Nuclear Instruments and Methods in Physics Research A 636 (2011) S83–S89

Contents lists available at ScienceDirect

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

Testing of bulk radiation damage of n-in-p silicon sensors for very highradiation environments
K. Hara p,n, A.A. Affolder h, P.P. Allport h, R. Bates e, C. Betancourt m, J. Bohm l, H. Brown h, C. Buttar e,
J.R. Carter b, G. Casse h, H. Chen a, A. Chilingarov g, V. Cindro i, A. Clark d, N. Dawson m, B. DeWilde o,
F. Doherty e, Z. Dolezal k, L. Eklund e, V. Fadeyev m, D. Ferrere d, H. Fox g, R. French n, C. Garcı́a q,
M. Gerling m, S. Gonzalez Sevilla d, I. Gorelov j, A. Greenall h, A.A. Grillo m, N. Hamasaki p, H. Hatano p,
M. Hoeferkamp j, L.B.A. Hommels b, Y. Ikegami f, K. Jakobs c, J. Kierstead a, P. Kodys k, M. Köhler c,T. Kohriki f, G. Kramberger i, C. Lacasta q, Z. Li a, S. Lindgren m, D. Lynn a, P. Maddock m, I. Mandić i,
F. Martinez-McKinney m, S. Martı́ i Garcia q, R. Maunu o, R. McCarthy o, J. Metcalfe j, M. Mikestikova l,
M. Mikuž i, M. Miñano q, S. Mitsui p, V. O’Shea e, U. Parzefall c, H.F.-W. Sadrozinski m, D. Schamberger o,A. Seiden m, S. Terada f, S. Paganis n, D. Robinson b, D. Puldon o, S. Sattari m, S. Seidel j, Y. Takahashi p,
K. Toms j, D. Tsionou n, Y. Unno f, J. Von Wilpert m, M. Wormald h, J. Wright m, M. Yamada paBrookhaven National Laboratory, Physics Department and Instrumentation Division, Upton, NY 11973-5000, USA
Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UKcPhysikalisches Institut, Universität Freiburg, Hermann-Herder-Str 3, D-79104 Freiburg, GermanydSection de Physique, Universite de Geneve, 24, rue Ernest Ansermet CH-1211 Geneve, SwitzerlandeDepartment of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
f
KEK, High Energy Accelerator Organization, INPS, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japang
Physics Department, Lancaster University, Lancaster LA1 4YB, UKhOliver Lodge Laboratory, Department of Physics, University of Liverpool, Oxfort St. Liverpool L69 7ZE, UKi
Jožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, SloveniajDepartment of Physics and Astronomy University of New Mexico, MSC07 4220 800 Yale Blvd. NE Albuquerque, NM 87131, USAkFaculty of Mathematics and Physics, Charles University in Prague, V Holesovickach 2 Prague 8, Czech RepubliclInstitute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Prague 8, Czech Republicm
SCIPP, UC Santa Cruz, CA 95064, USAn
Department of Physics and Astronomy, The University of Shefﬁeld, Hicks Building, Hounsﬁeld Road, S3 7RH Shefﬁeld, U.KoDepartment of Physics and Astronomy, Stony Brook University, Tony Brook, NY 11794-3800, USAp
School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japanq
IFIC (Centro Mixto CSIC-UVEG), Ediﬁcio Investigacion Paterna, Apartado 22085 46071 Valencia, Spainba r t i c l e in fo

abstract

Available online 28 April 2010

We are developing n + -in-p, p-bulk and n-readout, microstrip sensors, fabricated by Hamamatsu
Photonics, as a non-inverting radiation hard silicon detector for the ATLAS tracker upgrade at the superLHC (sLHC) proposed facility. The bulk radiation damage after neutron and proton irradiations ischaracterized with the leakage current, charge collection and full depletion voltage. The detectors
should provide acceptable signal, signal-to-noise ratio exceeding 15, after the integrated luminosity of
6000 fb  1, which is twice the sLHC integrated luminosity goal.
& 2010 Elsevier B.V. All rights reserved.

Keywords:
p-Bulk silicon
Microstrip
Charge collection
Radiation damage

1. Introduction
The silicon microstrip detector continues to play an essential
role in high-energy experiments for its capability for precision
tracking. The detectors at the planned super-Large HadronnCorresponding author.
E-mail address: hara@hep.px.tsukuba.ac.jp (K. Hara).

