Nuclear Inst. and Methods in Physics Research, A 924 (2019) 142–146

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Nuclear Inst. and Methods in Physics Research, A
journal homepage: www.elsevier.com/locate/nima

First bulk and surface results for the ATLAS ITk Strip stereo annulus sensors
R.F.H. Hunter a ,∗, S.H. Abidi l , A.A. Affolder k , J. Bohm j , J. Botte a , B. Ciungu l , K. Dette l ,Z. Dolezal i , C. Escobar g , V. Fadeyev k , J. Fernandez-Tejero b , C. García Argos e , D. Gillberg a ,K. Hara m , M. Hauser e , K. Jakobs e , J.S. Keller a , P. Kodys i , T. Koffas a , Z. Kotek j , J. Kroll j ,C. Lacasta g , V. Latonova j , J. Loenker d , D. Madaffari g , M. Mikestikova j , M. Miñano g , S.Y. Ng f ,R.S. Orr l , U. Parzefall e , D. Rodriguez g , U. Soldevila g , J. Stastny j , M. Stegler c , J. Suzuki m ,R. Teuscher l , Y. Unno h , S. Wada m , M. Wiehe e , L. Wiik-Fuchs e , F. Wizemann d , V. Zahradnik ja Physics Department, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6, CanadabCentro Nacional de Microelectronica (IMB-CNM, CSIC), Campus UAB-Bellaterra, 08193 Barcelona, Spain
Deutsches Elektronen-Synchrotron, Notkestraße 85, 22607 Hamburg, Germany
Experimentelle Physik IV, TU Dortmund, Dortmund, GermanyePhysikalisches Institut, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg-im-Breisgau, GermanyfInstitut für Physik, Humboldt-Universität zu Berlin, Newtonstraße, Berlin, Germany
g Instituto de Física Corpuscular (IFIC) - CSIC-University of Valencia, Parque Científico, C/Catedrático José Beltrán 2, E-46980 Paterna, Spainh IPNS, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japani Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, Prague, CZ18000, Czech RepublicjAcademy of Sciences of the Czech Republic, Institute of Physics, Na Slovance 2, 18221 Prague 8, Czech RepublickSanta Cruz Institute for Particle Physics (SCIPP), University of California, Santa Cruz, CA 95064, USAl
Department of Physics, University of Toronto, 60 Saint George St., Toronto M5S 1A7, Ontario, CanadamInstitute of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japancd

ARTICLEINFO

Keywords:
HL-LHCATLAS ITkn+ -in-p microstrip sensor
Stereo annulus
Bulk properties
Surface properties
Surface irradiation

ABSTRACT
A novel microstrip sensor geometry, the stereo annulus, has been developed for use in the end-cap of the ATLAS
experiment’s strip tracker upgrade at the HL-LHC. Its first implementation is in the ATLAS12EC sensors, a largearea, radiation-hard, single-sided, AC-coupled, n+ -in-p design produced by the ITk Strip Sensor Collaboration andfabricated by Hamamatsu Photonics in early 2017. The results of the initial testing of two ATLAS12EC batches
are presented here with a comparison to specification. The potential of the new sensor shape to reinvigorate
endcap strip tracking is explained and its effects on sensor performance are isolated by comparing the bulk
mechanical and electrical properties of the new sensor to the previous iteration of prototype, the conventionalbarrel ATLAS12A sensor. The surface properties of the new sensor are evaluated for full-size unirradiated sensors
as well as for mini sensors unirradiated and irradiated with protons up to a fluence of 2.2 × 1015 neq cm−2 . The
results show that the new stereo annulus ATLAS12EC sensors exhibit excellent performance and the expectedirradiation evolution.

1. Introduction and the stereo annulus geometry
In the mid-2020s the Large Hadron Collider (LHC) will be upgraded
into a High-Luminosity (HL-LHC) phase which will allow significant
increases in the instantaneous and integrated luminosity targets of theexperiment, up to 7.5×1034 cm−2 s−1 and 4 ab−1 , respectively. Occupancy,
readout, and radiation damage are the main concerns for the detectors
which sit poised to capture the order of magnitude more premier physics
data than was originally planned for the LHC. For the general purpose
ATLAS detector [1] the more intense collision environment requires thecomplete replacement of the innermost tracking component to be able
to operate for the full high luminosity phase.

