ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 565 (2006) 538–542
www.elsevier.com/locate/nima

Performance of irradiated nþ-on-p silicon microstrip sensors
K. Haraa,, A. Mochizukia, T. Munakataa, Y. Nakamuraa, K. Nakamuraa, K. Inouea,
Y. Ikegamib, T. Kohrikib, S. Teradab, Y. UnnobaInstitute of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Ten’nodai, Tsukuba, Ibaraki 305-8571, Japanb
High Energy Accelerator Research Organization (KEK), Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan
Received 3 April 2006; received in revised form 11 May 2006; accepted 5 June 2006
Available online 14 July 2006

Abstract
The silicon microstrip detector type with nþ readout fabricated on p bulk, nþ -on-p, is a candidate to be operational in radiation
environments much severer than in LHC. We characterized nþ -on-p detectors which were irradiated up to 1:1  1014 protons=cm2 10
years ago. The noise level and charge collection were evaluated using the ATLAS SCT readout electronics system. Radiation-induced
increase in the noise and loss in the charge collection are not signiﬁcant. The charge collection is not degraded in any particular point in
the inter-strip region when the detector is operated under partial depletion.
r 2006 Elsevier B.V. All rights reserved.
PACS: 29.40.Gx; 29.40.Wk
Keywords: Silicon microstrip detector; Charge collection; Irradiation; p bulk

1. Introduction
Silicon microstrip detectors have become one of thecommon apparatus in high-energy physics experiments for
tracking in a region close to the interaction point or even in
the entire ‘‘inner’’ tracking volume. The advantages ofsilicon detectors are, among many others, superior spatial
resolution and resistance against radiation if they aredesigned properly. The two large silicon microstrip
detector systems of ATLAS [1] and CMS [2] groups at
LHC employ a scheme of pþ readout strips fabricated in nbulk, or pþ -on-n. These detectors are expected to remain as
precision trackers up to the ﬂuence of 2  1014 neq cm2
anticipated for 10 years of LHC operation. The bulk will
behave as p-type by radiation damage and then the full
depletion voltage increases with ﬂuence. Since the inverted
pþ -on-p detector needs to be fully depleted to electricallyisolate the readout strips, the detector lifetime is determined by the high-voltage applicable to the detector.
Efﬁcient cooling system is also essential to suppress theCorresponding author. Tel.: +81 29 853 4270; fax: +81 29 853 4491.

E-mail address: hara@hep.px.tsukuba.ac.jp (K. Hara).
0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2006.06.051

radiation-induced leak current and to inhibit the reverse
annealing to turn on.
The ATLAS SCT (semiconductor tracker) group has
speciﬁed the high voltage of 350–500 V be applicable to thedetector modules, depending on their radial positions fromthe interaction point. In the super LHC environment where
the radiation becomes 10-fold severer, the high voltage
would reach a few kV if we employ the same pþ -on-nsensors. Our recent experience [3] shows that Hamamatsu
Photonics can fabricate high voltage resistant detectors but
to about 800 V, which is still far below the above
requirement.
The nþ -on-n detectors presently employed for ATLAS
and CMS Pixel devices have longer lifetime. The signal will
be read out from the junction side after the bulk inversion,
which allows the sensors to be operated at partial depletion
once the full depletion exceeds the applicable bias voltage.
With use of low noise ampliﬁers, such devices can be
operated with a sufﬁcient signal-to-noise ratio even for
reduced amount of signal charge. Since the nþ -on-ndetector requires backside processing and hence the sensor
fabrication is much expensive, it was not employed for the
strip detectors.

ARTICLE IN PRESSK. Hara et al. / Nuclear Instruments and Methods in Physics Research A 565 (2006) 538–542

Intensive study is being carried out for silicon detectors
for the super LHC to develop radiation hard bulk material
which does not show rapid increase in the full depletion
voltage [4]. As for the detector type, the nþ -on-p is
attractive for the advantage the nþ -on-n possesses and theprocessing is single sided. Ten years ago, we fabricated andirradiated some nþ -on-p sensors with 12-GeV protons up
to 1:1  1014 =cm2 [5]. The leak current increase and the full
depletion voltage evolution have been evaluated. The nþ on-p type was not selected for the current SCT because
experiences with high resistive p bulk silicon had not been
accumulated by that time. We re-visited the results andadded new measurements including noise and chargecollection evaluations.

