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Nuclear Instruments and Methods in Physics Research A 575 (2007) 353–389
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The ATLAS semiconductor tracker end-cap module
A. Abdesselamz, P.J. Adkinae, P.P. Allporto, J. Alonson, L. Andricekv, F. Anghinolﬁe,A.A. Antonovt, R.J. Apsimonae, T. Atkinsons, L.E. Batchelorae, R.L. Batesj, G. Beckp,
H. Beckerv, P. Belle, W. Bellj, P. Benešab, J. Bernabeuak, S. Bethkev, J.P. Bizzellae, J. Blockig,
Z. Broklováaa, J. Brožaa, J. Bohmac, P. Bookerae, G. Brights, T.J. Brodbeckm, P. Bruckmanz,
C.M. Buttarj, J.M. Butterworthq, F. Campabadala, D. Campbellm, C. Carpentierih,
J.L. Carrollo, A.A. Carterp, J.R. Carterd, G.L. Casseo, P. Čermákab, M. Chamizoi,
D.G. Charltonc, A. Cheplakovj, E. Chesie, A. Chilingarovm, S. Chouridouah, D. Chrenab,
A. Christineti, M.L. Chuai, V. Cindrok, A. Ciocion, J.V. Civeraak, A. Clarki, A.P. Colijnw,
P.A. Cookeo, M.J. Costaak, D. Costanzoaf, W. Dabrowskif, K.M. Danielseny, V.R. Daviesae,
I. Dawsonaf, P. de Jongw, P. Dervano, F. Dohertyj, Z. Doležalaa, M. Donegai, M. D’Onofrioi,
O. Dorholty, Z. Drásalaa, J.D. Dowellc, I.P. Duerdothr, R. Duxﬁeldaf, M. Dwuznikf,
J.M. Eastonae, S. Eckerth, L. Eklunde, C. Escobarak, V. Fadeyevn, D. Faschingal, L. Feldh,
D.P.S. Fergusonal, P. Ferrarie, D. Ferrerei, C. Fletaa, R. Fortine, J.M. Fosterr, C. Fowlerae,
H. Foxh, J. Freestoner, R.S. Frenchaf, J. Fusterak, S. Gadomskig, B.J. Gallopc, C. Garcı́aak,
J.E. Garcı́a-Navarroak, S. Gibsonz, M.G.D. Gilchriesen, F. Gonzalezak, S. Gonzalez-Sevillaak,
M.J. Goodrickd, A. Goriseke, E. Gornickig, A. Greenallo, D. Greenﬁeldae, S. Gregorys,
I.G. Grigorievat, A.A. Grilloah, J. Grosse-Knettere, C. Gryskav, A. Guipeti, C. Habern,
K. Haraaj, F.G. Hartjesw, D. Hauffv, S.J. Haywoodae, S.J. Hegemans, K. Heinzingerv,
N.P. Hesseyw, C. Heuschv, A. Hicheurae, J.C. Hilld, M. Hodgkinsonaf, P. Hodgsonaf,
T. Horažďovskýab, T.I. Hollinsc, L.S. Houai, S. Houai, G. Hughesm, T. Husey, M. Ibbotsonr,
M. Iglesiasi, Y. Ikegamil, I. Ilyashenkoag, C. Isseverz, J.N. Jacksono, K. Jakobsh, R.C. Jaredal,
P. Jarrone, P. Johanssonaf, R.W.L. Jonesm, T.J. Joneso, D. Joosh, J. Josephal, P. Jovanovicc,
O. Juskoab, V. Juskoab, J. Kaplone, S. Kazis, Ch. Kettererh, A.G. Kholodenkoad, B.T. Kingo,
P. Kodyšaa, E. Koffemanw, Z. Kohoutab, T. Kohrikil, T. Kondol, S. Kopernyf, H. Koukolab,
V. Králab, G. Krambergerk, P. Kubı́kaa, J. Kudlatyv, C. Lacastaak,, T. Lagouriaa, S.C. Leeai,
K. Leneyo, S. Lenzv, C.G. Lesterd, K. Liebicherv, M. Limperw, S. Lindsayo, V. Linhartab,
G. LLosáak, F.K. Loebingerr, M. Lozanoa, I. Ludwigh, J. Ludwigh, G. Lutzv, J. Lysn,
M. Maassenh, D. Macinai, A. Macphersone, C. MacWatersae, C.A. Magrathw, P. Maleckig,
I. Mandićk, M. Mangin-Brineti, S. Martı́-Garcı́aak, J.P. Mathesonae, R.M. Matsonae,
S.J. McMahonae, T.J. McMahonc, J. Meinhardth, B. Melladoal, J.J. Melonej, I.J. Mercerm,
I. Messmerh, B. Mikuleci, M. Mikužk, M. Miñanoak, V.A. Mitsouak, P. Modestoak, S. Moedi,
B. Mohna, S. Moncrieffs, G. Moorheads, F.S. Morrisae, J. Morrisp, M. Morrisseyae,
H.G. Moserv, A. Moszczynskig, A.J.M. Muijsw, W.J. Murrayae, D. Musketto, J. Nacherak,
K. Nagaip, I. Nakanox, R.B. Nickersonz, R. Nisiusv, O.K. Oyea, V. O’Sheaj, E. Paganisaf,
Corresponding author. Tel.: +1 34 96 3543490; fax: +34 96 3543488.

E-mail address: Carlos.Lacasta@iﬁc.uv.es (C. Lacasta).
0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2007.02.019

