Nuclear Instruments and Methods in Physics Research A 466 (2001) 354–358

Characteristics of irradiated silicon microstrip detectors withh1 0 0i and h1 1 1i substrates
T. Akimotoa, S. Araia, K. Haraa,*, T. Nakayamaa, Y. Ikegamib, Y. Iwatac,
H. Kobayasia, T. Kohrikib, T. Kondob, I. Nakanod, T. Ohsugic, M. Shimojimaa,
S. Shinmaa, R. Takashimae, S. Teradab, N. Ujiieb, Y. Unnob, K. Yamamotof,
K. YamamurafaUniversity of Tsukuba, Institute of Physics, Tsukuba, Ibaraki 305-8571, JapanIPNS, High Energy Accelerator Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
cHiroshima University, Department of Physics, Higashi-Hiroshima, Hiroshima 739-8526, JapandOkayama University, Department of Physics, Okayama, Okayama 700-8530, JapaneKyoto University of Education, Department of Education Fushimi, Kyoto 612-0863, Japanf
Solid State Division, Hamamatsu Photonics K.K., Hamamatsu, Shizuoka 435-8558, Japanb

Abstract
In a program of developing radiation-hard silicon sensors for the LHC-ATLAS experiment, we have irradiatedvarious types of silicon sensors with 12 GeV protons at KEK. Among the other properties, we made a comparative
study of characteristics of the sensors with two wafer planes h1 1 1i and h1 0 0i. The studied sensors are p-on-n type,which satisfy the ATLAS-SCT speciﬁcations. Possible dependence on the substrate orientation could result from
diﬀerent dangling-bond conﬁgurations. The compared characteristics are the charge collection eﬃciency, interstrip
capacitance, and noise levels. The noise levels were measured with a real ATLAS-SCT electronics system. A substantial
diﬀerence is observed in the interstrip capacitance at  10 kHz, while the diﬀerence is small at > 1 MHz. The
diﬀerences in the charge collection eﬃciency and in the noise levels appear to be small. # 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Silicon microstrip detector; Radiation damage; Wafer direction; h1 1 1i; h100i; Charge collection; Interstrip capacitance;
ATLAS SCT

1. Introduction
Silicon microstrip detectors will be extensively
used for tracking in the ATLAS experiment at theLHC. The ATLAS SCT (Semiconductor Tracker)
system is described in detail in Ref. [1]. The sensors*Corresponding author.
E-mail address: hara@hepsg1.px.tsukuba.ac.jp (K. Hara).

are AC-coupled and single sided with dimensions of 63:56 ðwidthÞ  63:96 ðlengthÞ  0:28
ðthicknessÞ mm3 . The electrode strips of 12 mm
width are made on the p side at a pitch of 80 mm.
The number of readout strips is 768. Two of suchsensors are wirebonded together, constructing a
readout strip length of  12 cm. The radiation
tolerance is one of the key issues to operate silicon
microstrip detectors in ATLAS, where the radiation

0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 5 8 8 - 5

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355

Table 1
Tested silicon microstrip sensors. * is either 0, 17 or 42,denoting the proton ﬂuence in unit of 1013 cm2Sensor
name

Maker

Resistivity

Wafer
plane

Fluenceðcm2 ÞHp4k *Hp1k *Sp4k *Hamamatsu
Hamamatsu
SINTEF

4 kO cm
1 kO cm
4 kO cmh1 1 1ih1 0 0i
h1 0 0i0; 17  1013
17; 42  1013
17; 42  1013

environment is estimated to correspond to 3 1014 multi-GeV proton equivalent per cm2 in 10years of operation.
In a program of developing radiation-hardsilicon sensors, we have fabricated various typesof silicon sensors and irradiated them with 12 GeV
protons at KEK. The irradiations were performed
for two separate sets of sensors to 1:7  1014 and
4:2  1014 cm2 . The present measurements weremade 6–12 months after the initial annealing wascompleted. The details of the irradiation conditions and performance tests which include theleakage current increase, change in the full
depletion voltage and drop in the charge collection
are reported elsewhere [2].
In this report, we examine the possible diﬀerence
between the sensors with the two wafer planes,
h1 0 0i and h1 1 1i. The compared sensors aresummarized in Table 1. All the sensors are thep-on-n type, satisfying the ATLAS-SCT speciﬁcations. Comparative studies are made on thecharge collection eﬃciency, interstrip capacitance, and noise performance. As pointed outin Ref. [3], diﬀerent behaviors between thesewafer orientations could result from the diﬀerent
amount of the dangling bonds at the Si–SiO2interface.

