Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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

Test beam evaluation of newly developed n-in-p planar pixel sensors
for use in a high radiation environment
K. Kimura v,n, D. Yamaguchi v, K. Motohashi v, K. Nakamura n, Y. Unno n, O. Jinnouchi v,
S. Altenheiner b, A. Blue c, M. Bomben f,k,l, A. Butter e,f,g, A. Cervelli a, S. Crawley c,A. Ducourthial f,k,l, A. Gisen b, M. Hagihara w, K. Hanagaki t,n, K. Hara w,x, M. Hirose v,
Y. Homma v, Y. Ikegami n, S. Kamada m, T. Kono s, A. Macchiolo r, G. Marchiori f,k,l, F. Meloni a,
M. Milovanovic j, A. Morton c, G. Mullier a, F.J. Munoz q, C. Nellist e,f,g, B. Paschen r,1,
A. Quadt d, T. Rashid e,f,g, J. Rieger d, A. Rummler f,h,i, K. Sato w, K. Sato w,x, N. Savic r,
H. Sawai v, K. Sexton c, M.E. Stramaglia a, M. Swiatlowski u, R. Takashima o, Y. Takubo n,
S. Terzo r, K. Todome v, J. Tojo p, K. Van Houten u, J. Weingarten d, S. Wonsak j, K. Wraight c,K. Yamamura maUniversität Bern, Laboratory for High Energy Physics, Sidlerstrasse 55, CH-3012 Bern, Switzerland
Experimentelle Physik IV, Technische Universität Dortmund, 44221 Dortmund, Germany
School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, Scotland, United KingdomdGeorg-August-Universität Göttingen, Friedrich-Hund-Platz 1, D-37081 Göttingen, GermanyeLAL, University Paris-Sud, Francef
CNRS/IN2P3, FrancegUniversité Paris-Saclay, Orsay, Franceh
LAPP, FranceiUniversité Savoie Mont Blanc, Annecy-le-Vieux, FrancejDepartment of Physics, University of Liverpool, Merseyside, Liverpool, L69 3BX, United KingdomkLaboratoire de physique nucléaire et de hautes energies (LPNHE), Univ. Paris-UMPC, 4 Place Jussieu, 75005 Paris, FrancelUniv. Paris Diderot, Francem
Solid State Div., Hamamatsu Photonics K.K., 1126-1, Ichino-cho, Higashi-ku, Hamamatsu-shi, Shizuoka 435-8558, JapannInstitute of Particle and Nuclear Study, KEK, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japano
Department of Science Education, Kyoto University of Education, Kyoto 612-8522, Japanp
Department of Physics, Faculty of Science, Kyushu University 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japanq
School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdomr
Max-Planck-Institut für Physik, Föhringer Ring 6, D-80805 München, Germanys
Ochadai Academic Production, Ochanomizu University, 2-1-1, Otsuka, Bunkyo-ku, Tokyo, 112-8610, Japant
Department of Physics, Osaka University, Machikaneyama-cho 1-1, Toyonaka-shi, Osaka 560-0043, Japanu
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, United States of AmericavInstitute 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 305-8751, Japanx
Center for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, Tsukuba, Ibaraki 305-8571 Japanbc

art ic l e i nf o

a b s t r a c t

Article history:
Received 4 December 2015
Received in revised form
29 March 2016
Accepted 1 April 2016

Radiation-tolerant n-in-p planar pixel sensors have been under development in cooperation with Hamamatsu Photonics K.K. (HPK). This is geared towards applications in high-radiation environments, suchas for the future Inner Tracker (ITk) placed in the innermost part of the ATLAS detector in the highluminosity LHC (HL-LHC) experiment. Prototypes of those sensors have been produced, irradiated, andevaluated over the last few years. In the previous studies, it was reported that signiﬁcant drops in thedetection efﬁciency were observed after irradiation, especially under bias structures. The bias structures
are made up of poly-Si or Al bias rails and poly-Si bias resistors. The structure is implemented on thesensors to allow quality checks to be performed before the bump-bonding process, and to ensure thatcharge generated in ﬂoating pixels due to non-contacting or missing bump-bonds is dumped in a