539

Table 1The sample sensors, categorized by two p-stop doping densities, and by
three radiation levels including non-irradiation
Samplep-Stop doping

Irradiation

H1
H2
L1
L2
N1
N2

1:0  1:014 =cm2
0:2  1:014 =cm2
1:0  1:014 =cm2
0:2  1:014 =cm2
1:0  1:014 =cm2
0:2  1:014 =cm2

1:1  1014 p=cm2
1:1  1014 p=cm2
0:43  1014 p=cm2
0:43  1014 p=cm2
Not irradiatedNot irradiated

2. Sample detectors and irradiation
The irradiation took place in July 1995 at KEK using 12GeV protons. The details of the irradiation and the sample
detectors are described in Ref. [5].
The sample detectors are nþ -on-p single-sided microstrip
detectors with dimensions of 60  34  0:3  mm3 , fabricated by Hamamatsu Photonics. The pitch of the nþ
readout strips is 50 mm. Each of the readout strips issurrounded by a ring of 26 mm wide p-stop implant. We
employed a ‘‘common p-stop’’ structure where the ends of
p-stop strips which extend the nþ strips in length areconnected together. The p-stop doping should be optimized
from the two competing requirements: the doping must be
high enough to isolate the strips, while too high density
creates high electric ﬁelds at the p-stop boundaries
lowering the detector breakdown voltage. We compared
the results of two p-stop densities of 1.0 and
0:2  1014 =cm2 .
The samples were irradiated to 0.43 or 1:11014 protons=cm2 . The temperature during and 12 days
after the irradiation was 10  C. Then the temperature was
kept essentially to 0  C except for some periods fortransportation and measurements [5]. The depletion
voltages ﬁrst measured 90 days after the irradiation
resulted in 150 and 154 V (210 and 226 V) for the samples
irradiated to 0.43 ð1:1Þ  1014 protons=cm2 . The full depletion voltages before irradiation was typically 153 and 155 V[6]. After the measurements, the samples have been kept ina refrigerator at 0  C. Unfortunately it then lost to control
the temperature due to power outages which happened
typically once a year for a weekend long period.
The samples used in this study are summarized inTable 1. These sensors are glued individually to PCB
frames since the pre-irradiation measurement. The biasring, backside contact, and three clusters of neighboring
AC pads, each consisting of ﬁve consecutive strips, arewirebonded individually to the pads on the PCB frame.
The pads are used to feed the voltage and measure the bulk
and interstrip capacitances.

Fig. 1. Bulk capacitance (nF) as a function of the bias voltage for the four
irradiated and one non-irradiated samples.

3. Results
3.1. Bulk and interstrip capacitances
The bulk capacitance between the bias-ring and thebackside was measured with an LCR meter. The LCR test
signal frequency was set at 1 kHz, where the impedancecontribution from the bias resistors in series is minimized.
The shape of the CV curves are dependent on thetemperature due to large radiation-induced leak current:
at lower temperatures the shape of the CV curves becomes
clearer to extract the full depletion voltage V FD . Fig. 1
shows the CV curves measured at 20  C for the four
irradiated and one non-irradiated samples. The full
depletion voltages determined from these curves are
330–350 and 200–205 V, respectively, for the samples
irradiated to 1.1 and 0:4  1014 =cm2 . It is 160 V for the
non-irradiated samples. The full depletion voltage of the
non-irradiated samples remains similar, but we see atendency that the full depletion voltages have increased in
the 10 years. The uncontrolled temperature rise may
explain the difference, although any qualitative estimate
is not possible without the temperature record in the past
10 years.
The interstrip capacitance was measured at 20  C andat 0.4 MHz LCR frequency. Among the ﬁve consecutive
strips, the capacitance between the central two strips was

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K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 565 (2006) 538–542

Fig. 2. Interstrip capacitance for irradiated and non-irradiated samples.

Fig. 3. The one-sigma noise levels as a function of bias voltage.