2. Charge collection
The charge collection could degrade due totrapping and recombination centers created byintense radiation. The surface charge accumulation could also modify the electric ﬁeld around thestrips and hence the charge collection eﬃciency. If
the inﬂuence associated with the wafer orientation
emerges itself only at the Si–SiO2 interface and notFig. 1. Charge collection curves of the three sensors irradiatedto 1:7  1014 protons cm2 . The vertical axis is normalized by
the charge collection plateau of an Hp4k-0 sensor.

in the bulk, we expect small diﬀerence at least in
the former eﬀect.
The charge collection curves were measured
using penetrating b-rays from a 90 Sr source [2].
The sample sensor was sandwiched between a pairof A‘ slits with a scintillation counter set underneath, which created gate signals to an ADC.
Three adjacent strips were read out separately. Thepeak ADC count of the summed signals is plotted
as a function of the bias voltage. Fig. 1 is anexcerption from Ref. [2], which shows the chargecollection curves of the three types of the sensors
irradiated to 1:7  1014 cm2 . The ordinate isnormalized by the charge collection plateau of a
non-irradiated Hp4k sample.
We note that the shapes of Hp4k-17 andSp4k-17 sensors are similar whereas the Hp1k-17
curve remains high at lower voltages. Thisdiﬀerence can be attributed to the initial resistivity
diﬀerence.
In Fig. 2 we plot the charge collection values atplateau for various samples we tested. Typicallythere are measurements for two samples at thesame condition (only one for SINTEF), which are356

T. Akimoto et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 354–358

Fig. 2. The charge collection of three types of the sensors as afunction of proton ﬂuence.

plotted by the same marks in the ﬁgure. Although
the set of samples is not complete, there seems
small diﬀerence between h1 1 1i and h1 0 0i wafer
orientations. As is pointed out in Ref. [2], though,n-on-n sensors are superior to p-on-n sensors in
the charge collection, since the junction side ofirradiated n-on-n sensors is on the strip side andthe electric ﬁeld becomes preferable for chargecollection.

3. Interstrip capacitanceThe characterization of the interstrip capacitance was based on an HP4192A LCR meter.
Several A‘ strips were wire-bonded to a printed
circuit board, which provided the connection tothe LCR meter through Lemo cables. Hereafter wedeﬁne the interstrip capacitance as that between
two neighboring strips with the adjacent strips
being at ﬂoating.
Fig. 3 shows the frequency dependence of theinterstrip capacitance. The bias voltages were seton the plateau in the charge collection curves.
Non-irradiated sample exhibits a cutoﬀ at a few
kHz, which is due to existence of bias resistors
parallel to the interstrip capacitance. The irradiated Hp1k h1 0 0i sensors show a small deviation at a frequency lower than 10 kHz. The curvesof Sp4k-17 and Sp4k-42 (not shown in the plot,
which behaves similar to Sp4k-17) show a larger
deviation than Hp1k sensors. However, thesecurves are essentially similar in shape, if we take
into account that the cutoﬀ of SINTEF sensors
resides in a smaller frequency region. On theFig. 3. Frequency dependence of the capacitance between
neighboring strips. The bias voltage is set on the plateau in
the charge collection curves.

contrary, the irradiated h1 1 1i sensors show anenhancement of  30% in a wider frequency range
below  100 kHz. They all, however, tend tobecome identical above  5 MHz. It can beconcluded that the frequency dependence of theinterstrip capacitance is not dependent on theproton ﬂuence in the range we investigated. Since amajor diﬀerence in the curves is observed between
h1 0 0i and h1 1 1i sensors, the wafer orientation
should be attributed to the changes.
We evaluated the bias voltage dependence of theinterstrip capacitance at 10 kHz, where the diﬀerence appears to be most distinctive. The results areshown in Fig. 4. While the interstrip capacitance atlower bias voltages seems diﬀerent between Hp1k17 and Sp4k-17 sensors, it converges into a similarvalue at the full depletion voltage of 300 V: The
diﬀerent behavior at lower voltages should berelated to the diﬀerence in the wafer resistivity. On
the other hand, the interstrip capacitance of Hp4k17 is high at lower voltages and continues todecrease across the full depletion voltage, resulting
in the interstrip capacitance being higher than theothers even at 300 V.
It is reported in Ref. [3] that the interstrip
capacitance is inﬂuenced more by irradiation if thefrequency is decreased (from 1 MHz to 100 kHz)and the bias voltage is lowered (from 500 to 300 V)
for h1 1 1i substrate with resistivity of 6 kO cm. It
also reports that there is essentially small change
for h1 0 0i substrate with resistivity of 1 kO cm.