Keywords:
Pixel detector
N-in-pRadiation hardnessnCorresponding author.
E-mail address: kimihiko@hep.phys.titech.ac.jp (K. Kimura).
1
Now at University of Bonn.

http://dx.doi.org/10.1016/j.nima.2016.04.004
0168-9002/& 2016 Elsevier B.V. All rights reserved.

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2
Bulk damage
Surface damage
Bias structure

controlled way in order to avoid noise. To minimize the efﬁciency drop, several new pixel structures havebeen designed with bias rails and bias resistors relocated. Several test beams have been carried out toevaluate the drops in the detection efﬁciency of the new sensor structures after irradiation. Newly developed sensor modules were irradiated with proton-beams at the Cyclotron and Radio-Isotope Center(CYRIC) in Tohoku University to see the effect of sensor-bulk damage and surface charge-up. An irradiation with γ-rays was also carried out at Takasaki Advanced Radiation Research Center, with the goal of
decoupling the effect of surface charge-up from that of bulk damage. Those irradiated sensors have been
evaluated with particle beams at DESY and CERN. Comparison between different sensor structures
conﬁrmed signiﬁcant improvements in minimizing efﬁciency loss under the bias structures after irradiation. The results from γ-irradiation also enabled cross-checking the results of a semiconductor technology simulation program (TCAD).
& 2016 Elsevier B.V. All rights reserved.

1. Introduction
A new pixel sensor has been developed by Hamamatsu Photonics K.K. (HPK) [1] in collaboration with the ATLAS Japan SiliconGroup [2–4]. R&D for this pixel sensor is geared towards applications in High Energy Physics experiments, such as for the future
Inner Trackers (ITk) of the ATLAS detector in the high luminosity
LHC (HL-LHC) experiment [5]. Since the pixel detectors have beendeveloped with the aim of tracking short-lived charged particles,
they are usually installed in the inner-most part of particle detectors. Thus, they are required to have high radiation-tolerance,such as
1.0 × 1016 neq /cm2 for the inner layers, and15
1.7 × 10 neq /cm2 for the outer layers of ITk in HL-LHC for instance[5]. In order to achieve this requirement, new pixel detectors haveadopted n-in-p type silicon structure. It was reported that bothn-in-p and n-in-n type sensors were able to collect sufﬁcientamount of charge after being irradiated to lifetime ﬂuence expected at the HL-LHC experiment [6,7], but n-in-p sensors have anumber of advantages in HEP applications: Signal electrons can be drawn towards and captured in the inter-pixel region, since the surface of such region may be positively charged by the ionizing dose.
 Since the bias rail, which is connected to ground level, is justabove the inter-pixel region, the electric potential in the interpixel region gets closer to the ground level. This further helps
the electrons to be captured in the inter-pixel region. Thus, the They do not undergo type-inversion when receiving non-ionizing bulk damage, while n-in-n type sensors do [8].

