Nuclear Instruments and Methods in Physics Research A 699 (2013) 78–83

Contents lists available at SciVerse ScienceDirect

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

Evaluation of novel KEK/HPK n-in-p pixel sensors for ATLAS upgradewith testbeam
R. Nagai a,n, J. Idárraga b, C. Gallrapp c, Y. Unno d, A. Lounis b, O. Jinnouchi a, Y. Takubo d, K. Hanagaki e,
K. Hara f, Y. Ikegami d, N. Kimura g, K. Nagai f, I. Nakano h, R. Takashima i, S. Terada d, J. Tojo j, K. Yorita g,
S. Altenheiner n, M. Backhaus r, M. Bomben l, D. Forshaw p, M. George m, J. Janssen r, J. Jentzsch n,T. Lapsien n, A. La Rosa q, A. Macchiolo k, G. Marchiori l, C. Nellist s, I. Rubinsky o, A. Rummler n,
G. Troska n, P. Weigell k, J. Weingarten maDepartment of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Laboratoire de l’Accélérateur Linéaire (LAL), Université Paris-Sud XI, Plateau du Moulon, 91400 Orsay, FrancecEuropean Organization for Nuclear Research (CERN), CH-1211 Gene ve 23, SwitzerlanddInstitute of Particle and Nuclear Study, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba-shi, Ibaraki 305-0801, Japane
Department of Physics, Osaka University, Machikaneyama-cho, Toyonaka-shi, Osaka 560-0043, Japanf
Institute of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba-shi, Ibaraki 305-8571, Japang
Research Institute for Science and Engineering, Waseda University, Japanh
Department of Physics, Okayama University, 3-1-1 Tshushima-naka, Kita-ku, Okayama-shi, Okayama 700-8530, Japani
Department of Education, Kyoto University of Education, 1 Fukakusa-Fujimori-cho, Fushimi-ku, Kyoto 612-8522, Japanj
Department of Physics, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japank
Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, GermanylLaboratoire de Physique Nucleaire et de Hautes Énergies (LPNHE), Barre 12-22, 1er étage - 4 place Jussieu - 75252 Paris Cedex 05, Francem
II. Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, GermanynTechnische Universität Dortmund, Experimentelle Physik IV, 44221 Dortmund, GermanyoDeutsches Elektronen-Synchrotron (DESY), Notkestraße 85, 22607 Hamburg, Germanyp
Department of Physics, The University of Liverpool, L69 7ZE Liverpool, United KingdomqSection de Physique (DPNC), Université de Gene ve, 24 quai Ernest Ansermet 1211 Gene ve 4, Switzerlandr
University of Bonn, Physikalisches Institut, Nussallee 12, D-53115 Bonn, Germanys
University of Manchester, School of Physics and Astronomy, Oxford Road, Manchester M13 9PL, United Kingdomba r t i c l e i n f o

a b s t r a c t

Available online 9 May 2012

A new type of n-in-p planar pixel sensors have been developed at KEK/HPK in order to cope with the
maximum particle ﬂuence of 1–3  1016 1 MeV equivalent neutrons per square centimeter ðneq =cm2 Þ inthe upcoming LHC upgrades. Four n-in-p devices were connected by bump-bonding to the new ATLAS
Pixel front-end chip (FE-I4A) and characterized before and after the irradiation to 2  1015 neq =cm2 .
These planar sensors are 150 mm thick, using biasing structures made out of polysilicon or punchthrough dot and isolation structures of common or individual p-stop. Results of measurements withradioactive 90Sr source and with a 120 GeV/c momentum pion beam at the CERN Super Proton
Synchrotron (SPS) are presented. The common p-stop isolation structure shows a better performance
than the individual p-stop design, after the irradiation. The ﬂat distribution of the collected charge inthe depth direction after the irradiation implies that the effect of charge trapping is small, at the
ﬂuence, with the bias voltage well above the full depletion voltage.
& 2012 Published by Elsevier B.V.

