Nuclear Instruments and Methods in Physics Research A 636 (2011) S111–S117

Contents lists available at ScienceDirect

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

Testing of surface properties pre-rad and post-rad of n-in-p silicon sensors
for very high radiation environment
S. Lindgren m, A.A. Affolder h, P.P. Allport h, R. Bates e, C. Betancourt m,n, 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. Ferre rre 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, K. Hara 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, D. Lynn a, P. Maddock m, I. Mandić i,
F. Martinez-McKinney m, S.Martı́i 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, 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-Street 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, Oxford Street, Liverpool L69 7ZE, UKi
Jožef Stefan Institute, 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, UKoDepartment 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 4 June 2010

We are developing n + -in-p, p-bulk and n-readout, microstrip sensors as a non-inverting radiation hardsilicon detector for the ATLAS Tracker Upgrade at the super LHC experiment. The surface radiation
damages of the sensors fabricated by Hamamatsu Photonics are characterized on the interstrip
capacitance, interstrip resistance and punch-through protection evolution. The detector should provide
acceptable strip isolation, exceeding the input impedance of the signal readout chip  1 kO, after the
integrated luminosity of 6 ab  1, which is twice the luminosity goal.
Published by Elsevier B.V.

Keywords:
p-Bulk silicon
Surface damage
Charge collection
Punch-through voltage

1. Introduction
The silicon microstrip detector continues to play an essential
role in high-energy experiments for its ability of precision
tracking. The detector at the planned Super LHC (large hadronnCorresponding author. Tel.: + 1 831 459 3567; fax: + 1 831 459 5777.
E-mail address: cbetanco@ucsc.edu (C. Betancourt).

0168-9002/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.nima.2010.04.094

collider) is required to remain operational up to the integrated
luminosity of 3000 fb  1 with the instantaneous luminosity of1  1035 cm  2 s  1. In order to cope with ten-fold increase ininstantaneous luminosity beyond the design value of the LHC,
currently under commissioning, the ATLAS collaboration isinvestigating an inner tracking system based fully on semiconductor devices. The segmentation is varied in radius R, theinnermost being the pixel, followed by short (2.4 cm) and long
(9.7 cm) microstrip detectors. The radiation activity [1] with a

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safety factor of two multiplied is (7 11)  1014 1 MeV neq./cm2
for the short strip (R¼38 cm) and 3 6  1014 1 MeV neq/cm2 forthe long strip (R ¼85 cm) regions, where the two numbers in theparentheses are the ﬂuence values at the central and forward
regions. The charged particle contribution to the ﬂuence is similar
to the neutral at R 28 cm in the central region but decreases
with R and in the forward region to typically 20% of the total. It istherefore important to investigate the damages due to bothcharged particles and neutrons. Both neutrons and protons
displace silicon atoms via non-ionizing energy losses, which
results in a bulk damage. Protons in addition ionize the atoms intheir path that leads to permanent damage at the sensor surface.
The ATLAS R&D group ‘‘Development of non-inverting silicon
strip detectors for the ATLAS ID upgrade’’ is formed to develop
radiation hard tracking detectors based on the p-bulk microstrip[2] technology. Since the radiation induced impurity in siliconacts as an acceptor, the n + -on-p device is non-inverting. This
allows us to operate the sensors at partial depletion when obliged.
The experience of adopting p-bulk silicon for particle tracker islimited. This is in part because additional strip isolation structure
is required for individual strip signal readout to prevent mobile
electrons to be accumulated between strips. The R&D group isevaluating the sensors fabricated by Hamamatsu Photonics using
commercially available p-type wafers. We report the surface
damage including the decrease in strip isolation and punchthrough voltage evolution. The bulk damage is reported in Ref. [3].
One of the most pressing issue for n-in-p strip sensors are theinterstrip characteristics before and after ionizing radiation, since
the electron accumulation layer on the surface needs to becompensated for large ﬂuences and dose levels.
Previous studies with p-type sensors, e.g. within the context of
RD50, were done using p-spray to isolate the n-strips [4]. This
study uses ‘‘mini-SSDs’’ ( 1 cm long) produced by Hamamatsu
Photonic (HPK) within the ATLAS upgrade program. The isolation
is done with p-stops of varying geometry, p-spray and both
combined with p-doses (concentration) varying from 0 up to2  1013 p/cm2.