The Inner Tracker (ITk) is the all-silicon replacement to the current
system. It utilizes two detection geometries, strips and pixels, thegranularity and layout of which have been optimized iteratively throughextensive data-driven simulation [2]. The strip portion of the ITk, called
the ITk Strip Detector, fills the majority of the volume; it is the full
6 m long extent of the inner cavity and extends from a radius of about
30 cm to the inner surface of the calorimeter assembly at a radius of
1 m. The Strip Detector layout is conventional for a collider experiment;
cylindrically symmetric with a four layer barrel in the central region(|𝑧| ≤ 1.4 m) and two endcaps each comprised of six disks filling theremaining volume on either end. The disks are made of 32 identical∗ Corresponding author.
E-mail address: robert.francis.hunter@cern.ch (R.F.H. Hunter).

https://doi.org/10.1016/j.nima.2018.08.031
Received 16 February 2018; Received in revised form 5 June 2018; Accepted 11 August 2018
Available online 16 August 20180168-9002/© 2018 Elsevier B.V. All rights reserved.

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Nuclear Inst. and Methods in Physics Research, A 924 (2019) 142–146

of constant lengths and angular pitch. The lengths ranging from 19 mm to
32 mm have been chosen for occupancy, radiation-hardness, and readoutconsiderations. The angular pitch of 193 μrad in the inner two segments
and 172 μrad in the outer two segments produce pitch variations of
73.5 μm to 83.9 μm across the sensor surface allowing 4360 strips to bearranged in manners compatible with the readout electronics assembly.
Float-zone, ⟨1 0 0⟩ p-type silicon wafers approximately 320 μm thick,are used. Further details of the ATLAS12EC design are presented inRef. [13].
Roughly 60 ATLAS12EC sensors from two batches are examined fortheir bulk character. The results are compared to their conventionallyshaped kin, the ATLAS12A, consisting of 120 sensors (Ref. [14]). They
are comprised of 5128 parallel strips arranged with a 74.5 μm pitch infour segments 23.9 mm in length within a bias ring area of 91.8 cm2 . The‘A’ denotes ‘axial’ the variant of ATLAS12 with the strips aligned along
one of the natural axis of the square sensor.
The ATLAS12EC full-size sensors are accompanied by 1 cm×1 cm
mini sensors. They come in several varieties with variant pitch. Threevalues of 70 μm, 75 μm and 80 μm (denoted narrow, default, and wide
respectively) are used to approximate the range on the full size sensors.
In Section 4 the surface results of 20 mini sensors, unirradiated andirradiated to NIEL fluences of 0.5, 1.1 and 2.2 × 1015 neq cm−2 arepresented along with the surface characterization of a few unirradiatedfull-size sensors. The irradiation is carried out with 70 MeV protons
from the CYRIC1 facility with all samples annealed for 80 min at 60 ◦ C
to the minimum of the annealing curve. For these sensors the main
surface damage mechanism is ionizing interactions in the AC-coupling
SiO2 layer. It can be noted that a CYRIC fluence of 1.0 × 1015 neq cm−2
corresponds to a TID in silicon of ∼80 Mrad [8]. These interactions canseed trapping sites in the oxide volume and at the SiO2 -Si interface. The
relative e− and h+ mobilities in SiO2 as well as the field configurationof the n+ -in-p type device leads to the buildup of positive space charge
in the oxide volume and in particular near the silicon interface [5,7].
This as well as the increase in trapping sites at the interface leads togreater charge exchange and a reinforced inversion layer which hasconsequences for the sensor operation such as reduced channel isolationand increased surface leakage current [8,12].
The ATLAS12EC full-size and mini sensors have been tested at 12
institutes. Unirradiated sensors are tested in cleanroom conditions of
20 ◦ C to 30 ◦ C and relative humidity of 10% to 40%. Dry storage (<5%relative humidity) is used between tests. The irradiated devices aretested and stored cold (−10 ◦ C to −30 ◦ C) to avoid further annealing.

