Nuclear Instruments and Methods in Physics Research A 765 (2014) 125–129

Contents lists available at ScienceDirect

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

Evaluation of KEK n-in-p planar pixel sensor structures for very highradiation environments with testbeam
K. Motohashi a,n, T. Kubota a, K. Nakamura b, R. Hori b, C. Gallrapp c, Y. Unno b, O. Jinnouchi a,
S. Altenheiner d, Y. Arai i, M. Hagihara j, M. Backhaus e, M. Bomben f, D. Forshaw g,
M. George h, K. Hanagaki i, K. Hara j, M. Hirose i, Y. Ikegami b, N. Ishijima i, J. Jentzsch c,K. Kawagoe l, N. Kimura m, T. Kono n, A. Macchiolo k, R. Nishimura p, S. Oda l, H. Otono l,
I. Rubinskiy o, A. Rummler d, R. Takashima p, Y. Takubo b, J.J. Teoh i, S. Terada b, K. Todome a,
J. Tojo l, J. Usui j, P. Weigell k, J. Weingarten h, D. Yamaguchi a, K. Yorita maDepartment of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Institute of Particle and Nuclear Study, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, JapancEuropean Organization for Nuclear Research, CERN CH-1211, Genève 23, SwitzerlanddTechnische Universität Dortmund, Experimentelle Physik IV, 44221 Dortmund, GermanyeUniversity of Bonn, Physikalisches Institut, Nussallee 12, D-53115 Bonn, GermanyfLaboratoire de Physique Nucleaire et de Hautes Énergies (LPNHE), Barre 12-22, 1er étage - 4 place Jussieu - 75252 Paris Cedex 05, FrancegDepartment of Physics, The University of Liverpool, L69 7ZE Liverpool, United KingdomhII. Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, GermanyiDepartment of Physics, Osaka University, Machikaneyama-cho, Toyonaka-shi, Osaka 560-0043, Japanj
Institute of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba-shi, Ibaraki 305-8571, Japank
Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, GermanylDepartment of Physics, Kyushu University, 6-10-1 Hakozaki Higashi-ku, Hukuoka 812-8581, Japanm
Faculty of Science and Engineering, Waseda University, 3-4-1 Ookubo Shinjuku-ku, Tokyo 169-8555, Japann
Department of Physics, Ochanomizu University, 2-1-1 Ootsuka Bunkyo-ku, Tokyo 120-0021, JapanoDeutsches Elektronen-Synchrotron (DESY), Notkestraße 85, 22607 Hamburg, Germanyp
Department of Education, Kyoto University of Education, 1 Fukakusa-Fujimori-cho, Fushimi-ku, Kyoto 612-8522, Japanb

art ic l e i nf o

a b s t r a c t

Available online 29 May 2014

Various structures for n-in-p planar pixel sensors have been developed at KEK in order to cope with thehuge particle ﬂuence in the upcoming LHC upgrades. Performances of the sensors with different
structures have been evaluated with testbeam. The n-in-p devices were connected by bump-bonding to
the ATLAS Pixel front-end chip (FE-I4A) and characterized before and after the irradiation to 1  1016
1 MeV neq =cm2 . Results of measurements with 120 GeV/c momentum pion beam at the CERN Super
Proton Synchrotron (SPS) in September 2012 are presented.
& 2014 Elsevier B.V. All rights reserved.

Keywords:
Pixel detector
N-in-pRadiation hardness

1. Introduction
In collider physics, the pixel detector is usually used as an
innermost subdetector of a multi-purpose detector, such as the
ATLAS [1] and CMS [2] detectors. The pixel detector is located atshort distance from the interaction point, O(1–10 cm), in a highradiation environment. After a high ﬂuence the full depletion
voltage may reach or exceed 1 kV [3]. Many concerns arising at
such high voltages (destructive sparking, large dark currents, cableisolation, power supplies, etc.) impose an operation under thepartially depleted voltage after irradiation. This is possible byE-mail address: kazuki.motohashi@cern.ch (K. Motohashi).
http://dx.doi.org/10.1016/j.nima.2014.05.092
0168-9002/& 2014 Elsevier B.V. All rights reserved.