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M.A. Parkerd, U. Parzefallh, J.R. Paterr, S.J.M. Peetersw, G. Pellegrinia, G. Pelleritii,
H. Perneggere, E. Perrini, P.W. Phillipsae, L.V. Pilavovau, K. Poltorakf, S. Pospı́šilab,
M. Postraneckyq, T. Pritcharde, K. Prokoﬁevaf, J.M. Rafı́a, C. Rainej, P.N. Ratoffm,
P. Řeznı́čekaa, V.N. Riadovikovad, R.H. Richterv, A. Robichaud-Véronneaui, D. Robinsond,
R. Rodriguez-Olieteak, S. Roee, A. Rudgee, K. Rungeh, A. Saavedras, H.F.W. Sadrozinskiah,
F.J. Sanchezak, H. Sandakery, D.H. Saxonj, D. Scheirichaa, J. Schieckv, A. Seidenah, A. Sfyrlai,
T. Slavı́čekab, K.M. Smithj, N.A. Smitho, S.W. Snowr, M. Solarab, B. Sopkoab, V. Sopkoab,
L. Sospedraak, E. Spencerah, E. Staneckag, S. Stapnesy, J. Stastnyac, V. Strachkoak,
A. Stradlingal, B. Stugua, D.S. Suai, P. Sutcliffeo, R. Szczygielg, R. Tanakax, G. Taylors,
P.K. Tengai, S. Teradal, R.J. Thompsonr, M. Titovh, B. Toczekf, D.R. Toveyaf, G. Tratzlv,
V.L. Troitskyu, J. Tsengz, M. Turalag, P.R. Turnero, M. Tyndelae, M. Ullána, Y. Unnol,
T. Vickeyal, E. Van der Kraaijw, G. Viehhauserz, E.G. Villaniae, T. Vitekab, T. Vu Anhi,
A.P. Vorobievad, J.H. Vossebeldo, M. Wachlerv, R. Wallnye, C.P. Wardd, M.R.M. Warrenq,
M. Webelh, M. Weberi, M. Weberae, A.R. Weidbergz, P. Weilhammere, P.S. Wellse, P. Wetzelv,
M. Whitleyo, M. Wiesmannv, I. Wilhelmaa, M. Willenbrocki, I. Wilmutae, J.A. Wilsonc,
J. Wintons, M. Wolterg, M.P. Wormaldo, S.L. Wual, X. Wui, H. Zhuaf, N. Bingeforsam,
R. Brenneram, T. EkelofamaCentro Nacional de Microelectrónica de Barcelona, CNM-IMB, CSIC, Barcelona, Spainb
Department of Physics and Technology, University of Bergen, N 5007 Bergen, NorwaycSchool of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UKdCavendish Laboratory, Cambridge University, Cambridge, UK
eEuropean Laboratory for Particle Physics (CERN), Geneva, Switzerlandf
Faculty of Physics and Applied Computer Sciences, AGH University of Science and Technology, Cracow, Polandg
The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, PolandhFakultät für Physik, Albert-Ludwigs-Universität, Freiburg, GermanyiSection de Physique, Universite de Geneve, 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 Organization, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japanm
Department of Physics and Astronomy, University of Lancaster, Lancaster, UKnLawrence Berkeley National Laboratory, Berkeley, California, USAo
Department of Physics, Oliver Lodge Laboratory, University of Liverpool, Liverpool, UKp
Department of Physics, Queen Mary and Westfield College, University of London, London, UK
q
Department of Physics, University College, University of London, London, UKr
School of Physics and Astronomy, University of Manchester, Manchester, UKs
University of Melbourne, Parkville, Victoria 3052, Australiat
NIIGraphite, Moscow, Russiau
NIITAP, Zelenograd, Moscow, Russiav
Max-Planck-Institut für Physik, München, Germanyw
NIKHEF, Amsterdam, The NetherlandsxOkayama University, The Graduate School of Natural Science and Technology, Tsushima-naka 3-1-1, Okayama 700-8530, Japany
The University of Oslo, Department of Physics, Oslo, Norwayz
Department of Physics, Oxford University, Oxford, UK
aaCharles University, Faculty of Mathematics and Physics, Prague, The Czech Republicab
Czech Technical University, Prague, The Czech Republicac
Institute of Physics of the Academy of Sciences of the Czech Republic, Prague, The Czech Republicad
IHEP, Protvino, Russia
ae
Rutherford Appleton Laboratory, Chilton, Didcot, UKaf
Department of Physics and Astronomy, University of Sheffield, Sheffield, UKag
Ioffe Physico-Technical Institute, St.Petersburg, RussiaahSanta Cruz Institute for Particle Physics, University of California, Santa Cruz, California, USAai
Institute of Physics, Academia Sinica, Taipei, Taiwan
aj
University of Tsukuba, Institute of Pure and Applied Sciences, 1-1-1 Tennodai, Tsukuba, Ibarai 305-8571, Japanak
Instituto de Fı´sica Corpuscular (IFIC), CSIC-Universidad de Valencia, Valencia, Spainal
Department of Physics, University of Wisconsin, Madison, Wisconsin, USAam
Uppsala University, Department of Nuclear and Particle Physics, P.O. Box 535, SE-75121 Uppsala, Sweden
Received 23 January 2007; accepted 7 February 2007
Available online 28 February 2007

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Abstract
The challenges for the tracking detector systems at the LHC are unprecedented in terms of the number of channels, the required readout speed and the expected radiation levels. The ATLAS Semiconductor Tracker (SCT) end-caps have a total of about 3 million
electronics channels each reading out every 25 ns into its own on-chip 3:3 ms buffer. The highest anticipated dose after 10 years operationis 1:4  1014 cm2 in units of 1 MeV neutron equivalent (assuming the damage factors scale with the non-ionising energy loss). The
forward tracker has 1976 double-sided modules, mostly of area 70 cm2 , each having 2  768 strips read out by six ASICs per side. The
requirement to achieve an average perpendicular radiation length of 1.5% X 0 , while coping with up to 7 W dissipation per module (after
irradiation), leads to stringent constraints on the thermal design. The additional requirement of 1500e equivalent noise charge (ENC)
rising to only 1800e ENC after irradiation, provides stringent design constraints on both the high-density Cu/Polyimide ﬂex read-out
circuit and the ABCD3TA read-out ASICs. Finally, the accuracy of module assembly must not compromise the 16 mm ðrfÞ resolution
perpendicular to the strip directions or 580 mm radial resolution coming from the 40 mrad front-back stereo angle.
A total of 2210 modules were built to the tight tolerances and speciﬁcations required for the SCT. This was 234 more than the 1976 required
and represents a yield of 93%. The component ﬂow was at times tight, but the module production rate of 40–50 per week was maintained
despite this. The distributed production was not found to be a major logistical problem and it allowed additional ﬂexibility to take advantageof where the effort was available, including any spare capacity, for building the end-cap modules. The collaboration that produced the ATLASSCT end-cap modules kept in close contact at all times so that the effects of shortages or stoppages at different sites could be rapidly resolved.
r 2007 Elsevier B.V. All rights reserved.
PACS: 29.40.Gx; 29.40.Wk
Keywords: Silicon; Module; Microstrip; ATLAS; SCT; LHC