 Since the p–n junction is formed on the electrode side and thedepletion zone grows from this side, charge collection is possible even without full depletion [1].
 Since the collected charge carriers are electrons, faster read-out
is possible than for the p-in-n sensors. Moreover, electrons areless affected by trapping than holes [8].
 Lithography is needed on only one side, thereby lowering the
manufacturing cost. This feature is vital for the application to
the areas larger than 1–2 m2 [8].
A schematic of the old-prototype sensor structure is shown in
Fig. 1. Each pixel electrode is separated by inter-pixel p-stop implants. The typical size of the pixel cells is 250 × 50 μm2. Sensors
are connected to the FE-I4 read-out chips [9] via bump-bonding.
The size of pixels on the FE-I4 is also 250 × 50 μm2. Each pixel
electrode is connected to virtual ground provided by the preampliﬁer when bump-bonded onto the FE-I4. SnAg or In (Ni/In)
solder is used for bump-bonding. Sensors have bias structures on
their surface with which I-V measurement can be done before the
bump-bonding process. This prevents faulty sensors from being
bump-bonded on the FE-I4s, and makes it possible to increase theoverall module yield. Bias rails were also grounded and connectedto each pixels via poly-Si resistors as shown in Fig. 2.
In a previous study [4], we investigated the hit and the chargecollection efﬁciency of HPK sensors and demonstrated that both
hit and charge-collection efﬁciency sharply dropped under thebias rail structures after irradiation with protons. The peak efﬁciency drop (1 − efficiency) observed was about 55% [4]. This result
suggested some possible explanations for the drop in efﬁciency:

Fig. 1. A schematic of an n-in-p planar pixel sensor proﬁle. (Old-prototype design).

Fig. 2. A schematic view of an n-in-p planar pixel sensor from the electrode side.
(Old-prototype design).

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magnitude of efﬁciency drop is larger on the bias rail side thanon the bump bonding side.
In order to minimize the loss of efﬁciency associated with thebias rails, several new structures have been designed based on thefollowing ideas: Large offset structure: a structure which locates the bias rails on

3varies depending on the sensor thickness. Schematic views of eachsensor structure are presented in Fig. 3. A detailed explanation oneach sensor type can be found in a summary paper on the Japanese effort in the ITk R&D [10].

3. Irradiations and test beams

the electrodes.

3.1. Proton irradiation at CYRIC

bands to shields the sensor bulk from the bias rails.

Proton-irradiation was carried out at CYRIC in Tohoku University. Sensor modules were aligned to make layers of DUTs in an
irradiation box. Samples were exposed to 70 MeV/c proton beams
up to a ﬂuence of 3 × 1015 neq /cm2. The hardness factor of 70 MeV
protons, which was used for the conversion to 1 MeV neutron
equivalent ﬂuence, was 1.4. The beam size was evaluated byscanning the beam current with thin Al bars in the vertical and thehorizontal directions. The typical resolution of the beam wasσ = 3 − 4 mm of Gaussian function (Gaussian sigma). During theirradiation, the samples were cooled down to  15 °C in order toavoid annealing due to the heat generated by the beam current.
Small pieces of Al foil of 1 × 1 cm2 were attached on the samples
for dosimetry purpose. Radiation dose was estimated after irradiation by measuring the 24Na yield of Al spallation process with a
Ge detector. After irradiation, all the samples were stored under 30 °C in order to mitigate the effect of annealing.

 Wide p-stop structure: a structure which has broadened p-stop
Furthermore, results gained from simulations conducted with a
TCAD program hinted at other possible reasons for the efﬁciency
drop [10]. The simulation triggered the investigation of sensors
irradiated with γ -rays, for the purpose of decoupling the effect of
surface damage from the other effects.
In order to evaluate the new sensor structures, sensor modules
were irradiated with proton beams at the Cyclotron and RadioIsotope Center (CYRIC) in Tohoku University, and with γ -rays at the
Takasaki Advanced Radiation Research Institute, Japan Atomic
Energy Agency (JAEA). Those irradiated modules have been tested
with beams of particles (test beams) at accelerator facilities suchas the Super Proton Synchrotron (SPS) at the European Organization for Nuclear Research (CERN) and DESY II at the Deutsches
Elektronen-Synchrotron (DESY).
Detailed information on the devices under test (DUTs) are given
in Section 2. Speciﬁc information on the proton- and γ -irradiations, followed by the details on the test beams, are presented in
Section 3. The data analysis method is explained in Section 4. The
analysis results are displayed in Section 5, and discussed in Section
6. Finally, conclusions are drawn in Section 7.