2. Samples and irradiation
The sample sensors were fabricated using 15 cm wafers with
/1 0 0S crystal orientation and 320 mm thickness. The wafers wereport in this paper are FZ grown (FZp wafers in Ref. [5]) having
fewer defects than normal FZ wafers. The R&D group continues toevaluate other commercially available p-type wafers [5]. The strip
pitch is 74.5 mm. Details of the design including strip isolation
structures are described in Ref. [2]. The performance of main
sensors, 97.5 mm2, is reported elsewhere [6]. The characteristics
of irradiated sensors are studied using miniature samples of
10 mm square, where there are 104 strips of 8 mm length.
The proton irradiation was performed at Cyclotron Radio
Isotope Center (CYRIC) of Tohoku University. Details of irradiation
facility and methods are described in Refs. [5,7]. 70-MeV protons
were uniformly irradiated by scanning periodically the sample
sensors. The irradiation took typically a few 10 min to a few hours
depending on the ﬂuence. The sensors were kept cooled at 101 C
during irradiation and the irradiated samples were immediately
stored in refrigerator to suppress any post-irradiation annealing
to take place. The samples measured in Japan were glued on
printed circuit board and biased at  100 V during the irradiation.
The samples measured elsewhere were irradiated as bare chips
with no bias applied. The ﬂuence we refer to is in 1 MeV neutron
equivalent value taking into account the NIEL factor of 1.4. The
ﬂuence uncertainty is determined by the 27Al(pn) reaction crosssection, which does not exceed 10%.

3. Strip isolation structures
It is thought that n-on-p detectors are more sensitive tosurface effects than p-on-n detectors. One concern is the risk thatthe ﬁxed oxide charges in the Si–SiO2 interface would lead to a
conductive layer of electrons at the surface [8]. Within the project
ATLAS07 for the ATLAS upgrade different structures for mini-SSDshave been produced. The different structures use the concept ofpreventing those damages by surface treatments, positive doped
implants (p-impurities) in form of p-stop or p-spray, or combinations of both.
The p-stops are implanted to the detectors with a mask while
p-spray is sprayed on over the whole sensor. Several doses andcombinations of p-stop and p-spray have been applied to different
sensors. Different structures to apply the p-stops have also been
used, which is indicated by different zone number and seen in
Fig. 1.
Detectors with zone 1 have no structure, i.e. they have only
p-spray since the p-stop mask was left out. Detectors with zone 2
have individual p-stops, i.e. each strip is surrounded by
p-implants in opposite to the other structures which only havea line of common p-implants between the strips. Zone 3 sharesthe p-stops between the strips and zone 4 has additional punchthrough protection structure, which will be discussed later in thepaper. Detectors with zone 5 have narrow metal, meaning thealuminum layer over the strips do not reach outside the strip itself
and ﬁnally zone 6 is similar to zone 3 but with a wider strip pitch.

4. Interstrip resistance and capacitance measurements
The interstrip resistance and capacitance are importantparameters used to characterize the effects of surface radiation
damage of silicon strip detectors. The interstrip resistance isimportant for strip isolation, so that a sufﬁciently high interstrip
resistance can prevent signal sharing between neighbors which
could lead to degradation of the position resolution. The interstrip
capacitance is the main contributor of noise in between strips.
A properly functioning detector should thus try to minimize theinterstrip capacitance in order to have a higher signal-to-noise
ratio, while maximizing the interstrip resistance to minimize
crosstalk between strips.
Interstrip resistance measurements were performed in a probestation. As seen in Fig. 2, the DC pad of a test strip was connected
to an Agilent 4156C Precision Semiconductor Parameter Analyzer
and grounded while a voltage V2 was applied to the DC pads of
two of its closest neighbors, which were also connected to theanalyzer. The detectors were biased using a Keithley 2410HV
Source Meter and each measurement was performed at several
bias voltages ranging from 5 to 300 V. The voltage V2 applied to
the neighbor strips was varied through the parameter analyzer
from  1 to 1 V in 100 mW steps. Each measurement was
performed at 221 C with nitrogen gas ﬂowing over the detector
for moisture control.
The idea is that the voltage V2 on the neighbors will induce acurrent I1 through the test strip. The interstrip resistance can thenbe determined by
Rint ¼ 2dV2 =dI1

