Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A
journal homepage: www.elsevier.com/locate/nima

Development of n þ -in-p large-area silicon microstrip sensors for very
high radiation environments – ATLAS12 design and initial results
Y. Unno i,n, S.O. Edwards a, S. Pyatt a, J.P. Thomas a, J.A. Wilson a, J. Kierstead b, D. Lynn b,
J.R. Carter c, L.B.A. Hommels c, D. Robinson c, I. Bloch d, I.M. Gregor d, K. Tackmann d,
C. Betancourt e, K. Jakobs e, S. Kuehn e, R. Mori e, U. Parzefall e, L. Wiik-Fucks e, A. Clark f,
D. Ferrere f, S. Gonzalez Sevilla f, J. Ashby g, A. Blue g, R. Bates g, C. Buttar g, F. Doherty g, L. Eklund g,
T. McMullen g, F. McEwan g, V. O'Shea g, S. Kamada h, K. Yamamura h, Y. Ikegami i, K. Nakamura i,
Y. Takubo i, R. Nishimura j, R. Takashima j, A. Chilingarov k, H. Fox k, A.A. Affolder l, P.P. Allport l,G. Casse l, P. Dervan l, D. Forshaw l, A. Greenall l, S. Wonsak l, M. Wormald l, V. Cindro m,
G. Kramberger m, I. Mandic m, M. Mikuz m, I. Gorelov n, M. Hoeferkamp n, P. Palni n, S. Seidel n,
A. Taylor n, K. Toms n, R. Wang n, N.P. Hessey o, N. Valencic o, Y. Arai p, K. Hanagaki p, Z. Dolezal q,
P. Kodys q, J. Bohm r, M. Mikestikova r, A. Bevan s, G. Beck s, S. Ely t, V. Fadeyev t, Z. Galloway t,
A.A. Grillo t, F. Martinez-McKinney t, J. Ngo t, C. Parker t, H.F.-W. Sadrozinski t, D. Schumacher t,
A. Seiden t, R. French u, P. Hodgson u, H. Marin-Reyes u, K. Parker u, S. Paganis u, O. Jinnouchi v,
K. Motohashi v, K. Todome v, D. Yamaguchi v, K. Hara w, M. Hagihara w, C. Garcia x, J. Jimenez x,
C. Lacasta x, S. Marti i Garcia x, U. Soldevila xaSchool of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom
Brookhaven National Laboratory, Physics Department and Instrumentation Division, Upton, NY 11973-5000, USAc
Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdomd
DESY, Notkestrasse 85, 22607 Hambrug, GermanyePhysikalisches Institut, Universitt Freiburg, Hermann-Herder-Str. 3, D-79104 Freiburg, Germanyf
DPNC, University of Geneva, 24, Quai Ernest-Ansermet, CH-1211 Genve 4, Switzerlandg
SUPA – School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United KingdomhSolid State Div., Hamamatsu Photonics K.K., 1126-1, Ichino-cho, Higashi-ku, Hamamatsu-shi, Shizuoka 435-8558, Japani
Institute of Particle and Nuclear Study, KEK, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japanj
Department of Science Education, Kyoto University of Education, Kyoto 612-8522, Japank
Physics Department, Lancaster University, Lancaster LA1 4YB, United KingdomlOliver Lodge Laboratory, Department of Physics, University of Liverpool, Oxford St., Liverpool L69 7ZE, United KingdommJosef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, SlovenianDepartment of Physics and Astronomy, University of New Mexico, MSC07 4220, 1919 Lomos Blvd. NE, Albuquerque, NM 87131, USAo
Nikhef, Science Park 105, 1098 XG Amsterdam, NetherlandspDepartment of Physics, Osaka University, Machikaneyama-cho 1-1, Toyonaka-shi, Osaka 560-0043, JapanqCharles University in Prague, Faculty of Mathematics and Physics, V Holesovickach 2, Prague 8, Czech Republicr
Academy of Sciences of the Czech Republic, Institute of Physics, Na Slovance 2, 18221 Prague 8, Czech RepublicsSchool of Physics and Astronomy, Queen Mary University of London, London E1 4NS, United Kingdomt
SCIPP, University of California, Santa Cruz, CA 95064, USAu
Department of Physics and Astronomy, The University of Shefﬁeld, Hicks Building, Hounsﬁeld Road, S3 7RH Shefﬁeld, United Kingdomv
Institute of Science and Engineering, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japanw
Institute of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-9751, Japanx
IFIC (Centro Mixto CSIC-UVEG), Ediﬁcio Investigacion Paterna, Apartado 22085, 46071 Valencia, Spainb

art ic l e i nf o

a b s t r a c t

Available online 14 July 2014

We have been developing a novel radiation-tolerant n þ -in-p silicon microstrip sensor for very highradiation environments, aiming for application in the high luminosity large hadron collider. The sensors
are fabricated in 6 in., p-type, ﬂoat-zone wafers, where large-area strip sensor designs are laid out

Keywords:
Silicon stripnCorresponding author.
E-mail address: yoshinobu.unno@kek.jp (Y. Unno).

http://dx.doi.org/10.1016/j.nima.2014.06.086
0168-9002/& 2014 Elsevier B.V. All rights reserved.

Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90
n þ -in-p
P-typeRadiation-tolerant
HL-LHC
PTP

81

together with a number of miniature sensors. Radiation tolerance has been studied with ATLAS07
sensors and with independent structures. The ATLAS07 design was developed into new ATLAS12 designs.
The ATLAS12A large-area sensor is made towards an axial strip sensor and the ATLAS12M towards a
stereo strip sensor. New features to the ATLAS12 sensors are two dicing lines: standard edge space of910 μm and slim edge space of 450 μm, a gated punch-through protection structure, and connection oforphan strips in a triangular corner of stereo strips. We report the design of the ATLAS12 layouts andinitial measurements of the leakage current after dicing and the resistivity of the wafers.
& 2014 Elsevier B.V. All rights reserved.

1. Introductionþ

We have been developing n -in-p silicon microstrip sensors
with p-type silicon wafers aiming for a cost-effective andradiation-tolerant solution for covering an area over 100 m2 invery high radiation environments. A speciﬁc application of such asilicon microstrip sensor is a replacement of the outer part of theinner tracker in the ATLAS detector [1] for the high luminosity
large hadron collider (HL-LHC) [2].
The current inner tracker of the ATLAS detector consists of a pixel
tracker (Pixels) at radii of 5–12 cm, a silicon microstrip tracker (SCT/
Strips) at 30–51 cm (Barrel cylinders) and at 28–56 cm (Endcap discs)
and a transition radiation tracker (TRT) at 56–107 cm. The siliconsensor area is 2.7 m2 and 62 m2 in Pixels and Strips, respectively. For
the HL-LHC, not only the entire inner tracker is to be replaced but also
the TRT is to be replaced with Strips [3]. The latest layout of the innertracker is Pixels at 4–25 cm and Strips at 40–100 cm. The silicon area is
8.2 m2 and 193 m2 (122 Barrel and 71 Endcap per m2), respectively. In
both Pixels and Strips, the areas are increased to approximately threetimes that of the original ATLAS detector.
With the integrated luminosity of 3000 fb  1 and a safety factor 2, a
number of particles passing through an unit area in the tracker volume
has been simulated [4], as shown in Fig. 1. The typical ﬂuences
are approximately: 2.2  1016 1 MeV-neutrons equivalent (neq )/cm2
at 3.7 cm for the Pixels, and 1  1015 neq=cm2 at 31 cm and 5 1014 neq=cm2 at 60 cm for the Strips. Charged particles dominate inside
and neutral particles, mainly neutrons, dominate outside a radius of
25 cm. Our goal of the radiation-tolerant silicon microstrip sensor is tocope with a ﬂuence of Z 1  1015 neq=cm2 and to be robust against
both charged and neutral particles.

2. Radiation-tolerant n þ -in-p silicon strip sensors
The sensor of n þ -implant readout in p-type silicon substrate(n -in-p) is intrinsically more radiation-tolerant. This is becauseþ