adopting a n-in-p approach. The n-in-p planar pixel sensor is oneof the candidates of high radiation tolerant sensors for the futurehigh luminosity experiments. The n-in-p type has the following
major advantages:

 It does not show type inversion and can be operated partiallydepleted during their whole lifetime because p–n junction isformed on the readout side.
 It needs only single sided lithograph processing and therefore
is potentially inexpensive compared with a typical n-in-n typesensor.
 It collects electrons which are, due to their higher mobility, lessprone to trapping. Trapping is the factor limiting the operationof any silicon sensor in a very high radiation environment.

126

K. Motohashi et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 125–129

A schematic view of an n-in-p planar pixel sensor is shown in
Fig. 1.

2. Devices under test
The n-in-p pixel samples used in the beamtest were manufactured by HPK (Hamamatsu Photonix K.K.) in collaboration with the
ATLAS Japan silicon group [4–6]. All samples were made from Float
Zone (FZ) wafers. The designs of HPK pixel sensors are characterized by the biasing and separation structures.
The measured wafer resistivity was  6 kΩ=cm.
2.1. Biasing structure and separation structure
Biasing structures are implemented in order to test the sensors
before being bump-bonded to the FE chips. As the sensor and the
FE chip are the most expensive components of a pixel module theyboth should be tested before joining them by the bump-bonding
process. The biasing structures enable sensor to apply bias voltage
by connecting all the pixels to the ground. The biasing structuresof HPK sensors are either made of ‘Poly-Silicon resistor (denotedby PolySi)’ or ‘Punch Through (denoted by PT)’. The PolySi structure
consists of a bias rail which is laid out on the short pixel sideboundary and poly-silicon resistor which is laid out to achieve
higher enough resistance to separate between the pixels and tominimize the noise. The PT structure consists of a bias rail and apunch through dot. If a bias voltage is applied on the bias rail, thepotential of the pixels will be ﬁxed via the punch through mechanism from the bias rail to the pixel implant.
In the n-in-p case, free charge carriers, electrons, are accumulated on the surface of the silicon bulk between the adjacent n þimplants by positive charge-up in the surface of the oxide layer.
The omnipresent electron inversion layer would shorten the n þimplants without further precautions. Separation is usually provided by a p-type boron implant between the pixels, forming alateral p–n junction. The separation structures of HPK sensors arecalled either the ‘common P-stop’ type or the ‘P-spray’, dependingon the structure of this isolation implant. The boundary ofadjacent pixels is divided by the single-line P-stop in the common
P-stop type and the area between the pixels is entirely covered by
P-spray in the P-spray type.
2.2. Speciﬁcation of HPK sensors
To investigate the combinations of biasing and separation structures, four options are designed and manufactured. The four candidates are ‘PolySi, common P-stop’, ‘PolySi, P-spray’, ‘PT, common
P-stop’ and ‘PT, P-spray’. A schematic view of four candidates is
illustrated in Fig. 2.
In order to communicate with the sensors via FE chip, the sensors
are mounted on the FE-I4A [7] single chip card (SCC). The size of

Fig. 1. A schematic view of an n-in-p planar pixel sensor.

pixel cells of FE-I4A is 250  50 μm2 . The speciﬁcations of the HPK
sensors are listed in Table 1. The three out of seven samples were
irradiated with neutron as high as 1  1016 1 MeV neq =cm2 at
Ljubljana reactor [8].