ð1Þ

where the factor of 2 comes from the fact that two neighboring
strips are used. The resulting IV curve is shown in Fig. 3.
To measure the interstrip capacitance a slightly different
set-up was used. Instead of using the DC pads, the test strip andits neighbors were connected to an Agilent E4980A Precision LCR
meter via the AC pads. In addition, the next two neighbors (twostrips away from the test) were grounded to act as a shield from

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Fig. 1. Different p-stop structures as indicated by the different zone numbers.

Fig. 3. The current through the test strip vs. the voltage applied to the neighbors
shown at different bias votlages. The interstrip resistance is given by the inverse of
the slope.

5. Punch-through protection

Fig. 2. Measurement set-up for interstrip resistance. Visual illustration of themeasurement (top), showing the connections to the strips via the DC pads and tothe bias ring. An equivalent circuit diagram (bottom) is also displayed.

the outer laying strips. The frequency of the AC signal from the
LCR meter was varied, and each measurement was taken at
10 kHz, 100 kHz, 1 MHz, and 2 MHz. The bias voltage ranged from0 to 800 V with a step size of 50 V. The measurements wereagain performed at 221 C with nitrogen gas ﬂowing over thedetector.

Ac-coupled sensors are susceptible to very large voltages
between the metal readout traces (held to ground through thefront-end electronics) and the strip implants in the case of large
charge accumulation in the bulk, for instance in the case of beamlosses [9]. When sufﬁcient charge is deposited in the detector, theelectric ﬁeld can collapse causing the implants and backplane of
the sensor to ﬂoat to unknown voltages. These large voltages on
the implants can reach the order of half the bias voltage, and thus
can exceed the speciﬁcation for the hold-off voltage of thecoupling capacitor, which are typically tested to 100 V. In order toprevent these large voltages, the punch-through (reach-through)effect is used [10], where implants in close proximity will
effectively be shorted together if the voltage between themexceeds a geometry dependent voltage. This provides an effective
over-voltage protection for single strips, which get shorted to thebias line in the case of voltages in excess of the punch-thoughprotection (‘‘PTP’’) voltage.
Protection against large voltages between strip implants andthe metal traces can be achieved with the punch-through effects
between strips and bias ring. This is trivially done for p-spray, but
requires sophisticated structures in the case of p-stops. In thispaper, the effects of several PTP structures on the PTP voltage

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have been studied. In particular, zone 4 detectors were designedwith complicated PTP structures, as seen in Fig. 4.
Unirradiated detectors were measured at Lancaster University
in a probe station kept at 211 C. Measurements on irradiateddetectors were carried out at the University of Tsukuba wheredetectors were placed two at a time on printed circuit boards
which were then placed in a thermostat chamber set at 201 C
with nitrogen gas ﬂowing in for moisture control.
For PTP measurements, the effective resistance is measured
between the DC pad of a strip and the bias ring. The detectors
were reverse biased to 300 V with voltage being applied to the
backplane and the bias ring grounded. A test voltage Vtest is thenapplied to the DC pad, and the subsequent induced Itest current
was measured between the pad and the bias ring. The neighboring
strips were left ﬂoating. The effective resistance is then given by
Reff ¼ dVtest =dItest :ð2Þ

The effective resistance can be thought of as the bias resistor inparallel to another resistance, which we call the punch-throughresistance. The effective resistance can then be written as
Reff ¼ ð1=Rbias þ 1=RPT Þ1

ð3Þ

where Rbias is value of the bias resistor and RPT is the punchthrough resistance.
The punch-through voltage VPT is then taken to be the voltage
where
RPT ¼ Rbias

ð4Þor alternatively the point where the effective resistance is equal tohalf the value of the bias resistor. The effective resistance as afunction of the test voltage for an unirradiated detector is shownin Fig. 5a while irradiated detectors are shown in Fig. 5b.