0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2010.04.090

Collider (LHC) are required to remain operational up to an
integrated luminosity of 3000 fb  1 with an instantaneous
luminosity of 1  1035 cm  2 s  1. In order to cope with this tenfold increase in instantaneous luminosity beyond the design valueof the LHC, currently under commissioning, the ATLAS collaboration is investigating an inner tracking system based fully on
semiconductor devices. The segmentation will be varied in radius
R with the innermost layers being pixels, followed by short(2.4 cm) and long (9.7 cm) microstrip detectors. The expected

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radiation ﬂuence [1] including a safety factor of two is(7–11)  1014 1-MeV neq/cm2 for the short strip (RZ38 cm) and(3–6)  1014 1-MeV neq/cm2 for the long strip (RZ85 cm) regions,where the ranges within the parentheses are the highest ﬂuencevalues at the central and forward regions, respectively. The
charged particle contribution to the ﬂuence is similar to theneutral at R  28 cm in the central region but decreases with R andin the forward region to typically 20% of the total. It is therefore
important to investigate the damage due to both charged particles
and neutrons.
The ATLAS R&D group ‘‘Development of non-inverting silicon
strip detectors for the ATLAS ID upgrade’’ was formed to develop
radiation hard tracking detectors based on p-bulk microstrips [2].
Since the radiation induced impurity in silicon contributes
dominantly acceptor like states, n + –on–p devices are noninverting. N-strip readout allows us to operate the sensors atpartial depletion, if required. However, the experience of employing p-bulk silicon for particle trackers is limited. This is in part
because additional strip isolation structures are required forindividual read-out strips to prevent electron accumulation
between strips effectively shorting them together. The R&D groupis evaluating sensors fabricated by Hamamatsu Photonics using
commercially available p-type wafers. We report here on the bulk
damage aspects, including the increase of leakage current, thedecrease in charge collection (CC) and the full depletion
voltage (FDV) evolution. The surface damage aspects are reported
in Ref. [3].

9 min and annealing effect during irradiation can be ignored. The
irradiated samples were stored in a refrigerator.
We compare the charge collection of Hamamatsu sensors with
Micron sensors [8,9]. Micron samples were irradiated in addition
using the 26-MeV protons at Compact Cyclotron, Karlsruhe and
24 GeV/c protons at CERN Proton Synchrotron.

3. Annealing
The irradiated sensors are known to show beneﬁcial annealing,followed by degradation such as in reduced signal and FDV increase[10]. Fig. 1 shows the FDV evolutions measured with three samples[11], which were irradiated with protons to 2  1014 neq cm  2. The
data for two neutron irradiated FZp samples are shown in Fig. 2.
Both proton and neutron data are consistent with a FDV minimum
after around 80 min at 60 1C, which is similar to the annealing
curves measured for n-bulk sensors [10].

4. Leakage current
The leakage currents of proton irradiated sensors are plotted inFig. 3 as a function of the bias voltage. The samples were annealed