double-sided ‘petals’ (annular segments) each with nine sensors per sidein six designs.
The ITk Strip Detector will utilize an entirely new sensor geometry,the stereo annulus, which possesses several advantages over the conventional configuration. In detectors like ATLAS the magnetic field isaligned along the beam pipe causing charged particle trajectories tobend in the transverse plane. In the endcaps the strips are oriented
radially to allow the most sensitive feature on the sensors, the strip
pitch, to capture the most significant dimension, the bending angle. To
avoid an insufficient resolution in the radial distance from the beam
pipe (obtained by centimeter long strips), the strips are rotated by asmall degree, called the stereo angle. In this manner the strips from thesensors on the two sides of a disk make a diamond shaped grid allowing
a much more precise measurement.
Conventionally these tracking requirements are filled by taking
a symmetric trapezoidal wedge shaped sensor and rotating it while
placing it on the support structure. This is the approach used in thecurrent ATLAS strip tracker. Since the sensors are mounted on circularannular disks the straight tangential edges of the conventional approach
are suboptimal leaving incomplete coverage at the radial extrema of thedisk and requiring unequal strip lengths even in a single sensor segment.
Furthermore the rotated placement of the sensors leads to increased
logistical load as they must be distributed in manners that are congruent
with strict hermeticity and radiation length requirements. These issues
are elegantly solved when moving to the aptly named stereo annulus
design. The radial edges and the strips on the surface have been rotatedby the stereo angle while the tangential edges have been replacedby circular arc segments concentric on the beam pipe center. From ageometrical standpoint this design is optimized for the annular disk
endcap environment yielding the best coverage and channel uniformity
with the most efficient stereo angle implementation. A schematic viewof the stereo annulus geometry is shown in Fig. 1 along with a depictionof its first implementation in the ATLAS12EC.
In this document the initial test results for the first-ever stereo annulus sensor fabrication are discussed. These are the ATLAS12EC a third
iteration of novel, large-area, radiation-hard sensor designed by the ITk
Strip Sensor Working Group and the first endcap prototype. Producedin early 2017 by Hamamatsu Photonics [3] the design originated in2012 and is similar to the contemporary barrel prototype; the ATLAS12
fabricated in 2014 [4]. This kinship makes a comparison of the bulk
characteristics of the ATLAS12 and ATLAS12EC perfect for isolating theeffect of the stereo annulus geometry on sensor performance.
The 4 ab−1 integrated luminosity target involves unprecedented particle fluences resulting in ∼ 1.6 × 1015 neq cm−2 non-ionizing energy loss
(NIEL) and ∼ 67 Mrad total ionizing dose (TID) in the ITk Strip Detector
volume (includes 1.5 safety factor). Surface as well as bulk damage
is expected [5–9]. In this document the surface characteristics of the
ATLAS12EC wafers before and after proton irradiation are contrasted
while Ref. [10] contains the bulk irradiated results.

3. Bulk sensor characteristics
3.1. Mechanical properties
The ATLAS12EC technical specifications place a number of restrictions on the mechanical character of the sensors, among these are thesensor flatness (<200 μm unstressed), thickness ((310 ± 25) μm), and edge
quality (no chips or cracks extending 50 μm or more inwards). The last ofthese is checked by a visual inspection of the sensor under magnification,
which all sensors passed. The first two quantities are obtained with noncontact (optical) measurement techniques with accuracy of a few micron(≤5 μm) in all three dimensions.
The sensor flatness (or bow) is measured in a grid of 100–130 points
across the surface. The data is analyzed and corrected to remove thetilt of the freely sitting sensor. The flatness or bow is extracted as
the difference between the highest and lowest points on the sensor.
The obtained profiles for 40 measured sensors follow the dome-like
expectation (higher in the center of the wafer). The average bow and
its RMSE are 51.18 μm and 9.30 μm respectively. These values are wellwithin the specification and compare quite closely with a similarly
measured sample of 100 ATLAS12A (51.72 μm and 12.36 μm) [14].

2. Test samples and irradiation
The ITk Strip sensor prototypes use a radiation-hard AC-coupled, n+ in-p design. This allows the cost-effective use of single-sided sensors
that are resistant to bulk leakage current increase and that can still
operate effectively under partial depletion [4]. The one major disadvantage of the n+ -in-p material, that of strip isolation, is overcome
with p+ traces (p-stops) whose doping level and geometry have been
optimized iteratively [11,12]. Also implemented are optimized punchthrough protection (PTP) structures, see Refs. [4,8,12], which protect
the coupling dielectric in the event of large charge liberation in the bulk
(in the case of beam splash).
The ATLAS12EC has a large surface area of ∼ 90.0 cm2 in a design
which mimics the sensor closest to the beam pipe (the Ring-0 or R0).
The annular edges of this stereo annulus sensor are obtained with 16 flatcuts each. A 20 mrad stereo angle is implemented in four strip segments

1

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Cyclotron and Radioisotope Center, Tokohu University, Japan.

