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The barrel modules of the ATLAS semiconductor tracker
A. Abdesselamv, T. Akimotoae, P.P. Allporto, J. Alonson, B. Andersonaf, L. Andricekr,
F. Anghinolﬁe, R.J. Apsimonaa, G. Barbieri, A.J. Barrd,af, L.E. Batcheloraa, R.L. Batesj,
J.R. Batleyd, G.A. Beckz, P.J. Belle,p, A. Belymamz, J. Bernabeuah, S. Bethker, J.P. Bizzellaa,
J. Bohmx, R. Brennerag, T.J. Brodbeckm, Z. Broklovaw, J. Brozw, P. Bruckman De
Renstromf,v, C.M. Buttarj, J.M. Butterworthaf, C. Carpentierih, A.A. Carterz, J.R. Carterd,,D.G. Charltonc, A. Cheplakovj, E. Chesie, A Chilingarovm, S. Chouridouab, M.L. Chuad,
V. Cindrok, A. Ciocion, J.V. Civeraah, A. Clarki, P. Coev, A-P. Colijns, T. Cornelissens,
D.P. Cosgroveab, M.J. Costae, W. Dabrowskig, J. Dalmauz, K.M. Danielsenu, I. Dawsonac,
B. Demirkozv, P. Dervanac,o, Z. Dolezalw, M. Donegai,e, M. D’Onofrioi, O. Dorholtu,
J.D. Dowellc, Z. Drasalw, I.P. Duerdothp, M. Dwuznikg, S. Eckerth, T. Ekelofag, L. Eklundag,
C. Escobarah, V. Fadeyevn, L. Feldh, P. Ferrarie, D. Ferrerei, L. Fiorinid, R. Fortine,
J.M. Fosterp, H. Foxh, T.J. Fraseraf, J. Freestonep, R. Frenchac, J. Fusterah, S. Gadomskif,
B.J. Gallopc,aa, C. Garcı́aah, J.E. Garcia-Navarroah, M.D. Gibsonaa, S. Gibsonv,
M.G.D. Gilchriesen, J. Godlewskie,f, S. Gonzalez-Sevillaah, M.J. Goodrickd, A. Goriseke,
E. Gornickif, A. Greenallo, C. Grigsonac, A.A. Grilloab, J. Grosse-Knettere, C. Habern,
K. Haraae, F.G. Hartjess, D. Hauffr, B.M. Hawesv, S.J. Haywoodaa, N.P. Hesseys,
A. Hicheuraa, J.C. Hilld, T.I. Hollinsc, R. Holtaa, D.F. Howellv, G. Hughesm, T. Huseu,
M. Ibbotsonp, Y. Ikegamil, C. Isseverv, J.N. Jacksono, K. Jakobsh, P. Jarrone, L.G. Johansenb,
T.J. Joneso, T.W. Jonesaf, P. de Jongs, D. Joosh, P. Jovanovicc, S. Kachiguineab, J. Kaplone,
Y. Katoae, C. Kettererh, H. Kobayashiae, P. Kodysw, E. Koffemans, Z. Kohouty, T. Kohrikil,
T. Kondol, S. Kopernyg, G. Krambergerk, P. Kubikw, J. Kudlatyr, T. Kuwanoae, C. Lacastaah,D. LaMarrai, J.B. Laneaf, S.-C. Leead, C.G. Lesterd, M. Limpers, S. Lindsayq,o, M.C. Llatasi,
F.K. Loebingerp, M. Lozanoa, I. Ludwigh, J. Ludwigh, G. Lutzr, J. Lysn, M. Maassenh,
D. Macinai,e, A. Macphersone, C. MacWatersaa, S.J. McMahonaa, T.J. McMahonc,
C.A. Magraths, P. Maleckif, I. Mandićk, M. Mangin-Brineti, S. Martı́-Garcı́aah,
G.F.M. Martinez-Mckinneyab, J.M.C. Mathesonaa, R.M. Matsonaa, J. Meinhardth,
B. Mikuleci, M. Mikužk, M. Minagawaae, J. Mistryz, V. Mitsouah, P. Modestoah, S. Moëdi,
B. Mohnb, G. Moorheadq, J. Morinz, J. Morrisz, M. Morrisseyaa, H-G. Moserr, A.J.M. Muijss,
W.J. Murrayaa, K. Nagaiz, K. Nakamuraae, Y. Nakamuraae, I. Nakanot, A. Nicholsaa,
R. Nicholsonac, R.B Nickersonv, R. Nisiusr, V. O’Sheaj, O.K. Oyeb, M.J. Palmerd,
M.A. Parkerd, U. Parzefallh, J.R. Paterp, S.J.M. Peeterss, G. Pellegrinia, H. Perneggere,
E. Perrini, A. Phillipsd, P.W. Phillipsaa, K. Poltorakg, S. Pospisily, M. Postraneckyaf,
T. Pritcharde, J.M. Raﬁa, P.N. Ratoffm, P. Reznicekw, R.H. Richterr, D. Robinsond, S. Roee,
F. Rosenbaumab, A. Rudgee, K. Rungeh, H.F.W. Sadrozinskiab, H. Sandakeru, D.H. Saxonj,
Corresponding author. Tel.: +44 1223 337235; fax: +44 1223 353920.

E-mail address: jrc1@hep.phy.cam.ac.uk (J.R. Carter).
0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2006.08.036

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J. Schieckr, K. Sedlakv, A. Seidenab, H. Sengokuae, A. Sfyrlai, S. Shimmaae, K.M. Smithj,
N.A. Smitho, S.W. Snowp, M. Solary, A. Solbergb, B. Sopkoy, L. Sospedraah, E. Spencerab,
E. Staneckaf,e, S. Stapnesu, J. Stastnyx, M. Stodulskif, B. Stugub, R. Szczygielf, R. Tanakat,
G. Tappernaa, G. Taylorq, P.K. Tengad, S. Teradal, R.J. Thompsonp, M. Titovh, B. Toczekg,
D.R. Toveyac, A. Tricoliv, M. Turalaf, P.R. Turnero, M. Tyndelaa, M. Ullána, Y. Unnol,
E. Van der Kraaijs, I. van Vulpens, G. Viehhauserv, E.G. Villaniaa, V. Vorobelw, M. Vosah,
R. Wallnye, M.R.M. Warrenaf, R.L. Wastiev, M. Weberaa, A.R. Weidbergv, P. Weilhammere,
P.S. Wellse, M. Wilderab, I. Wilhelmw, J.A. Wilsonc, M. WolterfaCentro Nacional de Microelectrónica CNM-IMB (CSIC), Barcelona, Spain
Department of Physics and Technology, University of Bergen, N 5007 Bergen, NorwaycSchool of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UKdCavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, UKeEuropean Laboratory for Particle Physics (CERN), 1211 Geneva 23, SwitzerlandfInstitute of Nuclear Physics PAN, Cracow, Polandg
Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Cracow, PolandhFakultät für Physik, Albert-Ludwigs-Universität, Freiburg, Germanyi
DPNC, University of Geneva, CH 1211 Geneva 4, SwitzerlandjDepartment of Physics and Astronomy, University of Glasgow, Glasgow, UKkJožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, Slovenial
KEK, High Energy Accelerator Research Organisation, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japanm
Department of Physics and Astronomy, University of Lancaster, Lancaster, UKnLawrence Berkeley National Laboratory, Berkeley, CA, USAoOliver Lodge Laboratory, University of Liverpool, Liverpool, UKp
The School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UKq
University of Melbourne, Parkville, Vic. 3052, Australiar+ Physik (Werner-Heisenberg-Institut), Fohringer++
Max-Planck-Institut fur
Ring 6, D-80805 Munchen,
Germanys
NIKHEF, Amsterdam, The NetherlandstOkayama University, The Graduate School of Natural Science and Technology, Tsushima-naka 3-1-1, Okayama 700-8530, Japanu
Department of Physics, P.O. Box 1048 Blindern, N-0316 Oslo, Norwayv
Physics Department, University of Oxford, Keble Road, Oxford OX1 3RH, UK
w
Charles University in Prague, Faculty of Mathematics and Physics, V Holesovickach 2, CZ 18000 Prague, Czech RepublicxInstitute of Physics of the Academy of Sciences of the Czech Republic, Na Slovance 2, CZ 18221 Prague 8, Czech RepublicyCzech Technical University in Prague, Zikova 4, CZ 16636 Prague 6, Czech Republicz
Department of Physics, Queen Mary University of London, Mile End Road, London E1 4NS, UKaa
Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 OQX, UK
abSanta Cruz Institute for Particle Physics, University of California, Santa Cruz, CA, USAac
Department of Physics and Astronomy, University of Sheffield, Sheffield, UKadInstitute of Physics, Academia Sinica, Taipei, Taiwan
aeUniversity of Tsukuba, Institute of Pure and Applied Sciences, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japanaf
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UKag
Uppsala University, Department of Nuclear and Particle Physics, P.O. Box 535, SE-75121 Uppsala, SwedenahInstituto de Fı´sica Corpuscular (IFIC), Universidad de Valencia-CSIC, Valencia, Spainb

Received 24 July 2006; accepted 4 August 2006
Available online 11 September 2006

Abstract
This paper describes the silicon microstrip modules in the barrel section of the SemiConductor Tracker (SCT) of the ATLAS
experiment at the CERN Large Hadron Collider (LHC). The module requirements, components and assembly techniques are given, as
well as ﬁrst results of the module performance on the fully assembled barrels that make up the detector being installed in the ATLAS
experiment.
r 2006 Elsevier B.V. All rights reserved.
PACS: 29.40
Keywords: ATLAS; SCT; Silicon; Microstrip; Barrel; Module; LHC

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region. The SCT endcap modules are described elsewhere[5].

1. Introduction
The ATLAS experiment [1] is being constructed to
explore the physics of 14 TeV proton–proton collisions at
the CERN Large Hadron Collider (LHC) [2], with ﬁrst
beam expected in 2007. The ATLAS Inner Detector (ID)[3] tracks charged particles coming from the interaction
region, and consists of a pixel detector (Pixel), surroundedby the SemiConductor Tracker (SCT), which is itself
surrounded by a gaseous/polypropylene foil transition
radiation tracker (TRT). The overall ID is 2.3 m indiameter and 7 m in length. For analysing the momentaof charged particles, a 2 T uniform magnetic ﬁeld isprovided by a superconducting central solenoid [4] whichis integrated inside the cryostat of a liquid argon
electromagnetic calorimeter. A quadrant view of the ID
together with the solenoid is shown in Fig. 1. Because of
the high energy of the proton–proton collisions, large
numbers of particles are generated in one interaction, andmultiple interactions are expected in one crossing of the
proton bunches. The main requirements for the ID are
precision tracking of charged particles in the environmentof numerous tracks, capability of bunch-crossing identiﬁcation, tolerance to large radiation doses, construction withthe least possible material, and a capability for electron
identiﬁcation within the ID.
The ID consists of barrel and endcap regions in order tominimize the material traversed by particles coming fromthe interaction region at its centre. The barrel region ismade of co-axial cylindrical layers and the endcap of disk
layers. The Pixel and SCT detectors use silicon semiconductor technology for precision measurement. In the barrel
region there are three Pixel and four SCT layers, each ofwhich is able to read out a position in two dimensions. This
paper describes the SCT detector modules of the barrel

2. Design speciﬁcations
2.1. Overview of SCT barrel module requirements
The four SCT cylinders in the barrel region (termed
Barrels 3, 4, 5 and 6) have radii between 299 and 514 mm
and a full length of 1492 mm. Their surface areas are tiled
with segmented detector elements, the SCT barrel modules,
to provide complete four-layer digitization coverage forparticles coming from a length of 776 mm about thenominal interaction point on the central axis. This is the
expected 72 sigma length of the beam interaction point.
The barrel cylinder parameters and the numbers ofmodules are summarized in Table 1. The design adopted
for the barrel module, illustrated in Fig. 2, is to use fournear-square silicon microstrip sensors, two on the top andtwo on the bottom side, with the readout hybrid placednear the centre of the unit. The design has minimum
structure near the end edges to allow overlap of theTable 1
SCT barrel cylinder parameters and the number of modules
BarrelcylinderRadius(mm)

Length (full)(mm)

Tilt angle inf(deg)

Number ofmodules

Barrel 3
Barrel 4
Barrel 5
Barrel 6
Total

299
371
443
514
—

1492
1492
1492
1492
—

11
11
11.25
11.25
—

384
480
576
672
2112

Note: Tilt angle is the angle of the modules relative to the local tangent tothe surface of their supporting cylinder.

Y(R)
SOLENOID COIL

eta=1.0

eta=1.5

CRYOSTAT
PPF1

R1150
R1080

715
839

PPB1

2713
eta=2.0

R999

RADIUSTRT

CRYOSTAT

TRT
R635
R560

R559
R514
R443
R371
R299

SCT (Endcap)

SCT (Barrel)

R408

R439

R338

PIXEL SUPPORT TUBE

R270

R122.5
R88.5
R50.5
0

R230

R150

PIXEL
0

BEAM PIPE

R89

401
495

580
650

746

1084
847.5
934

eta=2.5

R36

1262
1377

1747

2072

2462

Z

2727

Fig. 1. A quadrant view of the inner detector (ID) together with the central solenoid inside the cryostat of the liquid argon electromagnetic calorimeter.