2. Devices under test
The modules evaluated in this analysis are listed in Table 1. The
newly developed structures can be characterized by the bias railmaterial and the position of the bias structure. Of all the sensors
investigated in this study, the wide p-stop structure is applied only
on the type13. Both Al and poly-Si were used for the bias material.
Each sensor structure is denoted by a sensor structure ID. The
thickness of the sensor is also an important feature for evaluation,
since the full-depletion voltage and the amount of collected charge
Table 1
List of evaluated samples. “ch” denotes the ROC number of four-chip modules.
KEK39 and KEK93 are single-chip modules. Each ROC has 80  336 pixels.
ID

Structure ID Bias rail material sensor
structureThickness (μm) (sensor/
ASIC)

KEK39 type10

Poly-Si rail
Large offset

320/150

KEK46 type10
ch2

Poly-Si rail
Large offset

150/150

KEK49 type13
ch3

Poly-Si rail
Wide p-stop

150/150

KEK53 type19
ch3

No bias rail
(Reference structure)

150/150

KEK71 type12ch3

Poly-Si rail
Small offset

150/150

KEK84 type2
ch1

Al rail
Large offset

150/150

KEK93 type8

Al rail
No offset

320/150

3.2. γ -ray irradiation at Takasaki
In order to replicate the expected surface damage in collider
experiments, and to decouple the effect of surface damage fromthe other causes, a γ -ray irradiation was carried out at Takasaki.
Samples were irradiated with 60Co γ -source. At ﬁrst, the dose rate
was estimated by transmission degradation of small pieces ofpolymethyl methacrylate resin after three hours of radiation. The
estimated dose rate was about 10 kGy/h. The irradiation lasted 10days during which the samples received an ionizing dose of
2.4 MGy to be compared with the expected ionization dose of
7.7 MGy in the innermost layer of the ITk [5]. The samples at the
proton irradiation also receive certain ionization dose. A ﬂuence of1 × 1016 neq /cm2 by 70 MeV protons corresponds to 8.3 MGy from adE/dx calculation. The effect of surface damage saturates at a
speciﬁc level, and 2.4 MGy is already in the range of saturation[11].
3.3. Testbeams
The effect of radiation damage has been investigated with test
beams, where a beam of particles produced by an accelerator istracked with the help of the beam telescope. The data used in thispaper were taken at the DESY II accelerator in DESY and the SPS in
CERN, between 2013 and 2015.
The ACONITE telescope has six telescope planes, which arearranged behind each other in the test beam setup. These planes
are realized in the form of MIMOSA26 sensors [12]. These aremonolithic pixel sensors with a pixel size of 18.4 × 18.4 μm2. Irradiated DUTs were installed in a cooling box located in between
3rd and 4th plane of the telescope in order to keep the DUTs at atemperature low enough to ensure equally sufﬁciently low leakage
currents, and to minimize the effect of annealing. The typicaltemperature on the sensor surface was  40 °C.
A non-irradiated DUT plane was installed outside the coolingbox as a reference plane. This is because the readout methods
applied for the beam telescope and for FE-I4 are different: while
the telescope uses the rolling-shutter method and takes 115.2 μs tocomplete the readout phase per trigger, the FE-I4 accumulates the

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Fig. 3. Schematic views of sensor structures evaluated in this paper.
Table 2
List of proton-irradiated samples. “BC”, which is the abbreviation of “bunch crossing”, is a unit of ToT. 1 [BC] corresponds to 25 ns.
ID

KEK39

Bias voltage(  V)

Fluence200, 300, 400,

4.91 × 1015

(neq /cm2)

500, 600, 700
KEK46150, 200, 250,

ch2

300, 350, 400

KEK49200, 400,

ch3

600, 800

KEK53200, 400,

ch3

600, 800

KEK71

200, 400,

ch3

600, 800

KEK84

200, 400,

ch1

600, 800

Threshold (e)ToT (BC at ke)

