001 Elsevier Science B.V. All rights reserved.
PACS: 29.40.g; 29.40.W; 85.30.D; 61.80
Keywords: ATLAS; Tracking; Detector; Silicon; Pixel; Radiation hardness; Radiation damage
1. Introduction
The ATLAS detector will be one ofthe two
omni-purpose detectors which are being built for
the Large Hadron Collider at CERN, Geneva.
Besides the high energy ofthe colliding protons of
2 7 TeV the high event rate ofaround 25
minimum bias events per bunch crossing every
25 ns challenges the technical realization ofall
detector components. To separate the different
events and their vertices within one bunch crossing
$ Work supported by BMBF under Contract 05H8PEA1.
*Corresponding author. Tel.: +49-231-7553-544; fax: +49-
231-7553-688.
E-mail address: wunstorf@physik.uni-dortmund.de
(R. Wunstorf).
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0168-9002(01)00568-X
the Inner Detector ofATLAS will have, besides
the TRD and SCT at outer radii, a high resolution
pixel detector [1,2].
The ATLAS pixel detector will measure true
space points in the two barrel layers at 10.1 cm and
13.2 cm radius plus the so-called B-layer at a radius
of4.3 cm and 5 disks up to z ¼ 92:6cm
and |Z|=2.5. The B-layer was added later to the
pixel system in order to allow better impact
parameter resolution for B-physics in the first 3
years at reduced luminosity. The decision to build
the B-layer with a silicon pixel detector was taken in
1996, because pixel detectors can operate in
this environment ofextremely high particle rates
without ghost hits and with good S/N ratios due to
the small pixel size of50 mm 300 mm. The
challenge to build a silicon pixel detector, which

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R. Wunstorf / Nuclear Instruments and Methods in Physics Research A466 (2001) 327–334
will work in the LHC environment was taken up by
the ATLAS pixel collaboration. The focus of this
article is the development ofthe radiation tolerant
silicon pixel sensors for the ATLAS experiment.
Fig. 1. Illustration ofthe strategy followed by the development
ofradiation tolerant silicon sensors.
Table 1
Overview ofthe sensor requirements for the ATLAS pixel detector
The overall strategy, as shown in Fig. 1, was to
follow the different tasks concerning the design
and material separately. From earlier systematic
radiation damage studies ofsilicon detectors the
radiation induced material changes were known
(e.g. Ref. [3]) and could be taken into account for
the development ofthe pixel sensor design. Parallel
to this work within the ATLAS collaboration,
members ofthe pixel collaboration were, from the
very beginning actively involved in the world wide
initiative on developing more radiation hard
silicon material, the ROSE collaboration [4]. The
results from the two separate R&D tasks, pixel
sensor design and radiation tolerant silicon material
for detectors were taken together for the
prototyping, which finally led to the production of
ATLAS pixel sensors.
2. Design considerations
Requirements Design features
A number ofdifferent requirements have to be
taken into account when designing a silicon sensor
for such a challenging application as the ATLAS
pixel detector. Table 1 gives an overview ofthe
different but sometimes interrelated issues using a
simplified classification. This long list shows that it
is not possible to just order the suitable devices offthe-shelf.
Especially, none of the existing experiments
until now required such radiation-tolerant
Module issues Small pixel size fitting the read-out cell 50 mm 400 mm (B-layer: 50 mm 300 mm)
Small sensor thickness 250 mm (B-layer: 200 mm)
No dead area between FE-chips Elongated resp. ganged pixels (B-layer: uni-sized pixels
with MCMD)
Compatible with bump bonding Silicon nitride passivation with 12 mm diameter opening
Quality assurance Testability on wafer level Bias grid with punch-through connection to every
pixel cell
Coping with missing bumps Bias grid with punch-through connection to every
pixel cell
Radiation tolerance Edges on ground potential after type conversion n–in–n sensor with guard rings only on the p-side
Increase ofoxide charge without excess current
or noise
p-spray isolation
High break down voltages before and after
irradiation
Moderated p-spray isolation

R. Wunstorf / Nuclear Instruments and Methods in Physics Research A466 (2001) 327–334 329
sensors. Therefore, it was the task of the ATLAS
pixel sensor group to start the necessary R&D
work on the basis ofthe known radiation induced
effects.