3. Beamtest at CERN SPS
In September 2012, a beamtest was held at H6 beam line of the
CERN super proton synchrotron (SPS), using 120 GeV π þ particles at
an average trigger rate of  650 Hz per cycle. The EUDET telescopes,
composed of six monolithic pixel sensor planes, were used for tracking
beam particles, with a pointing resolution of s  3 μm. In the ﬁrst
batch of the beamtest, the four non-irradiated (NR) samples were
tested. In the second batch, three out of four samples were irradiatedand the rest one was non-irradiated to be used as a reference.
Table 2 shows the run conditions. The threshold and the timeover-threshold (ToT) values were tuned to achieve uniform performance of pixels on the FE-I4A. Although no reliable calibrated
conversion values from ToT to charge are available for the FE-I4A
chip, the average calibration between the ToT and the Vcal wasavailable for each sample at the beginning of the beamtest by
seeing the ToT distributions as a function of the Vcal. The Vcal is a
DAC value which is linear to the injection charge, while the ToT
may not be linear to it. This Vcal calibration enables to evaluate therelative charge values of hit clusters. The relation between Vcal
and collected charge, Q, is described as
Q ½e ¼

C cal ½F  ða ½V=DAC  Vcal ½DAC þb ½VÞ;elementary charge ½Cð1Þwhere C cal is the capacitance of the injection capacitor which islocated on the analog circuit of FE-I4A for calibration, a and b are
the constant values which are corresponding to the slope andoffset of the Vcal DAC, respectively. These variables are different ineach FE-I4 chip, and therefore it is necessary to measure thesebefore a beamtest. However, a circuit to measure the injection
capacitance is not implemented for FE-I4A and we cannot know
absolute value of collected charge.

4. Results
4.1. Hit efﬁciency
Fig. 3 shows the overall hit efﬁciency for each sensor as afunction of the bias voltage.
The efﬁciency is deﬁned as a probability of the existence of a hit
within a window of 7 1.5-pixel size in both row and column
directions from the track position extrapolated from the EUDET
tracking. The hit efﬁciencies at the full depletion voltages are listed
in Table 3. In order to investigate the structure dependence, the hit
efﬁciencies inside the pixel were calculated with small granularityand mapped on 2D histograms, which are denoted by pixel hit
efﬁciency maps. The pixel hit efﬁciency maps at  400 V in batch
1 and  1200 V in batch 2 are shown in Fig. 4 and 5, respectively.
In the pixel hit efﬁciency maps, the efﬁciencies are measured with1  1 μm2 granularity. The plot represents one pixel plus parts of
its eight neighbors.
By comparing the hit efﬁciencies of sensors with various
biasing and separation structures before and after the irradiation,
a number of differences are observed. For the non-irradiatedsamples, the hit efﬁciencies of P-spray separation are inferior tothat of P-stop separation even at the full depletion voltage. In the
P-spray, the large inefﬁciency regions are observed at the corners
on the bias rail side. In the PT biasing structure, the inefﬁcientregion is larger than that of PolySi.

K. Motohashi et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 125–129

Pixel Electrode

Common P-stop

Bias Rail

Poly Silicon Resistor

127

P-spray
Punch Through Dot

Fig. 2. A schematic view of four candidates.

Table 1The properties of the KEK/HPK n-in-p pixel sensors. ‘BC’, which is a unit of thetime-over-threshold (ToT), is short for ‘Bunch Crossing’. 1 BC is corresponding to
25 ns. In the ﬁrst batch of the beamtest, the four non-irradiated samples were
tested. In the second batch, three out of four samples were irradiated and the rest
one was non-irradiated to be used as a reference.
Silicon crystal
Sensor thickness
Pixel size

p-type FZ, 4–7 kΩ cm
150 μm
50 μm  250 μm

ID

Bias st.
Isolation st.