measured with other strips set ﬂoating. The results areshown in Fig. 2. The detailed shape in the small voltage
region actually depends on the temperature but the shape ishardly affected in the region above 50 V bias voltage. Theinterstrip capacitance drops within a few 10 V, implyingthat the strips are electrically isolated above that voltage.
The asymptotic value at high voltages is 3.6–3.8 pF. These
characteristics are not different among the irradiated andnon-irradiated samples, and among the different p-stop
densities.
3.2. Noise performance
The increase in the detector leak current potentiallydeteriorates the noise performance. The magnitude, however, should be dependent also on the readout electronics
and it is essential to use the same, or similar, electronics
foreseen in the future experiments. We used the SCT
readout chip, ABCD3T [8], to evaluate the noise levels. The
ABCD3T, providing 128-ch binary hit data, consists ofmainly ampliﬁer-shaper-comparator chains and 132 deep
digital pipeline buffers. The chip was read out using the
VME-based SCTdaq system through the SCT hybrid [7].
Ten consecutive strips of each sample were wirebonded tothe same channels of the same chip so that the noise
affected by irradiation and p-stop doping can be directlycompared.
The noise levels were evaluated in the SCT standard
procedure, where the ampliﬁer response is measured forthree different test charges to extract the gain andequivalent noise charge (ENC) at 1 fC threshold. The noiselevels at the comparator input were measured up to 500 V
bias, which is the limit speciﬁed to the sensor. For somesamples, the bias was raised up to 650 V, the limit being
given by the hybrid. The sample was cooled to 6 to 8  Cmonitored at the cooling block attached underneath the
PCB frame for the sample. The temperature monitored on
the hybrid was typically 4  C. Fig. 3 shows the results.
The rms noise values are calculated from the six central
channels out of the ten wirebonded channels, excluding
two channels each side to suppress effects from un-bonded
channels at neighbors. The mean noise distributes between

11 and 14 mV with typical rms spread among the six
channels being 0.6 mV. Typically, the 12 mV spreadcorresponds to 1700 e ’s ENC. The reproducibility of themeasurement was examined for one sample by cutting
the wires, re-wirebonding and repeating the measurement.
The ratios averaged over six channels are found to be
0.94–1.00 at different bias voltages. Taking these into
account, the typical uncertainty is less than 1 mV. The
irradiated samples show the results consistent within 2 mV.
There is no systematic difference between the two irradiation ﬂuences. The detector leakage reached at most 0.5 mA,or 1 mA per strip in these measurements. The same
electronics is sensitive to micro-discharges with this strip
leakage and the noise is degraded substantially. Therefore,
the electronics is far less sensitive to the radiation induced
current. The un-irradiated samples show 2–3 mV larger
noise especially at higher voltages above the full depletion.
We see a tendency that the noise is smaller by 1 mV for the
samples with 0:2  1014 =cm2 p-stop doping than 1:0 1014 =cm2 doping.
3.3. Charge collection
The charge collection was evaluated by injecting
Nd:YAG laser [9] of 1064 nm wavelength and about 10 ns
base width. Since the laser width is shorter than the charge
integration time 20 ns and the laser absorption in silicon[10] can be regarded moderate in the 0.3 mm thick bulk,
this laser system can simulate the passage of charged
particles. The laser intensity was adjusted to generate asignal corresponding to passage of a single high-energy
charged particle. Unlike the charged particle, though, thelaser is affected by sensor’s local structures by additional
absorption and reﬂection. The laser was focused to 2 
2 mm on the sensor surface. The response is found not to beaffected by the precision of focusing: the focusing was
adjusted better than 1 mm while the response changes by
only 4% per 10 mm shift along the focal direction. The
sensor and hybrid were placed in an aluminum box which
has a window for laser injection. The box was cooled toARTICLE IN PRESSK. Hara et al. / Nuclear Instruments and Methods in Physics Research A 565 (2006) 538–542

5  C to suppress the leakage current of the irradiatedsensor.
Fig. 4 shows the threshold curves of Sample H2
measured at different bias voltages. At each thresholdsetting 500 laser pulses were injected at 1 kHz. The numberof hits is plotted in the ﬁgure as a function of the thresholdvoltage. Since the signal polarity of nþ -on-p detectors isreversed, we made use of the trimming feature of the
ABCD3T chip: we set the offset voltage at the discriminator (trim level) to its maximum for the discriminatorworks only with positive signals. Under this condition, thediscriminator output turns from ‘‘1’’ to ‘‘0’’ on the presenceof large negative signals out of nþ -on-p. The ABCD3T
examines the hit pattern in the three consecutive clock
cycles at 40 MHz about trigger timing. We deﬁne the hit if‘‘0’’ to ‘‘1’’ transition is present in these three clocks.
Therefore, the discriminator senses the voltage in the risingpart of the shaper output, e.g. the tail of the shaper output.
The left part of the trapezoidal curves represents the signal
pulse height distribution, the voltage at 50% efﬁciency
corresponding to the mean pulse height. The input charge,
or collected charge, is extracted from the mean pulse heightminus that at zero bias, using the measured ABCD3T gain,
which is typically 50 mV/fC. The collected charge for
Sample H2 (1014 =cm2 p-stop doping, 1:1  1014 =cm2
irradiation) is plotted in Fig. 5. Also plotted in the ﬁgureis the data for non-irradiated sample N2 of the same p-stop
doping. The laser was focused at 13 mm from one of the
strips and the charges from this strip and the neighboring
strip are added. The collected charge increases with the bias
and reaches the maximum around 130 V (300 V) for the
non-irradiated (irradiated) sample, which are consistent
with the full depletion voltage of 150 V (350 V). Since themaximum collected charge are consistent with each other,
we can conclude that the additional charge trapping due toirradiation is little in the nþ -on-p sensors. In a previous
study, we measured that the charge is lost by typically
10–15% for pþ -on-n sensors irradiated to a similarradiation level [11]. We note that the charge collection
at lower voltages is moderate in the nþ -on-p sensors
while substantial fraction of charge is lost in the pþ -on-nFig. 4. Threshold curves measured at various bias voltages, 0–400 V.