1. Introduction
The ATLAS experiment [1] at the CERN Large Hadron
Collider (LHC) [2] is a general purpose detector, aiming at
a TeV-scale reach for new physics. The LHC is expected tostart with luminosities ramping ﬁrst to 1033 cm2 s1 in its
initial running and gradually increasing up to 1034 cm2 s1(corresponding to integrated luminosities of up to 105 pb1
per year). The semiconductor tracker is required toreconstruct isolated leptons with a transverse momentumof pT 45 GeV with 95% efﬁciency out to jZjp2:5, tomeasure momentum even at pT ¼ 500 GeV with better
than 30% precision, to track back to the vertex zcoordinate with better than 1 mm accuracy, achieve two
track resolution of better than 200 mm at 30 cm radius andrepresent no more than 20% X 0 in total [3].
To achieve this, a design consisting of four barrels of2112 silicon modules in total and two sets of nine disks(each set comprising 988 end-cap modules) was adopted by
ATLAS. Roughly speaking, the barrel region covers jZjp1
on its own, with the disks needed to extend the coverage to
jZjp2:5 while minimising the material seen by the highly
inclined tracks in these directions.
While the barrel only requires one module type [4], theend-cap region has four module types and ﬁve different
sensor types. The end-cap sensors have also been purchased from two different suppliers. However, all the endcap modules, as seen in Figs. 1 and 2, have, located at one
end, identical high density Cu/Polyimide hybrid ﬂex
circuits [5] to house the ABCD3TA read-out ASICs [6].
They also share the same thermal design, with the sensors
sandwiched either side of a thermal pyrolytic graphite(TPG) [7] spine, which conducts heat away from thesensors to the cooling block contacts. The two-sided hybrid
circuit is laminated around a high conductivity carbon–
carbon (CC) substrate which contacts the main cooling
block directly, such that no heat from the read-out chips
should ﬂow into the sensors. The hybrid and the sensors
are, in addition, separated by a thermal break bridged by
fan-in structures used to connect the sensors to the ASICs.
This is important since the sensors need to be kept cool,7  C, to freeze out radiation damage annealing effects [8]and to keep the irradiated sensor currents below 0:5 mA
each. For most module types, two sensors per side aredaisy-chained together to give 12 cm strip length perchannel. The requirement of negligible shot-noise contribution and the physics of the annealing process (whichleads to additional degradation with time unless cooled)drive the sensor temperature requirements. A further
demand on the thermal design comes from the need to
avoid silicon self-heating since this will drive up the leakage
current, in turn leading to further self-heating and possible
thermal runaway.
Mechanical tolerances for the building of the 1976—plus
spares–end-cap modules of the ATLAS SCT are very tightto ensure that the intrinsic spatial resolution of the silicondetectors is in no way compromised. The complexity andextreme requirements for 10 years LHC operation as a highresolution, high efﬁciency, low noise tracking system haveresulted in demanding and time consuming quality
assurance (QA) procedures for every module. The assembly and testing has been distributed over 14 institutes basedin Australia, the Czech Republic, Germany, the Netherlands, Poland, Spain, Switzerland and the United Kingdom. The ﬁnal assembly of modules onto their disks
was carried out for End-cap A in the Netherlands and forARTICLE IN PRESS
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Fig. 1. Exploded view of an SCT end-cap module showing the different components.

Fig. 2. ATLAS SCT end-cap modules. From left to right: outer, middle and inner. There is a fourth type, the short-middle, that follows the design of a
middle module with only one pair of silicon sensors.

End-cap C in the UK, prior to both assemblies beingtransported to CERN for integration into the ATLAS
Experiment.
2. Layout of the ATLAS silicon tracker
This section describes the arrangement of tracking subdetectors employed in ATLAS to measure charged particle
3-momenta, determine their charge, reconstruct secondary
and tertiary vertexes, and identify interactions with thematerial in the tracking volume.
2.1. Physics goals of the ATLAS silicon tracker
The ATLAS Inner Detector (ID) is 2.3 m in diameter,
7 m in length and consists of the Semiconductor Tracker(SCT), with the Pixel detector within it and the gaseous/polypropylene foil Transition Radiation Tracker (TRT)
surrounding it [3]. The ID sits within a 2 T magnetic ﬁeld,
provided by the superconducting central solenoid, integrated inside the cryostat of the Liquid Argon electromagnetic calorimeter.

The very high interaction rate at full luminosity and thehigh energy of jets, lead to requirements of both highgranularity and high spatial resolution. These effectively
determine the strip dimensions, given also that the ABCD3TA [6] read-out chips give a binary read-out with a variable
threshold typically set to 1 fC for unirradiated operation. The
resolution requirements for isolated tracks are set by thetransverse momentum precision of p30% at 500 GeV
discussed above. In practice, this is readily achieved bymeasuring four space points along the track with the 16 mm
resolution in the f coordinate provided by the 80 mm pitch(two sided with 40 mrad stereo) of the SCT modules, even
without the charge interpolation possibilities which would
come from analogue read-out. More severe are the requirements on the granularity to maintain typical hit occupancies
of order 1% or below, even within high pT jets. Also, because
the capacitive load on the ABCD3TA chips cannot exceed
18 pF if noise of 1500e ENC is to be achieved, the length of
the strips cannot exceed 12 cm for 1.2 pF/cm capacitance(when also allowing for fan-in and bonds).
The layout described below was chosen to ensure at least
four space points in the SCT (where the orthogonal
coordinate to f measurement comes from the small angle

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stereo between front and back strips) on any track from theprimary interaction region. The geometry was chosen tosatisfy this for interaction points up to two standard
deviations away from the sensor centre along the z-axis.
The standard deviation of the interaction point in z is
expected to be 76 mm at nominal LHC operation.
2.2. Operating conditions of the ATLAS silicon tracker
The modules of the ATLAS SCT have been designed towithstand doses of up to 2:14  1014 neq =cm2 normalisedusing the non-ionising energy loss (NIEL) cross-sections to
the expected damage of 1 MeV neutrons [3]. This assumes a
3 year start up at lower luminosity followed by 7 years at
design luminosity and includes a 50% additional safety
factor on the simulated particle ﬂuences. The radiation
environment largely determines the other operating conditions which are designed to ensure the modules remain
performant to the end of such an operating scenario.
Key to retaining good performance is limiting the reverse
annealing of the sensors. It has been one of the key
observations of operating irradiated high-resistivity silicon
micro-strip sensors that their depletion voltage, V dep , andhence their required operating voltage for high efﬁciency,
varies with time long after exposure to radiation. Furthermore, while the radiation induced currents fall with time[9], the behaviour of the effective doping concentration,
N eff , (which determine the V dep value) shows a more
complicated behaviour [10].
The ﬁrst observation is that N eff initially drops with dosefor pþ -in-n sensors, but after about 2  1013 neq =cm2 thematerial effectively inverts space-charge from being n-type
and acts as if it is increasingly p-doped with dose [11].
Furthermore, the effective doping after inversion tends togrow with time, reverse anneals, but in a way that isstrongly dependent on operating temperature. This motivates the requirement of an average operating temperatureon the sensors of 7  C which effectively freezes out thereverse annealing, at least during data taking. A uniformityof better than 5  C across the silicon is also required.
Finally, once one takes account of realistic maintenancescenarios [3] it is required that one allows for V dep values

357

of up to 300 V and corresponding operating voltages of
up to 350 V.
The requirements of cold operation of the sensors, given
the 7 W module power dissipation after irradiation, lead tothe need for coolant temperatures of down to 25  C. Thisin turn requires the dew-point in the detector environment
to be well below this value, leading to the need to ﬂush the
SCT with nitrogen or very dry air. The requirement tooperate cold, as well as dry, is reﬂected in the testing of themodules discussed below.
2.3. End-cap mechanical layout
A view of the ID is shown in Fig. 3. As shown in the
picture, the ATLAS SCT consists of both barrel and endcap regions to minimise the material seen by traversing
particles as a function of polar angle. The barrel region ismade of four cylindrical layers and the end-caps consist ofnine disk layers. The geometry is determined by the four
space point requirement above, with a coverage in jZj out
to 2.5 (allowing for the z dispersion of the primaryinteractions). The resulting tracking detector has 63 m2 ofsilicon micro-strip sensors with just under 40% in the two
end-caps.
2.4. Spatial resolution requirements
The physics speciﬁcation for the pT resolution of
500 GeV tracks in the inner detector is sðpT Þ=pT o30% at
jZjo2, relaxed to 50% for jZj up to 2.5 [12]. The pT
resolution of the ﬁnal inner detector design is shown in
Fig. 4 at 20 and 1000 GeV. Clearly it depends on the spatial
resolution of all three sub-detectors; pixels measuring threepoints with 12 mm resolution, SCT measuring four points
with 16 mm resolution and TRT measuring 36 points with
170 mm resolution. If this speciﬁcation were the only one
that mattered to the SCT an increase in the pitch would bepossible. However, the argument for limiting the pitch toaround 80 mm is based primarily on occupancy.
Pattern recognition performance in the tracker isfundamentally limited by occupancy. In the case of the
SCT with binary readout, the only way to resolve two

Fig. 3. A quarter section view of the ATLAS Inner Detector (ID).