2. Samples and irradiation
The sample sensors were fabricated using 15 cm wafers with
/1 0 0S crystal orientation and 320 mm thickness. The new data
we report on in this paper are ﬂoat-zone grown wafers (FZp
wafers in Ref. [4]), showing better leakage current characteristics
than other p-type FZ wafers (FZs wafers in Ref. [4]) available toHamamatsu. The initial FDV is 200 V. The R&D group continues toevaluate other commercially available p-type wafers including
magnetic Czochralski grown wafers (MCZ in [4]). The strip pitch is74.5 mm. Details of the design, including strip isolation structures,are given in Ref. [2]. The performance of the main sensors,
97.5 mm square, is reported elsewhere [5]. The characteristics ofirradiated sensors are studied using miniature samples of 10 mm
square, where there are 100 strips of 8 mm length.
The proton irradiation was performed at Cyclotron Radio
Isotope Centre (CYRIC) of Tohoku University. Details of theirradiation facility and methods are provided in Refs. [4,6]. The
sample sensors were uniformly irradiated by 70-MeV protons
through scanning them periodically in the beam. The irradiation
took typically tens of minutes to a few hours depending on the
ﬂuence. The sensors were kept cold at  7 1C during irradiationand the irradiated samples were immediately stored in arefrigerator to prevent any post-irradiation annealing from taking
place. The samples measured in Japan were glued onto a printed
circuit board and biased at  100 V during the irradiation. The
samples measured outside Japan were irradiated as bare chips
with no bias applied. The ﬂuence we refer to is the 1-MeV neutron
equivalent value, taking into account the non-ionising energy loss
(NIEL) correction factor of 1.4. The ﬂuence uncertainty isdetermined by the 27Al(pn) reaction cross-section, which is no
more than 10%.
The neutron irradiation was carried out at TRIGA reactor in
Ljubljana, Slovenia [7]. The spectrum averaged NIEL factor is 0.88
with the overall ﬂuence uncertainty of 10%. The irradiation wasmade at a rate of 1.9  1012 neq cm  2 s  1 with no bias to thesensors. The irradiation to 1015 neq cm  2 is thus completed inFig. 1. Evolution of full depletion voltage as a function of annealing time, withsamples kept at 60 1C. The full depletion was determined from C–V curves andcharge collection curves to infrared laser [4]. These samples were irradiated with
70-MeV protons to 2  1014 1-MeV neq/cm2.

Fig. 2. Evolution of full depletion voltage of two FZ samples (irradiated withneutrons to 2  1014 1-MeV neq/cm2) as a function of annealing time, with samples
kept at 60 1C. The full depletion was determined from C–V curves and chargecollection curves to 90Sr b-ray.

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5. Charge collection (CC)
5.1. Measurement system and requirement in CC
The charge collection (CC) resulting from penetrating 90Sr

b-rays was evaluated at four sites independently using different

Fig. 3. Leakage current vs bias voltage of the samples irradiated with protons. The
numbers attached to the curves are the charged particle ﬂuence corrected to neq/cm2.

systems. The Liverpool and Ljubljana groups employed ananalogue electronics chip (SCT128) [13] clocked at 40 MHz thathas 25 ns shaping time. The Valencia group has used the LHCb‘‘Beetle’’ ASIC [14] with analogue signal output. Conventional
discrete ampliﬁers (10 ns peaking time) and CAMAC ADC systemwere used by the Tsukuba/KEK group. These groups deﬁne thecollected charge as the most probable charge of the analogue
signal distributions. The UCSC group had a binary readout system
based on PTSM (100 ns shaping time) [15]. In all the sites the
samples were cooled to  20 to  30 1C and scintillation counterswere used for triggering. Fig. 5 shows that the charge collection of
non-irradiated samples is consistently measured at four of thesites.
The electronics noise of the new 250 nm ABCN chips [16],which is the ﬁrst version of the readout ASIC planned for use for
the ATLAS strip upgrade is under evaluation [17,18]. The
measurements indicate a noise of around 600 ENC for short strip
and of 950 ENC for long strip sensors. In order to re-use thepresent ATLAS infrastructure, mainly the cables, it is preferredthat reasonable signal-to-noise ratio (S/N 410) be achieved at
maximum rated voltage of 500 V bias. Taking into account the
maximum ﬂuence, including a safety factor of two, the collectedcharge is required to be larger than 6 ke  at 1015 neq/cm2 (short
strips) and 9.5 ke  at 6  1014 neq/cm2 (long strips) at 500 V bias
in order to achieve S/N 410.

5.2. Annealing in CC curve

Fig. 4. Leakage current at (ﬁlled marks) 500 V and (open) 800 V irradiated with
protons and neutrons (p or n attached). The lines are drawn to guide the eye. The
samples are from two productions.

for 80 min at 60 1C. The temperature during the measurementis  20 1C. The ﬂuence ranged from 1.4  1012 to 1.12  1015
1-MeV neq/cm2. The leakage current increase seen at the lowest
ﬂuence is not consistent with micro-discharge [12], and isunderstood as resulting from surface current. The increase
disappears for the samples irradiated to 1013 neq/cm2 and above.
The leakage current values at 500 and 800 V are plotted in Fig. 4
for proton and neutron irradiated samples. We note that there islittle difference between proton and neutron irradiations. The
damage constant known for n-bulk, (3.9970.03)  10  17 A/cm
[10], can be translated to 17.3 mA at 1015 1-MeV neq/cm2 foreffective area of 80 mm2 and 0.32 mm thickness. Here the energy
gap of 1.12 eV is taken for the temperature correction. The
measured values are in reasonable agreement taking into accountthat the FDV is approximately 700 V. We conclude that the p-type
wafers exhibit a damage constant similar to that of n-type wafers.
We note that FZp wafers show a pre-irradiation leakage current
similar to that of Hamamatsu n-bulk sensors.