the p–n junction develops from the readout implants before andafter radiation damage as the radiation-induced levels in thesilicon band-gap are primarily acceptor states (p-type) [5]. The
fact leads to a number of virtues: (1) the sensors can be operated
under partially depleted conditions when the full depletion
voltage is extremely high after radiation damage; (2) the signals
are generated by collecting electrons and are larger, as comparedwith collecting holes, because of a faster drift velocity and lesscharge-trapping in silicon; (3) the sensors can be fabricated withmask-processes only on the segmented side and with diffusion
processes on the backside, i.e., the so-called “single-side process”.
The fabrication cost in this case is 30–40% less than the “doubleside process” where both sides are processed with masks; (4) the
sensors can be robust against handling because of the absence ofsegmentation and junctions on the backside. Points (1) and (2) are
not applicable to p þ -in-n sensors, while they are possibly applicable to n þ -in-n sensors. Points (3) and (4), however, ensure thatn þ -in-p single-side process sensors are more cost-effective thann þ -in-n double-side process sensors.
2.1. ATLAS07 sensors
We have fabricated large-area strip sensors of 9.75  9.75 cm2,
known as the ATLAS07 sensor, together with 1  1 cm2 miniature
sensors, in 6 in., p-type, ﬂoat-zone (FZ), 320 μm thick wafers [6].
The large-area main sensor is segmented into four blocks ofshort strips with a strip length of 2.39 cm each in order to reduce
the number of particles simultaneously impinging on a strip(its fraction to total impinging particles known as “occupancy”).
Initial evaluation of the ATLAS07 sensors was reported for leakage
currents, depletion voltages, and interstrip resistances and capacitances before and after radiation damage [7–10].
Further irradiations have been carried out at CYRIC [11] for
70 MeV and KIT [12] for 23 MeV protons, at PSI [13] for 300 MeV
pions, and at Ljubljana [14] for reactor neutrons. Irradiatedsamples have been kept cold (  20 1C) and “annealed” by heating
approximately 80 min at 60 1C for minimizing radiation damage
effect. Charge collection has been measured before and after“annealing”, typically on the AliBaVa system [15] using a 90Sr β
source. The latest results of collected charges after “annealing” areshown in Fig. 2 for two bias voltages. The ﬂuences of different
projectiles and energies are scaled with the non-ionizing energy
loss factors (NIEL) [16]. The collected charges at a ﬂuence of 1 1015 neq=cm2 are approximately 14,000 and 17,000 electrons at 500
and 900 V bias voltages, respectively. With a typical input noise of
600-800 e, the signal-to-noise ratios will be  18–23 and  21–28,
respectively.
2.2. Edge space and punch-through protection (PTP) studies

Fig. 1. Expected particle ﬂuences from simulations in the inner tracker of the
ATLAS detector for the HL-LHC [4]. The transition from the pixel to the strip
detector is at r¼ (approximately) 30 cm.

An edge space in a silicon sensor is a required space to hold abias voltage but is a dead space. A required edge space was studied
in 4  4 mm2 square diodes independent of the ATLAS07 sensors
by varying the space in one edge out of four [17]. An edge space of82

Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90

450 μm was derived to be the minimum for holding a bias voltageof 1 kV initially and as radiation damage accumulated.
The PTP structure is to protect the AC-coupling capacitor
formed with the strip implant and the strip metal when thevoltage of the strip implant has departed from the voltage of the
strip metal. Such a case would happen, e.g., when an accidental
beam splash deposited a very large amount of charge in the siliconbulk, and the resulting current ﬂowing through the bias resistor
generated a large voltage drop to the strip implant. The PTP
structure creates a short-circuiting current path in parallel to thebias resistor to limit the voltage drop to the strip. A novel conceptof “gated” PTP structure was introduced by placing the potential of
the bias ring over the PTP gap [18,19]. The “gated” PTP structures
were effective to improve the PTP performance [17]. Among thestructures studied, the “full-gate” PTP structure was shown to havethe smallest onset voltage, the sharpest decrease of resistance andthe smallest resistance at saturation.

3. ATLAS12 wafer layouts
Developing from the ATLAS07 sensors, we have designed a new
set of strip sensors, the ATLAS12A and the ATLAS12M large-area
main sensors and miniature sensors in 6-in. wafer. The wafer
layouts of the ATLAS12A and the ATLAS12M sensors are shownin Fig. 3. Both ATLAS12A and ATLAS12M large-area main sensors
are made to have four “blocks of short strips” (segment). The
ATLAS12A main sensor is prototyping an “axial” strip sensor where
the strips are running parallel to the sensor edges. The ATLAS12M
main sensor is prototyping a “stereo” strip sensor, having “stereo”
geometry in the bottom two segments where the strips are
inclined to the sensor edges at an angle of 40 mrad. In the
peripheral area of each layout, a number of miniature sensors
are implemented with variations of the features in the main
sensors and with prototypes of the endcap sensors with a fan
geometry (see Appendix). Speciﬁcations of the ATLAS12 sensors
are summarized in Table 1. New features developed from the
ATLAS07 into the ATLAS12 main sensor are the implementation of
two dicing lines, a gated PTP structure, an arrangement of thewire-bonding pads for the new readout ASIC, and readout ofshorter strips in a triangular corner of the stereo layout.

Table 1
Speciﬁcations of ATLAS12 n þ -in-p large-area silicon microstrip sensor.

Fig. 2. Collected charges measured by ATLAS07 miniature sensors after radiation
damage with 70 MeV (square) and 23 MeV (diamond) protons, pions (triangle), andneutrons (circle) at bias voltages of 500 V (ﬁlled) and 900 V (open). The ﬂuences
are scaled to a unit of 1-MeV neutrons equivalent/cm2 using the NIEL factors.