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Nuclear Inst. and Methods in Physics Research, A 924 (2019) 142–146

Fig. 1. The concept of the stereo annulus design is shown schematically in the left of this figure. A photograph of its first implementation in the ATLAS12EC is shown at right. Note thatthe schematic has the stereo angle skewed left while the ATLAS12EC has it skewed right.

The thickness of the sensors is measured in a minimum of fivelocations on the sensor. Measurements of a few sensors at the test sites
have yielded thicknesses in the range 320 μm to 325 μm, values which
are safely within specification and closely match the supplier’s results.
3.2. Depletion capacitanceThe bulk capacitance of the sensors is obtained as a function ofreverse bias voltage to determine the full depletion voltage, 𝑉𝐹 𝐷 , of thesensors. An LCR meter (such as the Wayne Kerr 6440B) in R-C seriesmode is used to apply the AC probe voltage at a frequency of 1 kHz.
This frequency is chosen as it lies on the flat part of the bulk capacitanceversus frequency curve as measured for the ATLAS12 [14].
The CV curves of 49 unirradiated, full-size ATLAS12EC sensors areshown in Fig. 2. The inverse square of the bulk capacitance is plottedon the 𝑦-scale to allow easy extraction of the full depletion voltage.
Extremely high consistency is obtained in the 49 sensors with theoutlier points at high voltages attributable to probe contact issues. The
histogrammed results for the extracted 𝑉𝐹 𝐷 are shown in Fig. 3 for
58 sensors, with an average of 303.04 V and an RMSE of 4.17 V. The
difference in the number of sensors entering these plots arises from aparasitic capacitance at one site which affected the capacitance values
but not the full depletion voltage.
Taking the bias ring enclosed area as the active area and assuming
a parallel plate capacitor model allows one to obtain estimates of
the active thickness and bulk resistivity from the measured 𝑉𝐹 𝐷 andcapacitance at full depletion [4]. The active thickness calculated was
302.3 μm or 93% of the full thickness, a value that agrees qualitatively
with the edge-TCT measurements in Ref. [10]. The calculated bulk
resistivity was (3.24 ± 0.03) kΩ cm which meets the specification of≥3.0 kΩ cm.
Although the ATLAS12 have a different wafer resistivity (∼2.5 kΩ cm)a meaningful comparison can still be made by looking at the RMSE of
the 𝑉𝐹 𝐷 and the active thickness percentages respectively. The ATLAS12𝑉𝐹 𝐷 obtained had a RMSE of 8.61 V for a sample of 98 sensors [14]. The
lower RMSE of the ATLAS12EC indicates a better processing consistency
as could be expected from its later fabrication. The active thickness
calculated for the ATLAS12 (∼94%) is in very close agreement with the
ATLAS12EC indicating similar high quality processing of the backside
implant.

Fig. 2. Inverse square bulk capacitance versus reverse bias voltage for 49 unirradiated,
full-size ATLAS12EC sensors. The outlying point on the high voltage plateau can be
attributed to poor contact.

cm cmFig. 3. Histogram of the ATLAS12EC 𝑉𝐹 𝐷 . An average of 303.04 V and an RMSE of 4.17 V
is obtained for 58 unirradiated, full-size sensors.

before the measurement is taken. For the unirradiated ATLAS12EC thespecifications of a leakage current less than 2 μA cm−2 at 20 ◦ C and 600 V
as well as stability up to 600 V are used.
The leakage current curves for 59 unirradiated, full-size ATLAS12EC
sensors are shown in Fig. 4. Here the current has been normalized by
the bias ring area and to 20 ◦ C using the well-established temperaturedependence, Ref. [15]. A minority of sensors showing early breakdown
required ‘training’ in which they were ramped much more slowly before
a normal measurement was made. The plot shows that the majority of
sensors exhibited ideal IV character showing a stable current plateau
and a small kink at the full depletion voltage attributable to surface

3.3. Reverse bias leakage current
The reverse bias leakage current has been obtained as a functionof voltage to ensure that it is stable and within specification. Data is
obtained with pico-amp precision source measurement units (SMUs)
such as the Keithley 2410 with the reverse bias voltage ramp in stepsof 10 V from 0 V to 1 kV. A 10 s settle period is allowed at each step144

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Nuclear Inst. and Methods in Physics Research, A 924 (2019) 142–146

cm cm

Fig. 4. Reverse bias leakage currents as a function of reverse bias voltage for 59
unirradiated, full-size ATLAS12EC sensors, normalized by the bias ring enclosed area andto 20 ◦ C.