541

Fig. 5. Collected charge (fC) as a function of the bias voltage for (ﬁlled
diamonds) irradiated Sample H2 and (open squares) non-irradiatedsample N2.

Fig. 6. Collected charge as a function of the distance from the left strip(strip pitch 50 mm) for Sample H2. The charge read out from the left
(right) strip is shown in dashed (solid) curves, the sum in ﬁlled diamondsfor 300 V bias and in open diamonds for 100 V bias.

sensors once the bias is lowered below the full depletion
voltage [11].
The charge collection between the strips was measured
by scanning the sensor across the strips at a step of 1 mm.
Fig. 6 shows the collected charges from the left and rightstrips separately, together with their sum (ﬁlled diamonds),
as a function of the distance from the left strip. The sensor
is Sample H2 operated at 300 V. The sum taken at 100 Vbias is also shown (open diamonds) to compare the chargecollection distributions at and below the full depletion. The
aluminum electrodes 6 mm wide are located at 0 and 50 mm,
which are not transparent for the laser. The dips around
729 mm are due to the boundary of the oxide insulators,
and the dip at the center is caused by the p-stop implant
26 mm wide. The distribution turns to be not perfectly
symmetric about the center because the sensor structure
itself is not symmetric due to the ﬁnite mask registration
precision at sensor processing. The distributions are similarin shape for the 300 and 100 V data. This ensures that
there is no particular region where the charge collection
is inferior when the sensor is operated under partial
depletion.

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K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 565 (2006) 538–542

4. Summary
We have characterized nþ -on-p sensors irradiated up to
1:1  1014 protons=cm2 with two different p-stop doping
densities of 0.2 and 1:0  1014 =cm2 . The strip isolation isachieved below 30–50 V bias voltage, the interstrip
capacitance reaching 3.7 or 0.6 pF/cm. There is a small
difference between the two irradiation ﬂuences andbetween the two p-stop doping densities. The noise level
was found also not dependent on the ﬂuence, while thenoise for 0:2  1014 =cm2 tends to be somewhat smaller
than for 1:0  1014 =cm2 .
The collectable charge is not degraded even for the
sample irradiated to 1:1  1014 protons=cm2 . The irradiated detectors can be operated below the full depletion
without showing any region of inferior charge collection.
References[1] F. Campabadal, et al., Nucl. Instr. and Meth. A 538 (2005) 384.
[2] The Tracker Project, Technical Design Report, CERN/LHCC 98-6
CMS TDR 5 (1998): Addendum to the CMS Tracker TDR, CERN/
LHC 2000-016 (2000).

[3] T. Akimoto, et al., IEEE Trans. Nucl. Sci. NS-51 (2004) 2215.
[4] See, for example, F. Campabadal, et al. (RD50 Collaboration)
CERN-LHCC-2004-031 (2005).
[5] S. Terada, et al., Nucl. Instr. and Meth. A 383 (1996) 159.
[6] The quoted full depletion voltages are for different individual
samples. The evolution of the full depletion voltage with the ﬂuenceis precise to 10 V, which is the typical variation among the samples.
We are conducting a new set of systematic measurements to coverwider ﬂuence range.
[7] Y. Unno, et al., Nucl. Instr. and Meth. A 541 (2005) 286.
[8] W. Dabrowski, et al., Proceedings of Sixth Workshop on Electronics
for LHC Experiments, Cracow, Poland, 11–15 September 2000,
CERN, Geneva, 25 Oct. 2000.
[9] K. Hara, et al., Nucl. Instr. and Meth. A 541 (2005) 122;
Y. Unno, et al., Nucl. Instr. and Meth. A 383 (1996) 238.
[10] A.H. Johnston, IEEE Trans. Nucl. Sci. NS-40-6 (1993) 1694 The
absorption length from the ﬁgure in this reference is about 0.5 mm for1064 nm infrared pulses in silicon.
[11] T. Nakayama, et al., IEEE Trans. Nucl. Sci. NS-47 (2000) 1885.