Doseðneq =cm2 ÞThreshold (e)ToT (BC at ke)

Batch

KEK18

PT
P-StopPolySiP-StopPT
P-Spray
PolySiP-Spray
PolySiP-StopPT
P-Spray
PolySiP-Spray0

1600
10 at 101600
10 at 101700
9 at 101600
10 at 101600
9 at 51600
6 at 51600
6 at 5

1

KEK19
KEK20
KEK21
KEK32
KEK33
KEK34

0
0
0
1  1016
1  1016

1  10

16

1, 2
1
1
2
2
2

Table 2Run conditions of the beamtest at the CERN SPS in September 2012.
Batch

Bias voltage (  V)

Temp. (1C)1
26, 14, 25, 40, 57, 100, 200, 400
300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200

þ 20
 50

For the irradiated samples, the inefﬁcient regions are observed
at four corners of a pixel, especially near the bias rail side. The
shapes of PT and bias rails structures are clearly observed. In orderto evaluate the differences in the region around bias rail, the hit
efﬁciencies are projected in the long pixel direction. The range
used for the projections is the region between 45 μm and 55 μm inthe short side direction and the projected hit efﬁciencies at
1200 V are shown in Fig. 6. According to the designs of KEK32
and KEK34, illustrated in Fig. 2, the routes of the poly-silicon
resistors near the bias rail are different. The poly-silicon resistors
are laid out near the bias rail in KEK32 which adopts common
P-stop separation, while it is not the case for KEK34 which adopts
P-spray separation. These results imply that the poly-silicon
resistors near the bias rail intensify the inefﬁciency.

4.2. Charge collection efﬁciency
Since the hit signals are formed by the deposited charges of
injected particles, studying the charge collection efﬁciency (CCE) is
necessary. The CCE of PolySi types is especially of large concern as
the results of the efﬁciency analysis proved that the poly-silicon
resistors create the inefﬁciencies after 1  1016 neq =cm2 neutron
irradiation. The CCE is calculated byðCCEÞ ¼

Vcalcollected
Vcalexpectedð2Þ

The expected Vcal is the average Vcal collected on the central
region of a pixel at  400 V for batch 1 and 1200 V for batch 2,
assuming all of the generated charge is collected there with such ahigh bias voltage. The central region of a pixel is deﬁned as aregion between 150 μm to 350 μm in the long side direction, andbetween 45 μm to 55 μm in the short side direction of Fig. 4 and 5.
The CCE provides the information where the charges are lost.
In order to investigate the reason of the decreases of the hit
efﬁciencies near the poly-silicon resistors after the irradiation,
the CCE of the middle region is projected in both long and shortpixel directions as the hit efﬁciency analyses. The middle region
projected in long pixel direction is the region between 45 μm and55 μm in the short side direction and that in short pixel directionis the region between 150 μm and 350 μm in the long side
direction.
The projected CCE along the long and short pixel directions isshown in Figs. 7 and 8, respectively.
The regions around bias rail cause inefﬁciency of chargecollection. This behavior is the same as that of the hit efﬁciency
analysis. However, inefﬁciency regions are seen in the otherboundaries between pixels even for non-irradiated sensors. Thiscan be explained by effect of charge sharing with neighbor pixel.
When a track passes at the boundary of two pixels, charge is
generated in the silicon bulk and induces charges in two pixels.
Sometimes the ratio of shared charge is partial to one of the two
pixels and the amount of collected charge by another pixel may beless than the threshold. As a result, the charge information is lostfor the latter pixel and the CCE becomes smaller than the actual
value effectively. The depth (inefﬁciency) and the width (s) of the
inefﬁciency valley at 125 μm in Fig. 7 are summarized in Table 4.
The inefﬁciency and s are found as the results ﬁtting thefollowing inverted Gaussian:!
ðx  pmean Þ2;
ð3Þ
poffset  pinefficiency  exp 
p2s
where the variables with p are the parameters of the ﬁtting.

128

K. Motohashi et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 125–129

100
95

Efficiency [%]

90
85
80
KEK18: PT / common P-stop

KEK19: PolySi / common P-stop

70

KEK20: PT / P-spray

KEK21: PolySi / P-spray65

KEK32: PolySi / common P-stop

KEK33: PT / P-spray

75

60
KEK34: PolySi / P-spray

55
200

0

400

600

800

1000

1200

Bias Voltage [-V]
Fig. 3. The overall hit efﬁciency of the DUTs in batch 2. The open symbols denote non-irradiated sensors and the ﬁlled symbols show irradiated sensors.