Fig. 5. Interstrip resistance, 𝑅𝑖𝑛𝑡 , evolution with proton fluence at 400 V reverse bias andnormalized to −20 ◦ C. Comparison to previous prototype iterations, the ATLAS07 and
ATLAS12, is made.

generated currents as the depletion front encompasses the backplane
contact [4]. A small number of sensors show signs of instability. An
average of 1.95 nA cm−2 with an RMSE of 1.87 nA cm−2 is obtained. Thisis three orders of magnitude within specification. It also compares wellto an average of 14.72 nA cm−2 with RMSE of 59.35 nA cm−2 obtained for
118 ATLAS12 [14].
The level of instability observed in the ATLAS12EC IV curves is very
reasonable when compared to the ATLAS12. For the ATLAS12EC, 7%
and 15% of the measured sensors show this feature at voltages below
600 V and 1000 V reverse bias respectively. For the 118 conventionallyshaped ATLAS12 the values are 14% and 28% respectively, giving
confidence to the conclusion that the instability is not related to thestereo annulus shape. In fact a number of results (not shown) havebeen gathered which support the hypothesis that surface charges are
the culprit, the main evidence being the humidity sensitivity of the
IV characteristics. Complicated relations between microdischarge onset
and long term instability with the ambient humidity are observed forboth generations of sensor and are the subject of further studies.

4.3. Interstrip resistance
The interstrip resistance, 𝑅𝑖𝑛𝑡 , is the resistance between neighboringn+ implants. For effective DC isolation of the channels through thesilicon bulk 𝑅𝑖𝑛𝑡 must be much larger than 𝑅𝑏𝑖𝑎𝑠 . A specification of> 10 × 𝑅𝑏𝑖𝑎𝑠 at a strip region depleting bias voltage of 300 V is applied.
𝑅𝑖𝑛𝑡 is measured by contacting three neighboring implants and applying
a small test voltage (up to ±5 V in 1 V steps) to the outer ones while
reading the current at the center from the voltage across its bias
resistor. High impedance, high isolation devices such as the Keithley
6517 and 2000 are used. A small number of strips on a few full-size
sensors have been measured with all results residing above 10 GΩ. A
flat bias voltage evolution is observed in all unirradiated sensors. For
irradiated mini sensors, a decrease in the interstrip resistance withfluence is expected from the oxide’s positive space charge and interface
trap build-up as well as, less importantly, from NIEL damage to thebulk [8,12]. This decrease is observed in conjunction with a positive
reverse bias voltage dependency which is also expected (the inversion
layer shrinks as bias voltage increases). The strip isolation specificationis already met at ∼50 V for the highest fluence tested indicating morethan sufficient performance of the optimized p-stops implemented in
the ATLAS12EC n+ -in-p design. The temperature dependence of 𝑅𝑖𝑛𝑡 isunderstood (Ref. [8]) and is used to normalize the 400 V reverse bias
point to −20 ◦ C for comparison to previous sensor generations. The
updated plot of Fig. 10 from Ref. [8] is shown in Fig. 5 where the
ATLAS12EC find good agreement.

4. Sensor surface properties
4.1. Strip coupling capacitanceThe coupling capacitance, 𝐶𝑐𝑜𝑢𝑝 , is the capacitance between the strips
and the implants. It must be large to allow the signal to be extracted
efficiently and consistent for optimal channel response. A specificationof ≥20 pF cm−1 at 1 kHz is set. Measurements of the coupling capacitancehave been conducted on a full strip segment of 1026 strips with a resultof (25.21 ± 0.02) pF cm−1 as well as for a few strips across the sensor
surface for a small number of sensors with all results falling in the range
24.89 pF cm−1 to 28.37 pF cm−1 . Measurements of 𝐶𝑐𝑜𝑢𝑝 have also been
conducted on the irradiated mini sensors with negligible change after
proton irradiation up to the maximal fluence.