Silicon wafer diameter
Wafer type
Crystal orientation
Resistivity
Thickness (uniformity)
Number of sections of strip segments
Number of strips per section
Strip pitch (θ: stereo angle)
Strip length (θ: stereo angle)
Angle, θ, of stereo strips
Bias resistance (polysilicon)
Signal readout
Microdischarge onset voltage

6 in. (150 mm)
p-type FZ〈100〉
4 4 kΩ cm (accepting 43 kΩ cm)
3107 25 μm ( 7 5 μm)
41282
74:5  cos θ μm
23.9/cos θ mm
40 mrad
1.5 70.5 M Ω
AC coupling
4 600 V

Fig. 3. ATLAS12A (left) and ATLAS12M (right) wafer layout in 6-in. wafer. In ATLAS12A, all four sections of strip segments, 1–4 of the main sensor are made with “axial” strips
where the strips are parallel to the sensor edge. In ATLAS12M, the top two strip segments (1 and 2) are “axial” strips and the bottom two (3 and 4) are “stereo” strips where
the strips are inclined at an angle of 40 mrad to the sensor edge. The miniature sensors around the perimeter are labeled with the position numbers as 1–24 and the8  8 mm2 diodes as P1–P4.

Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90

83

Fig. 4. Dimensions and PTP structure of ATLAS12A and ATLAS12M large-area sensors. The ﬁducial mark “F” at the corners identiﬁes the orientation of the sensor.

3.1. “Standard” and “slim” dicing
The ATLAS07 sensors were made with an edge space ofapproximately 980 μm (the “standard” to the HPK design).
ATLAS12 sensor is designed with two dicing lines, one with the“standard” edge space and the other with the narrow space ofapproximately 450 μm (the “slim” edge space) in full circumference as shown in Fig. 4. Dicing the sensors with the slim edge
space (“slim” dicing) will verify the applicability of the minimum
width obtained with the test diodes to a real large-area sensor thathave a 100 times longer length in the circumference than thelength studied in the diodes.
3.2. “Full-gate” PTP structure
The ATLAS07 main sensor had no PTP structures implemented,
where the strip-end structure was that of BZ3F in Section A.1. The
ATLAS07 miniature sensors had included the PTP structures of
BZ3E and BZ4B2 (but without an explicit overlap of the bias
resister). No explicit “gated” structure was implemented in the
ATLAS07 sensors. The ATLAS12 main sensors are implemented
with a “full-gate” structure as shown in the inset of Fig. 4. The
ATLAS12 main sensors are the ﬁrst-time implementation of a“gated” PTP structure in a large area sensor to conﬁrm no adverse
effect to the radiation tolerance shown in the ATLAS07 sensors.

Variation of “gated” PTP structures is implemented in the miniature sensors of the ATLAS12 layout for understanding the radiation
tolerance of the structures further (see Section A.1).
3.3. Wirebonding pads in strips
With a new design of 256 channel ASIC, ABCN130 [20], thereadout of strips can be arranged so that the strips in two blocks ofstrips, e.g., the segments 1 and 2 (or 3 and 4) are read out with asingle ASIC, as shown in Fig. 5(a). The scheme reduces the numberof rows of the ASIC's from 4 to 2, thus reducing the material
associated with the ASIC's and the hybrids. Since the pitch of thepads in the ASIC is narrower than the allowed one by the wirebonding machine, the pads will be separated into 4 rows in the
ASIC. The staggering of the pads in the sensor and the ASIC is
mirrored so that the wire-bonds can be separated in height. Thus
the staggering of the pads in the segments 1 and 2 should be thesame. In addition, we design to connect the strips in the segments
1 and 2 to form longer strips. The connection requires thestaggering of the pads in the segments 1 and 2 should be mirrored,as shown in Fig. 5(b). A solution is to have triple rows of staggered
pads in one of the segments as shown in Fig. 5(c). The rows andstaggering of pads in the segments 3 and 4 are 1801 rotationsymmetric relative to the segments 1 and 2 to allow rotation of thesensor. The existence of DC-contact pads at the strip ends to test

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Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90

Fig. 5. (a) Wire-bonding between ASIC and strips to readout two segments of strips with one ASIC. (b) Wire-bonding of strips in two segments of strips to form longer strips.
(c) A solution of triple rows realized in the main sensor in the strip-segments 1 and 2. AC-pads are those for the wire-bonding that are connected to AC-coupled strip metals.
DC-pads are connected to the strip implants directly for quality assurance of the strip implants.

Fig. 6. Connection of orphan strips in the triangular corner of stereo strips that are not connected to readout electronics to readout strips in the segment 4 of ATLAS12M
large-area main sensor.

the continuity of n þ strip implant has constrained the minimum
distance of the wire-bonding pads between the segments 1 and 2(or 3 and 4).