As predicted in 1990 and later on, observed
n-type silicon bulk converts, at room temperature
at a fluence ofaround 10 13 neq/cm 2 , to a p-type
behavior [5,6]. The main consequence ofthis is that
the sensor for the ATLAS Pixel Detector will be
built as n + –in–n devices with only one guard-ring
structure on the p-side. This allows the whole n-side
including the edges ofthe sensor to be kept on
ground potential and prevents sparking from the
sensor bias (up to 600 V) into the FE-chip which
may be as close as 10 mm to the sensor. Measuring
the radiation induced change ofthe silicon resistivity,
e.g. with a four-point probe, in thermal
equilibrium, shows that the resistivity increases
reaching a value close to intrinsic [7]. This high
resistivity is the main reason that silicon detectors
work after radiation induced type conversion without
a guard-ring structure on the n-side [8].
The n + –in–n sensor requires, ofcourse, a
p-isolation on the structured n-side. Therefore,
different isolation options ofthe p-stop and pspray
techniques were investigated systematically.
Device simulations were performed, especially
concerning the electric field distribution and the
development under irradiation to minimize the
risk ofearly breakdown [9]. Both isolation
techniques were realized within the wafer layout
ofthe first prototype wafer. As discussed in
Ref. [10], for p-stop designs the electric field
strength increases under ionizing irradiation, while
it decreases for p-spray isolation. This means that
for p-spray isolated devices the worst case with the
highest electric field can be tested directly after
delivery and under ionizing irradiation the risk of
breakdown will even decrease. A different aspect
ofusing p-spray isolation, is the possibility of
implementing a bias-grid. As successfully tested
beforehand on a simple pixel test structure, such a
bias-grid connects every pixel via punch and thus
allows one to test, with only two contacts, the
quality ofthe whole pixel array proved to be
essential for quality assurance.
Different designs ofthe pixel cell itselfhave
been studied with device simulations and were
implemented in the prototype wafers [9,11]. Fig. 2
shows a photo ofan ATLAS pixel sensor wafer of
the first prototype series, which includes two full
size tiles, one with p-stop isolation and one with
p-spray isolation and to study different design
variation, the wafer includes several so-called
single chip devices fitting to one FE-chip.
3. Material considerations
The radiation induced crystal damage leads to a
degradation ofthe detector behavior in three
aspects: increase ofthe bulk generation current,
increase ofacceptor-like defects and increase of
trapping centers. The higher generation current
can be limited by low operation temperatures and
together with small pixel sizes, these lead to
relatively low input currents which the front-end
electronics can compensate [12]. The limiting
factor for the operation of silicon is due to the
increase ofacceptor like defects which cause type
conversion and the further increase of the depletion
voltage. For practical reasons, the applicable
bias voltage is limited to 600 V in the ATLAS pixel
detector. Consequently, after high fluences the
sensors have to be operated only partially depleted,
causing the signal heights to decrease
drastically. Compared with this signal loss due to
the smaller depletion depth, the signal loss caused
by charge trapping ofup to 20% is still smaller.
Fig. 2. Photo of the first prototype sensor wafer for the
ATLAS pixel detector.

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As crystal defects are responsible for the radiation
induced changes ofthe sensor parameters, the
approach ofthe ROSE collaboration was to alter
the defect kinetics by controlled introduction of
individual impurities in the silicon crystal [13,14].
After the first step of getting the necessary
processing developed and silicon detectors produced,
the ROSE collaboration tested them under
irradiation with high energy neutrons and charged
hadrons. While almost no difference in the radiation
induced parameters were found under neutron
irradiation, much improvement was observed
under irradiation with charged hadrons for silicon
with high oxygen content [15]. The improvement
concerns only the radiation induced change ofthe
doping concentrations, while the current and
charge collection parameters are the same.
The different responses ofoxygenated silicon to
neutrons and charge hadrons indicates that the
improvement due to point defects, may be caused
by an introduction ofdonor-like defects compensating
part ofthe ‘unwanted’ acceptors, which
would therefore be cluster related. Although the
defect kinetics of oxygen in silicon is not fully
understood and it might be a different mechanism,
it is interesting that already, several years ago,
oxygen was thought to be a candidate for
improvement, when it was observed that close to
the surface, where the oxygen diffuses in during a
normal oxidation process, the silicon does not
convert to p-type behavior, even though the bulk
has been converted [16].