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Fig. 2. A 3D view of the ATLAS SCT barrel module. The overall lengthof the module is 128 mm.

sensitive regions of adjacent modules on a barrel. A core
sheet, known as the module baseboard, provides thethermal and mechanical structure. It is sandwiched
between the top and the bottom sensors, and extends
sideways to include the beryllia facing regions shown in
Fig. 2. The hybrid assembly bridges over the sensors and isheld clear of their surfaces by feet that are glued to theberyllia facings. The module is attached to the support
structure at three points, two in the large (cooling side) and
one in the small (far side) facing regions. The large facing
contacts a cooling element that runs along the length of themodules on a barrel.
The required tracking precision is obtained using silicon
microstrip sensors with a readout pitch of 80 mm and abinary (on–off) readout scheme. The back-to-back sensor
pair in a module has a stereo rotation angle of 40 mrad. A
module is mounted on a barrel with its strips on one sideparallel to the barrel axis (z), resulting in a precision of
17 mm in the r–f coordinate and 580 mm in the z coordinate
from the correlation obtained through ﬁtting. The mechanical tolerance for positioning sensors within the back-toback pair must be better than 8 mm transverse to the strip
direction.
The high accumulated radiation levels at the LHC havesevere consequences for silicon sensors, causing increased
leakage current and type inversion, and give rise to theneed to operate the sensors at about 7 1C. The maximum
expected integrated ﬂuence after 10 years of operation in
the SCT is 2  1014 1 MeV-neutron-equivalent/cm2 (atthe upper limit of uncertainty of 50% coming from thetotal cross-section and particle multiplicity). The corresponding sensor bias voltage required for high chargecollection efﬁciency will be in the range 350–450 V,
depending upon SCT warm-up scenarios. This will resultin a total leakage current of 0.5 mA for an individual
sensor operated at 7 1C at a bias voltage of 450 V. The
leakage current is strongly dependent on temperature,roughly doubling every 7 1C. The heat generation istherefore a strong function of the temperature of the645

sensors in the module. The power consumption of thefront-end ASICs is expected to be 5.5 W nominal and7.5 W maximum per module. These values are larger thanoriginally anticipated [3], and have been among the driving
factors in a signiﬁcant evolution of the overall SCT design,
and in particular that of its cooling system.
Thermal considerations, and especially the danger ofthermal run-away, lead to a module design where theeffective in-plane thermal conductivity must be increased
beyond that of silicon. This is achieved by the use of highthermal conductivity material in the baseboard, which islaminated as part of the detector sandwich. The SCT will
undergo temperature cycling over the range 20 to +25 1C
in a controlled sequence, and it must survive, in the eventof cooling or local power ﬂuctuations, up to temperaturesapproaching 100 1C. This requires the module to have asmall coefﬁcient of thermal expansion and to be capable oflimited elastic deformation, as the precision of the tracking
measurement depends on the return to a stable proﬁle after
changes in operating conditions. The SCT modules are in
the tracking volume and are therefore required to haveminimal mass.
2.2. Specifications of the components to provide thenecessary module performance
2.2.1. Silicon microstrip sensors
Full details of the SCT barrel silicon microstrip sensors
are given elsewhere [6,7]. Their ﬁnal speciﬁcation wasreached after several years of R&D. The principal
requirements were to match the parameters of the chosenbinary readout electronics [8] (Section 2.2.3), to accommodate the high levels of radiation within ATLAS, tomaximize the sensitive area of the sensor, to minimize
material and to provide modules with very high tracking
efﬁciency and low noise occupancy, both before and after
irradiation. The design was also required to be simple,
using single-sided processing, for economic reasons. The
choice was p-in-n microstrip sensors. All 10,650 SCT barrel
sensors were provided by Hamamatsu Photonics [9] andmade from standard high resistivity 4-in. silicon wafers.
The sensors are identical throughout the barrel SCT, with arectangular geometry and 768 AC-coupled readout strips
at a pitch of 80 mm. Table 2 summarizes their principal
parameters, pre- and post-irradiation. The sensors are to be
operated at about 150 V bias voltage initially. After 10
years of LHC operation, those in the innermost regions are
expected to be operated at about 450 V bias voltage, withover 90% charge collection efﬁciency.
A corner detail of a sensor is shown in Fig. 3, where thepolysilicon bias resistors, the bond pad layout and the edge
bias and guard structures are visible. Some of the ﬁducial
marks used for the mechanical alignment of sensors during
module assembly (Section 3.2.1) are also shown. Apartfrom pads used for bonding and probing, the front sides ofthe sensors are fully passivated. The passivation, together
with stringent requirements on the quality of the cut edge

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Table 2The major parameters of the SCT barrel silicon microstrip sensors
Parameter

Value and description

LengthWidthEdge quality
Thickness
Uniformity of thickness within a sensor
Flatness
Wafer
Implanted strips
Read-out strips
Strip pitch
Implant strip width
Read-out strip width
Bias resistors
Rinterstrip
Interstrip capacitance (pre-irradiation)
Interstrip capacitance (post-irradiation)
Ccoupling
Reach-through protection
Sensitive region to cut edge distanceHigh voltage contact
Read-out pad
Passivation
Identiﬁcation
Maximum operating voltage
Total leakage current (pre-irradiation)
Total leakage current (post-irradiation)
Microdischarge (pre-irradiation)
Microdischarge (post-irradiation)
Bad strips (pre-irradiation)
Bad strips (post-irradiation)

63,960725 mm ﬁnish (64 mm nominal centre cutting line to cutting line)
63,560725 mm ﬁnish (63.6 mm nominal centre cutting line to cutting line)
No edge chip or crack to extend inwards by 450 mm
285715 mm
10 mm
Flat when unstressed to within 200 mm
n-type, 44 kO high resistivity silicon, /1 1 1S or /1 0 0S orientation
768+2 strips, p-implant, o200 kO/cm
768 strips, aluminium, o15 O/cm, capacitively coupled with implant strips
80 mm
16 mm
22 mm
Polysilicon, 1.2570.75 MO
42  RBIAS at operating voltage after correcting for bias connection
Nearest neighbour on both sides, o1.1 pF/cm at 150 V bias measured at 100 kHz
Nearest neighbour on both sides o1.5 pF/cm at 350 V bias, measured at 100 kHz
X20 pF/cm, measured at 1 kHz.
5–10 mm gap from end of implanted strip to grounded implant
1 mm
Large metalized contactable n-layer on back.
200  56 mm bond pads, Xtwo rows, daisy-chainable
Passivated on the strip side and un-passivated on the backplane
Every 10th strip, starting at 1 for the ﬁrst read out strip
500 V
o20 mA at 15 1C up to 350 V bias voltage
o250 mA at 18 1C up to 450 V bias voltage (on the sensor)
None below 350 V bias
o5% increase in the noise of any channel with bias increase from 300 to 400 V
A mean of X99% good readout strips per sensor, with all sensors having 498% good strips
Number of bad strips at 350 V bias satisfying the above pre-irradiation bad strip speciﬁcation

During pre-series production, both /1 1 1S and /1 0 0S
orientations for the silicon substrate were evaluated, andfound to be equally good for use in the SCT. The series
production was with /1 1 1S silicon, for reasons ofavailability of supply. Pre-series sensors were used to make61 (2.6%) of the SCT barrel modules. Passivation in theedge region was increased between the pre-series and mainseries sensor production.

Fig. 3. Photograph of a corner of an SCT barrel silicon microstrip sensor,
showing the guard structure, a selection of ﬁducial alignment marks, thebias ring, polysilicon resistors, and the metallization above implant strips,
including wire-bonding pads.

(the latter being at the backplane bias voltage), areimportant to reduce the risk of creating accidental highvoltage shorts during module construction or operation.

2.2.2. Baseboards
The baseboard is the central element of the module,
providing its thermal management, mechanical integrityand precision attachment to the barrel. Within the module
the baseboard is sandwiched between the two planes ofsilicon sensors. To minimize the overall material within amodule, the baseboard is designed to have the least
possible mass compatible with both the mechanicalrequirements and with providing the necessary internal
heat transfer and the interface to the external heat sink.
The thermal load comes from the 12 ASICs and, after
irradiation, the silicon sensors (Sections 2.1 and 2.3.2).
The requirements are well met by the customized
thermal–mechanical baseboard shown in Figs. 4(a) and(b), developed speciﬁcally for this project using newprocesses [10]. These include directly encapsulating

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Fig. 4. (a) Baseboard upper surface, (b) baseboard lower surface.

380 mm thick anisotropic thermal pyrolytic graphite sheets[11] with epoxy, and at the same time interfacing 250 mm
thick beryllia facing plates [12] into the structure. The
highly ordered graphite crystal plane is parallel to thebaseboard surface, providing the high thermal conductivity
path for the heat ﬂow, and the intrinsic fragility of suchgraphite plates is circumvented by the encapsulation
process. The result is a baseboard with the necessary
robustness, mechanical integrity and external electricalinsulation, and one that is easily handled in the module
construction process. The production of some 3000 baseboards for the ATLAS SCT project was carried out at
CERN by a collaboration of ATLAS members [13]. Detailsof the baseboard fabrication and processing are given in
Ref. [14].
The larger of the two pairs of beryllia facing plates
within each baseboard have holes that are aligned withsimilar holes in the graphite substrate to provide throughholes for attaching modules to the cylinders that support
them within ATLAS. The precision alignment in theoverall SCT structure comes from aluminium washers thatare attached with epoxy to the upper beryllia facing plateof the baseboard. The washers are positioned above thethrough-holes using jigs that set the relative separation of
their centres and also the orientation of their line of centres[14]. One washer has a circular hole of diameter 1.800 mm,
tolerance (0.005, +0.010) mm, which determines a
precise spatial location for the module, and the second
washer maintains similar precision in the direction
perpendicular to the module sensor strips while its opening
is slotted in the other direction to provide for mechanicaltolerances in assembly. These washers can be seen in Fig.
4(a). Module attachment to the cylinder within ATLAS iscompleted through a clamped third mounting point at theedge of the small beryllia facing.
Electrical high-voltage bias contact to the back surfaceof the attached silicon wafers is through areas where theencapsulation is removed from the baseboard surfaces
during production, and electrically conducting epoxy is
applied during the wafer attachment process (Section
3.2.1). The bias connectivity is completed using theelectrical conductivity of the graphite core, via small holes,

647

ﬁlled with electrically conducting epoxy [14], in the small
and large facings within the upper surface of the baseboard(see Fig. 4(a)). The holes in the beryllia facings have goldplated surround pads that subsequently allow connection
to be made with wire bonds to the bias potential supplied
from the module hybrid (Section 3.2.4). These features canbe seen in the picture of the upper side of a baseboard in
Fig. 4(a), while beryllia facings without bias contacts or
washers are shown on the lower side of the baseboard in
Fig. 4(b), and are inclined by 40 mrad with respect to thoseon the upper surface for purposes of silicon sensor
mounting (Section 2.3.1).
When assembled in the SCT, the conducted heat leaves
the module via the larger beryllia facing on the baseboard
lower surface, which is interfaced to an aluminium block
via a layer of thermal grease, approximately 100 mm thick,and a copper-kapton shunt-shield. The block itself issoldered to a cooling pipe in which the heat is used as latent
heat to evaporate liquid C3F8 at temperatures close to25 1C.
The baseboard properties are summarized in Table 3.
2.2.3. ASICs
The ABCD3TA chip provides all functions required forprocessing the signals from 128 strips of a silicon strip
detector in the ATLAS experiment. It is a single chip
implementation of a binary readout, using DMILL
technology. The main functional blocks are front-end,
input register, pipeline, de-randomizing buffer, command
decoder, readout logic and threshold and calibration
control sections. The die size of the ABCD3TA chip is6550 mm  8400 mm. The full details of the ASIC aredescribed elsewhere [8].
The main requirements of the chip are summarized inTable 4 and the pad layout is shown in Fig. 5. The frontend section performs charge integration, pulse shaping andamplitude discrimination. The threshold value for theamplitude discrimination is provided as a differential
voltage from an internal programmable DAC. The outputsof the discriminators are latched either in the edge-sensing
mode or in the level-sensing mode. At the start of eachclock cycle the chip samples the outputs from thediscriminators and stores these values in a pipeline until adecision can be made on whether to keep the data. Upon
receipt of a Level 1 Trigger signal the corresponding set ofvalues, together with their neighbours in time, are copied
into the readout buffer serving as a de-randomizing buffer.
The data written into the readout buffer are compressed
before being transmitted off the chip. Transmission of datafrom the chip is by means of token passing. The chipincorporates features that, with the hybrid circuit, enable
the system to continue operating in the event of a single
chip failure. It is a system requirement that less than 1% ofdata will be lost due to the readout buffer on the chip being
full, provided that the average occupancy of the silicon
strip detectors is below 2% at an average trigger rateof 100 kHz. This is to be compared with the expected

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Table 3Speciﬁcations of the SCT barrel baseboard
Parameters

Central value

Tolerance limit

Thermal pyrolytic graphite substrate:(i) in-plane thermal conductivity(ii) transverse thermal conductivity
Encapsulated baseboard thickness in sensor area
Baseboard thickness in beryllia facing area
Beryllia facing thermal conductivity

1650 W/m K at 20 1C
6 W/m K
430 mm
930 mm280 W/m K at 20 1C

41450 W/m K at 20 1C
750 mm
770 mmTable 4
Requirements of the ABCD3TA chipParameter

Description

Signal polarity
Input protection
Gain at the discriminator input
Peaking time
Noise (ENC) on fully populated modules

Positive signals from p-type strips
Voltage step of 450 V of either polarity with a cumulative charge of 5 nC in 25 ns
50 mV/fC
20 ns
Typically 1500 electrons r.m.s. for an unirradiated module
Typically 1800 electrons r.m.s. for an irradiated module
o5  10–4
0–12.8 fC with 0.05 fC step
16 ns (1.25–10 fC with 1 fC threshold)50 ns for a 3.5 fC signal followed by a 3.5 fC signal
o6 W nominal

Noise occupancy at 1 fC threshold
Threshold setting range
Timewalk
Double pulse resolution
Power consumption (of fully populated module)

6550

detgnddetgnddetgndGNDA

pads: 200 x 140
pitch: 300

131 input pads: 43x120
pitch in rows: 96
row separation: 80

detgndpads: 200 x 140
pitch: 300

GNDA
VCC

VCC
VTHN
VTHP
ring_a
D_ISH
IP_PR
IS_PR

to be bonded to analog gnd
external decoupling of ISH line

VTHN
VTHP
ring_acal0

to be bonded to analog gnd

cal1
cal2
cal3

pads: 100 x 140
pitch: 200

pads: 100 x 140
pitch: 200

ABCD3TA

8400

tokenoutBPB
tokenoutBP

pads: 120 x 180
pitch: 330

datainBPB
datainBP

tokeninBP

VDD
pads:
200 x 180
pitch: 270

pads:
120 x 180
pitch: 180

pads:
160 x 180
pitch: 200

pads:
120 x 180
pitch: 180

dataoutBP

pads:
160 x 180
pitch: 200

pads: 120 x 180
pitch: 330

pads:
120 x 180
pitch: 180

dataoutBPB

DGND

spare pads fortesting only

tokeninBPB

implementation of an individual threshold correction in
every channel using a 4-bit digital-to-analogue converter(TrimDAC) per channel. The TrimDAC has four selectable
ranges to cope with the spread of threshold offsets, which
increases by a factor of three after a ﬂuence of 3  1014 protons/cm2.
Each channel has an internal Calibration Capacitor
connected to its input for purposes of simulating a hit strip.
The Calibration Capacitors are charged by an internal
chopper circuit that is triggered by a command. Every
fourth channel can be tested simultaneously, with group
selection determined by two binary coded Calibration
Address inputs. The voltage step applied to the Calibration
Capacitors by the chopper is determined by an internal
DAC. A tunable delay of the calibration strobe withrespect to the clock phase covers approximately two clock
periods.