3.4. Connection of orphan strips in the stereo layout
In designing stereo strips in a square sensor there is atriangular corner of shorter strips that may not be connected toreadout electronics. Those orphan strips can be readout by connecting the strips to neighbor strips that are connected to readout
electronics, the so-called “ganging”. Although ganging creates
ghosts of space points, the ghosts can be resolved by reconstructing tracks with other layers. A “ganging” is implemented in the
ATLAS12M main sensor in the segment 4, being composed of 11
strips as shown in Fig. 6. The connection is made to the AC readout
metals in this fabrication (AC-ganging). The connection between
strip implants (DC-ganging) is implemented in miniature sensors
of endcap geometry (see Section A.2).

4. Initial performance of the ATLAS12 sensors
4.1. Leakage current
The wafer processing of 120 ATLAS12A and 45 ATLAS12M
wafers was completed using wafer lots of 6 in., p-type, FZ, and320 μm thick. Out of these wafers, 30 ATLAS12A and 26 ATLAS12M
wafers were diced using stealth dicing technology [21]. For the
ATLAS12A dicing, 25 wafers were given standard and 5 slim dicing.
For the ATLAS12M, 3 wafers were given standard and 23 slim
dicing; thus, in total, 28 standard and 28 slim dicing samples wereobtained. The leakage currents as a function of the bias voltage ofthe large-area main sensors after dicing measured at room
temperature are shown in Fig. 7(a) and (b) with standard dicing
and slim dicing, respectively.
Most of the sensors demonstrated no breakdown in leakage
current up to 1000 V with the leakage current o 8 μA at 1000 V.
Two sensors with standard dicing exhibited microdischarge(MD) breakdown at approximately 750 V, which is still above

Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90

the speciﬁcation of MD breakdown 4 600 V. The ATLAS12M
sensors with slim dicing show a tendency for an increasing leakage current above an onset voltage of approximately 800 V, while
the ATLAS12M sensor with standard dicing and the ATLAS12A
sensor with slim dicing do not. The observed differences may havearrived from subtle differences in the wafer lots and the fabrication processes. Inclusion of the slim dicing, the gated PTP structure, and the ganging has not deteriorated the initial performanceof the large-area sensors.
An upward bump in the leakage current is observed at onset
voltages of approximately 200 V for the ATLAS12M sensors andapproximately 300 V for the ATLAS12A sensors. These onset
voltages are corresponding with the full depletion voltages
observed in Fig. 8, but independent of the edge space of thestandard or slim dicing. The effect would be a surface generated
current in the backside silicon–metal barrier associated with
depletion of the backside implantation when the depletion had
reached the backside.

85

diode implantation once it is fully depleted, the capacitance iscalculated to be 14.3 pF for a thickness of 320 μm. The capacitanceof a thickness W (μm) would be
C  14:3  ð320=Wð μmÞÞ ðpFÞð4Þand its inversion squared is
1=C 2  4:9  10  3  ðW ðμmÞ=320Þ2 ðpF  2 Þ:

ð5ÞThe bias voltage dependence of the capacitance of the diodes
was measured with a LCR meter at a frequency of 1 kHz [22]. The

4.2. Characteristics of wafers
Characteristics of wafer such as resistivity are evaluated by using
8  8 mm2 square diodes from the ATLAS12A and ATLAS12M
wafers, implemented at four places near edge of wafer, denotedby P1–P4 in Fig. 3. The resistivity of the wafer, ρ, can be calculated
from the following equation:ρ ¼ W 2 =ð2 μϵV FD Þ

ð1Þ

 10:6  ðWðμmÞÞ2 =ðV FD ðVÞÞ ðΩ cmÞ

ð2Þ

where W is the active thickness of the diode at full depletion, VFD isthe full depletion voltage, μ is the charge carrier mobility for p-type
wafer (450 cm2 V  1 s  1) and ϵ is the dielectric constant of silicon(11.9ϵ0 ¼ 1.05 pF/cm).
The full depletion voltage, VFD, can be estimated from themeasurement of the capacitance of the diode as a function of
the bias voltage. The capacitance increases as the depletion region
expands as a function of bias voltage and saturates once the diodeis depleted fully. Since the capacitance of diode ispﬃﬃﬃﬃ
C p 1=w p 1= V
ð3Þwhere w is the depletion depth, the VFD can be estimated from thecross point of two straight lines of the 1=C 2 curve as a function of
the bias voltage; one in the depletion expansion and the other inthe depletion saturation.
The active thickness W can be estimated from the saturation
capacitance. By assuming the active area of the diode being
6.6  6.6 mm2 with a lateral depletion of 300 μm outside theFig. 8. Capacitances of the diodes of 8  8 mm2 from ATLAS12A (b–d) and
ATLAS12M (a) wafers as a function of bias voltage measured at a frequency of
1 kHz.