4.4. Interstrip capacitance
The interstrip capacitance, 𝐶𝑖𝑛𝑡 , is the capacitance between neighboring top metal strips. In this AC design it is the dominant sourceof noise at the charge-sensitive front-end readout chip and must be
minimized. A value of <0.9 pF cm−1 at 300 V and 100 kHz is specified.
From a few strips measured on a small number of sensors values of
𝐶𝑖𝑛𝑡 ranging from 0.61 pF cm−1 to 0.89 pF cm−1 have been obtained. Theinterstrip capacitance dependency on reverse bias voltage at 100 kHz
is shown in Fig. 6 for at least one strip from each segment of a fullsize sensor. The variant peak at very low bias voltage is attributable tothe variable surface state at the start of measurement (dependent upon
sensor bias and humidity history). Matching expectations (see Ref. [8]),the measurements on the mini sensors depict an almost flat bias voltage
evolution of 𝐶𝑖𝑛𝑡 for the irradiated devices saturated at the pre-irradiatedfull depletion value.

4.2. Bias resistance
The resistance of the polysilicon traces connecting the implants
to the bias rail, 𝑅𝑏𝑖𝑎𝑠 , is measured to ensure that the value is large
enough for effective bias dispersion and isolation of the channels. Highconsistency is also required in this feature to avoid any transverse
field gradients in the bulk. As such (1.5 ± 0.5) MΩ is specified for
the ATLAS12EC design. Like the coupling capacitance this value has
been measured for all 1026 strips of a single segment as well as fora number of strips across the surface of a few other full-size sensors.
Here the results are (1.59 ± 0.01) MΩ for the segment and 1.47 MΩ
to 1.52 MΩ for the other tests with a flat bias voltage dependency.
For irradiated mini sensors a small positive fluence dependency aswell as a negative temperature dependency were observed, matching
expectations [8,12]. The essentially flat dependency versus reverse bias
voltage is maintained.

4.5. Punch through protection
In the event of miscontrolled beam during HL-LHC activity large
charge liberation in the bulk may result in the voltage on the implants
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Nuclear Inst. and Methods in Physics Research, A 924 (2019) 142–146

cm cm

Acknowledgments
The research was supported and financed in part by Canada Foundation for Innovation, the National Science and Engineering ResearchCouncil (NSERC) of Canada under the Research and Technology Instrumentation (RTI) grant SAPEQ-2016-00015; the Ministry of Education,
Youth and Sports of the Czech Republic coming from the projectLM2015058—Research infrastructure for experiments at CERN; theFederal Ministry of Education and Research, BMBF, Germany; USA
Department of Energy, Grant DE-SC0010107; the Spanish Ministry of
Economy and Competitiveness through the Particle Physics National
Program, ref. FPA2015-65652-C4-4-R (MINECO/FEDER, UE), and cofinanced with FEDER funds.

Fig. 6. Interstrip capacitance evolution as a function of bias voltage at a frequency of
100 kHz for at least one strip from each segment of a full-size sensor (Test and Bond pad
indicate measurements done at different ends of the strips of that segment).

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increasing up to half of the bias value [12]. With the breakdown voltageof the coupling capacitor dielectric layer specified as 100 V the useof the PTP structures [4,8] which allow a controlled soft breakdown
between implant and bias rail is necessary. The PTP effectiveness isquantified by the onset voltage of breakdown which should be muchsmaller than 100 V, the rate of decrease of the gap’s effective resistanceafter breakdown which should be high, and the current flow at 100 V
which should be maximal. These values are measured directly between
implant and rail. A typical onset voltage of less than 20 V with a steep
decrease in the effective resistance and a large current flow even at 40 V
are obtained for the unirradiated sensors. The PTP was also measured forthe proton irradiated mini sensors. The largest effect is a moderation in
the decrease rate of the effective resistance after breakdown. For thehighest fluence point the effective resistance still falls below 0.2 MΩ
before 30 V indicating excellent PTP performance.
5. Summary and conclusions
The initial results for the bulk and surface characteristics of the
ATLAS12EC n+ -in-p sensor have been reported along with the surface
properties of proton irradiated mini sensors. The sensors showed excellent consistency in both bulk and surface quality with all structures
performing as expected and within specification. The ATLAS12EC sensors are unique as they are the first to ever use the stereo annulus
geometry, a feature which provides considerable advantages over theconventional design. Comparison to the more conventionally designed
ATLAS12 sensors showed that no deleterious effects on sensor operation
were introduced by the optimized stereo annulus geometry.

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