Keywords:
Pixel detector
n-in-pATLAS
HL-LHC

1. Introduction
The High Luminosity LHC (HL-LHC) upgrade is planned for 2022 inorder to extend the discovery potential for new physics
and improving the data statistics for high precision measurements.

n

Corresponding author.
E-mail address: nagai@hep.phys.titech.ac.jp (R. Nagai).

0168-9002/$ - see front matter & 2012 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.nima.2012.04.081

Its speciﬁcations include an increased peak luminosity of up to1
5  1034 cm2 s1 , and a total integrated luminosity of 3000 fb
which represents a ﬁve times larger peak and a 10 times higher
integrated luminosity than foreseen for the current ATLAS detector [1].
Once such upgraded run starts, the present ATLAS Pixel detectors [2] might sustain elevated radiation damage from the maximum particle ﬂuence of 1–3  1016 1-MeV equivalent neutrons per
square centimeters ðneq =cm2 Þ for the innermost layer. The
increased luminosity will make it necessary to implement a new

R. Nagai et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 78–83

pixel detector which is able to cope with the increased occupancy
and the ﬂuence. Several different sensor technologies have been
proposed to instrument the upgraded ATLAS Pixel detectors, as forexample, silicon 3D [3], silicon planar [4–6], and diamond pixel [7]detectors. In this paper, we present an evaluation of the novel
n-in-p planar sensors manufactured by Hamamatsu Photonics(HPK) in collaboration with KEK et al. [8,9].
The n þ readout in the p-type silicon bulk (n-in-p type) has anumber of advantages over the conventional n-type sensors [10].
The n þ readout is always formed in the pn-junction side because
the type of the bulk does not invert even if high radiation damage
is sustained. This makes it possible the operations without full
depletion, thus highly radiation tolerant. It allows to achieve ahigher charge collection efﬁciency, by collecting fast electron
carriers in the strong electric ﬁeld in the junction side, thus thetrapping effects in the bulk material, which are expected toincrease with the radiation damage, could be reduced [11]. The
single sided lithographic process is expected to reduce fabrication
cost thus allowing larger detector coverage with pixel sensors. On
the other hand, the n-in-p sensors have additional requirements
e.g. the need for an inter-pixel isolation and the coating of thesensor edges, where a high bias is present, to prevent sparks withthe chip at ground potential [9].

79Table 1The properties of the KEK/HPK n-in-p pixel sensors.
Silicon crystal
Sensor thickness
Pixel size

p-type FZ, 427 kO cm
150 mm
50 mm  250 mm

ID

Bias st.

Isolation st.

First test

Second test

SCC93
SCC94
SCC95
SCC96

PolySiPolySiPolySiPTLA

Individual p-stop
Common p-stop
Common p-stop
Individual p-stop

NR
NR
NR
NR

IR(N/A)NR
IRIR

SCC, single chip card; st., structure; PolySi, polysilicon resister; PTLA, punchthrough dots; NR, unirradiated; IR, irradiated to 2  1015 neq =cm2 ; N/A, notavailable.

smaller than in the common p-stop in order to create space fortwo p-stop lines. The punch-through dot has an implant on siliconbulk from the bias rail. Neither the PolySi nor the bias rail has animplant underneath it. We needed the PTLA dots, not at every
four-corner of the pixels, but at alternate four-corner, as one dot
acts for the surrounding 4 pixels.