Table 2Wafer parameters as derived from the capacitance of 8  8 mm2 diodes.
Group Sample id's

(a)
(b)

(c)

(d)

ATLAS12M wafers
W03, 07, 08
ATLAS12A wafers
W602-P1, P2, P4
W623-P1, P4; W648P3,P4
ATLAS12A wafers
W602-P3, W623-P2,
P3
W648-P1
ATLAS12A wafer
W648-P2

C (pF) W
1=C 2(μm)(10  3 pF  2)

V FD
(V)

ρ
(k Ω cm)

 4.68

 14.6  313  225  4.5

 4.38

 15.1  303  330  2.9

 4.88

 14.3  320  330  3.3

 6.02

 12.9  355  330  4.0

Fig. 7. Leakage currents as a function of bias voltage of the ATLAS12A and ATLAS12M large-area main sensors after dicing: (a) standard dicing and (b) slim dicing.

86

Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90

capacitances, 1=C 2 , are shown in Fig. 8 as a function of bias voltage.
The samples from the ATLAS12M wafers are in the group (Fig. 8(a)). Those from the ATLAS12A wafers are scattered in three groups(Fig. 8(b)–(d)). The diodes of the ATLAS12A wafers were sampledfrom wafers W602, W623, and W648. No noticeable systematic
dependence on the wafers nor in the position of the diodes isapparent from the groups and the sample id's as in Table 2. The
source of the observed systematic deviations is as yet unknown.
From Fig. 8, the full depletion voltages of the ATLAS12A andATLAS12M wafers are VFD approximately 330 V and 225 V, respectively. With the full depletion voltage VFD and the active thickness
W from the capacitance C at saturation, the resistivity ρ are derived
and summarized in Table 2. The resistivity of the ATLAS12A andATLAS12M wafers is approximately 3 and 4.5 k Ω cm, respectively.
The sensor manufacturer has conﬁrmed the usage of different
batch of wafers that are traced to come from different ingots. The
homogeneity of resistivity at edge is conﬁrmed with the measured
data of the diodes. The resistivity over the wafer, the difference
between center and edge, is measured by the wafer manufacturer
in sampled wafers. The full depletion voltage of ATLAS07 wafer isapproximately 200 V and is consistent with the ATLAS12M wafers.

Table 3
Miniature sensors with or without a PTP structure and their position in ATLAS12
wafer layouts. Identiﬁers are, B for miniature sensors of axial geometry with a strip
pitch of 74.5 μm, Z1 for narrow PTP gap (10 μm) without p-stop isolation, Z2 forindividual p-stop, Z3 for default geometry, Z4 for atoll-type PTP structure, Z6 forwide strip pitch of 100 μm, EC for endcap sensors of stereo-strip fan geometry, ECsmall pitch for square stereo sensor with a strip pitch of 64–65 μm, EC-large pitch
for square stereo sensor with a strip pitch of 102–105 μm, EC-skewed: skewed
stereo sensor with a strip pitch of 65–71 μm.
Id

BZ1
BZ2C
BZ3A
BZ3B
BZ3C
BZ3C-unpassivation
BZ3D
BZ3E
BZ3F
BZ4B2
BZ6C
BZ6E
EC-small pitch-C

5. Summary
We have been developing novel radiation-tolerant n þ -in-psilicon microstrip sensors for very high radiation environments,
aiming for an application in the inner tracker of the ATLAS
detector upgrade for the HL-LHC. Radiation tolerance has beenstudied with ATLAS07 sensors and with independent structures.
Incorporating the results obtained for the minimum edge space forholding 1 kV bias voltage and the novel concept of “gated” PTP
structure, the ATLAS07 design is developed into new ATLAS12ATLAS12A

EC-small pitch-E

EC-large pitch-C
EC-large pitch-E
EC-skewed-C
EC-skewed-E

AC gang
DC gang
No gang
AC gang
DC gang
No gang
AC gang
DC gang
AC gang
DC gang

ATLAS12M

PTP

No PTPPTPNo PTP

––––2, 4, 12, 14, 15
6, 16
–
–
–
–
–
–

1, 11
–
–
–
–
–
–
3, 5, 13
–
–
–

–
2, 14
1, 13
3, 15
6, 18
–
9, 20
–
–
4, 16
5, 12
–

7, 19
–
–
–
–
–
–
10
11
–
–
8, 17

7
17
–
–
–
–
9
19
–
–
21
–

–
–
–
8
18
–
–
–
10
20
–
22

–
–
21, 23
–
–
–

–
–
–
–
–
22, 24

–
–
–
–
–

–
–
–
–
–

Fig. 9. Miniature sensors of axial geometry with or without a PTP structure in ATLAS12 wafers.

Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90

designs. The ATLAS12A large-area main sensor has 4 stripsegments of all axial geometry for prototyping an axial strip
sensor. The ATLAS12M main sensor has stereo-strip segments for
prototyping a stereo strip sensor. New to the ATLAS12 layouts are
two dicing lines: standard edge space of 910 μm and slim edge
space of 450 μm, a gated PTP structure at the end of strips, and87

connection of orphan strips to readout strips in a triangular cornerof the stereo-strips segment.
The wafer processing of 120 ATLAS12A and 45 ATLAS12M
wafers is completed using wafer lots of 6 in., p-type, FZ, and320 μm thick. Out of these wafers, 30 ATLAS12A and 26 ATLAS12M
wafers are diced to this report, resulting in 28 large-area main

Table 4ATLAS12 miniature sensors of axial geometry, BZ1–BZ6.
Inner dimension of bias ring length  width (area)
Number of strips BZ1  BZ4/BZ6Strip pitch BZ1  BZ4/BZ6Strip width of implant/metal

8.14  7.82 mm2 (0.637 cm2)
104/7774.5/100 μm
16/22 μm

Id

Strip length (mm)Features

BZ1
BZ2C
BZ3A
BZ3B
BZ3C
BZ3D
BZ3E
BZ3F
BZ4B2
BZ6C
BZ6E

8.06
8.04
8.05
8.05
8.05
8.05
8.05
8.00
8.00
8.05
8.05

No p-stop, PTP gap 10 μm
Individual p-stop, full gate-slit
Common p-stop, half gate-continuous
Common p-stop, full gate-continuous
Common p-stop, full gate-slit
Common p-stop, bias resistor gate
Common p-stop, no gate
Common p-stop, PTP gap 70 μm
Common p-stop, PTP dotþ bias resistor gate
Common p-stop, full gate-slit
Common p-stop, no gate

Fig. 10. Excerpts of the layout of endcap “square” stereo miniature sensor of fan geometry in the
ATLAS12A wafer layout with narrow strip pitch (left ﬁgure) and wide strip pitch (right ﬁgure). The
orphan strips at the top-left corner are connected to neighboring strips.

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Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90

Table 5ATLAS12 endcap miniature sensors of fan geometry.
Endcap “square” stereo miniature sensor with small strip pitch (EC-small pitch)
Inner dimension of bias ring length  wide/narrow width (area)
Number of strips at wire-bonding pads
Strip length (θ: stereo angle)
Strip pitch wide/narrow
Angle, θ, of stereo strips

8.14  8.52/8.33 mm2 (0.686 cm2)
127
8.05/cos θ mm
65.06/63.55 μm
20 mradEndcap “square” stereo miniature sensor with large strip pitch (EC-large pitch)
Inner dimension of bias ring length  wide/narrow width (area)
Number of strips at wire-bonding pads
Strip length (θ: stereo angle)
Strip pitch wide/narrow
Angle, θ, of stereo strips

8.14  8.58/8.37 mm2 (0.690 cm2)
79
8.05/cosθ mm
104.69/102.05 μm
20 mradEndcap “skewed” stereo miniature sensor (EC-skewed)
Inner dimension of bias ring length  wide/narrow width (area)
Number of strips per segment
Top segment
Strip length
Strip pitch wide/narrow
Bottom segment
Strip length
Strip pitch wide/narrow
Skew angle

17.68  18.38/16.73 mm2 (3.10 cm2)258
8.77 mm70.98/67.90 μm
8.80 mm67.67/64.60 μm
20 mradFig. 11. Excerpts of the layout of endcap “skewed” miniature sensor of fan-geometry in the ATLAS12A wafer layout.

Y. Unno et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 80–90

sensors of standard and 28 of slim dicing. Most of the sensors
exhibit no breakdown in leakage current up to 1000 V. The leakage
currents are o8 μA at 1000 V, measured at room temperature.
Inclusion of the new features has not deteriorated the initial
performance of the large-area sensors. By using capacitancemeasurement of the diodes, the full depletion voltages of the
ATLAS12A and the ATLAS12M wafers are approximately 330 V and
225 V, and the resistivity of the wafers is derived to be approximately 3 and 4.5 k Ω cm, respectively.

Acknowledgments
The research was partly supported by the Ministry of Education,
Youth and Sports of the Czech Republic (Grant number LG13009),
the German Federal Ministry of Education and Research, and theHelmholtz Association, the Japan Society for Promoting Science
KAKENHI A (Grant number 20244038) and KAKENHI C (Grant
number 20540291), the Japan MEXT KAHENHI for Research on
Priority Area (Grant number 20025007) and for Scientiﬁc Researchon Innovative Areas (Grant number 23104002), the Slovenian
Research Agency, the Spanish National Program for Particle Physics(under Grants FPA2009-13234-C04-01 and FPA2012-39055C02-01), the ﬁnancial support of the State Secretariat for Education,
Research, and Innovation, the Swiss National Science Foundationand the Canton of Geneva, Switzerland, the UK Science andTechnology Facilities Council (under Grant PP/E006701/1), and theUnited States Department of Energy, Grant DE-FG02-04ER41286.