Batch 1 (  400 V)

Batch 2 (  1200 V)
Eff. (%)

ID

Eff. (%)

KEK18
KEK19
KEK20
KEK21

99.517 0.01
99.53 7 0.01
97.157 0.03
98.647 0.02

KEK19
KEK32
KEK33
KEK34

99.87 70.00
96.28 70.02
97.09 7 0.01
97.58 7 0.01

Coordinate along short side of pixel (µm)

ID

100
50

100

Coordinate along short side of pixel (µm)

Table 3The overall hit efﬁciencies of non-irradiated and irradiated sensors from the ﬁrst
and the second batch. The run conditions are summarized in Table 2.

0

50
0
50
0
50
0
500
0
100
200
300
400
Coordinate along long side of pixel (µm)

100
90
80
70
60
50
40
30
20
10
0

Fig. 4. Local tracking efﬁciency maps of a pixel of batch 1. (a) KEK18-NR, (b) KEK19NR, (c) KEK20-NR and (d) KEK21-NR. The dashed box shows the region of a pixel.

0

5. Conclusion
Newly developed n-in-p pixel sensors were evaluated using
testbeam at CERN SPS. Four combinations of structures were

100
90
80
70
60
50
40
30
20
10

(b)

(c)

50
0

(d)

50
0
0

100

200

300

400

0

500

Coordinate along long side of pixel (µm)
Fig. 5. Local tracking efﬁciency maps of a pixel of batch 2. (a) KEK19-NR as areference, (b) KEK32-IR, (c) KEK33-IR and (d) KEK34-IR. The dashed box shows theregion of a pixel.

1
0.9
0.8
0.7
KEK19: PolySi / common P-stop

0.6

KEK32: PolySi / common P-stop
KEK33: PT / P-spray
KEK34: PolySi / P-spray

0.5
0In Fig. 8, two peaks of inefﬁciency are observed at the twoboundaries, at 25 μm and 75 μm. By comparing the peak of two biasing
structures, PT and PolySi, signiﬁcant charge losses are observed at theboundaries which have poly-silicon resistor. The results of the CCE
analysis strongly support the results of the hit efﬁciency analysis, where
the poly-silicon resistors cause the decrease of the hit efﬁciency. The
potential of the poly-silicon resistors which are at ground weakens theelectric ﬁeld in the silicon under the poly-silicon resistors. The inefﬁciency and s of the inefﬁciency valley at 25 μm in Fig. 8 are summarizedin Table 5. These values are found as the results ﬁtting with Eq. (3).

[%]

50

[%]

0

(a)

50

100

200

300

400

500
X [µm]

Fig. 6. The projection of efﬁciencies of the regions with 45 o yo 55 to long pixel
direction (x) for the bias rail effect measurement. The results with bias voltage at
 1200 V in batch 2 are plotted.

compared. Overall hit efﬁciency of 97.6% was conﬁrmed at
 1200 V after irradiation of 1  10 16 neq =cm2 . It is found that pspray caused the inefﬁciency region around the PT dots even
before irradiation; the bias rail and the PolySi resistor caused
the inefﬁciency after irradiation, which were caused by inefﬁcient charge collection due to the weakened electric ﬁeld
under the bias rail and the PolySi resistor. In the next production, several new designs are prepared. Bias rails will be moved
outside the pixel boundary; PolySi resistors will be placed on
top of the pixel electrode.

K. Motohashi et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 125–129

1.4

Table 4The inefﬁciency and s of the inefﬁciency valley at 125 μm in Fig. 7.