6. Results
To ﬁrst order, the interstrip resistance does not seem todepend on the speciﬁc zone, but instead depends only on the total
amount of p-dose applied to the surface (through p-stop or
p-spray), as seen in Fig. 6. There is an obvious correlation between
the total amount of p-dose applied and the value of interstrip
resistance after irradiation, with a higher total p-dose resulting in
better post-rad strip isolation (higher interstrip resistance), which
is illustrated in Fig. 7.

Fig. 4. The different PTP structures designs that have been tested for the paper.

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Fig. 7. Interstrip resistance vs. the total p-dose applied for detectors irradiatedwith protons to 1  1013 neq. Higher p-doses lead to higher interstrip resistanceafter irradiation.

Fig. 5. The effective resistance vs. the test voltage applied for (a) unirradiateddetectors and for (b) detectors irradiated with protons to 1.2  1013 neq. The
horizontal arrow indicates where the punch-through resistance is equal to thevalue of bias resistor, and the vertical arrow indicates the corresponding punchthrough voltage.

Fig. 6. Interstrip resistance for irradiated series 3 detectors. There is a clear
dependence on the total p-dose applied after irradiation.

The interstrip capacitance shows little change after irradiation,and seems to be dependent on the speciﬁc zone, but not really on
the amount of total p-dose applied. Still, the dependence is weak,Fig. 8. Scatter plot of the interstrip resistance vs. interstrip capacitance fordetectors irradiated to 1  1013. Zone 5 (black circle) tends to perform the worst
compared with the other zones.

and no predictions could be made about whether adding a p-stopmask would increase or decrease, or by which amount, theinterstrip capacitance. Zone 5 is the only exception to this,consistently showing an increase in capacitance after irradiation.
It is helpful to plot the interstrip resistance vs. the interstrip
capacitance. This helps determine which detectors have the mostfavorable behavior (the highest resistance and the lowest
capacitance). Detectors performing the best should lie in theupper-left corner of the graph, while the least preferable behavior
is seen in the lower-right of the graph. Fig. 8 shows the interstrip
resistance vs. capacitance for several detectors of different zones
and total p-dose. One can see that zone 5 tends to have the worst
interstrip properties after irradiation.
The leakage current as a function of the bias voltage wasmeasured in order to determine if good strip isolation came withthe disadvantage of a lower breakdown voltage. The IV curves forvarious irradiated detectors taken at room temperature are shownin Fig. 9. It can be seen that all most detectors have a breakdown
voltage in excess of 1000 V, while three detectors (all zone 3)
exhibit breakdown at about 900 V. The high breakdown voltage
should not signiﬁcantly affect our measurements, as the interstrip
resistance is taken at 300 V, and the capacitance at 800 V, which
are both safely below the breakdown voltage. It is also important
to note that all detectors exhibit breakdown at a much higher

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Fig. 9. Leakage current vs. bias voltage for various detector types irradiated to1  1013 neq. taken at 22 1C. Zone 3 breaks down the earliest at  900 V, which isstill greater than the 600 V breakdown speciﬁed by the manufacturer.
Fig. 11. Punch-through voltage for zone 3 measured at various ﬂuences. Total pdose¼ 4  1012 cm  2, ﬂuence values are in neq.

It is interesting to note that the punch-through voltage forzone 3 detectors and zone 4 detectors is similar before and after
irradiation. This implies that the complicated punch-throughprotection structures are not needed, as zone 3, which has a
distance of 70 mm from the n+ implant to the bias rail, seems toprovide adequate protection. Further, as seen in Fig. 11, zone 3
detectors exhibit adequate protection even at high ﬂuences,having a punch-through voltage in the range of 7–20 V. Thisfurther suggests that complicated punch-through protection
structures are not necessary.