Data taking
period1800

November 2013

7 at 10

4.18 × 1015

1800

November 2013

7 at 5

3.20 × 1015

1800

March 2014

to adjust the threshold and the time-over-threshold (ToT) to deposited charge relation of each pixel to the calibration value. ToT
denotes the duration of signal over threshold expressed as an unitof bunch crossing clock [9]. Data-taking was carried out under
different bias voltages in order to investigate the bias voltage dependence of the detection efﬁciency. Table 2 lists detailed information on the tested samples.
In addition to the bulk effect, the effect of surface charge-up
has been evaluated with data taken with a 120 GeV/c pion beam at
the CERN SPS. The typical pointing resolution in this test beam was
8–9 μm. Details are presented in Table 3.

7 at 52.35 × 1015

1800

March 2014

4. Analysis

7 at 53.08 × 1015

2400

April 2015

7 at 53.03 × 1015

3000

April 2015

7 at 5hit information for 400 ns for every trigger. Therefore, if two or
more particles pass through the test beam setup, there may beevents with more than one hit on the telescope and only one hit
on DUTs. In order to avoid this readout timing issue, only the
tracks penetrating the reference plane are used in the analysis[13].
Proton-irradiated sensors were tested at DESY in 2013, 2014
and 2015. In 2013 and 2014, data was taken with 4 GeV/c positron
beams, and in the 2015, with 4 GeV/c electron beams. The typical
position resolution of tracking on the DUT planes were 26 μm in2013 and 2014, and 30–35 μm in 2015. In order to achieve uniform
performance of sensors, DUTs were calibrated before data-taking
Table 3
List of γ -irradiated sample.
ID

Bias voltage(  V)Dose(MGy)

Threshold (e)ToT [BC at ke]

Data taking
PeriodKEK93100, 200, 400

2.4

3000
7 at 10

JulY 2015

The analysis of test beam data can be divided into two parts:
Reconstruction and hit/charge-collection efﬁciency analysis. In theformer part, particle tracks are reconstructed from hit informationin raw ﬁles, exploiting the EUTelescope reconstruction framework1[14,15], which is realized as a couple of Marlin processors. This
software makes clusters out of information of charge-collection
and generates hit maps by applying geometrical information. Italigns the telescope and DUT planes at sub-micron level, so as tobuild ﬁnal tracks with good precision. Finally, it builds the ﬁnal
tracks and generates the track data-collection.
In the latter part of analysis, an ofﬂine analysis framework
TBmon22 is used. This software loads the output ﬁle from the
EUTelescope package and, making use of its track information,
calculates the detection efﬁciency of DUTs. Only tracks which passthrough the reference plane are used. Hit efﬁciency is deﬁned as
the ratio of the number of tracks associated with hits within certain radius to the total number of tracks. The track-hit ﬁnding
radius (match width) varies depending on the pointing resolution.
Evaluation of the hit efﬁciency is carried out at two different levels: overall hit efﬁciency and sub-pixel resolved hit efﬁciency.
Overall hit efﬁciency is displayed in the granularity of the sensor
size, while the sub-pixel resolved hit efﬁciency visualizes the12

http://eutelescope.web.cern.ch/
https://bitbucket.org/TBmon2/tbmon2/wiki/Home

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5

Fig. 4. A sub-pixel resolved efﬁciency map (upper part) and a projected pixel efﬁciency map (lower part) created from KEK71 ch3 (  800 V) data. The rectangle drawn in the
sub-pixel resolved efﬁciency map indicates the area of one pixel.