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 353–389

Fig. 4. pT momentum resolution as a function of jZj for simulated muons of various momenta. Results are shown for a solenoidal ﬁeld without (circles)
and with (squares) a beam constraint and for a uniform ﬁeld without a beam constraint (triangles).

tracks is to see them as two hit strips with at least one
empty strip in between. The noise occupancy of the SCT isvery low ðo0:1%Þ by design, and its occupancy due tounderlying events at full luminosity is also low at around
0.5%. These levels do not pose a signiﬁcant problem for
track ﬁnding. However, the occupancy of a module hit by ab jet is typically 1.5% and this can make track ﬁnding
difﬁcult. Fig. 5 shows track reconstruction efﬁciency as afunction of distance from the jet axis. The efﬁciency isalready down at 89% (compared with 98% for isolatedmuons) because we are dealing with pions with pT down to
1 GeV. The efﬁciency drops further to 78% near the core of
the jet, indicating pattern recognition difﬁculties. An
indication of the role of the SCT in track ﬁnding in jets
is that the average efﬁciency of the whole tracker is 89.5%
with a fake rate of 0.24%. If one layer of the SCT isremoved, the efﬁciency only drops to 89.0%, but the fakerate doubles to 0.46% [3]. These numbers show that highoccupancy is already a cause of some tracking inefﬁciency
with 80 mm SCT pitch and would be worse with larger
pitch. Lower pitch would give lower occupancy but is ruledout on grounds of cost and power density.
There is a small stereo angle between the front and backsensors of an SCT module. As stereo angle is reduced, it
reduces the number of allowed combinations of a f strip
with a stereo strip, but at the same time it degrades thespace-point resolution in the radial direction making it
more difﬁcult for pattern recognition software to choose
the right combination. Simulation studies showed thatthese competing effects largely cancel, so that patternrecognition performance is weakly dependent on stereo
angle in the range 40–80 mrad. We chose to use 40 mrad
because it reduces the size of the corner regions, where thecorner of a stereo sensor sticks out beyond the sensitive
area of a f sensor, or vice versa. Small corner regions makeit easier to tile an area with modules efﬁciently and makepattern recognition less sensitive to module misalignments.
Having designed a high resolution tracker, it is requiredthat any misalignment of its components should lead toFig. 5. Track reconstruction efﬁciency as a function of distance from thejet axis. The distance is measured in the pseudorapidity-azimuthal angle
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
space and is deﬁned as DR ¼ D2 Z þ D2 f.

negligible loss of resolution. It was chosen to deﬁnenegligible as a 20% loss of track parameter resolution,
relative to the performance of a perfectly aligned innerdetector. A simulation study was performed to ﬁnd a set of
misalignment tolerances appropriate to each sub-detector,
such that the resolution of the whole ID was within 20% of
its ideal value. All ﬁve helix track parameters wereconsidered in this study but in fact only the pT and impact
parameters turned out to be relevant. The alignment
required for the SCT end-caps is shown in Table 1. These
tolerances apply to the total errors, made up from theuncertainty about the module position combined withuncertainty about the position of the four sensors within
the module. The former uncertainty will be by far the most

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 353–389

difﬁcult to control, so most of the error budget is assigned
to it and one third of the budget is left for the tolerance on
the relative positions of sensors within the module.
2.5. Description of the module geometries
The geometries of the four types of end-cap module areclosely coupled to the constraints of processing on 4 in.
wafers, the z-position of disks in the end-cap and theengineering design of the support disks.
The physics requirements of 4 rf=r hits up to jZj ¼ 2:5
together with the overall detector active volume constraints
on the radius 275 mmoro 560 mm and jzjo2800 mm leadto a layout consisting of nine disks. Each disk has one, twoor three rings of modules, named Outer, Middle and Inner.
All modules belonging to a particular ring type are
identical. The active length of the modules for the outerand middle rings is 120 mm whilst that of the inner ring is
55 mm. Each module consists of two planes of sensors
glued back-to-back around a central spine. In the case ofOuter and Middle modules, each side contains two daisy
chained sensors to achieve the required active length. Thereis a relative angular rotation between the two planes of
sensors of 40 mrad to give the required position resolutionsin rf ð16 mmÞ and r ð500 mmÞ.
The coverage of each disk is required to be fully hermetic
for tracks above a transverse momentum of 1 GeV, except
for the unavoidable dead area between the two sensors ineach plane for outer and middle modules. Moreover, thelayout allows sufﬁcient overlapping active area between
neighbouring modules for the module alignment parameters to be efﬁciently determined. As a consequence, theshape of the modules is trapezoidal, resulting in a variable
strip pitch.

Table 1
Alignment accuracy required for the SCT (mm r.m.s.)rfz

Total error budget

Detector alignment within module

12
200

4
67

359

The number of modules in a given ring was determinedin the process of deﬁning the module geometries byrequiring that the total number of modules, and therefore
silicon sensors and hence cost, was minimised. This isequivalent to using the largest sensor areas that could beaccommodated on a wafer.
The active area overlap between neighbouring modules
in rf is achieved by staggering modules in z by 1.5 mm
about the mean z of the ring, allowing to classify themodules in a ring as upper or lower. The design of the disk
is such that the outer and inner rings are mounted on theside towards the interaction point (front) whilst the middle
ring modules are mounted on the side away from theinteraction point (rear), as seen in Fig. 6. The distance in z
between upper modules on either side of the disk is
34.0 mm.
The ﬁnal geometries for the ﬁve individual siliconsensors needed were determined in an iterative fashion
starting with a nominal physics layout. The geometry of thetwo sensors making up the outer ring module wasdetermined using the outer active radius in ATLAS of
560.0 mm and the constraint that there must be no featureextending beyond a radius of 44.72 mm from the centre of
the undiced 4 in. wafer. The outermost sensor is denoted
W32 and its partner is W31. The innermost edge of W31
deﬁnes the limit of coverage of the outer module.
The effective radius of the inner corners of W31 together
with the requirement of hermeticity deﬁnes the outer radiusof the middle ring. This is ﬁxed by extrapolating a line
corresponding to an inﬁnite momentum track, originatingfrom z ¼ 2sz (sz the beam spread in z) over the 34 mm
gap that separates the middle and outer rings at the z
position corresponding to disk 1. As above, the geometryof the two sensors was determined by the 44.72 mm
processing constraint. Here the sensors are denoted W21
and W22. The inner edge of W21 deﬁnes the limit ofcoverage for the middle ring. Similarly, the geometry of theinner ring was determined from the inner active edge ofW21 and the 34 mm front-to-back module separation. This
time the limiting case is disk 6. Again, the limiting
processing radius then deﬁnes the inner edge of the innersensor, denoted W11.

