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Nuclear Instruments and Methods in Physics Research A 591 (2008) 178–180
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Comparison of charge collection efﬁciency of segmented silicondetectors made with FZ and MCz p-type silicon substrates
G. Cassea,, A. Affoldera, P.P. Allporta, A. Chilingarovd, A. Greenalla, K. Harae,
B. Hommelsb, T. Kohrikic, Y. Ikegamic, T. Meguroe, S. Teradac, Y. Unnoc
aPhysics Department, University of Liverpool, Oxford Street, L69 7ZE Liverpool, UK
Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UKcINPS, High Energy Accelerator Research Organisation, KEK Tsukuba, Ibaraki 305 0801, JapandLancaster University, Physics Department, LA1 4YB Lancaster, UKeInstitute of Pure and Applied Sciences, University of Tsukuba, Ibaraki 305 8571, Japanb

Available online 25 March 2008

Abstract
High-resistivity p-type silicon has emerged as one of the most promising materials for the ﬁnely segmented detectors to be used inparticle physics experiments where high levels of radiation damage are expected. Beside the standard high-purity ﬂoat-zone (FZ) silicon,
relatively high-resistivity magnetic Czocharlski (MCz) is now available from industry. This material has been proposed as possibly moreradiation hard than the standard FZ. This work shows a comparison of these substrate materials in terms of charge collection efﬁciency
measurements performed with 40 MHz analogue electronics, before and after irradiation.
r 2008 Elsevier B.V. All rights reserved.
PACS: 29.40.Gx; 29.40.Wk; 61.80.Hg; 61.82.Fk
Keywords: Silicon microstrip; Radiation hardness; SLHC; Charge collection efﬁciency

1. Introduction
Plans for a staged upgrade of the LHC to a factor of 10higher luminosity are well advanced (the super LHC,SLHC) [1]. Silicon detectors are the strongest candidate forthe tracker upgrade of the SLHC experiments due to their
low mass and high position resolution. The increased
luminosity leads to a tremendous level of irradiation in thedetector volume, due to the particles emerging from theinteraction region and due to backscattered albedo
neutrons [2]. The irradiation dose that the tracker detectors
will suffer depends on the radial distance to the interaction
region. The sensors need to be qualiﬁed to the dose they
will receive at the end of the physics program. Chargecollection efﬁciency (CCE) measurements of the signal
generated by minimum ionizing particles are the mostdirect method for qualifying the performances of theCorresponding author. Tel.: +44 151 7943399; fax: +44 151 7946932.

E-mail address: gcasse@hep.ph.liv.ac.uk (G. Casse).
0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2008.03.090

detectors before and after irradiation. Recent CCE results
show that sufﬁcient signal can be obtained using highresistivity ﬂoat-zone (FZ) p-type silicon substrate devices
with n-side segmented electrodes after irradiation to thepredicted doses of the innermost microstrip layer of thepreliminary layout of the upgraded ATLAS detector at theSLHC [3].
An alternative substrate to the high-grade FZ ismagnetic Czocharlski (MCz) silicon. This technology is areﬁnement of the standard Czocharlski (Cz) method for
growing silicon single crystal with resistivity higher thanthe original method. Higher resistivity silicon is required tokeep the initial full depletion voltage sufﬁcient to ensure
maximal charge collection at an achievable bias voltage,
given system constraints. For example, the largest applicable bias voltage in ATLAS is anticipated to be 500 V for
microstrip detectors. This requires p-type substrates with aminimum resistivity of 2 kO for a depletion voltage of
450 V in 300 mm thick sensors before irradiation. It has
been suggested that the higher oxygen content of the MCz

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The radiation damage in the tracker volume of theupgraded ATLAS detector will be caused by both charged
particles emerging from the interaction region and byalbedo neutrons.
Simulations [2] show that the two contributions areequal at a radial distance of about 20 cm from the beamaxis. The silicon microstrip detectors for the barrel tracker
are planned to be at a radii between 25 and 100 cm [5]from the beam axis. At these radii, the neutron contribution to the radiation damage ranges between 53% and87%. The neutron damage is therefore more relevant to themicrostrip detectors for the ATLAS upgrade. The dose
expected for the innermost layer, corresponding to3000 fb 1 of integrated luminosity, is 5  1014 cm 2
1 MeV neutron equivalent.
Two sets of MCz and FZ p-type silicon detectors wereirradiated with neutrons to this ﬁnal dose in order toevaluate the radiation resistance of the substrates. Thedetectors, fabricated by Hamamatsu Photonics, were1  1 cm2, with 128 AC coupled strips with 75 mm pitch.
The strips are electrically isolated by dedicated isolation
structures. Fig. 1 shows the different fabricated structures
studied ranging between no isolation structures to various
types of implanted p-stops. All the prototypes withisolation implants had interstrip resistance in excess of1 GO.
3. Results
Pre-irradiation measurements were performed to evaluate the capacitance–voltage, reverse current, and interstrip
capacitance characteristics of the FZ and MCz samples.
The CV curves presented in Fig. 2 display a much higher
full depletion voltage (VFD) for the MCz sensors (400 V)
compared to the FZ ones (190 V), due to the lower initialresistivity of the MCz wafers. As shown in Fig. 3, the0.0014
1/C2 (1/pF2)