Appendix A. Miniature sensors in the ATLAS12 wafer layouts
The ATLAS12A and ATLAS12M wafer layouts (Fig. 3) include
miniature sensors composed of 20 1  1 cm2 and two 2  2 cm2sensors and 24 1  1 cm2 sensors. Two dicing lines, the standard
and slim dicing, are implemented as in the case of the main sensor.
These miniature sensors are to be used for understanding andoptimizing the designs against radiation damage further.
A.1. PTP structure variants
Variation of “gated” PTP structures is implemented in theminiature sensors of ATLAS12A and ATLAS12M wafers. Those
structures in the miniature sensors of axial-strip geometry are
shown in Fig. 9. The strip pitch, the PTP gap between strip end andbias ring, and the strip isolation method are 74.5 μm, 20 μm, andcommon p-stop, respectively, other than the strip pitch of 100 μm
in BZ6C and BZ6E. Identiﬁcation of the miniature sensor with andwithout a PTP structure and corresponding location per layout aresummarized in Table 3.
The “full gate” structure (BZ3C) is the structure implemented in
the main sensor. The “half gate” structure (BZ3A) where the gate iscovering just over the p-stop structure is to compare with the “fullgate” structures (BZ2C, BZ3B, BZ3C, BZ6C). The structures (BZ2C,
BZ3C, BZ6C) have a “comb-like” slit at the corner of the p-stop
structure to test whether the cut-out may improve the breakdown
voltage while the sensor is reverse-biased. The structures (BZ3D,
BZ4B2) with a “narrow and full gate” of the polysilicon bias
resistor over the PTP gap are to test the difference in narrow andwide gate structures. The structures (BZ1, BZ3E, BZ3F, BZ6E) haveno gate structure to compare with the gated structures. The BZ3F
is the structure of the ATLAS07 main sensor. These variants may
clarify the points in the structures susceptible to radiation damage.
The geometrical speciﬁcations of the miniature sensors of axial
geometry are summarized in Table 4.

89

A.2. Endcap “square” stereo miniature sensors
In order to arrange silicon strip sensors on discs of the endcap
region, the sensor geometry must be trapezoidal and the strips
must be laid in a fan geometry. A symmetrical geometry is a“square” trapezoid. In the same way as the stereo strips of the
ATLAS12M main sensors, the strips in a fan geometry are inclined
at an angle to the sensor edges to make a stereo sensor. An angle of
20 mrad is chosen so that the stereo angle of 40 mrad can beachieved with a pair of a stereo sensor. The radial strips can bearranged by rotating the pair of sensors by 20 mrad. Two types ofthese sensors are laid out with strips of small and large pitches:
64.04 μm and 102.78 μm at wire-bonding pads, respectively.
Excerpts of the layout of the small and large pitch sensors areshown in Fig. 10. The ganging of orphan strips is made either by
connecting the strip implants (DC-ganging) or connecting the ACcoupling strip metals (AC-ganging). The geometrical speciﬁcationsof the endcap square stereo miniature sensors of fan geometry aresummarized in Table 5.
A.3. Endcap “skewed” stereo miniature sensors
The orphan strips of the endcap square stereo sensors can beeliminated by arranging the sensor edges parallel to the stereo
strips in a fan geometry. Consequently, the trapezoid is skewed at
the inclination angle of the stereo strips. Excerpts of the layout of
the endcap skewed stereo miniature sensor are shown in Fig. 11.
The perimeters of the strip ends have been made circular, alongwith the row of wire-bonding pads, to prototype a circular sensor
for accommodating a smooth transition in the radial transition
region with a different quantization of the number of sensors inadjacent rings in a disc. The dicing lines of the miniature sensors
are kept straight for this fabrication for simplicity. The sensor ispositioned in a 2  2 cm2 area in the ATLAS12A wafer layout. The
geometrical speciﬁcations of the endcap skewed stereo miniature
sensors of fan geometry are summarized in Table 5.
A.4. Special miniature sensor without passivation
A very special sensor, labeled by BZ3C-unpassivation in Table 3,
is a sensor where the edge region between the guard ring and theedge metal is without SiO2 passivation. This is to make dicing at apoint immediately outside of the guard ring, e.g., by the scribe andcleave technology [23].
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