VDD

DGND
tokenout0B
tokenout0
datain0B
datain0
clk0B
clk0
clk1B
clk1
id<4>
id<3>
id<2>
id<1>
id<0>
master
com1B
com1
com0B
com0
selectresetB
ledB
led
dataout0
dataout0B
tokenin0
tokenin0B
DGND
DGND

VDD
VDD

Fig. 5. The pad layout of the ABCD3TA chip.

worst-case strip occupancy averaged over strips and time,
which is only 1%.
The ABCD3TA design provides uniformity of thresholds, a critical parameter for the binary architecture, by

2.2.4. Hybrids
The SCT barrel hybrid carries electrical circuitry and 12
ABCD3TA readout chips, six on the top side (termed
link0) and six on the bottom side (link1). It is also required
to have mechanical rigidity, a high thermal conductivity fortransferring generated heat from the ASICs, and a small
fraction of a radiation length of material. Since the hybrid
is bridged over the sensors, with a gap in between, thebridge material must be rigid enough to make ultrasonicwedge bonding of aluminium wire possible.
The design uses a Cu/polyimide ﬂexible circuit for theelectrical functions and a carbon–carbon substrate for the

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mechanical and thermal functions. The Cu/polyimide
ﬂexible circuit technology has been widely used in industry.
The use of laser vias and build-up layers allows the hybrid
to be both small and thin. A beneﬁt of using a ﬂexible
circuit is that it enables the hybrid and cable sections to bemade as a continuous piece so that vulnerable connections
between the two are eliminated. The technology used inthis hybrid is described in detail elsewhere [15].
The dimensions of the SCT barrel hybrid are displayedin Fig. 6. The electrical schematics are shown in Figs. 7 and
8, where Fig. 7 is for the six chips on the top side of themodule, link0, and Fig. 8 for the six chips on the bottomside of the module, link1. Each link has one master chip,
which is responsible for the transmission of data off themodule. The last chip of a link is set to an end function,and the rest of the chips are set to a slave function. The
data and tokens are passed between the adjacent chips with‘‘datain0/dataout0’’ and ‘‘tokenout0/tokenin0’’ pads at the
back-end of the chips (Fig. 5). In the case of a failure, achip is skipped by using the ‘‘datain1/dataout1’’ and‘‘tokenout1/tokenin1’’ pad connections at its side. The
digital and analogue ground connections are made on the
hybrid, at the side of every ASIC.
The hybrid is made of four copper layers, whosefunctions are summarized in Table 5. The middle twolayers, L2 and L3, extend through the ﬂexible circuit fromthe connector to the far-end of the hybrid, and L3 iscontinuous to make the resistivity of the grounds as small
as possible. The thickness of the copper is 12 mm. The
ground and power planes of layers L1 and L4 are meshed649with 50% openings to compensate for an increase inmaterial from the metallization of through-holes and vias
causing thickening to 30 mm. The thickness of the ﬂexible
circuit at the hybrid section is 280 mm.
The Cu/polyimide ﬂexible circuit is reinforced with abridge to provide good thermal conductivity and highYoung’s modulus, with only a small fraction of a radiation
length of material. The bridge substrate is made ofcarbon–carbon material with uni-directional ﬁbres
along the length of the hybrid to maximize the thermal
conductivity. It is connected electrically to the groundof the hybrid and ASICs. The properties of carbon–
carbon are summarized in Table 6 and the speciﬁcation of
the bridge is shown in Fig. 9. The surface of the bridge
is coated with a polymer, parylene, to a thickness of
10 mm, to provide insulation and improve reliability forhandling. The surface of this coating is roughened with alaser where adhesion is required, and the coating isremoved where electrical and thermal conduction arenecessary.
The pitch of the input pads of the ASIC is 48 mm,
whereas that of the silicon microstrip sensors is 80 mm. In
order to make simple parallel wire bonding, a pitch adapter
is used in front of the ASICs. Because of the ﬁne pitch, thepads and traces are fabricated on a glass substrate with athin aluminium deposition. The size of the pitch adapter is
63 mm (long)  2.7 mm (wide)  0.2 mm (thick). The thickness of aluminium is in the range 0.9–1.0 mm. The
fabrication details for the pitch adapter are described inRef. [15].
Pitchadaptors(2-0.2x2.7x63mm3)193.01
74.6

8.4

74.6

4-ø0.65

1.67

25.09

21.3

35.41

Size 1608 components(37)ABCD3 chips(12-0.5x6.55x8.4mm3)Size 3216 components(13)Cu/polyimid hybrid
Hybrid + glue + bridge

Maximum component height

32.69
35.41

74.6

8.4

Carbon/carbon bridge(0.3/0.8x21x74.6mm3)Fig. 6. Dimensions (in mm) of the SCT barrel hybrid.

74.6

1.5

1.1

4.87

0.6

Connector(3x4.5x26mm3)

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Fig. 7. Circuit diagram of SCT barrel hybrid (connector and link0 circuitry).

Fig. 8. Circuit diagram of the SCT barrel hybrid (link1).

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671
Table 5Function of each of the four Cu layers of the SCT barrel hybrid
Layer(from top)

Function

L1

Pads for wire-bonding, traces to the pads from the bus
lines along the hybrid in L2
Bus lines along the hybrid, power supply planes in thecable section
Analogue and digital ground planes continuous from theconnector to the far-end of the hybrid
Power supply planes for the analogue and the digital
sections

L2
L3
L4

In order to transfer the heat from the ASICs and toconnect their ground to the carbon–carbon bridge, a set of
17 through-holes per chip is placed in the hybrid ASIC
analogue section. The through-holes are 300 mm indiameter, plated with Cu of 20 mm thickness, and ﬁlledwith silver-loaded electrically conductive adhesive. The
effective thermal conductivity of these ‘‘pillars’’ is estimated to be about 40 W/m K.
Two thermistors, one per link, are equipped to monitor
the temperature of the hybrid. The temperature is readily
calculated from the measured resistance, R, as
R ¼ R25 expðBð1=T þ 1=T25ÞÞ

Table 6
Properties of the hybrid carbon–carbon substrate material
Parameter

Description

Carbon ﬁbre direction

Unidirectional
700720
3575
294
0.8

Thermal conductivity (ﬁbre direction) (W/m K)
Thermal conductivity (transverse to ﬁbres) (W/m K)
Young’s modulus (ﬁbre direction) (GPa)Thermal expansion coefﬁcient (CTE) (ﬁbre direction)(ppm/K)
Thermal expansion coefﬁcient (CTE) (transverse toﬁbres) (ppm/K)
Electrical resistivity (ﬁbre direction) (O m)Density (g/cm3)
Percentage of radiation length (%Xo)

10
2.5  10–6
1.9
0.26

Openings for electrical contact
6 - 2 mm x 4 mm
11.7

5x10.24=51.2

11.7

2-ø1.8421±0.1

7.5

2

8.9

5.6

10.24

2.3
72.9

(1.74)

74.6±0.2

T=0.30 ± 0.03

4±0.2

(66.6)

0.8 ± 0.05

Surface roughened
21mm x 74.6 mm

4±0.2

Surface roughened 2 sections
2 - 4 mm x 21 mm

Fig. 9. Speciﬁcation of the hybrid carbon–carbon bridge (dimensions in
mm).

651

(1)

where the temperature, T, is given in kelvin and R25 andT25 are the nominal resistance and the temperature (inkelvin) at 25 1C, respectively. The thermistors have R25 ¼
10 kO  1% and B ¼ 3435 K  1%.
2.2.5. Adhesives
The barrel modules are constructed from the four major
components described above; the silicon microstrip sensors, baseboards, ASICs and hybrids. They are joined
together with thermally conductive and electrically conductive epoxy adhesives. Epoxy adhesives are chosen since
they are known to be radiation-tolerant up to a very highﬂuence.
A thermally conductive adhesive is required in order totransfer the heat generated in the sensors into thebaseboard. This is critical, especially after accumulating alarge ﬂuence of particles which damage the silicon bulk andinduce an increase of many orders of magnitude in theleakage current, together with increased full depletion
voltage. The sensors may run away thermally throughpositive feedback of the leakage current and the temperature unless the heat from the sensors is transferred to thebaseboard efﬁciently. Thermally conductive epoxy is also
used for other joints where thermal conduction is required,such as the attachment of the hybrid to the beryllia facingsof the baseboard (Section 2.2.2).
An electrically conductive epoxy is required because thebaseboard, made of carbon, is used for the electricalconductive path from the bias line on the hybrid to the
backplane of the sensors. The readout ASICs are also
attached to the hybrids with electrically conductive epoxy.
The thermally conductive epoxy used is a two part, room
temperature curing epoxy, known as Araldite 2011 [16]. In
order to enhance its thermal conductivity, a boron-nitride(BN) ﬁller is added [17]. BN was chosen in preference toalumina for this ﬁller as tests with alumina showed thatsome increase was caused in the leakage current of theglued sensors. The resin, hardener and ﬁller are mixed byweight in the proportions 38.5%, 30.75% and 30.75%,
respectively. The thermal conductivity of the mixture isestimated to be about 1 W/m K.
The electrically conductive epoxy used is a two part, lowtemperature curing epoxy, Eotite p-102 [18]. Although thelisted curing temperature is above 50 1C for this product,

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652

the epoxy will cure at room temperature after sufﬁcient
time (for example, 24 h at 23 1C).

Table 7
Some SCT barrel module mechanical, electrical and thermal speciﬁcations
Parameter

Description

Silicon outer dimension
Construction

63.56 mm  128.05 mm (cut-edge)
Four 63.56 mm  63.96 mm p-in-nsingle-sided sensors to form backto-back glued sensors
Back-to-back: o8 mm (in-plane
lateral, X), o20 mm (in-planelongitudinal, Y), o70 mm (out-ofplane, Z, deviation from theaverage proﬁle)Fixing point: o40 mm (X), o40 mm(Y)
126.09 mm (2.090 mm dead in the
middle)720 mrad (0, 740 mrad on
support structure)
768 per side, 1536 total
80 mm
One-piece hybrid wrapped aroundthe module
5.5 W nominal, 7.5 W maximum
460 V (on the detector), up to
500 V in the module7 1C (average)o5 1C
1 W total at 7 1C, Heat ﬂux(285 mm): 120 mW/mm2 at 0 1C

2.3. Specifications for the assembled module
2.3.1. Mechanical specification
Each barrel module contains four 63.96 mm  63.56 mm(cut-edge to cut-edge) single-sided silicon microstrip
sensors. The geometrical alignment of the two sensors on
one side, to form a 128 mm long unit, is shown in Fig. 10.
The strips of the two sensors are wire bonded to form
126 mm long strips in a later stage of the assembly (Section
3.2.4). The pitch of the strips is 80 mm and there arephysically 770 strips, with the ﬁrst and the last being
connected to the strip bias potential for electric ﬁeld
shaping and for deﬁning the strip boundary. The sensors
have been designed to have a minimum guarding region
around their edges, consistent with their high-voltage
performance requirements. The distance between thesensor cut edge and sensitive region is 1 mm (Section
2.2.1), which results in a dead region of 2 mm length in
the centre of the module.
Some principal barrel module parameters are summarized in Table 7. The sensors and hybrid on the two sides arerotated around the ‘‘module physics centre’’, by 720 mrad.
The ‘‘module physics centre’’ is the geometrical centre of
the four sensors. The support structure of the module on
the barrel cylinder rotates the module around the ‘‘module
physics centre’’ by a further 720 mrad so that strips on one
side of the module are parallel to the axis of the barrel
cylinder and those on the other side are at 740 mrad to theaxis, the sign alternating for successive barrels.
The nominal thicknesses of the module are: 1.15 mm in
the sensor area, 0.93 mm in the beryllia facing area,
3.28 mm in the blank hybrid area, 6.28 mm in the highest
hybrid component area, 4.48 mm in the ASIC area, and5.08 mm in the highest wire-bond area. Since wire bonds
have height variations, at least a 1 mm stay-clear distance isrequired in elevation, and so the module stay-clear
thickness in the highest wire-bond area is 7.08 mm. The
wrap-around part of the hybrid interconnect cable extends

126

61.44063.5600.130

62.00062.00063.96063.960Fig. 10. Geometrical alignment of the two silicon microstrip sensors in the
SCT barrel module. The shading represents their sensitive area. Units arein mm.

Mechanical alignment tolerance

Strip length
Strip directions
Number of readout strips
Strip pitch
HybridHybrid power consumption
Maximum sensor bias voltage
Operating temperature of sensor
Uniformity of silicon temperature
Maximum irradiated sensor power
consumption

to a distance of between 40.0 and 41.22 mm from themodule centre, depending on the shape at the wrap around.
There is a maximum error of about 500 mm in theconnector position on the hybrid ‘pigtail’ cable (Fig. 2),
coming from both the error in the connector placement
within the hybrid and that of the hybrid placement in themodule. The ﬂexible pigtail cable has to connect to a lessﬂexible opto-harness on the barrel cylinder [19]. The hybrid
pigtail cable is sufﬁciently long to allow its route to be
adjusted to achieve the correct mating between the twoconnectors.
The cut edge of the sensor is conductive and at the highvoltage of the backside of the sensors. Thus conducting
debris between the cut edge and ground could cause highvoltage shorts. A stay-clear distance of at least 1 mm is
imposed between the sensor edge and any other ground
potential when the module is mounted on the barrel, whichassures a high-voltage breakdown of 3 kV to ground at sea
level in air [20]. The centres of adjacent modules are
separated by 2.8 mm in height on the cylinders. Thisdistance leaves a nominal stay-clear distance of 1.65 mm
between a cut edge and the opposing sensor surface, after
allowing for 400 mm in thickness tolerances and 200 mm in
non-planarity tolerance of overlapping modules.
The layout of modules mounted on a barrel is illustrated
in Fig. 11. The visible features include the overlap of thesilicon of neighbouring modules, the stereo angle of thelower silicon sensors within a module and the hybrid pigtail
connection.

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2.3.2. Thermal specification
The module design must be safe against thermal
runaway of the silicon sensors throughout the lifetime of
the SCT. The requirement is also to maintain the sensors at
a uniform temperature of about 7 1C to reduce the reverse
annealing of the silicon and the bulk leakage current afterradiation damage (Section 2.1). The module design was
therefore developed with the help of thermal Finite
Element Analysis (FEA) simulations. The bulk heat653

generation after 10 years of operation at LHC is estimatedto be 120 mW/mm2 at 0 1C in the silicon microstrip sensors
in the worst case (Table 7). The calculated thermal proﬁleof the module is shown in Fig. 12 for a hybrid power
consumption of 6.0 W and a heat sink temperature of14 1C at the cooling contact (which is the large beryllia
facing that contacts the cooling block on a barrel, Fig. 11,
through thermal grease and a copper-kapton shunt-shield).
The highest temperature of the module is at the ASIC in

Thermal Runaway
04 / 04 / 2001

Top left corner temperature [K]

278

274

270

266

262
0

Fig. 11. A photograph of modules mounted on a barrel cylinder. The
large beryllia facings of the modules are connected mechanically by PEEK
clips and thermally by grease to heat sinks, which are aluminium blocks
with copper-kapton shunt-shields, soldered to the visible Cu/Ni coolingpipes (which have 70 mm wall thickess).

40

80

120

160

200

240

280

320

q(Si) at 273K [uW/mm2]

Fig. 13. Temperature of the hottest point in the sensors as a function ofheat generation for a hybrid power of 6 W, with the heat sink temperaturesof 14 1C (upper curve) and 17 1C (lower curve).

Fig. 12. An FEA simulation of the thermal proﬁle of the SCT barrel module, with the maximum heat generation in the silicon microstrip sensors, 120 mW/
mm2, a hybrid power consumption of 6 W, and a heat sink temperature of 14 1C at the cooling contact. The highest and the lowest temperatures are+5.7 and 13.8 1C, respectively.