The charge collection of the irradiated samples was measured
either with or without annealing. We studied the change in CC
curves due to annealing in order to compare the results from allsites. Fig. 6 shows that two samples irradiated with protons to
1.12  1015 1-MeV neq/cm2 exhibited identical CC before
annealing, and 10–30% larger CC after annealing for 80 min at
60 1C. The CC for the samples irradiated with neutrons to 2  1014
and 5  1014 1-MeV neq/cm2 showed also an increase of 10–30%,
as shown in Fig. 7.

Fig. 5. Charge collection in kilo-electrons of non-irradiated samples, measured
independently at four sites.

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Fig. 6. Charge collection measured before (open marks) and after (ﬁlled) the
annealing of 80 min at 60 1C. The two samples were irradiated with protons to
1.12  1015 1-MeV neq/cm2. Typical uncertainty is shown as an error bar.

Fig. 8. Charge collection of proton irradiated samples: (top) 1.3  1015 and (above)6  1014 1-MeV neq/cm2. The horizontal bars correspond to S/N ¼10 (see text).

the maximum expected ﬂuence for the short and long strip
sensors. The horizontal bars correspond to the S/N¼10 criteria forshort and long strip sensors. In both cases, S/N of 15–18 should beachievable at 500 V.
Fig. 7. Charge collection measured before (open marks) and after (ﬁlled)annealing. The two samples were irradiated with neutrons to 2  1014 and5  1014 1-MeV neq/cm2.

In the following comparisons, we multiply by 1.2 the CC datameasured without annealing in order to emulate the effects ofannealing.
5.3. Charge collection of proton irradiated samples
Fig. 8 shows the CC of proton irradiated samples measured atvarious sites. The two plots are approximately for the expected
maximum ﬂuence for the short and long strip sensors. The
horizontal bars correspond to the S/N¼ 10 criteria for short andlong strip sensors. In both cases, however, S/N of 18–20 should beachievable at 500 V.
5.4. Charge collection of neutron irradiated samples
Fig. 9 shows the CC corresponding measurements for neutron
irradiated samples. The two plots correspond approximately to

5.5. Comparison with other strip detectors
The charge collection of various sensor types fabricated byMicron has been evaluated by Liverpool group [8,9]. The collectedcharge at 500 V bias is shown in Fig. 10, together with the present
data for Hamamatsu sensors. Since no annealing was included in
the Micron data, the collected charge from the previous plots(Figs. 8 and 9) was reduced by 20 710% for direct comparison.
Among these, the p-in-n sensors show much lower CC for both FZ
and MCz bulk material. The FZ n-side read-out sensors from
Hamamatsu and Micron are similar both for proton and neutron
irradiations.

5.6. Dependence on irradiation sources
Fig. 11 shows the collected charge of Hamamatsu and Micron
n + –in–p FZ sensors at 500 V bias for various irradiation sources.
The Micron data are from [9] and the Hamamatsu data correspondto the condition without annealing. At 500 V, the differences are
small between the different irradiation sources.

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Fig. 10. Charge collection at 500 V bias in comparison with Micron sensors: (top)proton irradiated samples and (above) neutron irradiated samples. Hamamatsu
data are shown in open squares. See text for annealing corrections.
Fig. 9. Charge collection of neutron irradiated samples: (top) 1  1015 and (above)5  1014 1-MeV neq/cm2. The horizontal bars correspond to S/N ¼ 10 (see text).