7. Summary and discussion

Fig. 10. Punch-through voltage for (a) unirradiated detectors and (b) detectors
irradiated to 1.2  1013 neq. Both pre and post-rad detectors show a dependence on
the total p-dose applied.

voltage than the breakdown voltage of 600 V that was speciﬁed by
the manufacturer.
The punch-through voltage for unirradiated detectors is
between 10 and 30 V, as seen in Fig. 10a. There seems to be aclear wafer correlation on the value of the punch-through voltage,i.e. the voltage depends on the total p-dose and type (p-spray,
p-stop). After irradiation, a dependence of the punch-throughvoltage on the total p-dose and type is still evident, which is seenin Fig. 10b.

We are designing radiation hard silicon microstrip detector for
the ATLAS Inner Detector Upgrade at the super LHC. P-bulk sensor
is a good candidate for such a very high radiation environment
reaching 1  1015 1 MeV neq/cm2.
The interstrip resistance decreases and interstrip capacitanceincreases after irradiation. To ﬁrst order, the interstrip resistancedoes not depend on the speciﬁc zone of the detector, but instead
depends on the total dose of the p-impurities applied. The higher
the total doses of impurities, the better the strip isolation that canbe achieved after irradiation. The interstrip capacitance shows
little change after irradiation and is dependent on the speciﬁczone. Speciﬁcally, zone 5, which has narrow metal, tends to havethe highest interstrip capacitance after irradiation, without
providing any beneﬁts such as a higher breakdown voltage. The
higher interstrip capacitance for zone 5 is not surprising given the
shorter distance between readout strips compared with otherzones.
Even if the interstrip resistance is low, one would only expect
to see a loss in signal and increase in noise if the values were on
the same order of magnitude as input impedance of the signal
readout chip, which is typically about 1 kO. The signal getscoupled to the metal strip and to the ampliﬁer instead of being
shorted to the neighbor via the DC conductance. Therefore, as longas the interstrip resistance is large compared to the input
impedance of the chip, the signal will end up in the ampliﬁer.
Further, there has not yet been any observed signal degradation ornoise increase due to a decrease in the interstrip resistance.
The breakdown voltage for zone 3 detectors tends to start atslightly lower voltages than other detectors, but the voltage isabove 900 V and is much greater than the 600 V breakdown
voltage speciﬁed by the manufacture.

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Measurements of the punch-through voltage reveal that there
is a clear wafer correlation. In particular, when looking at a
speciﬁc conﬁguration (p-spray only, p-stop only, p-spray +stop)
wafers with the highest p-dose have the highest punch-throughvoltage, which holds true even after irradiation.
After irradiation, zone 3 detectors have similar punch-throughvoltages as zone 4 detectors, which have a much more
complicated punch-through protection structure. The acceptablepunch-through voltage of zone 3 detectors with p-stops of
4  1012 cm  2 extends to proton ﬂuences beyond 1015 p/cm2.
Thus, punch-through protection is achievable with zone 3 which
has no dedicated structure.
It is important to mention that Eq. (2) is not the only way todeﬁne Reff. An alternate deﬁnition would be to take the integral
form of Eq. (2). This would have the advantage of incorporating
the total current that can be drained from the strip to the bias railand providing the effective resistance for charges to escapethrough. But the integral deﬁnition is less sensitive to the onset ofpunch-though, and thus less suitable for ﬁnding the onset of thepunch-though voltage. This leads to a higher punch-throughvoltage, culminating in differences between 5 and 30 V from thedeﬁnition used in this paper.

Acknowledgements
We would like to express our thanks to K. Yamamura and
S. Kamada of Hamamatsu Photonics K.K. for helpful comments on

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the design of the sensor. The team from CYRIC, Tohoku University
for conducting excellent irradiations. The research was partly
supported by Ministry of Education, Youth and Sports of the Czech
Republic, the German Federal Ministry of Education and Research,
the Japan Grant-in-Aid for Scientiﬁc Research (A) (Grant no.
20244038), (C) (Grant no. 20540291), and on Priority Area (Grant
no. 20025007), the Slovenian Research Agency, the Spanish
National Program for Particle Physics, the UK Science andTechnology Facilities Council [under Grant PP/E006701/1], theUS Department of Energy and the US National Science Foundation[under Grant PHY0652607].

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