3. Devices under tests and experimental setup
2. n-in-p sensor design
The three sensor designs examined are illustrated in Fig. 1: thecommon p-stop and the polysilicon resistor (PolySi), the individual
p-stop and PolySi, and the individual p-stop and the punch-throughdot (PTLA) [9]. The n-in-p sensor requires an isolation structure tointercept the inversion layer between neighboring n þ pixels due tothe positive charge-up in the surface interface of the silicon and theoxide. A bias structure is implemented to bias the pixels for testing
the sensors prior to the bump-bonding to the readout electronics.
The structure has to have a resistance high enough to isolate thepixels and to minimize the parallel noise.
The common p-stop has only one p-stop line for all pixels,
which runs through adjacent pixels. The individual p-stop has an
isolated p-stop line for each pixel, thus the pixel size is slightly

Table 1 shows the properties of the KEK/HPK pixel sensors. The
pixel sensors were bump bonded to FE-I4A readout chips [12] andmounted on the single-chip test card (SCC). In 2011, two beamtests [13] were held at H6 beam line of the CERN super proton
synchrotron (SPS), using 120 GeV p þ particles at an averagetrigger rate of  650 Hz per cycle. The EUDET telescope, composed of six planes, was used for tracking beam particles [14],with a pointing resolution of s  3 mm. In the ﬁrst beamtest, allsamples were unirradiated (NR). Afterwards, three samples,
SCC93, SCC95 and SCC96, were irradiated at Karlsruhe to 2 1015 neq =cm2 with 23 MeV protons. In the second beamtest, after
irradiation, SCC93-IR developed a data-acquisition problem andwas removed from the beam, thus the results after irradiation are
based on the measurements of SCC95-IR and SCC96-IR. SCC95-IR
and SCC96-IR were also measured with a collimated 90Sr b sourcein the laboratory with an external trigger.
During the beamtests the samples were characterized withparticles imprinting at normal incidence and at 41 angle. The test
using a normal incidence angle aimed to understand the efﬁciency and the charge collection for the samples before and after
irradiation. The measurements at small grazing angle allowed tounderstand the charge collection in the depth of the sensor.
Table 2 shows the run conditions. The threshold of each pixelof the FE-I4A was tuned to a threshold of either 1600 e or 3200 e,
while the time-over-threshold (ToT) value was tuned to ﬁve for acharge of 10 ke. As there is no reliable ToT to charge calibration
available for the FE-I4A chip, at the time of this paper, all quoted
charges are given in ToT. Although there is a certain amount of
uncertainty between the measurements of the samples, we notethat the measurements within one sample using the same tuning
are not affected by this uncertainty. In this analysis, dead (no hit)and noisy (occupancy 4103 ) pixels were masked. The occupancy was deﬁned as number of random hits per pixel per trigger.

4. Results
Fig. 1. Designs of the n-in-p pixel structure (top: SCC94 and SCC95, middle:
SCC93, bottom: SCC96). A pixel implant is shown in the centers, being isolatedwith common p-stop (top) or individual p-stop (middle and bottom) structure. A
bias rail, running vertically in the left side, provides the reference ground voltage
to two adjacent pixels through the polysilicon resistor (top and middle) or thepunch-through dot (bottom).

4.1. Results for normal incidence angle
Fig. 2 shows the overall tracking efﬁciency (global) for eachsensor as a function of the bias voltage. We deﬁned the efﬁciency

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R. Nagai et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 78–83

Table 2Run conditions (bias voltage, threshold (Th), temperature (T) and ToT for 10 ke(ToT)) of the beam (ﬁrst and second) and laboratory (lab) tests.

Table 3
The global tracking efﬁciencies of non-irradiated and irradiated sensors from theﬁrst and the second beamtest. The run conditions are summarized in Table 2.