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

the middle of the hybrid, and is about 20 1C above the heatsink temperature. The maximum (Tsi max) temperature of
the silicon sensors is about 10 1C above the heat sinktemperature, at the top left corner of the sensor. The Tsi
max temperature as a function of the bulk heat generation,
normalized at 0 1C, is shown in Fig. 13 for a hybrid power
consumption of 6.0 W and heat sink temperatures of 14
and 17 1C. The simulation shows that thermal runawayof the silicon sensor would occur at 280 mW/mm2 at 14 1Cheat sink temperature, which gives a safety factor of 2.3
against thermal runaway for the estimated ﬁnal heatgeneration of 120 mW/mm2. For a hybrid power consumption of 8 W, in excess of the anticipated maximum of
7.5 W, a slightly lower heat sink temperature of 17 1C isrequired to give a similar safety factor, with thermal
runaway at 290 mW/mm2. A temperature of 17 1C at themodule cooling interface can be achieved by operating the
SCT evaporative cooling system at 25 1C, which is within
its design speciﬁcation.
2.3.3. Mass specification
The module has to be designed to minimize the materialpresented to particles. The target ﬁgure within the initial
overall ATLAS detector design was 1.2% of a radiation
length, averaged over the module area, for particles atnormal incidence to the silicon [3]. The calculated radiation
length fractions and masses of the components for the ﬁnal
module design are summarized in Table 8. The actual
weights of components have been measured and the results
match the estimated values to within a few percent. The
maximum contribution comes from the silicon sensors, andtheir thickness was reduced from a standard 300 to 285 mm(Section 2.2.1) to reduce the material budget (and also thefull depletion voltage). The overall mass of the module is
25 g and the percentage radiation length averaged over thesilicon sensor area is 1.17% Xo. This therefore matches the
ATLAS requirement for the module. The total material in
the as-built SCT barrel detector averages 3% Xo perlayer, and is dominated by the module services.
2.3.4. Electrical performance
The criteria for the module to be classiﬁed as electricallygood are shown in Table 9. The strip ‘micro-discharge’
criterion relates to the fact that the anticipated radiationlevels during ATLAS running are a factor of two larger for
Barrel 3 than for Barrel 6. After 10 years of ATLAS
operation (with the LHC as presently approved) and thecorresponding radiation damage and type inversion of thesilicon sensors, it is expected that the modules on Barrel 3
will need to be operated at 450 V bias for 90% chargecollection efﬁciency, while those on Barrel 6 will operate at
250 V bias. The initial operation within ATLAS is
expected to be at 150 V bias for all barrels. The electricﬁeld conﬁguration and high-voltage properties of sensors
are different after type inversion following irradiation. All
SCT sensors are speciﬁed to operate up to 500 V postirradiation (Section 2.2.1), without strip micro-discharge
breakdown [6] and this has been veriﬁed by samplingmeasurements. The initial sensor leakage currents ondelivery are, however, speciﬁed only up to 350 V, and1.5% of sensors have some micro-discharge between 350
and 500 V in their initial state. This micro-discharge arises

Table 8
Radiation length percentages and masses estimated for the SCT barrel module
Component

Percentage of a radiation length (%Xo)

Mass (g)Fraction of total module mass (%)

Silicon sensors and adhesives
Baseboard and BeO facings
ASICs and adhesives
Cu/polyimide/CC hybrid
Surface mount components
Total

0.612
0.194
0.063
0.221
0.076
1.17

10.9
6.7
1.0
4.7
1.6
24.9

44
27
4
19
6
100

Table 9
Electrical speciﬁcations of the assembled modules. See text for explanation of the barrel assignments. In addition the module must be digitally functionalin all respects
Measured quantity
Average noise occupancy per channel at 1 fC threshold (operating both at temperatures of27 and 0 1C on the hybrid thermistor)
Number of bad readout channels per module (operating both at temperatures of 27 and
0 1C on the hybrid thermistor)
Number of consecutive bad readout channels on a module
Strip micro-discharge (see text)
Leakage current at 20 1C and 500 V bias if no micro-discharge

Limit for an electrically good module
5  10–4
p15 (1% of total)
p7
None to 500 V bias for mounting on any ATLAS Barrel
None to 350 V bias for ATLAS Barrel 5 or Barrel 6 only
o4 mA

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655

from the initial high electric ﬁeld region close to the edgesof the implanted strips in the sensors causing local
breakdown, which rapidly decays with time, the plateau
value of the current being similar to that for a normal
sensor. The deﬁnition of acceptable micro-discharge (asopposed to other mechanisms, such as bonding damage,resulting in high currents) used by the SCT is that thecurrent approaches a plateau of less than 1 mA within anhour when held under bias at the onset of the microdischarge voltage.
The presence of micro-discharge between 350 and 500 V
bias has no implication for the operation of a module at theinitial bias of 150 V within ATLAS. However, modules
need to be tested during the assembly of the SCT up to
their ﬁnal bias voltage anticipated after 10 years of ATLAS
operation to ensure that there is no high-voltage breakdown coming, for example, from conducting debris shorting to ground. Thus the modules are grouped into twoelectrical categories on the basis of their leakage current
characteristics:

It should be noted that the excellent quality of thesensors with respect to bad channels and leakage current
allow limits to be placed on the modules that areconsiderably tighter than those indicated by the contractual sensor speciﬁcation (Table 2).
With the ASICs powered, the difference in the thermistor
temperatures on the two sides of the hybrid was required tobe o2 1C for a module to be accepted for Barrels 3, 4 or 5.
This was a check that the hybrid was properly glued to theberyllia baseboard facings, with a good thermal interface(Section 3.2.3). For Barrel 6, the criterion was relaxed too4 1C, because of the less stringent thermal requirements in
the region of the lowest radiation levels.
Each module was subjected to a long-term cold test
(24 h, at the ATLAS operating temperature of 0 1C on the
hybrid thermistors), and was required to operate stably
throughout this period, with the expected analogue anddigital performance and leakage current.

(a) Good for any Barrel—modules with good IV characteristics (module leakage current for the sum of the four
sensors versus bias voltage) up to 500 V bias after
construction, which can therefore be tested initially for
HV integrity up to 500 V and so are suitable formounting on the innermost barrels.
(b) Good for Barrel 5 or Barrel 6—modules with good IV
characteristics up to 350 V (the sensor speciﬁcation)
after construction, but with micro-discharge between
350 and 500 V. They are not tested for HV integrity
above 350 V after mounting on the barrel, and so aresuitable for Barrels 5 or 6, where the operating voltage
in ATLAS is never expected to exceed 350 V. (Inpractice, all modules selected for mounting on Barrel 5
could be operated up to 400 V, Section 5.)

3.1. Acceptance of the module components before assembly

The criteria in Table 9 relating to bad readout channels
are based on the deﬁnitions of such channels listed inTable 10.

3. Module assembly procedures

3.1.1. Silicon microstrip sensors
The 10,650 silicon sensors used in the assembly of the
SCT barrel modules were all fabricated by Hamamatsu
Photonics [9] and delivered over a 3-year period from 2000
to 2003. They were received by three SCT Institutes [21],where their quality was checked on a sampling basis, andfrom there they were forwarded for use at the appropriate
module assembly site. A small sample of sensors (0.3%)
was extracted during the series production and irradiatedat the CERN PS with 24 GeV/c protons to a ﬂuence of3  1014/cm2, to conﬁrm that the speciﬁcations for performance post-irradiation were continuing to be satisﬁed(Section 2.2.1).
The full sensor QA procedures are described in Ref. [7].
Detailed QA was carried out by the supplier, and theresults provided for each individual sensor on delivery,
both on paper, and also as an upload to the SCT
production database [22]. Table 11 summarizes the sensor

Table 10Channel faults causing the individual channel to be classiﬁed as bad in the readout
Channel defectChannel is dead in readout (stuck off)
Channel is stuck on
Sensors un-bonded to ASIC channel
Sensors part-bonded (missing bond between them)High noise channelHigh gain channel
Low gain channelHigh offset channel
Low offset channelHigh noise occupancy channel
Channel threshold cannot be trimmed

Criterion

ENCo800 electrons
ENCo950 electrons if hybrid thermistor To15 1C; ENCo1100 electrons if T415 1C
Channel noise 41.15  (mean noise of 128 channels in ASIC)
Channel gain 41.25  (mean gain of 128 channels in ASIC)
Channel gain o0.75  (mean gain of 128 channels in ASIC)
Channel offset4(mean offset of 128 channels in ASIC+80 mV)
Channel offset o(mean offset of 128 channels in ASIC 80 mV)
Noise occupancy of channel 45  104

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656

Table 11
Sensor QA measurements provided by the supplier and checks at the SCT receiving institutes
Checked by supplier and data
provided for every siliconsensor as applicable

Checked by SCT receiving
Institute for each sensor

Serial number, also recorded on scratch pads on
sensorVisual inspection
Sensor thickness
Substrate identiﬁcation
IV data, 10 V steps, with temperature ofmeasurement
Leakage current stability over 24 h period at 150 V
bias
List of strip numbers with AC-coupling oxide
pinholes with 100 V across the oxide
List of strip numbers with strip metal
discontinuities
List of strip numbers with strip metal shorts toneighbours
List of strip numbers with implant breaks
Polysilicon bias resistor range
List of strip numbers with defective polysilicon bias
resistors
Depletion voltage

O

O

O
O
O
O to 350 V bias

O

Checked by SCT receiving
Institute on a 5% sampling
basis

O

O to 500 V bias
O

O

O

O

O

O

O

O

O
O
O

O

O

800
700
600
500
400
300
200
100
0
395 405 415 425 435 445 455 465 475
Baseboard Thickness in central area (microns)

Baseboards/5 microns

Baseboards/5 microns

Sensor property

700
600
500
400
300
200
100
0
890 900 910 920 930 940 950 960 970
Baseboard Thickness across beryllia
facings (microns)

Fig. 14. The measured baseboard thicknesses: left, across the central region of the baseboard and right, across the region of the beryllia facings.

properties checked against the speciﬁcation by the supplierand at the Institutes.
In addition, the Institutes carried out occasional
diagnostic tests of the sensor interstrip capacitance, interstrip resistance, metal strip resistance, and coupling
capacitance between strip metal and strip implant. Resultsof all these measurements are contained in Ref. [7].
The post-irradiation tests of the small sample ofirradiated sensors were carried out at 18 1C, following a
7-day anneal at 25 1C. They included an IV measurement
to 500 V, and measurement of strip quality and comparative charge collection efﬁciency from a Ru106 beta source
after bonding the sensor to readout electronics [6].
The sensors are of uniformly excellent quality, and fully
satisfy all aspects of the SCT speciﬁcation. They have low,stable leakage currents (typically 150 nA at 350 V bias, at
20 1C), and more than 98% of sensors maintain thisperformance to 500 V bias. Over 99.9% of all readout
strips are good.

3.1.2. Baseboards
The baseboards were fabricated to the speciﬁcations
given in Section 2.2.2, and were subject to the following
quality assurance (QA) checks before dispatch to themodule assembly sites: visual inspection for mechanical integrity;
 thickness measurements in both the central region used
for sensor interfacing, and in the regions of the large andsmall beryllia facings (see Fig. 14, the means andstandard deviations of the distributions are 430710
and 930712 mm, respectively); general ﬂatness;
 washer hole diameters, and washer-to-washer separation;
 electrical conductivity of the epoxy-ﬁlled facing holes forthe sensor backplane bias connections (resistance o3 O); electrical conductivity to the substrate of the openings
in the encapsulation required for sensor backplaneconnections.

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There were also checks for thermal integrity bymonitoring thermal proﬁles on a sampling basis throughout the series production of nearly 3000 baseboards [14].
This was done using dummy heat loads and an externallycooled heat sink attached to the large facings. It allowed
checks to be made of the thermal conductivity of thegraphite substrate and the thermal interface between thesubstrate core and the beryllia facings. All results weresatisfactory, with thermal conductivities measured to bewithin the speciﬁed range.
After these QA checks, the baseboards were transported
to module assembly sites in customized solid boxes thatprovided protection against external impact. At the module
assembly sites, each baseboard was inspected by microscope to check for damage, and again for acceptableﬂatness, and to ensure that washer separations and holesizes were compatible with site-speciﬁc assembly jigging.
After satisfying these checks a baseboard was available foruse in the ﬁrst stage of module assembly, namely theassembly of four silicon sensors with one baseboard toform a sensor–baseboard sandwich (Section 3.2.1).
3.1.3. Front-end ASICs
All ABCD3TA ASICs were fully tested at the wafer levelat three sites in the SCT collaboration [23]. Full details are
given in Ref. [8]. The measurements performed included:657

because of the low ASIC yield, the remaining 12% ofbarrel modules were made with some of the 12 ASICs
having one bad analogue channel ﬂagged in the wafer test,out of the 128 channels.
3.1.4. Hybrids
The barrel hybrids and their components were subject to
the QA steps described below before being delivered to themodule assembly clusters with their passive components
mounted.
The Cu/polyimide ﬂexible circuits were produced according to the speciﬁcation described in Section 2.2.4 [24].
QA was carried out by the vendor as follows: visual inspection for all circuits; sampling tests for mechanical tolerance on the outerdimensions, bonding pads/gap widths, plating thickness; integrity tests of lines: open/short tests for all circuits,
and resistance measurements for samples.
The carbon–carbon (CC) bridges were delivered as aﬁnished product [25]. QA was carried out by the vendor
for: mechanical tolerance of the outer dimensions;
 the Young’s modulus and tensile strength for samples tobe greater than 90% of the speciﬁed values; analogue

front-end performance (gain, noise andcomparator offsets for every channel); digital functions (control register, addressing, communication, pipeline, output buffer); sensitivity of digital functionality to clock frequency andsupply voltage;
 linearity of internal DACs; I/O signal properties (timing, amplitudes, duty cycles); power consumption.

 thermal conductivity of the material to be greater than
600 W/m K; electrical resistance less than 25 mO between the twofarthest openings.
QA on the Al-glass pitch-adapters covered: visual inspection for mechanical ﬁnish and tolerance,
and for opens/shorts of the traces for all pitch adapters;

 tape-peel test of the aluminium traces on a sampling
The wafers were diced in industry after measurement,
and the good ASICs picked for use in module production.
It was not practical to screen for infant mortality at thewafer level or before assembly of the ASICs onto hybrids.
However, an extended electrical test was performed oneach assembled hybrid and the few ASICs that failed were
replaced at that stage, before assembly into modules(Section 3.2.2).
The radiation hardness of each fabrication lot of ASICs
was certiﬁed by the foundry based upon their X-ray testingof process monitor devices on a sample of wafers from eachlot. As part of the SCT QA programme, X-ray and neutron
irradiations were performed on a small sample of ASICs
from each lot to the full radiation speciﬁcation. Electricaltests were performed after the irradiations and the lot
qualiﬁed based upon the sample results. Details of the QA
programme can be found in Ref. [8].
The yield of perfect ASICs on the wafers was onlyaround 20%. The large majority (88%) of the barrel
modules were constructed using 12 perfect ASICs, butbasis;

 wire-bond pull strength test on a sampling basis, withthe requirement for the pull strength to be 46 g for abond height/distance ratio (H/L) setting of 30%.
The Cu/polyimide/CC hybrids were assembled andtested in industry [26]. The ﬁrst process was to glue theCC bridges to the Cu/polyimide ﬂexible circuits. The QA
checks were: visual inspection for excess adhesives, residues on thesurface, and mechanical tolerance for alignment andthickness, for all pieces; bows of the hybrid section at room temperature, to beo75 mm in both the longitudinal and transverse directions, for all pieces.
The second process was to solder on the surface-mountcomponents (SMD) and the connector, and to attach thetwo pitch-adapters with glue. The SMD components wereARTICLE IN PRESS
658