reg
ε loss=Sloss
Electrode
Lreg × αeff

10.9Overall Hit Efficiency

contrast in efﬁciency among different pixel structures. In order toinvestigate the effect of pixel structures, the pixel efﬁciency map is
produced as shown in the upper part of Fig. 4. The area displayedin the pixel efﬁciency map corresponds to 2  2 pixel-size. This
pixel map shows the sub-pixel resolved efﬁciency in 1 × 1 μm2resolution.
The bottom part of Fig. 4 presents the projected efﬁciency along
the X-direction. This projection map is created by dividing the
projected hit map with the projected track map. The efﬁciency inthe electrode region is deﬁned as the offset value of doubleGaussian plus offset ﬁt function.
In order to evaluate the efﬁciency drop quantitatively, a nureg
is introduced as folmerical value, efﬁciency loss per pixel εloss
lows:

0.8

0.7

0.6

This value is calculated from the projected pixel efﬁciency map.
First, the area of each Gaussian function is calculated in order toobtain the total amount of efﬁciency drop Sloss along the X-direction.
Second, the calculated area of Gaussian function is divided by twopixel length Lreg to uniformly average the inefﬁciency in 2 × 2 pixel
map. Finally, the averaged value is normalized with the efﬁciency inElectrode . This value represents averaged ratio of
the electrode region αeff“the total inefﬁciency in each pixel boundary” to “the hit efﬁciency inthe electrode region”. Hence, the name, “efﬁciency loss per pixel”. The
inefﬁciencies of each sensor structure are compared in the next
section exploiting this analysis method [16].

5. Results
5.1. Proton-irradiated modules
The dependency of the overall efﬁciency on the bias voltage ispresented in Fig. 5. The data used in this plot is from test beams in

KEK39 Type10

KEK46 ch2 Type10

KEK49 ch3 Type13

KEK53 ch3 Type19

0.5
0

(1)

KEK41 Type10 (non-irrad.)

100

200

300

400

500

600

700

800

900

1000

Bias Voltage [-V]

Fig. 5. Bias voltage dependence of overall hit efﬁciency of DUTs in DESY test beams
held in 2013 and 2014 [16]. The ﬁlled black dot represents a non-irradiated reference sensor, KEK41.

2013 and 2014. KEK41 is a Type10 sensor installed as a non-irradiated reference, which achieved an efﬁciency of 99.48170.009% at
 200 V. The overall efﬁciency of KEK49 ch3 and KEK53 ch3 reached98.62070.024% and 99.32070.066% respectively at  800 V. The
errors are calculated making use of the Bayesian statistics.
Since there was no reference plane installed in the November
2013 test beam, the overall efﬁciencies of KEK39 and KEK46 ch2
are 67.831 ± 0.049% (  700 V) and 76.992 ± 0.051% ( 400 V) respectively, which are lower than that of the other sensors [16].
The efﬁciency loss per pixel of those sensors is calculated withthe method introduced in Section 4, and Fig. 6 displays its bias
voltage dependency. However, since the thickness and the radiation ﬂuence of the samples differ, it is not possible to directlycompare the efﬁciency loss. The plots in Fig. 6 were scaled by using
the efﬁciency loss on the bump-bonding side, since all sensor

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6

0.02

0.06

0.018
0.016

Efficiency Loss per Pixel ε

Efficiency Loss per Pixel ε

0.05

0.04

0.03

0.02

0.014
0.012
0.01
0.008
0.006
0.004

0.01
0.002
0

0
0

200

400

600

800

1000

0

1200

100

200

300

Bias Voltage [-V]

Fig. 6. Bias voltage dependency of efﬁciency loss per pixel of DUTs in DESY test
beams held in 2013 and 2014 [16].

500

600

700

800

900

Fig. 8. Bias voltage dependency of efﬁciency loss per pixel of DUTs in DESY test
beams held in 2015. The efﬁciency loss per pixel in the bump bonding side (denoted by BB side) and bias rail side (denoted by BR side) is displayed separately.