Fig. 6. Modules mounted on the disk. The left picture (a) shows the outer and inner rings on one side of the disk and the picture on the right (b) shows the
middle ring on the other side of the disk.

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360

Table 2
Sensor dimensions, module locations on the disks, their tolerances and rf
overlap between neighbouring modules
1
Rin
Rout rf
Module Sensor Sensor
2 length(mm) (mm) overlap
type
centre (mm) (mm)
ðmmÞ

Inner

rf
tolerance

W12

304.550

29.5500 275.00 334.10 0.442

67

Middle W21
W22369.163429.063

31.5625 337.60 400.73 0.588
26.2375 402.83 455.30 0.588

99
99

Outer

470.555
532.22331.7900 438.77 502.35 1.437
27.7775 504.45 560.00 1.437

286
286

W31
W32

Table 3Module distribution between disks
Disk

1

2

3

4

5

6

7

8

9

z (mm)
853.8 934 1091.5 1299.9 1399.7 1771.4 2115.2 2505 2720.2
Outer
52
52 52
52
52
52
52
52 52
Middle
40
40 40
40
40
40
40
Short middle
40
Inner
40 40
40
40
40

Following the basic deﬁnition of the module geometry,further studies with stiff tracks were used to reﬁne thepositions of the disks and to optimise which disks have
inner rings of modules. These studies showed that the best
overall tracking performance was achieved with a layoutthat is only fully hermetic for disk 3 onwards. The layout
resulting from the process is summarised in Tables 2
and 3 [13,14]. Table 3 shows the z position of each disk andthe numbers of each module types assigned to them. At thez position of disk 8, a line corresponding to Z ¼ 2:5 would
cut a standard middle module in half. To reduce cost the
short-middle module was developed. This uses just the W22
sensor and has an inner active radius of 402.83 mm. Also,in order to give a disk 9 position within the allowed z
constraints whilst maximising the coverage in Z, disk 9 isrotated about the vertical axis so that the outer modulering is to the rear.
3. Overview and motivation for the ATLAS end-cap moduleThis section describes the technical choices made todeliver a detector system with the required physics
performance. Many of the constraints come from the very
high luminosity of the LHC, with its 25 ns beam-crossing
interval, very high number of tracks per event, and extreme
radiation environment.
3.1. Electrical and optical interfaces
The sensors are readout by 12 ASICs mounted on the
hybrid, a double sided, Cu/Polyimide ﬂexible circuit
laminated onto a CC substrate. The chips should beradiation resistant, up to 10 mrad, fast in order to

efﬁciently identify the beam crossings, low noise, lowpower and capable of keeping the data in on-detector
buffers during the ﬁrst level trigger latency time.
The Atlas Binary Chip (ABCD3TA [6]) has been
designed to meet the imposed requirements. It has been
fabricated using the radiation hard DMILL technology[15] and implements a binary readout architecture in asingle chip. Binary readout offers advantages in terms ofrequiring lower data transmission bandwidth compared toother read-out modes, less stringent requirements on thequality of the data links and simpler off-detector electronics. On the other hand, special care was taken in thedesign and grounding of the system to avoid problems due
to external electromagnetic interference. In addition, theproper threshold setting and channel-to-channel matchingis crucial for a binary system, as it is impossible todistinguish between large noise ﬂuctuations and genuine
signal after the discriminator.
The ABCD3TA chip has 128 channels and comprises
front-end circuitry (employing a bipolar transistor at the
input stage), discriminators, binary pipeline, derandomising buffer, data compression and readout control logic aswell as calibration circuitry. The 25 ns peaking time is shortenough to keep the time-walk in the range of 16 ns and the
double peak resolution below 50 ns, ensuring that thefraction of events shifted to the wrong beam crossing is
below 1% and that less than 1% of the data will be lost at
the highest occupancies. To compensate the expected dropof the DC current gain after irradiation, a 5-bit DAC has
been implemented in the chip to adjust the collector currentof the input stage and optimise the noise performance. In
addition, the bias current of the following stages is also
controlled by another 5-bit DAC. The preampliﬁer-shaperstage is followed by a discriminator with a common
threshold for all the channels that is controlled by an 8-bit
DAC. To maintain the channel-to-channel variation of thethreshold below 4%, especially after irradiation, theADCD3TA implements an individual threshold correctionin each channel with a 4-bit DAC with four selectable
ranges. Data from the discriminator output are latched in
the input register every 25 ns, either in edge sensing or levelmode, and clocked into a 132-cell pipeline that matches theﬁrst level trigger latency time. Upon reception of a trigger,
the data are transferred from the pipeline to the second
level buffer, eight events deep. Data are then compressedby the data compression logic and read out via a token
ring, allowing for the readout of the six chips in a hybridside through a single data link.
The ABCD3TAs can be conﬁgured to three different
operation modes; master, end and slave. The default
conﬁguration is realised by connecting the appropriate
bond pads on the ASICs to the digital ground potential. In
addition, this settings can be changed through control
commands. The six chips on each side form a chain withthe master and the end chip at the ends of the chain. Each
chain is read out serially using the aforementioned tokenbased scheme. In such a token-based readout scheme,

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single chip failure can cause the loss of all data from thefollowing chips. To avoid this, a bypass scheme isimplemented in the hybrid to cope with single chip failures
without loosing the data of the remaining chips. The only
limitation is that the failing chips must not be immediateneighbours.
The sensor bias voltage is ﬁltered in a dedicated networkat the hybrid as shown in Fig. 7. The high voltage is thensupplied by the hybrid at a pad in an extension of thehybrid ﬂex (finger) that contacts a metal trace on one of the
AlN cross pieces of the spine, onto which the sensors
are glued with a conductive epoxy. At the interface of thecooling block the hybrid implements a shielding schemethat will shunt any noise from the cooling block into
the main analogue ground. For the sensors a shunt shieldalso needs to be implemented. This is done using thespine TPG acting as a conductive layer between sensors
and cooling blocks. The TPG is electrically insulated
from both and, in order to behave as a shunt shield,
connected to the analogue ground through a capacitor(see Fig. 7 at the top). In order to have the TPG and thesensors at the same DC level, the TPG is connected to thesensor bias through a 1 MO resistor. This connection isdone with another ﬁnger similar to the one used for thesensor bias.
The electrical interface between the end-cap modules andthe disk services occurs mainly at the hybrid level. Powersupply currents and DC levels, needed to operate the
ASICs and to bias the sensors, are carried by power tapesthat connect the module to the periphery of the disk withminimal radiation length. As for the grounding, themodules’ power return is shorted to the cooling tubes at
the module mounting point. The connection is made bymeans of an additional small kapton ﬁnger with a copper
trace which is to be soldered to each module end of thepower tape.
The data transmission off detector is also an important
issue given the huge data rates expected. A system based on
optical ﬁbres [16,17] has been designed because of its low
mass and the absence of electromagnetic interference.