A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

soldered manually because it was necessary to avoid hightemperature processes (460 1C) after the gluing of the
ﬂexible circuit to the CC bridges. The QA steps carried out
for all hybrids were: visual inspection for component placement, solder
ﬁllets, surface contamination and residues; electrical measurement from the connector of thetermination resistances and the capacitances between
the analogue and digital voltages and their grounds;
 a wire-bond pull test using test pads placed for thispurpose, with the requirement of a pull strength of 46 g
for a 30% H/L ratio setting.
After delivery, the ﬁnished product (termed a PCHybrid) was thermally cycled ﬁve times between –20 and+50 1C. Following this, the QA checks were: visual inspection for component loss, cracks;
 mechanical tolerances for thickness and bows; electrical tests of resistance, capacitance and leakage
current in low and high-voltage lines, the latter at 500 V.
The tested PC-Hybrids were distributed to the hybrid
assembly sites [27]. Upon receipt, visual inspection wasmade to check for transport damage. Pull tests ofaluminium wire bonding on hybrid gold pads and on thealuminium pitch-adapter pads were made. The minimum
pull strength for acceptance was again 6 g for the 30% H/L
ratio setting. There were signiﬁcant problems with thequality and bondability of the aluminium on the glass pitch
adapters during the early series production of these pieces,with whiskers being produced around the wire-bonding
feet. After lengthy investigation, this was found to becorrelated with the hardness of the aluminium. The
problem was solved by keeping the temperature of theglass low during the metallization, as described in Ref. [15],and the quality of the delivered product remained highthroughout the remaining production. However, the initialpitch adapter problem proved to be the limiting factor in
the rate at which barrel modules could be completed.
3.2. Assembly of the module components
The components were delivered to four clusters [28] for
their assembly into modules and for the tests of thecompleted modules. All assembly work was carried out inclean rooms, of class 10,000 or better. The modules werebuilt to the same speciﬁcation by all four clusters. The
assembly tooling followed common principles, but variedin detail between the clusters in order to make best use ofavailable local infrastructure.
3.2.1. Assembly of sensor–baseboard sandwiches
This was the process demanding the highest level ofmechanical precision. A pair of sensors was aligned in the
X–Y plane on the top surface and another pair on thebottom surface of the baseboard. The sensors were aligned
relative to one another and to the module reference points,
which were the washer dowel holes in the baseboard(Section 3.1.2). In this process, it was essential to prevent
any mechanical damage to the sensors, and important tominimize distortions out of the X–Y plane, i.e. in the Z
direction.
The assembly procedure was based upon having aprecision assembly station. This was equipped with two
sets of (X, Y, y) stages to move the two individual sensors
on one side, with a large (X, Y) stage to move the sets of (X,Y, y) stages, and with microscopes. A pair of sensors wasaligned to the pre-determined ‘‘module physics centre’’(Section 2.3.1) that was deﬁned relative to the dowel holesof the baseboard. After the sensor pair was aligned, it wastransferred to a pick-up jig with vacuum chucking. The
alignment precision was preserved during the transfer byswitching the vacuum off the stages of the assembly stationand the transfer jig, sequentially, to avoid asymmetricforces on the (X, Y, y) stages. The positions of the sensors
in the pick-up jigs were conﬁrmed through inspection holes
in the pick-up jigs before and after the transfer. The
surfaces of the silicon sensors were protected at all times
when in contact with jigs by the use of clean-room paperthat was renewed for each separate operation. The design
details of the assembly stations were separately optimized
by each of the four SCT barrel clusters.
The sensors and the baseboard were glued withthermally conductive epoxy (Section 2.2.5). A pre-deﬁned
volume of the epoxy was applied to the baseboard toensure a ﬁnal glue thickness close to 80 mm, using a glue
dispensing machine. In addition, there were two spots per
sensor where electrical connection was made between the
baseboard and the backside of the sensor throughelectrically conductive epoxy (Section 2.2.5).
The sensors in the module were numbered as 1 and 2 on
the top (link0) side, and 3 and 4 on the bottom (link1) side.
Longitudinally, the bridging hybrid overlapped sensors 2
and 4 (Fig. 2). The four sensors in a module were matchedaccording to their properties, such as the wafer orientation/1 1 1S or /1 0 0S, pre-series or series sensors, and those
without or with microdischarge (Section 2.3.4). Those
sensors with the smallest number of defective strips wereused as sensors 2 and 4 in order to maximize the strip area
in case the wire bonding to the defective strips had to be
removed in the daisy chain between the sensors (the ASICs
were bonded to the strips of sensors 2 and 4). A full set of(X,Y) metrology survey measurements was performed on
the sensor–baseboard sandwich, and the assembly wasclassiﬁed according to the mechanical speciﬁcations (Section 3.3).
3.2.2. Assembly of ASICs to hybrids
The ASICs were attached to their pads on the hybrid(Fig. 6) with electrically conductive epoxy [18] for goodthermal and electrical contact, although the ASICs were infact AC-coupled electrically to the pads because their back

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

surface was not metallized. The procedure was to apply thecorrect amount of epoxy in a deﬁned pattern with adispensing tool, to align the ASICs by using four ﬁducial
marks at the corners of the chip pads, and to press down
the ASICs to provide uniform glue coverage over their fullarea, with a smooth ﬁllet extending up their sides. The
bonding pads near the ASICs were masked to prevent
contamination. After the epoxy was cured, the electricalconnections on the hybrid (other than to the input pads)
were made by aluminium wire bonding. At this stage, avisual inspection and electrical tests were made on the
hybrid to ensure that all ASICs performed to speciﬁcation
and all wire bonds were functional. The electricalperformance of the ASICs was characterized at room
(27 1C), warm (37 1C) and cold (0 1C) temperatures, asmeasured by the thermistors on the hybrids. A temperatureof 0 1C on the hybrid corresponds to the operating point
anticipated within ATLAS.
In addition to the full ASIC characterization on the
hybrids, a test of longer duration was performed. The aimof this long-term test was to catch infant mortality
problems in the ASICs. The test consisted of a long run
at the highest and lowest temperatures, initially 90 h at
37 1C and 10 h at 0 1C. As no failure was found in the ﬁrst
300 hybrids tested, the time was subsequently reduced to
10 h at 37 1C and 10 h at 0 1C.
In the characterization, the ASICs were powered andtriggered at the nominal ATLAS Level 1 Trigger frequencyof 100 kHz. The currents drawn and the temperatures weremonitored every few minutes. In the long-term test, thecurrents and temperatures were monitored and thefunctionality of the ASICs was tested every few hours sothat the time structure of any failure could be observed.
An ASIC could be replaced on the hybrid if it had beendamaged or if its electrical performance was unsatisfactory.
The ASIC was heated, and when the epoxy was softened atwist was applied and the ASIC was detached. A total of
1.3% of the ASICs (affecting 11% of hybrids) were
replaced in this manner, and the remainder all performed
as expected from the measurements made on the wafer(Section 3.1.3).
The 2550 completed hybrids with ASICs (called ‘‘ASICHybrids’’) consisted of two types; the large majority madewith 12 perfect ASICs (88% of the total), and theremainder with some of the 12 ASICs having one badanalogue channel (Section 3.1.3). The assembly and test of
the barrel ASIC-Hybrids was carried out at three of thefour SCT barrel module clusters [27].
3.2.3. Mounting hybrids on to sensor–baseboard sandwiches
The hybrid is wrapped around the sensor–baseboard
sandwich, with the feet of its carbon–carbon bridge
attached to the baseboard facings. The procedure was tohold a complete ASIC-Hybrid in a folding jig. The functionof the jig was to apply a controlled volume of thermallyconductive epoxy (Section 2.2.5) to the feet of the link0
hybrid, to align the hybrid to the top surface of the659

sensor–baseboard by using the ﬁducial marks on the hybridsurface, to press the hybrid so that the epoxy extended over
the area of the foot and formed a smooth ﬁllet at its side,and ﬁnally to hold the assembly until the epoxy was cured.
The same procedure was repeated for the second side, withthe link1 hybrid folded over the bottom side of thesensor–baseboard assembly.
3.2.4. Wire bonding the modules
The electrical connections were made between the
hybrids and the sensors in two stages, using 25 mmaluminium wire with ultrasonic wire bonding. Firstly, thehigh-voltage bias supply and high-voltage return connections were made between the hybrid and the sensor–baseboard. The bias supply connections were made to the goldpads on the upper-side beryllia facings (Section 2.2.2).
These pads connected the bias supply to the graphite of thebaseboard, and from there to the backside of the sensors
via the holes in the encapsulation and the electricallyconducting epoxy. At least four bond wires were used foreach connection to the large and to the small facings, toprovide redundancy. The bias return connections weremade by wire bonding from the hybrid to the bias ring padsof the sensors. At least two wire bonds were made at eachof two points in both the upper and the lower sensors,
again for reasons of redundancy. The leakage current of
the sensors was measured up to 500 V at this stage on aregular sampling basis, to check that the assembly process
was not damaging the sensors.
Secondly, the high-density wire bonds were made fromthe ASICs to the pitch adapters, if this had not been doneat the ASIC-Hybrid stage, from the pitch adapters to thesensors, and between the daisy-chained strips of the two
sensors. There were a total of 4608 wire bonds per module,
which took two to three hours to complete, using
automatic bonding machines. All strips of the sensors were
wire bonded to the pitch adapters, and thus to the ASICs,
irrespective of any defects in the strips.
3.3. Module metrology measurements during assembly
The completed sensor–baseboard assemblies, and thenthe modules, were surveyed for mechanical precision. The
module was held by vacuum in a measurement frame atthree points. The precision was characterized by in-planeand out-of-plane parameters. For the in-plane survey, awell-deﬁned set of ﬁducial marks on the sensors was used.
For the out-of-plane survey, a matrix of points wasmeasured on the surface of the sensors and the beryllia
facings. The coordinate systems of the upper and the lower
measurement were correlated by the measurement of anumber of reference points observable both from the upper
and the lower sides, for example, transparent ﬁducial
marks on the frame, or corner edges of the upper and lower
sensors. The location of the connector on the pigtail sectionof the hybrid was critical for mating the connectors on thebarrels (Fig. 11). The measurement of the hybrid position

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660

f1
f2 f3
(mid-point of front pair)C1

midxf, midyf

7

5

6

Ym

15

14

a4

C4

a1

13
Xm

4
8

sep

12
16

Sensor1

half-stereo

f

a3

b
sep

Sensor2

C2

a2

C3

3

2

1

9

f4
f5

Xf

11

10

Yf
f9
f8

f10
f11

f7

f12

f15
f14

f6

back
b6

b17
b7
b16

b8
b9

b5
b4

52

53

b13
b12
b11

b14
b15

b10

59

60

66

Sensor4

Sensor3

57

56

65

64

55

61

62

54
58

Fig. 16. In-plane parameters of the SCT barrel module. The black circles,
C1–C4, are the geometrical centres of sensors. The origin of thecoordinates is the geometrical centre of the C1–C4, the ‘‘module physics
centre’’. The line connecting C1–C2 is the axis of the upper (front) sensors,
and the line C3–C4 the lower (back) sensors. The open circles are thecentres of the pairs. The offset of the front and the back pairs is deﬁned by
the coordinates (midxf, midyf) of the front pair. The orientations ofindividual sensors, a1–a4, are deﬁned from the axes of the pairs.

Table 12In-plane parameters of the SCT barrel module
Parameter

Design value

Speciﬁedtolerance

Dowel hole, mhx (mm)Dowel hole, mhy (mm)Dowel slot, msx (mm)Dowel slot, msy (mm)Mid-point of front pair, midxf (mm)Mid-point of front pair, midyf (mm)Separation of front pair, sepf (mm)Separation of back pair, sepb (mm)Sensor1 angle, a1 (mrad)
Sensor2 angle, a2 (mrad)
Sensor3 angle, a3 (mrad)
Sensor4 angle, a4 (mrad)
Half stereo angle, half-stereo (mrad)
Mid-point of front hybrid ﬁducial pair,
hymxf (mm)Mid-point of front hybrid ﬁducial pair,
hymyf (mm)Angle of front hybrid ﬁducial pair, hymaf(mrad)
Mid-point of back hybrid ﬁducial pair,
hymxb (mm)Mid-point of back hybrid ﬁducial pair,hymyb (mm)Angle of back hybrid ﬁducial pair, hymab(mrad)
Connector pin #1, conp1x (mm)Connector pin #1, conp1y (mm)

6500.06500.0
38,500.0
37,000.0
0.0
0.0
64,090.0
64,090.0
0.00
0.00
0.00
0.00
20.007698.5

40.0
40.0
140.0
40.0
20.0
8.0
20.0
20.0
0.13
0.13
0.13
0.13
0.13
200.0

154.0

200.0

63

Yb
Xb

(dowel slot)

f13

front

51

msx, mxymhx, mhy(dowel hole)

b2 b3
b1

Fig. 15. A survey frame and the survey points of the SCT barrel module(hybrid is not shown in the ﬁgure). Sensors 1 and 2 are on the top (link0)side and 3 and 4 the bottom (link1) side. Sensors 1 and 3 are on the leftwhen the module is held in the conventional orientation (i.e., hybrid on theright side). The mark ‘‘+’’ represents the ﬁducial marks on the sensors.
Points 1–16 and 51–66 are for the in-plane survey. For the out-of-plane
survey, points on a 5  5 matrix were measured for a sensor. The survey
frame has ‘‘peepholes’’ in the arms for the cooling facing and for the third
mounting point in the small facing (Section 2.2.2). The arm opposite to thethird mounting point was retracted in height so that the module was held
kinematically at the three points used to mount the module on a barrel.

deﬁned the location of pin number 1 of the connector for
the ideal case of a ﬂat and straight pigtail. A threedimensional (3D) measuring machine was used for thesurvey at each cluster, and the results were uploaded to the
SCT production database [22].
3.3.1. In-plane (XY) survey
The in-plane survey characterizes the relative positionsof the four sensors and the dowel hole and slot of thebaseboard washers, which deﬁne the module position on
the barrel. The X and Y coordinates of a sensor wereobtained from the measurement of eight ﬁducial marks ona sensor, 1–16 and 51–66 in Fig. 15. The reduced parameterset used (Fig. 16) did not, however, rely on this choice of20.003.1457698.5

200.0

154.0

200.0

20.00
3611.8
69451.1

3.145
480
200

ﬁducial marks. The centres of the dowel hole and slot wereobtained from the measurement of perimeters of therespective washers from the upper side. From the (X,Y)
coordinates measured, a reduced parameter set was

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

obtained as shown in Fig. 16. The origin of coordinates
was the geometrical centre of the four sensors, the ‘‘module
physics centre’’. C1–C4 were the geometrical centres of the
ﬁducial marks of the sensors 1–4, respectively. The stereo
angle was the angle between the axes of the front pair, C1
and C2, and the back pair, C3 and C4. The centre-line
along the X direction of points C1, C2, C3 and C4 was the
X-coordinate, Xm. The reduced parameter set is summarized in Table 12, with the design values and the tolerance
speciﬁcations. The positions of the hybrids were measured
using two ﬁducial marks on each on the hybrids (link0 andlink1). The link0 position was used to deduce the connector
position in the pigtail.