PeriodBias voltage (V)Th (e)

T (1C)ToTIDFirst beamtest

Second beamtest

Lab100, 200, 400, 500
600, 800, 1000
15, 2550, 75, 100, 125, 150
50, 100, 200, 300
400, 500, 600, 700
800, 900, 1000

1600

 30

5

1600
3200
1600

þ 20

5

SCC93
SCC94
SCC95
SCC96

99.7 70.01% (NR)a98.7 70.01% (NR)a99.7 70.01% (NR)a94.2 70.02% (NR)aN/A
99.67 0.01% (NR)b95.67 0.02% (IR)c94.97 0.02% (IR)c

 40

5

First
Seconda

Weighted averages and errors of (100, 125, 150 V).
Weighted averages and errors of (100, 200, 300 V).
c
Weighted averages and errors of (800, 900, 1000 V).
b

Efficiency

0.9
0.8
0.7

SCC93-NR

SCC94-NR

0.6
0.5

SCC94-NR

SCC95-IR

SCC95-NR

SCC96-IR

0.4
0.3

SCC96-NR

0

200

400

600
Bias voltage [V]800

Noise Occupancy

10-4

1.1
1

SCC94-NR
SCC95-IR
SCC96-IR
10-5

0

200

400

600
Bias voltage [V]

800

1000

1000

Fig. 2. The global tracking efﬁciencies of non-irradiated (NR) and irradiatedðIR : 2  1015 neq =cm2 Þ sensors as a function of the bias voltage. The run conditions
are summarized in Table 2.

by the existence of a hit within a window of 7 1-pixel size inboth row and column directions from the track position extrapolated from the EUDET tracking. The non-irradiated sensors
showed an efﬁciency plateau above 25 V and the irradiatedones above 200 V. The non-irradiated, SCC94-NR, recorded
an efﬁciency of 99:8 70:01% when the devices was biased at
200 V and the front-end chip was tuned at a threshold of1600 e, and the irradiated ones, SCC95-IR and SCC96-IR, recorded
efﬁciencies of 95:7 7 0:03% and 95:0 7 0:03% at 1000 V, respectively. Table 3 summarizes the global tracking efﬁciencies fromthe ﬁrst and the second beamtest. The values are weighted
averages and errors of three bias voltages in the footnote of
the table.
The efﬁciencies in the central region of a pixel ð150 mm o Xo350 mmÞ were 98:7 70:01% and 99:5 70:01% at a bias voltageof 1000 V for SCC95-IR and SCC96-IR, respectively. SCC95-IR was
noisy in the data taking, plausibly due to the readout chip andreadout chain.
Fig. 3 shows the noise occupancies obtained from the beamtest
data. The noise occupancy represents the number of noise hits per
pixel, per trigger. All hits outside the window of 72 pixel sizes
from the track position and the window of the trigger timing(LVL1) of 3 o LVL1 o10 were accumulated as noise hits,
per sensor. The noise hits were then divided by the number of
total triggers and the number of active pixels. The noise occupancies were approximately 4  105 both in the non-irradiatedand irradiated sensors. The decrease of the occupancy below
200 V in the non-irradiated sensors may indicate that the most of
the hits were caused by the real particles.
Fig. 4 shows the local tracking efﬁciency maps within a pixel ofthe unirradiated sensor, SCC94-NR, at 600 V and within the two
irradiated sensors, SCC95-IR and SCC96-IR, at 1000 V. The pixels
in a sensor were folded into a pixel in the center and adjacentones. The average number of entries per map-point was approximately 30. The unirradiated sample SCC94-NR in Fig. 4(a) showsan efﬁciency of  99%. Similar results were achieved for SCC95NR before irradiation, which has the same design as SCC94. The
irradiated sensors, SCC95-IR and SCC96-IR, indicate an efﬁciency

Fig. 3. The noise occupancies of non-irradiated and irradiated sensors as afunction of the bias voltage from the second beamtest.