Optical links are also used to distribute timing, trigger andcontrol (TTC) data from the counting room to the frontend electronics. There are two data ﬁbres per module. In
normal operation each ﬁbre reads out the data corresponding to one side of the module. The system contains
immunity to single point failure. The redundancy isimplemented in the module in two levels. For the data
links, when one link fails, all the data from that module canbe routed through the other ﬁbre. At the expected occupancies this will not lead to any loss of data. As for the
TTC data, the redundant lines are distributed electricallyfrom one module to the neighbour.
3.2. Thermal design, simulation and prototyping
Outer and middle modules are supported and cooled bytheir contact with two cooling blocks; the main block (area
230 mm2 ) is shared between the hybrid and the spine, while
the far block ð78 mm2 Þ cools only the spine. Inner modules
are cooled by the main block and in this case the far blockis only for mechanical support. Fig. 8 illustrates the heat
paths in an outer module.

Fig. 8. Heat ﬂow at the hybrid end of an outer module.

Shunt Shield
47nF/630V
spine TPG
1MOhm

AGND

AGND

5.1kOhm

5.1kOhm

10nF/630V
sensor backplanebias
10nF/630V
bias
returnsensor bias rail
1kOhm

Detector bias

361

AGND

Fig. 7. Filter network implemented in the hybrid for the high voltage supply.

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3.2.1. Thermal loads and interfaces
The electrical power input to the whole hybrid is
typically 5.4 W but it can be as high as 7.5 W in the worst
case, so the design was based on that value. The electrical
power input from the unirradiated sensor is negligible.
Thereafter it depends on irradiation and annealing history
and temperature. After the fast components of annealing
are complete, the leakage current is simply proportional toradiation dose and the constant of proportionality is takenfrom the tests on pre-production sensors. The voltage
needed to fully deplete the sensors also grows withradiation and is expected to be around 300 V in the worst
case after 10 years. However, the design was made for the
maximum voltage available from the power supply; 500 V
minus a 40 V drop in the ﬁlter. The outcome [18] is that the
maximum sensor power, including safety factors to allow
for uncertainties, is predicted to be 185 W=m2 at 0  C. The
temperature dependence of the sensor power is given by
 2



Eg 1
T
1

exp 
PðTÞ ¼ PðT 0 Þ(1)
T0
2k T T 0
where PðT 0 Þ is the power at T 0 ¼ 273 K, T is temperaturein Kelvin and k is the Boltzmann constant; for the energy
gap the value E g ¼ 1:20 eV, based on measurements ofirradiated sensors, is taken.
There is a heat load on the sensor part of a module due toconvection from the surrounding gas. This load wassimulated with Computational Fluid Dynamics (CFD) andmeasured in a mock-up of a full disk [19]. Both approaches
predict that the load is strongly dependent on position, being
maximal for the outer modules at the top of a disk. Based onthese results an upper limit on the sensors convective load
was estimated to be 0.8 W for middle and outer modules and
0.4 W for inner modules. This heat is coming mainly fromthe hybrids and the rest is from the power tapes.
Evaporative C3 F8 cooling has been chosen for the SCT.
The coolant is injected through capillaries into the Ø
3.6 mm cooling pipes, where it arrives mainly in the liquid
phase. Then it runs through the pipe on a wiggly circuit,passing through the cooling/mounting blocks of 10–13
modules, before exiting the SCT mainly in the gaseous
phase. By adjusting the pressure at the exhaust theoperating temperature can be tuned over the range
10–30  C, though it operates most efﬁciently near the
middle of this range. An important property of the coolant
is the heat transfer coefﬁcient (HTC) between the pipe walland the ﬂuid which shows a strong rise with the power
density and is moderately dependent on several otherfactors. In this system we ﬁnd that a conservative
parametrisation is to take HTC ¼ ð1800 þ 330  rÞ
W m2 K1 , where r is the power density in W cm2 [20].
3.2.2. Thermal specifications
The equivalent noise charge of the ABCD3TA readout
chip has a temperature dependence of six electrons per
Kelvin before irradiation and 24 electrons per Kelvin after

irradiation [21]. This temperature dependence is not
negligible compared with the design noise level of around
1500e ENC, so there is a motivation to keep the readoutchips reasonably cool.
There is a shallow optimum between harmful andbeneﬁcial annealing effects in the silicon sensors at atemperature of 7  C. It is likely that there will be some
accidental or planned warm-ups of the SCT for maintenance; therefore, we specify that the operating temperature of the sensors at maximum power should be below
7  C.
A stronger constraint on sensor temperature comes fromthe possibility of thermal runaway. Heat will be extracted
from the sensors mainly by conduction, which is linear withtemperature. But the heat generated in the sensors grows
exponentially with temperature, doubling every 7 K. Thiscan lead to an unstable situation called thermal runaway inwhich the sensor temperature rises by positive feedback
until limited by some external factor, in this case the HV
power supply limit at about 2.5 W. This is a hard failuremode, unlike the chip noise and sensor annealing, so we
add an extra 30% safety factor to the sensor power and we
specify that the module must not go into thermal runaway
when the sensor power is 240 W=m2 at 0  C and the otherloads are as speciﬁed in Section 3.2.1.
3.2.3. Split module and split block design
Thus there is a strong speciﬁcation on the temperature of
the sensors, which generate around 2 W, and a weakerspeciﬁcation on the temperature of the chips, which
generate around 7 W. So we can beneﬁt by providing
separate heat paths to the coolant from the sensors andfrom the chips. This leads us to our design of a module thatis thermally split between the sensor part and the hybrid
part. The glass substrate of the fan-ins and plastic base of
the location washer provide mechanical connection combined with thermal isolation between the two halves of themodule. The TPG spine and the CC substrate of the hybrid
provide low resistance heat paths to the cooling blockwithin their respective parts of the module. Having split themodule, we found that we could also beneﬁt by introducing
a thermal split into the main cooling block. This was doneby making the block from a CC-PEEK-CC sandwich,where the 1 mm layer of PEEK is aligned with the thermal
split in the module.
The thermal performance of modules has been simulatedwith FEA programmes and measured in several dummy
modules and one real irradiated module.
3.2.4. Hybrid thermal performanceFig. 9 shows the hybrid end of a full module thermal
simulation at maximum load. It illustrates the large
temperature gradient across the fan-in and the rather
uniform temperature of the near-by sensors. The hybrid issimulated in some detail, showing that the chip farthestfrom the cooling block has its thermal plug under the

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363

Hybrid Temperature
Zoom (-22,-2,-50,50)

50.0

12.0
11.0
10.0
9.00
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
-1.00
-2.00
-3.00
-4.00
-5.00
-6.00
-7.00
-8.00
-9.00
-10.0
-11.0
-12.0

40.0

Y

30.0

20.0

10.0

0.0
-20.0

-10.0

10.0

0.0

20.0

30.0

X
Fig. 9. The hybrid end of an outer module simulated at full power and with the coolant at 20  C. The simulation has a 2-fold symmetry (zero stereo
angle) so only half of the module is shown.