6. Full depletion voltage
The full depletion voltage (FDV) is an important parameter for
detector operation, although sufﬁcient signal charge can becollected even under partial depletion.
The FDV is evaluated from the bulk capacitance C–V
characteristics (fLCR ¼1 kHz) and from the charge collection
CC–V curves. Examples are shown in Fig. 12 for two proton
ﬂuence values: (a) 1.2  1013 and (b) 1.1  1015 neq/cm2. At
1.2  1013, the C–V curves exhibit characteristic distortions forthe samples with p-stop only while samples with p-spray inaddition show good agreement with the curve expected from CC–
V. Note that CC–V is consistent between the two sample groups.
The above distortion was observed only at 1.2  1013 neq/cm2.
Non-linear dependence in 1/C2 curves, as shown in Fig. 12b,
becomes more distinct at larger ﬂuence. The CC2–V curve,
however, increases linearly with the bias as expected.
The FDV is deﬁned from the CC2–V curve as the bias where thelinear line intercepts with the saturated plateau. At the highest
ﬂuence, where the plateau is not obvious, the plateau is taken as
the one derived from the lower ﬂuence data. The FDV from 1/C2–V
is determined similarly but two lines deﬁned in lower and higher
bias ranges are used and the difference was taken as theuncertainty. The central FDV value is taken as the one derived
from the lower bias range.
The FDV is summarized in Fig. 13 as a function of the ﬂuencefor proton and neutron irradiated samples. The agreement

Fig. 11. Charge collection at 500 V of Hamamatsu and Micron FZ sensors forvarious types of irradiation sources. See text for annealing.

between C–V and CC–V is noticeable but the FDV differs
between neutron and proton irradiations.
The difference in CC–V curves between proton and neutron
irradiation is shown in Fig. 14 where the charge collection iscompared between two proton and two neutron irradiatedsamples. The ﬂuence is similar, 5  1014 for neutrons and6  1014 neq/cm2 for protons, and all the samples were
processed for the same annealing. Although the collected charge
is similar at 500 V, it increases further with the bias for theneutron irradiated samples.

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K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 636 (2011) S83–S89

Fig. 14. Collected charge as a function of bias voltage for proton irradiatedsamples in comparison with neutron irradiated samples. There is an overall scale
uncertainty of 5%. The sensors were annealed for 80 min at 60 1C before
measurement.

Fig. 12. 1/C2 (lhs) and CC2 (rhs) vs bias voltage for the samples irradiated with
protons to: (a) 1.2  1013 and (b) 1.1  1015 n/cm2. In (a) 1/C2–V curves for two
samples are shown.

results previously obtained from FZ sensors produced by Micron.
Signal-to-noise ratio of 18–20 and 15–18 should be achievable forshort and long strip sensors, respectively, at the maximum ﬂuence
where a safety factor of two is included.
The full depletion voltage extracted from capacitance–voltage
studies rises at 1  1015 neq/cm2 to  700 V for proton irradiationand to nearly 1 kV for neutron irradiation. Operation in partial
depletion is therefore foreseen.

Acknowledgements
We would like to acknowledge K. Yamamura and S. Kamada ofHamamatsu Photonics K.K. for fruitful discussions on sensor
design. The teams of CYRIC, Tohoku University, Reactor Centre ofJožef Stefan Institute, Karlsruhe and CERN-PS are also acknowledged for conducting excellent irradiations. The research waspartly supported by Ministry of Education, Youth and Sports of
the Czech Republic, the German Federal Ministry of Education andResearch, the Japan Grant-in-Aid for Scientiﬁc Research (A) (Grant
no. 20244038), (C) (Grant no. 20540291), and on Priority Area
(Grant no. 20025007), the UK Science and Technology Facilities
Council (under Grant PP/E006701/1), and the US National Science
Foundation (under Grant PHY0652607).
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Fig. 13. Full depletion voltage as a function of the ﬂuence for proton and neutron
irradiated Hamamatsu FZ samples.

7. Summary
We are designing radiation hard silicon microstrip detectors
for the ATLAS inner detector upgrade at the super LHC. P-bulk
sensors are good candidates for such a very high radiation
environment, reaching 1  1015 1-MeV neq/cm2.
We evaluated the performance of Hamamatsu FZ grownsamples irradiated with protons and neutrons. The leakage
current increase and annealing property are in agreement withthe characteristics known for n-bulk sensors.
The collected charge was measured with different systems
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