Fig. 4. Local tracking efﬁciency maps of a pixel of non-irradiated (NR) andirradiated (IR) sensors: (a) SCC94-NR at a bias voltage of 600 V, (b) SCC95-IR at
1000 V and (c) SCC96-IR at 1000 V. The box shows the region of a pixel.

of  99% in the central region but with two larger regions ofdecreased efﬁciency at the left corners. In addition, SCC96-IR
shows a decreased efﬁciency in the right corners of the pixel dueto the individual p-stop structures.
Fig. 5 shows the charge collection as a function of the bias
voltage for SCC95 and SCC96, before and after irradiation. The
amount of collected charge may be decreased after irradiation.
The collected charge saturates for the unirradiated and irradiatedsamples at voltages above 50 V and 400 V, respectively. They
were also conﬁrmed by the measurements performed in laboratory by using a 90Sr b-source. The full depletion voltages were
estimated to be 44 7 10 V and 380 7 70 V [9]. The effective doping
concentrations were, then, derived to be ð2:6 70:6Þ  1012 andð2:2 70:4Þ  1013 cm3 , before and after irradiation, respectively.
Fig. 6 shows the charge collection maps for the unirradiated,
SCC94-NR, and the two irradiated sensors, SCC95-IR and SCC96IR. The charge at the track position is the induced charge in thecenter pixel. For SCC95-IR sample which has the common p-stop
structure with polysilicon bias resistor, the charge collection
maps show that it has slightly less tail into the adjacent pixels
than SCC96-IR which has the individual p-stop structure withpunch-through dots. The larger tail into the adjacent pixels
implies a larger charge sharing.

R. Nagai et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 78–83

7

Charge [ToT]

6
5
4
3

SCC95-NR
SCC96-NR
SCC95-IR
SCC96-IR
SCC95-IR (lab test)
SCC96-IR (lab test)

2
1
0

0

200

400
600
Bias voltage [V]

800

1000

Fig. 5. Charge collection in units of ToT of non-irradiated (NR) and irradiated (IR)sensors as a function of the bias voltage. The run conditions are summarized in Table 2.

Fig. 6. Charge collection map in a pixel of non-irradiated (NR) and irradiated (IR)sensors: (a) SCC94-NR at a bias voltage of 200 V, (b) SCC95-IR at 1000 V and(c) SCC96-IR at 1000 V. The box shows the region of a pixel.

Fig. 7. Local charge sharing map of a pixel of non-irradiated (NR) and irradiated(IR) sensors: (a) SCC94-NR at a bias voltage of 200 V, (b) SCC95-IR at 1000 V and(c) SCC96-IR at 1000 V. The box shows the region of a pixel.

Fig. 7 shows the charge sharing maps. The charge sharing ratiois the ratio of the charge of the pixel at the track position being
divided by the sum of the charges of all pixels in the cluster. A
slightly wider charge sharing in SCC96-IR is observed inFig. 7(c) than in SCC95-IR in Fig. 7(b).