Fig. 10. Measurement and simulation of the hybrid surface temperature proﬁle on a line passing through the six ABCD chips. Temperature is relative tothe cooling block.

analogue part where most of the heat is generated, while
the others are cooled by plugs under the digital part.
Fig. 10 shows a measurement of the chip temperaturesusing an infra-red sensor scanned in a straight line across a
hybrid operating at maximum power. The sensor sees anarea of 2  2 mm2 and is calibrated for the emissivity of thechips with an accuracy of 2 K. The result is comparedwith a simulated temperature proﬁle. We conclude that thechip temperatures are reasonably well understood and are
not excessively high.

Table 4 illustrates how the simulated total temperature
difference of 29 K between chip 2 and the coolant is built
up from smaller temperature differences along the heat
path. This apportioning of the temperature drop into a
series of steps can only be approximate since the real heat
path is three-dimensional.
3.2.5. Spine thermal performanceFig. 11 shows the simulated temperature of the sensor
part of an outer module at full power. It shows that the

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 353–389

sensor temperature is rather uniform, covering a range
from 9:1 to 11:7  C. More details of the behaviour of
inner and outer modules are shown in Table 5. A
consequence of the uniform sensor temperature is thatthe thermal behaviour of a module is accurately predictablefrom a single thermal resistance value, deﬁned as the areaaveraged sensor temperature, relative to the block, when apower of 1 W is applied uniformly to the sensors. This
resistance value was measured and simulated in both
middle and inner modules [22]. The simulation matched themeasurement within 20%, validating the simulation at thislevel of accuracy.
Finally we measured the thermal runaway of an
irradiated inner module. The radiation damage was notenough to bring the sensor power up to the speciﬁcationlevel of 185 W=m2 at 0  C; it could only reach 107 W=m2 at
0  C when biased to 500 V. By running the coolant at 5  C
we were able to bring this module into thermal runaway,
and a simple scaling law [20] allows us to predict the

behaviour of the same module at higher radiation damage
and lower coolant temperature. Fig. 12 shows these results.
3.3. Clearances and mechanical tolerances
Mechanical tolerances and clearances come, mainly,
from positioning requirements and envelopes. The former
is driven by the required physics performance, hermeticity,
resolution, overlap for alignment, etc., while the latter are
set to avoid physical interference of items.
As already mentioned in Section 3.2, the modules areattached to two blocks which support them at precise
locations [23]. The block at the hybrid end provides theaccurate positioning of the module, while the slot at the
far end provides the module rotation which is set atTable 5The performance of inner and outer modules, simulated with the coolant
at 20  C
Coolant at 20  C

Table 4
Showing how the temperature difference between the middle chip and thecoolant is built up
Step

Material

DT (K)

Chip to substrate below chip
Within substrate
Substrate to surface of block
Within block to pipe
HTC into coolant

AlN plug and two glue layers
Carbon–carbon composite
Thermal grease
Carbon–carbon composite
C3 F8

4
9
3
4
9

Total

29

Module type
Outer

Inner

Sensor temperature ð CÞ
Maximum
Average

9.1
10.1

6.4
7.1

Heat taken out (W)Hybrid part of main block
Sensor part of main block
Far end block

6.8
0.7
1.3

6.8
1.2

Runaway power ðW=m2 at 0  C)

280

290

Middle and outer modules are identical in terms of thermal properties.

Fig. 11. The sensor part of an outer module simulated at full power and with the coolant at 20  C.

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365

Table 6
Sensor properties
Barrel W12

W21

W22

W31

W32

Length (mm)64.000 61.060 61.085 54.435 65.540 57.515
Outer width (mm)63.360 55.488 66.130 74.847 64.636 71.810
Inner width (mm)63.630 45.735 55.734 66.152 56.475 64.653
Strip pitch ðmmÞ80
57–69 70–83 83–94 71–81 81–90
Interstrip angle ðmradÞ 0
207
207
207
161.5 161.5

Fig. 12. Measurement of thermal runaway in an irradiated inner moduleand extrapolation of the result to a lower coolant temperature.

either plus or minus 20 mrad, depending on the disk
number. The accuracy of the z position of the modules
will be determined by the tolerances on the block heights
while the rf position is determined by the precisionof placing rotation holes in the modules and locatorpins in the blocks. Also co-planar block surfaces are
essential to avoid distortion of modules when they aremounted to the disk, and to maintain cooling contact at theblock surfaces.
A given ring has alternating high and low modules withan overlap between neighbours that provides the required
hermeticity and allows for the relative alignment of themodules within the same ring using tracks that hit both
modules. This overlap in rf determines the tolerances inpositioning the module in that direction and thesetolerances are different depending on the ring radius, as
shown in Table 2 [24]. Mechanical interference constraints
require a clearance of 0.5 mm for the position of high to
low modules, while 1 mm is required between module
components at high voltage and disk services.

1000 mm distance from the sensitive area to the physicalcut edge. The guard region was optimised by eachmanufacturer according to their processing to guarantee
the current(voltage) requirements outlined below. The
implanted strips were required to have o200 kO=cm
and to be capacitively coupled ð420 pF=cmÞ to aluminium
read-out strips ðo15 O=cmÞ matching the implant
dimensions. The implants are biased by resistors of
1:25  0:75 MO.
The sensors were required to hold up to 350 V with lessthan 20 mA leakage current at room temperature, be able to
run to 500 V and to draw no more than 6 mA at the initial
operating voltage of 150 V. Good strip capacitors wererequired to hold 100 V and the total strip failure rate
(capacitor breaks, implant shorts or opens, metal shorts oropens, broken bias connections, etc.) were required to beless than 1%. These speciﬁcations were observed by the
manufactures and checked by the sensor reception centresof the collaboration. The measurements of the manufacturers were conﬁrmed with high accuracy. For thosesensors meeting our speciﬁcations, Table 7 and Fig. 13
show the statistics obtained.
Depending on the manufacturer, humidity dependent
breakdown effects, which also related to the storage historyof the sensors, were sometimes observed [27]. With suitableprecautions and after appropriate screening, these issues
were not found to affect the performance of any of themodules assembled onto the disks.

4. ATLAS end-cap module components
4.2. Hybrid design, specifications, testing and results
4.1. Sensor designs, specifications, testing and results
The ATLAS micro-strip sensors are fabricated using pþ
implanted 20 mm wide strips in high resistivity
ð44 kO=cmÞ n substrate, 285 mm thick, but with a numberof features to ensure high voltage operation to cope withthe substrate effective doping changes following heavy
irradiation. The sensors were manufactured by Hamamatsu [25] and CiS [26]. There are ﬁve different types of
forward silicon sensors. They are referred to as W12, W21,
W22, W31, W32. W21 þ W22 are used together to makethe middle modules, and W31 þ W32 form a pair for theouter modules. W12 is used by itself for the inner modules.
They all have 768 read-out strips plus two edge strips and awedge-like geometry with a strip pitch varying with theoverall device width (see Table 6) while allowing always

The end-cap electronics hybrid provides the electricalinterface between the module and the disk services. The
basic design of the hybrid is six layers of kapton wrappedaround a carbon core. It carries, as shown in Fig. 14, 12
ABCD3TA readout ASICs, six on each side of the hybrid,and two ASICs, on the front side, for optical communication: DORIC4A (for TTC signals) and VDC (for datatransmission off the module). All of them are provided
with analogue and digital supply voltages. The hybrid also
implements a bypass scheme to cope with single ABCD3TA chip failures in the token ring. The sensors areconnected via the hybrid to the HV power supplies.
A direct contact of the CC substrate to the coolingsystem efﬁciently removes the heat dissipated from thereadout chips.