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357

Fig. 5. Noise as a function of the bias voltage for the three
types of irradiated sensors, two sensors for each.
Fig. 4. Interstrip capacitance at 10 kHz as a function of bias
voltage.

The present results are qualitatively consistent
with their measurements.

4. Contribution to the electronics noise
The interstrip capacitance contributes dominantly to the detector capacitance. The actual
contribution to the electronics noise may not be
substantial since the increase is small in the range
above 1 MHz which is the frequency range
responsible for the electronics.
The contribution to the electronics noise wasevaluated by using a non-irradiated real SCT
readout system based on ABCD2T front-end chips[4]. One ABCD2T chip (128 readout channels) wasmounted on a ﬂexible Kapton-based hybrid.
Consecutive 6 cm long 16 strips were wirebonded to the ABCD2T chip through a pitchadapter. Another consecutive 16 strips were also
wire-bonded to the same chip inputs, thereby
simulating 12 cm long strips in the real SCT
conﬁguration. The sample was placed in athermostat chamber with the temperature being
kept at 108C. The same input channels of thesame ABCD2T chip are used in order to compare
the noise levels of the diﬀerent sensor types.
The ABCD2T is a binary readout system,
therefore the noise was evaluated from the sigmaof the error function which was ﬁtted to theeﬃciency vs injected charge relation at a given
comparator threshold voltage. The thresholdvoltage was adjusted to correspond to 2 fC of
the charge injection. Although the noise values of
the connected 16 channels are consistent to eachother, we calculated the average of the central
channels in order to avoid the possible inﬂuence
from the unconnected strips. We plot the averagesin Fig. 5. Two sensors were measured for each of
the three types irradiated to 1:7  1014 cm2 . The
noise levels at 300 V, where all the sensors are on
the full depletion plateau, show that Hp4k-17
h1 1 1i tends to be somewhat larger than Hp1k-17
and Sp4k-17 h1 0 0i by 5–7%. The signiﬁcance is,though, weak compared with the systematic
uncertainty of  3%. Note that the noise level
for a non-irradiated sensor is 0:20 fC.
In the ﬁgure we show the noise levels up to350 V. Above this voltage the noise level wasfound to increase. The sensors we used in thisstudy are based on ‘‘1997 design’’ where asubstantial micro-discharge is observed aroundthe guard rings. The current design (‘‘1998design’’) has modiﬁed the layout and such anincrease is not observed [5].

5. SummaryWe have evaluated the characteristics of silicon
microstrip sensors irradiated with protons andcompared the results in view of h1 1 1i and h1 0 0i
substrate orientations. The comparison was performed on the charge collection, interstrip capacitance, and the contribution to the electronics

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T. Akimoto et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 354–358

noise. Although an increase of 30% was observedfor h1 1 1i in the interstrip capacitance at 10 kHz, there are small diﬀerences in othercharacteristics. The noise levels measured with areal SCT electronics system agreed to each other toa level of 5–7%.

References[1] ATLAS Technical Design Report 5, CERN=LHCC=97–17,
1997.
[2] T. Nakayama et al., Radiation damage studies of silicon
microstrip sensors, Proceedings for 1999 IEEE Nuclear

Science Symposium, October, 24–30 , 1999, Seattle, WA,USA.
[3] S. Albergo et al., Comparative study of 1 1 1 and 1 0 0crystals and capacitance measurements on Si strip detectors
in CMS, Nuovo Cimento A 112A (1999) 1261.
[4] W. Dabrowski et al., The ABCD binary readout chipfor silicon strip detectors in the atlas silicon tracker,
Proceedings of the Fourth Workshop on Electronics for
LHC Experiments, Rome, September 21–25, 1998,
CERN=LHCC=98-36, p.175. Updates to the current version
ABCD3T can be found in: http:==scipp.ucsc.edu=groups=
atlas=elect-doc=ABCD3T Spec v1.1.pdf.
[5] D. Morgan et al., Characterization of p-in-n silicon microstrip detectors fabricated by Hamamatsu Photonics andirradiated with 24-GeV=c protons to 3  1014 p=cm2 ,Nuovo Cimento A 112A (1999) 1245.