Batch 1 (HV = -400 V)

1.2
1Charge collection ef

ciency0.8
KEK18: PT / common P-stop

0.6

KEK19: PolySi / common P-stop
KEK20: PT / P-spray

0.4

KEK21: PolySi / P-spray

1.4

0.8
KEK19: PolySi / common P-stop

0.6

KEK32: PolySi / common P-stop
KEK33: PT / P-spray

0.4

KEK34: PolySi / P-spray

100

200

300

400

500

Coordinate along long side of pixel ( m)Fig. 7. The projected collected charge efﬁciency along the long pixel direction ofboth batches 1 (upper) and 2 (lower). ‘KEK19’ in the lower plot is a non-irradiatedsample used as a reference.

s ðμmÞ

Batch 1 (  400 V)
KEK18
KEK19
KEK20
KEK21

13.6 7 0.5
21.7 7 0.5
43.8 7 0.3
63.9 7 0.2

5.0 7 2.5
3.2 7 0.7
5.4 7 0.6
4.7 7 0.3

Batch 2 (  1200 V)
KEK19
KEK32
KEK33
KEK34

12.9 7 0.0
71.9 7 0.0
44.1 70.0
51.17 0.0

4.7 7 1.1
6.2 7 0.2
5.7 7 0.3
4.3 7 0.2

ID

Inefﬁciency (%)

s ðμmÞ

Batch 1 (  400 V)
KEK18
KEK19
KEK20
KEK21

16.8 7 1.2
12.4 7 1.3
10.5 7 1.3
9.5 7 1.1

4.8 7 0.5
3.2 7 0.7
4.2 7 0.7
6.0 7 1.0

Batch 2 (  1200 V)
KEK19
KEK32
KEK33
KEK34

8.8 7 0.0
38.7 7 0.0
15.0 7 0.0
31.2 7 0.0

7.8 7 0.7
5.8 7 0.1
4.6 7 0.2
5.3 7 0.1

Acknowledgments
The authors would like to thank the accelerator and beam linecrews. The beam tests were supported by the European Commission under the FP7 Research Infrastructures project AIDA. The
authors also acknowledge the ﬁnancial support from the Center of
Excellence Program by MEXT, Japan through the ‘Nanoscience andQuantum Physics’ Project of the Tokyo Institute of Technology.

Batch 1 (HV = -400 V)

1.2

References

1[1] The ATLAS Collaboration, The ATLAS experiment at the CERN large hadroncollider, Journal of Instrumentation 08003.
[2] The CMS Collaboration, The CMS experiment at the CERN LHC, Journal of
Instrumentation 08004.
[3] G. Lindströ
m, et al., Nuclear Instruments and Methods in Physics Research
Section A 465 (2001) 60.
[4] Y. Unno, Nuclear Instruments and Methods in Physics Research Section A 569(2006) 41.
[5] R. Nagai, et al., Nuclear Instruments and Methods in Physics Research Section A
699 (2013) 78.
[6] Y. Unno, et al., Nuclear Instruments and Methods in Physics Research Section A
699 (2013) 72.
[7] M. Garcia-Sciveres, et al., Nuclear Instruments and Methods in Physics Research
Section A 636 (2011) S155.
[8] M. Ravnik, Technical description of TRIGA Mark II reactor at Jožef Stefan
Institute, 〈http://www.rcp.ijs.si/ric/description-s.html〉.

0.8

Charge collection ef ciency

Inefﬁciency (%)

Table 5The inefﬁciency and s of the inefﬁciency valley at 25 μm in Fig. 8.

1

1.4

ID

Batch 2 (HV = -1200 V)

1.2

0

129

KEK18: PT / common P-stop

0.6

KEK19: PolySi / common P-stop
KEK20: PT / P-spray

0.4

KEK21: PolySi / P-spray

Batch 2 (HV = -1200 V)

1.4
1.2
1
0.8

KEK19: PolySi / common P-stop

0.6

KEK32: PolySi / common P-stop
KEK33: PT / P-spray

0.4

KEK34: PolySi / P-spray

0

20

40

60

80

100

Coordinate along short side of pixel ( m)Fig. 8. The projected collected charge efﬁciency along the short pixel direction ofboth batches 1 (upper) and 2 (lower). ‘KEK19’ in the lower plot is a non-irradiatedsample used as a reference.