Several tests with different oxygenation processes
at different vendors and different starting
materials have confirmed the improvement of
silicon in the following three aspects [15,17]:
* reduction ofthe stable damage,
* reduction ofthe amount ofreverse annealing,
and
* deceleration ofthe reverse annealing.
Fig. 3 shows, for example, the improved behavior
ofthe effective doping measured directly after
irradiation, due to the reduction ofthe stable
Fig. 3. Comparison ofoxygen enriched silicon (DOFZ-silicon) with standard silicon in the radiation induced change ofthe effective
doping concentration measured after successive irradiation steps with 24 GeV protons. The DOFZ-silicon for the ROSE diodes used
here were simultaneous oxygen diffused with the second prototype ATLAS sensor wafers.

R. Wunstorf / Nuclear Instruments and Methods in Physics Research A466 (2001) 327–334 331
damage by oxygen diffused silicon (DOFZ-silicon).
The ROSE test diodes used here, are ofthe
same material as used for the second ATLAS pixel
sensor prototype and have undergone an identical
oxidation process, ofa 24 h diffusion under
nitrogen atmosphere at CiS, Erfurt.
The expected improvement using oxygenated
silicon, as shown in Fig. 4, has been calculated
using the damage parameters evaluated by the
ROSE collaboration [15]. These figures include the
prediction calculation for the ‘standard’ warm-up
scenario (3 days 208C and 14 days 178C) [1] and
also calculations for longer warm-up time (30 days
resp. 60 days 208C) which will be possible with the
use ofoxygenated silicon. The sensors for the Blayer,
for example, would survive not only the
expected 3 years at lower luminosity, but could
work the full ATLAS operation time of 10 years.
The radiation is largely dominated by pions,
leading to this impressive advantage ofusing
oxygenated silicon for the ATLAS pixel detector.
Fig. 4. Damage projections for the B-layer (a) and the first
layer (b) ofthe ATLAS pixel detector.
4. Prototyping
Before ordering the final sensor wafers, which
will go into the ATLAS detector, the ATLAS pixel
collaboration had two prototype series. The prototype
sensors were important for the development of
the sensors as well as for testing front-end electronic
chips and other module aspects [1]. The pixel
detector module utilizes high density interconnect
technologies such as bump bonding and MCMD,
which are using further photolithography processing
steps. These processing steps cannot be done
on diced parts but require full wafers. Therefore,
the sensor prototyping was already done with full
wafer layouts given to the vendors as GDS-II file
ready for the production of masks.
The main sensor issues under investigation in
the first prototype had been the different isolation
techniques and different designs ofthe pixel cell
itself. As shown in Fig. 2, the wafer includes two
full size tiles, and several small single chips fitting
with different design variation for both isolation
techniques. These prototype sensors have been
studied thoroughly by the ATLAS pixel collaboration
[11,18] and resulted in the optimal choice for
the radiation tolerant sensor design [19]. Moderated
p-spray has been shown to provide superior
current characteristics without any excess current,
very high breakdown voltages above 1000 V and
low noise [18,20]. Extensive studies ofsingle chip
sensors bonded to prototype ATLAS pixel electronic
chips have been performed in the H8 test
beam at CERN [21]. Even after 10 15 neq/cm 2 the
noise occupancy was below 10 7 measured with
low thresholds in the testbeam and allow e.g. a
direct measurement ofthe depletion depth with the
inclined particle tracks as shown in Fig. 5. The
optimal pixel cell design was the so-called ‘smallgap’
design, which showed in the test beam a signal
efficiency of99.1%, a flat top of22 mm by single
hits and with double hits 5 mm space resolution.
The analog charge measured across the pixel plane
using the time-over-threshold information is
shown in Fig. 6. Only the area ofa few mm 2
around the small bias dot shows a charge loss of
10%, which is still well above the threshold.
These good results left only two sensor issues to
the second prototype run: the yield oflarge tiles

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R. Wunstorf / Nuclear Instruments and Methods in Physics Research A466 (2001) 327–334
Fig. 5. Depletion depth measurement ofan unirradiated and an
irradiated ATLAS pixel sensor.
Fig. 6. Analog charge measurement across two adjacent pixel
cells.
and the use ofoxygenated silicon. In the first
prototype, the yield ofthe 16 times smaller single
chips was much higher than that ofthe full size
tiles. The reason for this could be randomly
distributed defects, like scratches introduced during
the different processing and handling steps or a
particular risky design feature. The most challenging
design feature is the small bias dot which
requires an alignment accuracy better than 2 mm.