1

0.02

0.98

0.018

0.96

0.016

Efficiency Loss per Pixel ε

0.94

Overall Hit Efficiency

400

Bias Voltage [-V]

0.92
0.9
0.88
0.86

KEK71 ch3 Type12

0.84

0.012
0.01
0.008
0.006
0.004

KEK84 ch1 Type2

0.82

0.014

0.002

0.8
0

100

200

300

400

500

600

700

800

900

0
0

100

200

Bias Voltage [-V]

Fig. 7. Bias voltage dependency of overall hit efﬁciency of DUTs in DESY test beams
held in 2015.

types have the identical geometry on that side, and the efﬁciency
should be the same if the thickness and the ﬂuence are the same.
In order to scale the plots, the inefﬁciency loss versus bias voltage
plot in the bump-bonding side was ﬁtted with complementary
error function 1 − Erf , since the efﬁciency as a function of bias
voltage should follow the integrated Gauss distribution when thebias voltage is sufﬁciently high. Then, the ﬁt function for eachsensor is scaled to reproduce the shape of reference plots so as toget scaling factors. Finally, those scaling factors are applied to thebias rail side. In Fig. 6, KEK46 ch2 is used as a reference point.
The KEK19 module in Fig. 6 has the old type sensor structure, astructure with neither a bias rail offset nor a wide p-stop. The
thickness of KEK19 is 150 μm . This old-type module was irradiatedup to 5.0 × 1015 neq /cm2 and was tested in the March 2013 test
beam at DESY. Comparison with the old-type structure and the
new structures is made in Section 6.1.
The overall efﬁciency of the other proton-irradiated sensors is
displayed in Fig. 7. Since at least one reference plane was installed in
the test beam at DESY 2015, the overall efﬁciency reached over 95%
at  800 V. KEK71 ch3, however, presents relatively lower overall
efﬁciency, even if the bias voltage is high enough for the
150 μm -thick sensors to be fully depleted. A possible reason for thisis that the ToT distribution of KEK71 ch3 is much broader than usual,and there could be a ﬁnite number of events that were lower incharge than the threshold value. In addition, the peak ToT value of
KEK71 ch3 is lower than that of KEK84 ch1, which indicates the relative threshold for the KEK71 ch3 is higher than that for KEK84 ch1.

300

400

500

600

700

800

900

Bias Voltage [-V]

Fig. 9. Bias voltage dependency of efﬁciency loss per pixel of γ-irradiated KEK93 in
CERN test beam held in 2015. The red plot represents KEK93 and the blue plots
represents proton-irradiated KEK84 ch1. (For interpretation of the references tocolor in this ﬁgure caption, the reader is referred to the web version of this paper.)

The efﬁciency loss per pixel of KEK71 ch3 and KEK84 ch1 isshown in Fig. 8. Unlike Fig. 6, the efﬁciency loss per pixel in the bumpbonding side and the bias rail side is calculated and displayed separately. A comparison between large and small offset structures ispossible from this ﬁgure, and this is discussed in detail in Section 6.2.
5.2. γ-irradiated modules
Bias voltage dependence of the efﬁciency loss per pixel of
KEK93 is presented in Fig. 9. The red plots represents the γ-irradiated KEK93. In order to compare the efﬁciency drop with a bulkdamaged module, proton irradiated data of KEK84 ch1 is shown as
the blue plots. One important point on this ﬁgure is that theseplots are not scaled even though the sensor thickness differs between KEK93 (320 μm) and KEK84 ch1 (150 μm). A detailed discussion can be found in Section 6.3.

6. Discussion
6.1. Comparison between new and old structures
Fig. 6 indicates that the efﬁciency of new sensor structures issigniﬁcantly improved compared with the old structures. Since thePlease cite this article as: K. Kimura, et al., Nuclear Instruments & Methods in Physics Research A (2016), http://dx.doi.org/10.1016/j.
nima.2016.04.004i