3.3.2. Out-of-plane (Z) survey
The out-of-plane module tolerances are constrained by
two factors. One is the requirement to keep the separation
between the surfaces of adjacent modules on a barrel to beat least 1 mm (Section 2.3.1). This sets the maximum
deviation of sensor surfaces from the nominal to beo200 mm. The other requirement relates to the deviation
in ﬂatness of a module. This arises because z and r–f
measurements are correlated on a barrel because modules
are mounted at a tilt angle of approximately 111 (Table 1).
This sets the deviation in Z-ﬂatness to be o70 mm from theexpected value, if the r–f resolution is not to becompromised.
The Z-coordinates of the surfaces of the sensors were
measured on the front and back of the module using 3D
measuring machines having a Z-precision of better than
10 mm. The upper and lower surface Z-coordinate systems
were tied together through the measurement taken aroundthe points 1, 7, 11, 13, 51, 57, 61 and 63 of Fig. 15 at thecorners of the sensors. These were visible on the two sidesof the sensor because of the stereo rotation and displacement, and were taken to be 285 mm apart in Z, the nominal
thickness of the sensors. The module plane was deﬁned
from the Z-coordinates of the three surface areas on thelower side of the facings around the dowel hole, the dowel
slot, and the third mounting point on the small facing. The
surfaces of each sensor were measured at the matrix of5  5 points in Fig. 15. The measurement points in the‘common proﬁle’, which was obtained from the average ofthe measurements of the constructed modules. The use of acommon proﬁle was a good method to ensure the stabilityof the construction process.
The surface of an individual module was parameterized
in the following way:

(1) The midplanes in Z of the left and the right sensors were
the average of the planes of the surfaces of the upperand lower sensors in the left and the right sides of themodule. These two (left, right) midplanes were eachﬁtted separately to the equation Z ¼ aX þ bY þ c. The
two sets of (a, b, c) parameters express any asymmetry
in the construction of the module, or non-planar
properties of the baseboard.
(2) The module thickness was deﬁned by the average
distance between the surfaces of upper and lower
sensors over the area covered by the baseboard.
(3) Common profile: The bowing of the sensors on an
individual module is given by the deviations of the 100
measured points from the module midplanes, having
subtracted half the module thickness. The average ofthese 100 deviations for the constructed modules
deﬁned the common profile of the sensor surfaces. Since
the bowing of the sensors is different for the two types
of wafer orientation used, /1 1 1S and /1 0 0S silicon,
separate common profiles were used for the /1 1 1S and/1 0 0S modules. That for the upper surface of /1 1 1S
modules is shown in Fig. 17.
(4) optimalMaxZerror and optimalRMSZerror: After theabove parameterization, the residuals from the common
profile were the errors in Z-ﬂatness for an individual
module. The maximum residual was called optimalMaxZerror and the r.m.s. of the residuals optimalRMSZerror. In the subsequent reconstruction of
tracks in ATLAS, the sensor surface of the ith of the2112 barrel modules, Z(X,Y: surface, side, i), can beobtained within these errors from the 100 points ofthe common profile, ZCP(X,Y: surface), the individual
module thickness, T(i), and the Midplane equation withthe parameters (a, b, c) of the left and right sides,
ZMID(X,Y: side, i):ZðX ;Y : surface; side; iÞ ¼ Z CP ðX ;Y : surfaceÞ þ Z MID ðX ;Y : side; iÞ  TðiÞ=2
areas of columns 9–15 and 10–14 on the front side, and
59–65 and 60–64 on the back side, were obscured by thehybrid on the module, and their sensor Z-coordinates wereinferred by interpolation from neighbouring points.
The maximum deviation of the sensor surfaces from thenominal was obtained directly from the surface measurements. Individual modules are not expected to be ﬂat towithin 70 mm, because of the intrinsic bows of the sensors.
Instead, the requirement was that the shape of anindividual module differed by o70 mm from a standard

661

(2)

where the surface and side denote the upper or lower
surface and the left or right side, respectively, and the
7 is for the upper/lower surface.
Further relevant parameters were obtained from thesurface measurements, in addition to the sensor shapes.
The surfaces of the far-end and the cooling beryllia facings
were measured at 17 and 15 points, b1–b17 on the lower
side and f1–f15 on the upper side, respectively (Fig. 15).
The upper side has two points less where the pigtail of the

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662

0.04
-31.5

0

15.8

-0.02

0.0
-15.8
31.5

-0.04
-0.06

0.02
X values (mm)0.02
Y values (mm)

-63.8
-48.0
-32.1
-16.2
-0.3

0.04
0
-0.02
-0.06

-0.08

-0.08

-0.1
-80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0

-0.1
-40.0

X mm

0.3
16.2
32.1

-0.04

48.0
63.8
-20.0

0.0

20.0

40.0

Y mm

Fig. 17. The common Z proﬁle (in mm) of the surface of the upper sensors for /1 1 1S modules (see text). The lower surface is very similar, but with the
opposite sign of Z. The left ﬁgure shows the proﬁle along the length of the module (X), at different transverse positions (Y), and the right ﬁgure the proﬁleacross the module (Y) at different longitudinal positions (X).

hybrid hides the surface. The third mounting point wasmeasured at three neighbouring points, b1–b3 in Fig. 15, toincrease the accuracy. These measurements gave thethickness of the baseboard tabs. The hole and slot area
surfaces were measured at four points each: b4, b5, b8 andb9, and b10, b11, b14 and b15. These points are 2 mm awayfrom the hole/slot washers so that the ﬁllet of adhesivearound the washers did not affect the measurement. The
average of (b4, b5, b8, b9) deﬁned the Z-coordinate of thehole, Z1, the average of (b10, b11, b14, b15) the Zcoordinate of the slot, Z2, and the average of (b1, b2, b3)
the Z-coordinate of the third mounting point, Z3. These
Z1, Z2 and Z3 deﬁned the lower cooling facing plane,
LoCoolingFacing, and by adding half the tab thickness, theModuleplane. The difference of the Z-coordinates of thecentre of the tabs and the centre of the sensor–baseboard
allows any asymmetry in the thickness of the adhesive
gluing the sensors to the upper and the lower sides of thebaseboard to be monitored. The angle of the lower beryllia
cooling facing across the module is an important parameter, since this facing has to make good thermal contact,
via thermal grease, with a ﬂat cooling block when themodule is mounted on the cylinder.
Parts of the module envelope were critical for mounting
the modules on the support cylinders. The most criticalcomponents were the three large capacitors on the hybrid
near the far-end tab. The surface measurement of thehybrids and the capacitors gave the envelope. In addition,these measurements showed the heights of the hybrids at
the tabs, and thus the thickness of the adhesives attaching
the hybrids to the facings. This thickness needed to be wellcontrolled, both for thermal and for mechanical reasons.
The principal module Z parameters derived from themeasurement of the sensors, the tabs and the hybrids are
summarized in Table 13, together with the nominal values
and tolerance speciﬁcations.
3.4. Thermal cycling
Every module was thermally cycled 10 times from 25 to+40 1C, unpowered, in an inert atmosphere. The module

was placed inside an environmental test chamber
which was purged with nitrogen for sufﬁcient time toprevent condensation when cold (typically three volume
changes within the chamber). The test cycle started andended at room temperature. The ramp up/down times wereapproximately 30 min (2–3 1C/min) and the soak timeabout 30 min at each temperature. The total test time wasabout 20 h.
The module metrology measurements were repeatedafter the thermal cycling, and compared with their initialvalues (Section 4.2.1).
3.5. Electrical tests of modules
Each SCT module was housed in its own metal box after
assembly and metrology, and remained in this box until
removed by a robot during the process of mounting onto
an ATLAS barrel. All electrical tests before mounting werecarried out with the module in its box. The module was
attached to the aluminium of the box through the holes in
the larger beryllia facing and a clip at one side of the small
facing, as illustrated in Fig. 18. This follows the principle
used for attachment to an ATLAS barrel. The moduleconnector was permanently attached to a transition PCB
mounted in the box (Fig. 18), to avoid damage due to its
repeated use, and the connector used during testing
was located at the edge of this PCB. The analogue groundof the hybrid was connected to the metal of the box. The
box could be mounted on a water-cooled plate to provide
the necessary cooling during ‘warm’ electrical tests. For
these, the temperature of the thermistors attached tothe upper and lower faces of the powered module hybrid
were typically 27 1C, about 15 1C above that of thecooling water. The module box had an inlet gas connection, and all electrical tests were carried out with themodule in a dry nitrogen atmosphere. For ‘cold’ tests, thebox was placed in a commercial freezer or a climate
cabinet, and operated with the thermistor on the powered
hybrid at 0 1C.
The individual modules in their boxes were tested using
custom readout electronics and low and high-voltage

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

663

Table 13
Z parameters of the SCT barrel module
Parameters

NominalTolerance

Description

maxZlower (mm)maxZupper (mm)moduleThickness (mm)optimalMaxZerrorLower (mm)optimalMaxZerrorUpper (mm)optimalRmsZerrorLower (mm)optimalRmsZerrorUpper (mm)coolingTabThickness (mm)farTabThickness (mm)loCoolingFacing a (mrad)
loCoolingFacing b (mrad)
loCoolingFacingConcavity (mm)hyb1NearH (mm)hyb1FarH (mm)hyb2NearH (mm)hyb2FarH (mm)hyb1Concavity (mm)hyb2Concavity (mm)hyb1CapMaxH (mm)

0
0
1.15
0
0
0
0
0.93
0.93
0
0
0
1.18
1.18
1.18
1.18
0
0
2.43

abso0.2
abso0.2
diffo0.1
abso0.07
abso0.07
abso0.025
abso0.025
o1.0
o1.0
abso0.5
abso5
abso0.03
0.25
0.25
0.25
0.25
0.125
0.125
0.30

Lower sensor maximum deviation from ModulePlane
Upper sensor maximum deviation from ModulePlane

hyb2CapMaxH (mm)

2.43

0.30

hybridMaxThickness (mm)capMaxThickness (mm)

3.28
5.78

0.44
0.66

Lower sensor maximum deviation from CommonProﬁleUpper sensor maximum deviation from CommonProﬁleLower sensor r.m.s. deviation from CommonProﬁleUpper sensor r.m.s. deviation from CommonProﬁleCooling-side tab thickness including baseboard and adhesive
Far-side tab thickness including baseboard and adhesive
Lower cooling facing angle along the module, 30 mm over 60 mm
Lower cooling facing angle across the module, 50 mm over 10 mm
Lower cooling facing concavity along X, 30 mm over dowel hole/slot
Height of the near-side surface of the upper hybrid from the upper facing
Height of the far-side surface of the upper hybrid from the upper facing
Height of the near-side surface of the lower hybrid from the lower facing
Height of the far-side surface of the lower hybrid from the lower facing
Concavity of the upper hybrid (+:away from sensors)
Concavity of the lower hybrid (+:away from sensors)
Maximum height of the large capacitors, C73, C53, C54, of the upper hybrid fromthe upper facing
Maximum height of the large capacitors, C74, C55, C56, of the lower hybrid fromthe lower facing
Maximum thickness of the module at surface of the hybrid
Maximum thickness of the module at surface of the large capacitors

Module fixing points

Nitrogen inlet
Connector for test
measurements

Module hybrid
connector

Fig. 18. A module housed within a module test and storage box. A metal plate closes the top of the box.

power supplies developed by the SCT for these functionality tests [29–32]. The ABCD3TA ASIC (Section 2.2.3)
binary readout architecture requires the parameters of thefront-end channel to be extracted by performing thresholdscans for different amplitudes of injected test pulses,
controlled by internal calibration circuitry. For each valueof the test charge, the result of the threshold scan is acomplementary error function with the 50% point corresponding to the signal amplitude at the discriminator input
and the transition width containing information on noise
spread. The threshold scans provide a response curve, thediscriminator threshold as a function of input charge, foreach individual channel. From the error functions, thechannel gain, discriminator offset and equivalent noise

charge (ENC) at the discriminator input can be extracted.
Details of the performance and operation of the ASICs are
given in Ref. [8].
Each module underwent a warm electrical characterization and acceptance test, and a 24 h cold electrical stability
test in the module assembly cluster. The cold test veriﬁedthat the module would function electrically at the ATLAS
operating temperature. The test consisted of an extended
run, with the ASICs being clocked and triggered, and withthe temperature of the thermistors on the hybrids around
0 1C. All currents were monitored every 5 min, and every
few hours a short electrical test was performed. At the end,
a full set of electrical tests was performed while the module
was kept cold.

ARTICLE IN PRESS
664

A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

4.2. Thermal performance of the modules

All electrical results were uploaded to the central SCT
database.

4.2.1. Thermally induced distortions
The changes in the measured parameters before andafter the steps of thermal cycling and the long-term
cold test have been measured for a subset of modules.
The module is very rigid in-plane, and there is no
measurable change in any of the in-plane parameters.
The module is less constrained mechanically out-ofplane. As illustrated in Fig. 21, the measured changes
in the maximum Z deviations after thermal cycling
are normally p10 mm, and a further change of up to
20 mm may be present after the long-term test. Changes
at the 10 mm level are comparable with the measurement
errors in Z, but it cannot be excluded that the larger
values correspond to real deformations. Nevertheless,these are small in comparison with the tolerances of theout-of-plane mechanical speciﬁcations; a maximum Z
deviation of o200 mm and the OptimalMaxZerror parameter o70 mm.

4. The performance of the assembled modules
4.1. Mechanical precision of the modules
A selection of typical in-plane (XY) survey results isshown in Fig. 19, and of out-of-plane (Z) survey results in
Fig. 20 for all modules, measured at room temperature
either after the assembly or after the long-term electricaltest. In each ﬁgure, the horizontal range of the distribution
is the tolerance speciﬁcation of the quantity, as given in
Tables 12 and 13. Nearly all parameters are well within thespeciﬁcation, and the standard deviations of their measured distributions are less than one-third of the tolerance,
as shown in Table 14. In total, 97% of the 2582 modules
entering the sensor–baseboard sandwich assembly step
satisﬁed the complete set of in-plane and out-of-planemechanical speciﬁcations.

modules/2.5 microns

modules/micron

600
500
400
300
200
100
0
-8

-6

-4

-2

0

2

4

6

1000
900
800
700
600
500
400
300
200
100
0

8

-20 -15 -10

midyf (microns)

700

modules/2.5 microns

modules/5 microns

800
600
500
400
300
200
100
0
-40 -30 -20 -10 0 10 20 30 40
mhy (microns)

modules/0.02 mrad

modules/0.02 mrad

1000
800
600
400
200
0
-0.13 -0.09 -0.05 -0.01 0.03 0.07 0.11

a1 (mrad)

1000
900
800
700
600
500
400
300
200
100
0
-20 -15 -10

-5 0
5 10
midxf (microns)

15

20

-5
0
5 10
sepf (microns)

15

20

800
700
600
500
400
300
200
100
0
-0.13 -0.09 -0.05 -0.01 0.03 0.07 0.11

stereo angle (mrad)Fig. 19. The measured distributions of the deviations from their design values of a selection of module in-plane metrology parameters, whose deﬁnitions
are shown in Fig. 16. Each plot is centred about the design value, with width 7 the speciﬁed tolerance (Table 12). The parameters are, from left to right:
top row, midyf, midxf; middle row, mhy, sepf; bottom row, angles a1 and half-stereo.