This question was investigated with the second
prototype, where three large tiles were implemented
in the wafer layout with different design
options ofthe bias dot, a small dot, a large dot,
and a bias grid without integrated dots. The
resulting yield statistic showed for two different
vendors no yield loss due to the small dot design.
Therefore, this design (see Fig. 7) was taken for the
production, which will have the optimum charge
collection.
Both vendors, CiS (Erfurt) and ITC (Trento),
produced halfofthe second prototype wafers with
oxygen diffusion of24 h at 11508C to allow a
comparison in the sensor performance. As already
shown before (Fig. 3) the material will be more
radiation tolerant to charged hadrons. Irradiations
ofoxygenated and not oxygenated second prototype
sensors have been performed up to 10 15 neq/
cm 2 with 55 MeV protons at LBNL, Berkely, and
with 24 GeV protons at CERN, Geneva, and are
currently under investigation. The I2V characteristics
show for both cases only the known increase
in the bulk generation current without any
indication ofbreakdown up to 1000 V. Irradiated
single chips have been bump bonded to FE-chips
and were successfully operated in the CERN
testbeam. As expected, the oxygenated silicon
shows full depletion and full charge collection at
much lower bias voltages. These pixel sensors can
be operated without any problems even after high
irradiations and at the same time allow the direct
measurement ofthe depletion depth (Fig. 5) and
the measurement ofthe analog signal. Therefore,
the pixel sensors allow for the first time the study
ofthe dependence ofthe depletion depth and the
charge collection from the applied voltage after
different fluences, especially after type conversion.
5. Production
The ATLAS pixel detector will need totally
more than 2000 modules for the outer two layers

and the 5 disks in each forward direction. The
modules for the B-layer are needed about 2 years
later allowing a separate production and the use of
the MCMD-technology, which will be optimized
according to the special B-layer requirements, e.g.
300 mm long cells. One important option is to have
equal-sized pixel cells all over the sensor without
elongated and ganged pixel as the cell geometry is
not dictated by the read-out chip and also the
implementation ofbricked pixels would easily be
possible without any additional cross-talk caused
by routing in one metal layer. These will improve
the spatial resolution in this important layer for
the impact parameter measurement and the
b-tagging. Therefore, the starting sensor wafer
production will be for tiles for the outer modules
which will be built with a flex hybrid. The wafer
layout has been finalized according to the results
ofthe prototype studies. The design ofthe pixel
cells is shown in Fig. 7, with the bias-grid structure
in the middle and the passivation opening for
bump bonding on the other side ofeach pixel.
Besides three tiles for the ATLAS pixel detector
modules, the wafer will contain six single chip
devices and a number oftest structures for quality
control. According to the Quality Assurance plan,
part ofthe control measurement will have already
been performed by the vendors, but these measurements
and additional tests will be performed
by the ATLAS institutes.
6. Conclusions
R. Wunstorf / Nuclear Instruments and Methods in Physics Research A466 (2001) 327–334 333
With the knowledge ofthe radiation effects in
silicon detectors as a good starting point, the
Fig. 7. Design ofthe pixel sensor cell for the ATLAS pixel detector.
ATLAS pixel collaboration began in 1996 to
develop silicon pixel sensors capable ofsurviving
the harsh radiation environment at the LHC. To
reach this challenging goal, a dual-track strategy
was followed. Besides the studies concerning the
sensor design and prototyping within the ATLAS
pixel collaboration, the investigations ofthe
radiation hardness ofsilicon were done within
the ROSE collaboration. The result is an n + –in–n
pixel sensor with moderated p-spray using oxygen
diffused silicon. Indispensable to the quality
control during production is the implemented bias
grid, which enables measurements ofthe I2V
characteristic with two contacts and prevents
problems due to missing bumps during operation.
Acknowledgements
These important results for the ATLAS Experiment
could not have been achieved without the
considerable engagement ofthe members ofthe
ATLAS Pixel Collaboration and the ROSE
collaboration and their cooperation. Therefore,
the acknowledgement goes to all who participated
in the sensor design, the sensor measurements, the
irradiations, the testbeam runs and the logistics,
sending the sensors on time to the different places.
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