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ﬂuence and the thickness of the sensors differed among the tested
samples, scaling was done as explained in 5.1. The ideas of large
offset and wide p-stop structures are considered effective in improving detector performance after irradiation. The amount of
efﬁciency loss differs among the new structures. If they are lined
up in descending order of efﬁciency drop, the order is as follows:
type13, type10, type19. In particular, type10, which has a large
offset structure, shows almost equivalent efﬁciency loss to type19,
which does not have a bias rail structure. This indicates that thelarge offset structure is close to the ideal case, because no-bias rail
structure should not be affected by the efﬁciency drop caused bybias structure.
6.2. Comparison between large and small structures
Fig. 8 indicates that the large offset structure presents thesmaller amount of efﬁciency loss. The difference is especiallyprominent on the bias rail side. This is reasonable because thesensor structure is the same on the bump bonding side. Putting
together the results in Fig. 6, the large offset structure can beconsidered as a strong candidate for the ﬁnal detector design. This
analysis, however, has been done with different bias rail materials:
KEK84 ch1 with Al rail and KEK71 ch3 with poly-Si rail. Thus,
further comparison between different material is still necessary to
decide on the ﬁnal design.

7

charge-up, which had been indicated by a TCAD analysis [10], wascarried out in order to decouple the effect of surface charge-up
from that of bulk damage. Data taken with a sensor irradiated with
γ-ray presented that the efﬁciency loss on the γ-irradiated sensor
is remarkably small compared with proton-irradiated sensors. This
indicated that the surface charge-up was not a contributory factor
to the efﬁciency drop.

Acknowledgments
The irradiations were performed, with protons at Cyclotron andRadioisotope Center (CYRIC), Tohoku University, with Y. Sakemi, T.
Wakui and M.Ito, and with γ's at Takasaki Advanced Radiation
Research Institute, Japan Atomic Energy Agency. The research was
supported and ﬁnanced in part by the Japan Society for Promoting
Science KAKENHI-A Grant number 20244038 and KAKENHI-C
Grant numbers 20540291 and 25400298, KAKENHI for Young
Scientists-B Grant number 26800156, the Ministry of Education,
Culture, Sports, Science and Technology-Japan KAKENHI for Research on Priority Area Grant number 20025007 and for ScientiﬁcResearch on Innovative Areas Grant number 23104002, and the
ENIGMASS Labex (France). We also thank the ITk pixel test beam
members that contributed to make these measurements possible.

6.3. Effect of surface damage
Fig. 9 points out that the efﬁciency loss of the γ-irradiatedsensor is only less than 0.5%. Still, as described in Section 5.2, thesensors compared in Fig. 9 have different thickness, and scaling isnecessary to compare the result quantitatively. However, the effectof sensor thickness enlarges the difference in the efﬁciency loss: if
the samples had the same thickness, 320 μm for the proton-irradiated sample for instance, the plots of proton-irradiated sensor in
Fig. 9 would shift towards the higher voltage direction. The effectof surface damage is caused by the Si–SiO2 interface-charge. The
charge density in the interface region increases as the ionizationdose grows, and is known to be saturated when the amount ofionization dose reaches the order of MGy [17]. This means that theharshest surface damage was replicated by 2.4 MGy of radiation
dose. For that reason, it is clear that the efﬁciency loss in the γirradiated sensor is remarkably small compared with the protonirradiated sensors. Thus, the effect of surface charge-up can beconsidered as notably smaller than that of bulk damage.

7. Conclusions
New sensor structures which are developed based on the indication from the previous study were evaluated at test beams at
DESY and CERN. Data from the test beams in 2013 and 2014 indicated that new sensor structures achieved signiﬁcantly smaller
efﬁciency loss compared with the old type sensors. Especially, thelarge offset structure achieved almost the same level of efﬁciency
loss as no bias rail structure, which was expected to show the least
efﬁciency loss. Furthermore, a comparison between the large offset structure and the small offset structure indicated that the larger offset structure presented better detection efﬁciency. Further
comparison between Al and poly-Si bias rail structure is still necessary to complete the optimization of the new sensor structure.
In addition, an investigation of the effect of Oxide-Si interface

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