ARTICLE IN PRESS
600

1400

500

1200

Modules/0.02 mm

400
300
200
100

800
600
400
200

0

16
0.

2

12
0.

0.

08
0.

0

04

-0
.2
-0
.1
6
-0
.1
2
-0
.0
8
-0
.0
4

1.
05
1.
07
1.
09
1.
11
1.
13
1.
15
1.
17
1.
19
1.
21
1.
23
1.
25

0

MaxZ Lower and Upper (mm)Module thickness (mm)

800

1200

700

1000
Modules/mrad

Modules/0.01 mm

665

1000

0.

Modules/0.01mm

A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

800
600
400
200

600
500
400
300
200
100

0

0
0

0.01 0.02 0.03 0.04 0.05 0.06 0.07

-5

OptimalMaxZerrorUpper (mm)

-4

-3

-2

-1

0

1

2

3

4

5

Cooling Facing angle b (mrad)Fig. 20. The measured distributions of a selection of module Z (out-of-plane) metrology parameters, which are deﬁned in Table 13. Each plot is centred
about the design value, with width 7 the speciﬁed tolerance (Table 13). The parameters are, from left to right: top row, module thickness, maxZlower andmaxZupper; bottom row, optimalMaxZerrorUpper, loCoolingFacing angle b.

Table 14
Standard deviation of the the measured distributions of mechanical parameters for all modules and their comparison with the speciﬁed tolerances
Measured standard
deviation

Standard deviation/
speciﬁed tolerance

Z (out-of-plane) parameter

Measured standard
deviation

Standard deviation/
speciﬁed tolerance

Dowel hole, mhx
Dowel hole, mhy
Dowel slot, msx
Dowel slot, msy
Mid-point of front pair,
midxf
Mid-point of front pair,
midyf
Separation of front pair,
sepf
Separation of back pair,sepb
Sensor angles, a1–a4
Half stereo angle, halfstereo

11.0 mm
9.1 mm28.3 mm
11.4 mm
3.1 mm

0.27
0.23
0.20
0.28
0.15

maxZlower
maxZupper
moduleThickness
optimalMaxZerrorLower
optimalMaxZerrorUpper

0.023 mm0.022 mm0.033 mm0.010 mm0.010 mm

0.11
0.11
0.33
0.14
0.14

2.2 mm0.27

optimalRmsZerrorLower

0.004 mm

0.15

2.9 mm0.14

optimalRmsZerrorUpper

0.004 mm0.15

3.0 mm0.15

loCoolingfacing a

0.145 mrad

0.29

0.031 mrad0.030 mrad

0.24
0.23

loCoolingfacing b
loCoolingFacingConcavity

1.250 mrad
0.007 mm

0.25
0.22

20

30

30

10

25

0

20

-10

15

-20

10

-30

optMaXZerror change after thermal cycling(microns)

0

0

5

20

-5

40

-10

60

-20

80

-15

100

90
80
70
60
50
40
30
20
10
0
-25

120

-30

140

Module sides/5 microns

Module sides/5 microns

XY (in-plane) parameter

optMaXZerror change after long-term test
(microns)

Fig. 21. Changes in module out-of-plane (Z) OptimalMaxZerror parameters after thermal cycling (left) and a cold long-term test (right).

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

4.2.2. Thermo-profile measurement of the hybrid
The temperature proﬁle was measured for a sample of (unirradiated) modules by using a thermo viewer. This measurement requires care because of practical difﬁculties such as: estimating or determining the reﬂectivity of the surface,20220170200026 after Thermal Cycle and Burn-in
Hybrid power = 5.3W

12
10
8
6
3.5

4
2

0

0.5

-2
-4

-1
7

5

3

-2.5

1

X[

mm

-1

]

-3

-5

-4
-7

Fig. 22. The temperature proﬁle of a powered hybrid measured using athermo viewer.

Table 15
Percentages of modules failing the electrical speciﬁcations
Good electrical acceptance criterion% of completed
modules failing thiscriterion

Noise occupancy at 1 fC threshold o5  104
p15 bad readout channels and p7 consecutive
bad readout channels
No strip micro-discharge below 350 V bias,
leakage current at 500 V (or at micro-discharge
voltage) o4 mA
Total failing the good electrical criteria

0.8
0.7

which depends on the surface and the material; shielding against infra-red light from the external
environment; the infra-red transparency of silicon.
The transparency of silicon to infra-red wavelengths
precluded the measurement of the temperature of thesilicon sensors. Thus the measurement was only effective
for estimating the temperature of the hybrids and theberyllia facings.
A measured temperature proﬁle of the hybrid is shown in
Fig. 22, with an ASIC power of 5.3 W. In the ﬁgure, thetemperatures are given with respect to that of the top
surface of the beryllia cooling facing. The highest
temperature on the hybrid was 11–12 1C, at the ASICs.
The temperature at the hybrid thermistor was about 6 1C.
The measurements are consistent with the thermal FEA
simulations (Section 2.3.2).
4.2.3. Thermal runaway measurement
A thermal module was built using silicon microstrip
sensors irradiated to the ﬂuence corresponding to 10 yearsof LHC operation. The sensors were assembled to a preseries baseboard and the module was completed by
mounting a pre-series Cu/polyimide/CC hybrid. The heatgenerated by the ASICs was simulated using 2.2 O siliconchip heaters. The module was equipped with a number ofPt100 thermal sensors that provided temperature readout

mm
]

2

10-12
8-10
6-8
4-6
2-4
0-2
-2-0
-4--2

Y[

Further studies on elastic and non-elastic thermallyinduced distortions were carried out on mechanicalmodules. A total of four modules were built, using nonelectrically working but thermally realistic components.
Each module was heated or cooled over a temperature
range of 17 to +39 1C. At ﬁve temperatures (17, 6, +7,+21 and +39 1C) the proﬁle was measured with a 3D
measuring machine.
Each module was then thermally cycled 10 times between
30 and +100 1C in a nitrogen atmosphere and the proﬁlere-measured. No variations in the in-plane XY measurements were observed. Out-of-plane Z variations were
measurable only at the unsupported corners of thedetectors. The average movement seen over all temperature
variations for the four modules, before thermal cycling,was 1.29 mm/1C. After thermal cycling, this average value
was 1.33 mm/1C, that is, essentially unchanged.
The systematic alteration in the out-of-plane shape of thesilicon sensors in the module between room temperature
measurements and cold operation in ATLAS will be taken into
account by applying an appropriate modiﬁcation to the single
parameterized common proﬁle of the modules (Section 3.3.2).

T [Deg.C]

666

5.2

6.7

of the two hybrids, three of the detector corners, the upper
cooled facing, the cooling block and the ambient gas
temperature. The cooling was provided by a mixture ofwater and antifreeze ﬂowing in a copper pipe, and thecooling interface was via thermal grease to a copper blockbrazed to the pipe.
The results showed stable running at a coolant temperatureof around 10.5 1C with a detector power dissipation of
about 130 mW/mm2, normalized to 0 1C. The FEA predicted
thermal runaway at such a coolant temperature at 135 mW/
mm2. Thus the results supported the safety factor against
thermal runaway with the envisaged coolant temperatures of
ATLAS given by the FEA calculations (Section 2.3.2).
4.3. Electrical performance of the modules
The electrical acceptance criteria for SCT barrel modules
to be classiﬁed as good are described in Section 2.3.4.

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

Overall, 93.3% of the completed modules satisﬁed thesecriteria. The percentages failing to meet the different
criteria are summarized in Table 15. The majority of thesefailing modules are appropriate to keep as spare modules
for ATLAS.
In the following sections, the results are summarized forthe electrical acceptance tests of the modules in theirindividual metal boxes (Section 3.5).
4.3.1. Module leakage currents
Fig. 23 shows a sample of IV (module leakage current
for the sum of the four sensors versus bias voltage) curves
for 100 individual modules, measured at 15 1C, taken with
voltage steps of 10 V and 10 s settling time at each voltage.
It is seen that the large majority of modules have good
behaviour up to 500 V bias, while a small number shows arise in current caused by strip micro-discharge above 350 V.
In total, 13% of all modules show the onset of strip
micro-discharge between 350 and 500 V.
In Fig. 24, the distributions of module leakage currents
at 350 and 500 V bias are shown, normalized to atemperature of 20 1C, for 2495 modules. The mean currents
are very low; 503 nA at 350 V bias and 540 nA at 500 V

1.0
0.9

+15 C

Leakage current [uA]

0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0

100

200

300

400

500

Bias voltage [V]Fig. 23. IV characteristics measured at 15 1C for a sample of 100 barrel
modules. Five modules show micro-discharge above 350 V bias.

bias. Modules showing micro-discharge above 350 V are
excluded from the 500 V bias data.
4.3.2. Module noise, gain and offset
As stated in Section 3.5, the channel gain, discriminator
offset and ENC at the discriminator input can be extracted
from the module response curves, obtained from thresholdscans.
The uniformity of the results for each channel of amodule has been illustrated in Ref. [8]. In Fig. 25 the
distributions of the average gain per module, and theaverage ENC are shown, with the modules operating warm(a hybrid thermistor temperature of 28 1C). The averagegain is 55 mV/fC, and the average ENC is 1615 electrons at
this operating temperature.
The ENC reduces with temperature by about 5 electrons
per degree [8]. The average value at the operating
temperature of ATLAS (0 1C at the hybrid thermistor)
is therefore 1470 electrons. This corresponds to asignal:noise ratio of 14:1, which is consistent with test
beam measurements for the modules [33].
4.3.3. Noise occupancy
In Fig. 26 the distribution of the mean noise occupancy
per channel for all channels in the module at 1 fC threshold
is shown for the modules, operating warm (about 28 1C at
the hybrid thermistors). The average value, 4.5  10–5, is anorder of magnitude less than the speciﬁed maximum value.
4.3.4. Bad readout strips
Fig. 27 shows the distribution of the number of badreadout strips per module, as deﬁned in Section 2.3.4. The
average is 2.6 per module, that is, 99.8% of the module
readout strips are good.
It is seen that there is a subsidiary peak in Fig. 27,
centred around 11 bad channels in a module. This is causedby the use of some hybrids that were loaded with ASICs
with 1 bad channel, as described in Section 3.2.2. Bad
channels in ASICs and localized damage to the silicon inwire bonding are the dominant causes of bad channels in
the modules. However, the achievement of 99.8% good
readout channels overall is well above the 99% speciﬁcation for the modules.

800
Modules/50 nA

Modules/50 nA

700
600
500
400
300
200
100

0
10
0
20
0
30
0
40
0
50
0
60
0
70
0
80
0
90
0
10
00
11
00
12
00

500
450
400
350
300
250
200
150
100
50
0

0
10
0
20
0
30
0
40
0
50
0
60
0
70
0
80
0
90
0
10
00
11
00
12
00

0

667

Module Leakage Current at 350V (nA)

Module Leakage Current at 500V (nA)

Fig. 24. Measured leakage currents of modules, normalized to 20 1C: left at 350 V bias, right at 500 V bias.

ARTICLE IN PRESS
A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

305
Modules/25 electrons

500
450
400
350
300
250
200
150
100
50
0

300
250
200
150
100
50

Average Module Gain (mV/fC)

50
16
00
16
50
17
00
17
50
18
00
18
50
19
00

00

15

50

15

00

14

14

60

58

56

54

52

50

0

48

modules/1mV/fC

668

Average Module ENC (electrons)

Fig. 25. Distributions of left: module gains, averaged over all channels in the module, and right: ENC, averaged over all channels in the module, atoperational temperatures of 28 1C at the hybrid thermistors.

60

no/0.00001

50
40
30
20
10

2
2.
2
2.
4

1.
4
1.
6
1.
8

2
1.

1

0
0.
2
0.
4
0.
6
0.
8

0

Average noise occupancy/strip x 10,000 for each module
at 1fC threshold
Fig. 26. Distribution of the average noise occupancy/channel at 1 fC
binary threshold for the modules, measured with a hybrid thermistor
temperature of 28 1C.

CERN, the beam is of high momentum (180 GeV pions),
and the modules can be placed within a 1.56 T magnetic
ﬁeld, with the possibility of rotating the angle of the face ofthe silicon with respect to the incident tracks over the 7201
range relevant to barrel modules within ATLAS.
Both irradiated and non-irradiated modules have been
evaluated in the beam tests. The irradiation of the modules
was uniform over their area and was carried out in the
CERN PS to a ﬂuence of 3  1014/cm2 24 GeV/c protons(that expected after 10 years of operation in ATLAS),
using the SCT irradiation facility [34,35]. The modules are
all kept cold, with the hybrids operating at about 0 1C, as
anticipated in ATLAS. A beam telescope is used to deﬁne
precise track positions at the module planes.
The procedures and results of the test-beam studies are
fully described in Ref. [33]. The principal conclusions
relating to the performance of the barrel modules are:

1200
1000
800
600
400
200

20

18

16

14

12

10

8

6

4

2

0

0

Number of Bad Readout Channels/module
Fig. 27. Distribution of number of bad readout channels per module forthe modules.