the n þ readout side to the back-side. As it reaches the back-side,
while increasing the bias voltage, the detector is said to be fully
depleted. The electron–holes created in the depleted region
generate the induced current in the electrode to be ampliﬁed.
In this setup, the track passes along the long side of the pixel
and spans 7–9 pixels, giving tracks of up to 2.25 mm. The peak of
the cluster size corresponded to the depth of 150 mm. In this way,
one can have conﬁgurations where a track goes through an
undepleted region, which results in a loss of charge collection
efﬁciency or, in the most dramatic case, a non-reactive pixel. At
the same time, the depletion depth can be estimated.
Fig. 8 shows a schematic view of the setup where the samples,
SCC94-NR and SCC95-IR, are located back to back. In a second
comparison, the sensor, SCC96, before and after irradiation isconsidered as well. Although clusters which were associated withthe reconstructed tracks with the telescope were used, neither
precision entry- nor exit-coordinate was used for the analysis.
Fig. 9 shows the collected charges in the depth direction for
SCC94-NR and SCC95-IR at different bias voltages, and Fig. 10 for
SCC96-NR and SCC96-IR. The depth is measured from the chargecollecting side, i.e., n þ side.
The ﬁrst-to-second pixel behavior can be best understood in
Fig. 9(e): SCC94 at 200 V and SCC95-IR at 1000 V. In the second
pixel, the collected charge is Q  7:4 ToT, a value 40% higher thanthe ﬁrst pixel. The particle enters the ﬁrst pixel at any point in thesurface with a ﬂat probability distribution as the beam is homogeneously distributed in all the sensor, thus the distance traversed is half of the pixel, in average. At the nominal 41, with along pixel size of 250 mm, the mean distance traversed in the ﬁrst
pixel is  130 mm. This is also true for the last pixel in the track.
In the inner pixels, the particle traverses  259 mm of silicon (Si),always. Hence, the charge collected in the ﬁrst and the last pixel
can be, in average, as low as half the charge in the second pixel.
The effect of the threshold and trapping modiﬁes this ratio.
The charge Q in the depth direction was calculated by using
the normalized pixel responses. In each sensor and at each bias
voltage, the responses of each pixel were normalized such thatthe mean value of the ToT’s of the pixel was equal to the meanvalue of all pixels. We assigned systematic errors to the normalized responses in a pixel to be the square-root of the product of
the standard deviation of the overall ToT distribution and thedeviation of the responses of the pixel from the mean value.
In Fig. 9(b), SCC94-NR at 10 V and SCC95-IR at 100 V, one canclearly see that SCC94-NR presents a dramatic loss in chargecollection at about 90 mm depth and eventually the last two pixels
toward the back-side are inactive. One
can calculate the depletionpﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
depth using the relation D ¼ 2EðV þV bi Þ=Ne [15], where
E ¼ 1 pF=cm is the dielectric constant in Si, V is the bias voltage,
V bi is the built-in voltage ð  0:5 VÞ, N is the doping concentration
which is approximately 2:6  1012 cm3 for this sensor, and e isthe electron charge. The depth, D is  72 mm at V¼10 V. This
result is comparable with what we observe experimentally within
the uncertainty. In the same ﬁgure, SCC95-IR at 100 V, presents
the same behavior but at a shallower distance,  70 mm. It
contains as well at least three completely inactive pixels. The
distance is consistent with the doping concentration of 2:2 1013 cm3 and the bias voltage of 100 V.

4.2. Results for 41 incidence angle
Measurements with a nominal angle of 41 were performed to
study the depletion and the charge collection in the depth of thesensor. In the n þ -in-p sensor, the depletion region extends from81

Fig. 8. Schematic of the beamtest setting.

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R. Nagai et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 78–83

11
10
9
8
7
6
5
4
3
2
1
0

SCC94-NR 5V
SCC95-IR 50V

10
9
8
7
6
5
4
3
2
1
0

Q (TOT)

10
9
8
7
6
5
4
3
2
1
0

10
9
8
7
6
5
4
3
2
1
0

Q (TOT)

SCC94-NR 10V
SCC95-IR 100V

SCC96-NR 7V
SCC96-IR 50V

9
8
7
6
5
4
3
2
1
0

SCC96-NR 15V
SCC96-IR 200V

9
8
7
6
5
4
3
2
1
0

SCC94-NR 20V
SCC95-IR 200V

SCC96-NR 100V
SCC96-IR 500V
0

10
9
8
7
6
5
4
3
2
1
0

20

40

60

80
100
depth (µm)

120

140

160

Fig. 10. Comparison of the amount of collected charge as a function of depth for
SCC96-NR and SCC96-IR.

box gives the cluster size in the most populated entry. The top
and bottom edges of the box represent the cluster size withentries greater and smaller than 10% of the center. The solid
vertical lines show the tails of the distribution where the entry
ends. We observe that the full depletion where the cluster size
arrives to its plateau arrives at 50 V for SCC94-NR and about
500 V for SCC95-IR. SCC96-NR and SCC96-IR exhibit a similar
behavior.