0.00120.001
0.00080.00060.00040.00020105090130 170 210 250
Bias Voltage (V)

290

330

0.00180.00160.0014
1/C2 (1/pF2)

2. Experimental method0.00160.00120.001
0.00080.00060.00040.00020200

0400600
Bias Voltage (V)800

1000Fig. 2. Capacitance–voltage (CV) characteristics of FZ (above) and MCz(below) detectors before irradiation.

1000Current (nA)crystals could lead to better radiation hardness with respect
to FZ silicon [4]. A comparison of irradiated FZ and MCzdetectors is required to determine the most suitable
substrate for applications where severe radiation damage
is a limiting factor.

179100FZ substrate detectors
MCZ substrate detectors

1010100

200300400
Bias (V)500600700

Fig. 3. Reverse current for FZ and MCz detectors before irradiation.

Fig. 1. Schematic showing the difference between strip isolation structures
studied.

reverse current of the MCz detectors is also systematicallyhigher than the FZ ones, due to the higher concentration ofimpurities in the MCz silicon crystal. The measured
interstrip capacitances (shown in Fig. 4) were about thesame for both FZ and MCz sensors. Fig. 5 shows themeasured CCE of non-irradiated FZ and MCz detectors.
The lower initial resistivity of the MCz crystals results inlower CCE for lower bias voltages (below VFD).
As shown in Fig. 6, the CCE measurements for FZ andMCz detectors were repeated after a 5  1014 neq cm 2

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G. Casse et al. / Nuclear Instruments and Methods in Physics Research A 591 (2008) 178–180

180Interstrip capacitance (pF)

2.0

4. Conclusions

1.8

FZ

1.6

FZ

1.4

FZ
FZ

1.2

MCZ

1.0

MCZ

0.8

MCZ

0.6
0.4
0.2
0.0
0100

200300400
Bias (V)

500

600

700

Fig. 4. Interstrip capacitance of FZ and MCz detectors before irradiation.

Collected charge (ke)30252015105FZ non-irradiated

The differences observed in the pre-irradiation characteristics between microstrip detectors made on FZ andMCz silicon are mainly due to the different values of theinitial resistivity. For complete charge collection, muchhigher bias voltage is required for MCz detectors. In
addition, higher reverse currents are measured for MCzdevices. The higher initial currents are not a concern forapplications where high radiation damage is expected,
because after irradiation the reverse current will bedominated by the bulk generated current. After irradiation,
the CCE curves are similar for both types of substrates,with slightly higher efﬁciency for the FZ substrates between
400 and 600 V. Before and after irradiation, the maximum
collected charge at the highest bias voltage is similar for theFZ and MCz sensors. The maximum collected charge of
16 Ke after 5  1014 neq cm 2 is also consistent withprevious measurements [6], but previously the saturation
value was reached at a much lower voltage (about 350 V).
The difference is thought to be due to the difference in theinitial resistivity, in excess of 10 kO for the results of Ref.
[6], suggesting that higher values are advisable for silicondetectors that will be exposed to signiﬁcant neutron
damage.

MCz non-irradiated

00200

400
Bias (V)600

800

References

Collected cherge (ke)Fig. 5. Charge collection efﬁciency (CCE) of non-irradiated FZ and MCz
detectors.

1816141210864200Open symbos: FZ

Full symbos: MCz

200600400
Bias (V)800

Fig. 6. Charge collection efﬁciency (CCE) of FZ and MCz detectors after5  1014 n cm 2.

irradiation. The CCE curves are very similar, with aslightly higher efﬁciency of the FZ detectors between 400
and 600 V applied bias voltage.

[1] F. Ruggiero, Eur. Phys. J. C 34 (2004) S433.
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facilities, ATLAS Tracker Upgrade Workshop, Liverpool, 6–8
December 2006, /http://www.liv.ac.uk/physics/AHLUTW/S.
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