4.4. Non-irradiated and irradiated module test-beam results
The performance of barrel modules has been evaluatedduring their development and their series production in the
H8 test beam at CERN, and also in a KEK test beam. At(a) The resolution per module side in a direction perpendicular to the strips has the expected value of (strip
pitch)/O12, that is, 23 mm, for tracks of normal
incidence. This provides a module precision of 17 mm
in r–f when using correlated hits from the sensors on
the two sides.
(b) The silicon sensors digitize with full efﬁciency to thecentres of the outermost readout strips. The deadregion in the centre of the module between the twosilicon sensors (Section 2.3.1) is 2060 mm, slightly lessthan the physical distance of 2090 mm between the endsof the p+ implants on the adjacent sensors.
(c) The median signal:noise value is 13:1 for the nonirradiated modules, and 10:1 for the irradiatedmodules, at 150 and 450 V bias respectively, for tracks
at normal incidence. The lower value after irradiation isprincipally due to higher noise, but is also affected bycharge trapping.
(d) Both irradiated and non-irradiated modules can be
operated at above 99% efﬁciency and below 5  104noise occupancy (the SCT speciﬁcations) at around
1 fC binary threshold, as required, at bias voltages of

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

150 V for non-irradiated modules and 450 V for fully
irradiated modules. The operating window in thresholdis larger for the non-irradiated than for the irradiatedmodules, because of the higher noise post-irradiation.
The test-beam results therefore indicate that the barrel
modules will operate satisfactorily, to speciﬁcation, within
ATLAS over the planned lifetime of the tracker.
5. Initial results from the assembled SCT barrels
The barrel modules are assembled to four carbon-ﬁbrebarrel structures within the SCT (Table 1). The mounting
and initial testing of all the 2112 modules on Barrels 3, 4, 5
and 6 took place over the period from June 2004 to August
2005 [36]. The process is described in detail elsewhere[37–40]. The four completed barrels were shipped to
CERN, where they have been assembled together, withina sealed thermal enclosure. The SCT four-barrel assembly
was then mounted within the barrel TRT to form the ﬁrst
part of the Inner Tracking Detector, ready for insertionand commissioning within the ATLAS experiment.
Fig. 28 shows the modules mounted on the largest SCT
barrel, Barrel 6. The 672 modules are arranged in 56 rows,
each consisting of 12 modules. A close-up of the detail ofthe overlapping modules is shown in Fig. 11. Sixteen of theFig. 28. Modules mounted on SCT Barrel 6, which is held within an
assembly frame.

669

modules for each barrel had small retro-reﬂectors glued
onto their upper beryllia facings in deﬁned positions andorientations before these modules were mounted inprescribed positions on the barrel. These will form partof the geodetic network for the Frequency Scan Interferometry (FSI) real-time alignment system under construction for the SCT [3].
The module mounting was successfully carried out using
two essentially identical robots [40,41]. Only three modules(0.15%) were physically damaged in the process, while afurther thirteen modules were replaced after their initial
functionality tests on the barrel, mostly due to readout
problems encountered with individual ASICs. All themounted modules then satisﬁed the SCT speciﬁcations,and the high mounting yield allowed modules near thelimits of the speciﬁcation to be kept back as spares. The
modules were assigned to the different barrels according totheir electrical quality, with the best being on the innerbarrels. These inner modules have to withstand the higher
levels of irradiation in ATLAS, and they also individuallysubtend larger solid angles at the interaction point. The
characteristic properties of modules mounted on eachbarrel are summarized in Table 16.
On each separate barrel, following assembly, themodules have been powered simultaneously using ﬁnal
SCT low-voltage and high-voltage power supplies, andcontrolled and read out using ﬁnal SCT readout electronics(Readout Drivers, Back of Crate optical interface cards,
Timing module) [42–44]. Initial results are encouraging. All
modules are fully functional and thermally well connectedto their cooling blocks, via thermal grease [45]. The
distribution of the mean hybrid thermistor temperaturefor each powered module is centred at 15 1C above thetemperature of the cooling pipe, which compares well withthermal FEA expectations (Section 2.3.2), and the modules
lie within 73 1C of this value. The distribution of thedifference in temperatures of the thermistors on the top
and bottom faces of the module hybrid is shown in
Fig. 29(a), for the 384 modules of Barrel 3, before being
mounted on the barrel. In Fig. 29(b), the distribution of thechange in this temperature difference is shown for each
module before and after mounting on Barrel 3. The
thermal integrity of all modules has been maintained, andhence also the mechanical integrity of the hybrid attachment, with all temperature differences still being less than
2 1C for this barrel.

Table 16
Properties of modules mounted on each of the four SCT Barrels
Barrel 3

Barrel 4

Barrel 5

Barrel 6

Minimum bias voltage for onset of sensor micro-discharge (Section 2.3.4) (V)Inclusion of modules with ASICs having one bad analogue channel (Section 3.1.3)

4500
No

4450
No

Temperature difference between thermistors on the two sides of the hybrid, whenpowered (1C)
Angle of the cooling contact of the baseboard beryllia facing, b (Table 13) (mrad)

o2

o2

4400
Yes (5% of
Barrel 5)
o2

4350
Yes (28% of
Barrel 6)
o3

o3

o3

o3

o4

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

(a)

70

80

60

70

30

Top-Bottom Hybrid Thermistor Temperature

(b)

2.
2

1.
8

1.
4

1

.4
-1

0.
6

.8
-1

.2

.2
-2

.4
-1
-0
.6
-0
.2
0.
2
0.
6
1
1.
4
1.
8
2.
2

.8

-1

.2

0
-1

10

0

0.
2

20

10

.6

20

40

-0

30

50

-1

40

60

-0

Modules/0.2C

50-2Modules/0.2C

670

Top-Bottom Hybrid Thermistor Temperature
Comparison before and after module
mounted on Barrel

6. Summary

10
0
15
0
20
0

0

50

-5
0

600
500
400
300
200
100
0
-2
00
-1
50
-1
00

Modules/25
electrons

Fig. 29. (a) The difference in temperatures, dT, of the thermistors on the top and bottom faces of the module hybrid for modules operated before being
mounted on Barrel 3 and (b) the change in recorded dT for these modules when being operated before and after mounting on Barrel 3.

Average Module ENC (electrons)
Fig. 30. The distribution of the difference in ENC, (barrel minus test box),
measured at a hybrid thermistor temperature of 28 1C, for all themodules mounted on the four SCT barrels.

The gain, offset and noise have been measured for all
modules on the barrels from threshold scans, and the noise
occupancy. Data have been taken with the modules
operated in small groups, and also with all modules
operated together, with synchronized triggering [38]. Thereis no evidence for performance degradation (coming, forexample, from coherent effects) in the large system. Also,
the performance of a module on a barrel is very similar tothat measured on the bench in its individual module box
before mounting. This is illustrated in Fig. 30, where the
distribution of the difference between the noise measured
on the barrel and in the module box is shown for all
mounted modules, both measurements being made at asimilar hybrid thermistor temperature of 28 1C. The
average ENC on the barrel is slightly lower, by just 16
electrons, and never more than 130 electrons higher thanthat measured in the box. Of the total of 3,244,032 readout
channels on the SCT barrels, 99.8% are good. All modules
on the barrels can be operated at the maximum speciﬁed
bias voltage (Table 16) for the barrel, with similar room
temperature leakage currents as before mounting. Thus theﬁrst tests of individually assembled SCT barrels haveshown that they meet the performance speciﬁcations andrequirements for ATLAS physics. The performance of thewhole system will be measured when it operates within thebarrel ATLAS TRT, and described in future publications.

The R&D, prototyping and construction phases havebeen successfully completed for the barrel module projectof the ATLAS SCT. A total of 2582 modules have been
constructed in four different SCT cluster locations during a
2-year period of series production. The overall yield of
modules with satisfactory mechanical and electrical performance is 90.5%. The required 2112 modules, to full
ATLAS electrical and mechanical speciﬁcation, have been
mounted on the four barrel structures of the SCT. Module
performance on the individual barrels, measured immediately after assembly, shows no degradation, with 99.8%
good channels in the readout, low leakage currents and thepredicted thermal performance.
Acknowledgements
We are greatly indebted to all the technical staff who
worked on the barrel module project from the ATLAS SCT
Institutes. We acknowledge the support of the funding
authorities of the collaborating institutes including the
Spanish National Programme for Particle Physics; theResearch Council of Norway; the Particle Physics andAstronomy Research Council of the United Kingdom; thePolish Ministry of Education and Science; the German
Ministry of Science; the Swiss National Science Foundation;
the State Secretariat for Education and Research and theCanton of Geneva; the Slovenian Research Agency and theMinistry of Higher Education, Science and Technology of
the Republic of Slovenia; the Ministry of Education,
Culture, Sports, Science and Technology of Japan; theJapan Society for the Promotion of Science; the Ofﬁce ofHigh Energy Physics of the United States Department ofEnergy; the United States National Science Foundation; theAustralian Department of Education, Science and Training;
Dutch Foundation for Fundamental Research on Matter(FOM); the Ministry of Education, Youth and Sports of the
Czech Republic; the National Science Council, Taiwan; theSwedish Research Council. The Research was supported in

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 642–671

part by a Marie Curie Intra-European Fellowship within the
6th European Community Framework Programme.
References[1] The ATLAS Detector and Physics Performance Technical Design
Report, CERN/LHCC 99-14, 1999.
[2] The LHC Conceptual Design Report—The Yellow Book, CERN/
AC/95-05(LHC), 1995.
[3] ATLAS Inner Detector Technical Design Report, vol. 1 and 2(ATLAS TDR 4 and 5), CERN/LHCC 97-16 and 97-17, 1997.
[4] ATLAS Central Solenoid Technical Design Report (ATLAS TDR 9),
CERN/LHCC 97-21, 1997.
[5] A. Abdesselam, et al., ATLAS SCT Endcap Module Production,
ATLAS Note, ATL-COM-INDET-2006-008, Nucl. Instr. and Meth.
A., to be submitted.
[6] D. Robinson, et al., Nucl. Instr. and Meth. A 485 (2002) 84.
[7] ATLAS silicon microstrip sensors, paper in preparation, Nucl. Instr.
and Meth. A., to be submitted.
[8] F. Campabadal, et al., Nucl. Instr. and Meth. A 552 (2005) 292.
[9] Hamamatsu Photonics Co. Ltd., 1126-1 Ichino-cho, Hamamatsu,
Shizuoka, Japan.
[10] A.A. Carter, R. de Oliveira, A. Gandi, A thermal device and methodof making such a device, Patents include USA, 6,514, 616 B1,European EPO 1095545, Chinese 99809437.4.
[11] Advanced Ceramics Corporation, Vale Business Park, Cowbridge, UK.
[12] CBL Ceramics Ltd, Milford Haven, UK.
[13] The baseboards were fabricated and supplied by ATLAS SCT groups
from the University of Cambridge, Queen Mary, London, and the
Rutherford Appleton Laboratory, together with CERN EST, using
processes protected by patents in Ref. [10].
[14] R.J. Apsimon, et al., Nucl. Instr. and Meth. A 565 (2006) 561.
[15] Y. Unno, et al., Nucl. Instr. and Meth. A 541 (2005) 286.
[16] Epoxy AW106/HV953U supplied by Ciba-Geigy.
[17] Boron nitride ﬁller supplied by DENKA, grade GP.
[18] Eotite p-102 supplied by Eon Chemie Co. Ltd.
[19] A. Abdesselam, et al., The ATLAS SCT Optoelectronics and the
Associated Electrical Services, ATLAS Note, ATL-COM-INDET2006-011, submitted to Nucl. Instr. and Meth. A.
[20] Spark gap voltages, based on results of the American Institute ofElectrical Engineers, Air at 760 mmHg, 25 1C, CRC Handbook ofChemistry and Physics, 61st ed., 1981, pp. E-55.
[21] The SCT Institutes responsible for the sensor acceptance tests were
Bergen and Cambridge Universities and KEK.
[22] A. Clark, et al., The ATLAS SCT Production Database, ATLAS
Note, ATL-INDET-2002-015.
[23] The SCT on-wafer ASIC testing was carried out at CERN, theRutherford Appleton Laboratory and the University of California,
Santa Cruz.
[24] Nippon Mektron, Co. Ltd., 1-12-15 Shiba-Daimon, Minato-ku,
Tokyo, Japan.
[25] Catalog No. NCC-AUD28, Nippon Mitsubishi Oil Corporation, 1-312 Nishi-Shinbashi, Minato-ku, Tokyo, Japan.
[26] Seiko Precision Inc., 1864 Ienaka, Tsuga-machi, Shimo-Tsuga-gun,Tochigi, Japan.

671[27] The assembly of ASICs to hybrids took place at the University ofBirmingham, Seiko Precision, Japan, and the University of California, Santa Cruz.
[28] The SCT barrel module assembly and test clusters were in Japan(KEK, Hamamtsu Photonics, Seiko Precision, the Universities ofHiroshima, Kyoto education, Okayama and Tsukuba), Scandinavia(the Universities of Bergen, Oslo and Uppsala), the UK (theUniversities of Birmingham and Cambridge, Queen Mary, London,
Rutherford Appleton Laboratory) and the US (Lawrence Berkeley
Laboratory, University of California, Santa Cruz). The numbers ofmodule assemblies started in each cluster were, respectively, 980, 320,
720 and 580.
[29] J. Bohm, et al., Power supply and power distribution sysytem for the
ATLAS silicon strip detectors, in: Proceedings of the Seventh
Workshop on Electronics for LHC Experiments, vol. 363, Stockholm, CERN-2001-005, 2001.
[30] E. Gornicki, P. Malecki, S. Koperny, Multichannel system of fully
isolated HV power supplies for silicon strip detectors, in: Proceedingsof the Sixth Workshop on Electronics for LHC Experiments, vol.
376, Cracow, CERN-2000-010, 2000.
[31] M.J. Goodrick, M.C. Morrissey, The MusTARD module, Cavendish
Laboratory preprint, Cavendish-HEP 04/21, 2004.
[32] M.C. Morrissey, The SLOG slow command generator, http://
hepwww.rl.ac.uk/atlas-sct/mm/slog/.
[33] F. Campabadal, et al., Nucl. Instr. and Meth. A 538 (2005) 384.
[34] C.M. Buttar, et al., Nucl. Instr. and Meth. A 447 (2000) 126.
[35] P.J. Dervan, et al., Irradiation of ATLAS SCT modules and detectors
in 2002, ATLAS Note, ATL-INDET-2003-001.
[36] All modules were mounted and tested on the barrels by members of
the SCT collaboration at the University of Oxford, UK.
[37] G. Viehhauser, Macro-Assembly of the ATLAS Barrel SCT,IEEE NSS Conference Records, 16–22 October 2004,
pp. 1188–1191.
[38] A.J. Barr, Calibrating the ATLAS semiconductor tracker front end
electronics, IEEE NSS Conference Records, 16–22 October 2004,
pp. 1192–1195, ATL-INDET-CONF-2006-001.
[39] P.W. Phillips, Functional testing of the ATLAS SCT barrels, in:Proceedings of the Seventh International Conference on Large Scale
Applications and Radiation Hardness of Semiconductor Detectors
October 5–7, 2005. Florence, Italy, Nucl. Instr. and Meth., to be
published.
[40] R.B. Nickerson, et al., Robotic Mounting of ATLAS Barrel SCT
Modules, Nucl. Instr. and Meth. A, accepted for publication.
[41] S. Terada, et al., Nucl. Instr. and Meth. A 541 (2005) 144.
[42] M.L. Chu, et al., Nucl. Instr. and Meth. A 530 (2004) 293.
[43] J. Butterworth, et al., TIM (TTC interface module) for ATLAS SCT
and pixel readout electronics, in: Proceedings of the Seventhth
Workshop on Electronics for LHC Experiments, vol. 222, Stockholm, CERN-2001-005, 2001.
[44] Information on the SCT DAQ and readout systems in use duringassembly is at http://www.hep.phy.cam.ac.uk/robinson/DisplayGUI.
html and http://www.hep.phy.cam.ac.uk/daq-bin/wiki.cgi.
[45] B. Mikulec, et al., Temperature behaviour and uniformity of SCT
barrels during assembly and reception testing, ATLAS Note, in
preparation.