SCC94-NR 50V
SCC95-IR 500V

10
9
8
7
6
5
4
3
2
1
0

SCC94-NR 200V
SCC95-IR 1000V

5. Conclusion

10
9
8
7
6
5
4
3
2
1
0

SCC94-NR 1000V
SCC95-IR 1000V
0

20

40

60

80
depth (µm)

100

120

140

160

Fig. 9. Comparison of the amount of collected charge between the unirradiatedsample SCC94-NR and the irradiated sample SCC95-IR.

For an unirradiated sensor with the bias voltage over 50 V andan irradiated one over 500 V, in Fig. 9(d)–(f), we observe a plateau
in the collected charge at the inner pixels (depth at 0:0220:13 mm) where all pixels collect the same charge. Thisimplies that the effect of charge trapping in depth was small
in the irradiated sensors with the present ﬂuence once thebias voltage is well above the full depletion voltage. If we assume
a be of 3:3  1016 , after annealing and scaled to the temperature
at 40 1C, and a ﬂuence of 2  1015 , for a saturation drift velocity
of 107 cm=s, we get a charge trapping distance of approximately
150 mm [11], that is of the order of the sensor thickness, and thus
consistent with our observation.
In Fig. 11, we compare the cluster size as a function of the bias
voltage for SCC94-NR and SCC95-IR (Fig. 11(a)), and for SCC96-NR
and SCC96-IR (Fig. 11(b)). We present the cluster size using acandle stick plot. The solid horizontal line in the middle of each

We have carried out beamtest and b-source measurements
for three different designs of thin, 150 mm, n-in-p pixel sensors
before and after the irradiation to 2  1015 neq =cm2 with 23 MeV
protons.
The analyses of global hit efﬁciencies, noise occupancy, chargecollection, and the charge collection in depth show consistent full
depletion voltages of approximately 50 V and 400 V, before andafter irradiation, respectively. The full depletion voltages were
estimated to be 447 10 V and 380 7 70 V, and the effective
doping concentration, ð2:6 70:6Þ  1012 and ð2:2 7 0:4Þ  1013
cm3 , before and after irradiation, respectively.
The ﬂat distribution of the collected charge in depth after theirradiation implies that the effect of charge trapping in depth issmall with the bias voltage well above the full depletion voltage.
This is consistent with the estimated charge trapping distance of
150 mm for the ﬂuence of 2  1015 neq =cm2 which is comparablewith the thickness of the sensor.
The common p-stop shows, before and after irradiation, abetter performance than the individual p-stop, concerning the hit
efﬁciency. The punch-through design shows, before and after
irradiation, a lower efﬁciency in the region of the implantation
dots than the polysilicon bias resistor design.
Due to the low statistics of only two irradiated samples and dueto the missing charge calibration for the FE-I4A samples, further
measurements are required to conﬁrm the results concerning thedifferent charge collection in the common and the individual
p-stop structures, and to understand if there are signiﬁcantperformance differences between the different designs. In orderR. Nagai et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 78–83

83

his dedicated support and work done, and D. Muenstermann of
CERN, A. Dierlamm and F. Bögelspacher, the team of Karlsruhe
Institut für Technologie for the irradiation. R. Nagai acknowledges
the ﬁnancial support from the Global Center of Excellence Programby MEXT, Japan through the ‘‘Nanoscience and Quantum Physics’’
Project of the Tokyo Institute of Technology. This research was
supported by the Grant-in-Aid for scientiﬁc research (A) (Grant No.
20244038) by Japan Society for the Promotion of Science and theGrant-in-Aid for scientiﬁc research on advanced basic research(Grant No. 23104002) by the Ministry of Education, Culture, Sports,
Science and Technology, of Japan. The beamtests were supportedby the European Commission under the FP7 Research Infrastructures project AIDA, grant agreement no. 262025.

References

Fig. 11. Comparison of cluster size as a function of the bias voltage using a candle
stick plot.

to assure the performance in an environment as the ATLAS pixel
detector at the HL-LHC operation, additional irradiation to higher
ﬂuences need to be performed in the upcoming years.

Acknowledgments
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