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366

Table 7
Statistics of the sensor characteristics, showing average values for the percentage of defective strips per sensor and measured currents at 150 and 350 V forthe different manufactures: Hamamatsu and CiS
Number built

W12
W21
W22
W31
W32

I ðmAÞ at 150 V

Defective strip/sensor

I ðmAÞ at 350 V

CiS

Ham.

All

CiS

Ham.

All

CiS

Ham.

All

CiS

Ham.

All

776
860
1114
373
363

692
757
810
2645
2596

1468
1617
1924
3018
2959

2.3
2.1
2.2
3.5
4.1

0.3
0.5
0.4
0.5
0.4

1.4
1.3
1.4
2.1
2.3

0.751
0.532
0.689
0.759
0.838

0.087
0.124
0.166
0.286
0.255

0.438
0.339
0.443
0.536
0.563

2.532
1.265
2.176
1.857
1.255

0.310
0.257
0.699
0.817
0.692

1.485
0.790
1.480
1.367
0.990

Fig. 13. Leakage current distribution of all the sensors when biased with(a) 150 and (b) 350 V.

4.2.1. Design
To achieve the required speciﬁcations on electricalstability, mechanical rigidity and thermal performance,
the hybrid was laid out as a ﬂexible printed circuit board

folded around and glued onto a unidirectional CC
substrate with high thermal conductivity. Details on thesubstrate properties are presented in Table 8.
In the area where the substrate is mounted onto thecooling block, the ﬂex circuit is cut out to ensure aclose thermal contact between the cooling system andthe hybrid substrate. To compensate for this cut-out
in the analogue ground plane the substrate in the
cut-out region is plated with a 22 mm copper layer in agalvanic process. This copper layer is soldered to theanalogue ground layer (layer six) at the backside of
the ﬂex circuit during the lamination of the ﬂex to thesubstrate, effectively forming one continuous analogue
ground layer.
It is vital to avoid any feedback from digital switching
into the analogue part of the ABCD3TA. This requiredvery stable supply voltages and led to a six layers design
with two solid planes for analogue ground and power. To
further improve the connection of analogue ground andpower between front- and back-side of the hybrid, there is
not only a connection across the wrap around, but also asolid solder connection between the tabs or ﬁngers at the
opposite side of the wrap around. Analogue and digital
power are decoupled on both sides of each chip andadditionally on the front side as well as on the backside to
improve the electrical stability of the ABCD3TAs at low
thresholds.
To improve the heat ﬂow away from the chips, the ﬂex
has small cutouts underneath each chip. These recesses are
ﬁlled with ceramic inlays made from aluminium nitride(AlN) with high thermal conductivity (150–180 W/m K).
The inlays are glued directly onto the CC substrate using a
cut-to-shape thermally conductive (3.7 W/m K) boron
nitride ﬁlled glue foil. These thermal plugs are staggeredso that the ABCD3TAs furthest from the cooling block
have their thermal plugs underneath the analogue part ofthe chip, which dissipates more heat than the digital
part. The other two chips share the same heat path withthermal plugs under the digital part of the chip. This
ensures that the outermost chips have their own heat pipealong the ﬁbres of the substrate, resulting in a more eventemperature of all chips. Simulations of the thermal
behaviour and a comparison to measured data can befound in Section 3.2.4.

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367

Fig. 14. (a) Front side of hybrid. (b) Back side of hybrid.

Table 8Properties of the carbon–carbon substrate
Thermal conductivity in ﬁbre direction
Thermal conductivity perpendicular to ﬁbre
direction
DensityYoung’s modulus in ﬁbre direction
Tensile strength in ﬁbre direction
CTE in ﬁbre direction
CTE perpendicular to ﬁbre direction

600–650 W/m K
20–30 W/m K
1.9 g/cm3
300 GPa
300 MPa(1:0–0:5) ppm/K(10–20) ppm/K

4.2.2. Hybrid construction
Electrically conductive silver loaded epoxy1 was chosen
to attach the ASICs to the ﬂex. In order to have a good
contact between the ASICs and the substrate the coverageof the thermal plugs with conductive glue had to be 100%,
while the bottom of the ASICs had to be covered with glue
on more than 80% of the die area. The ABCD3TAs had tobe placed relative to ﬁducial marks on either side of theirchip pad with a precision of 50 mm. The serial number ofthe ABCD3TA mounted at each of the 12 positions on the
hybrid was recorded and uploaded to the production
database (see Section 6.3).
1

Eotite P102.

For the wire connections from the chips’ pads to theﬂex circuits’ pads Al wedge–wedge bonding was chosen
since this process allows for the bond wire connections to
be made at room temperature. The wire thickness was
25 mm. The SMD and bond pads on the hybrid consistof copper, which is protected against oxidation by anickel layer and thin ﬂash of gold deposited in theelectroless Nickel Gold (ENIG) process. The gold layer
prevents the nickel surface from oxidising. The actual bondis made through the gold layer to the nickel surface.
Pull strength tests were carried out regularly on samples
and showed good adhesion of the bond wire. The averagepull strength during production was measured to bearound 8 g. After wire bonding, the hybrids were thermallycycled to probe for weak wire bonds, using 10 cycles
between 30 and þ50  C. The hybrids were then electrically tested as described in Section 4.2.4, and hybrids
passing these tests were delivered to the hybrid QA
centres for more thorough electrical tests as described in
Section 5.5.
4.2.3. Mechanical tests
Before gluing the prepared copper-plated substrate andbare ﬂex together, the thickness and the ﬂatness of thesubstrate were checked using a simple jig. Flex, glue foil
and substrate were aligned using precision pins in the

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Fig. 15. The gauge to test the opto-connector position.

lamination jig together with precision holes in the glue foil
and bare ﬂex. After reﬂow soldering the SMD components
to the bare laminated ﬂex, the correct positioning of the
opto-connector was checked using a space model ofthe opto-plugin, as shown in Fig. 15. The test was passed
if the gauge ﬁtted onto the connector and could be pushed
all the way down to the surface of the hybrid without
touching other connectors. The assembly was stopped incase of failure.
Next, the VCC and AGND tabs from each side, visibleat the top of Fig. 14a, were soldered and it was veriﬁed thatthey did not protrude beyond the ﬂex surface on either side.
After coating the HV parts on the hybrid withpolyurethane, it was subjected to bending measurementswith a measuring microscope to ensure that the bare hybrid
complied with the hybrid envelope. It was also veriﬁed thatthe thermal plugs did not protrude over the chip pad
surface to keep the hybrid inside the envelope and to avoid
any tilting of the ABCD3TAs after die bonding.
4.2.4. Electrical tests
The electrical integrity of the hybrid was checked forshorts and correct SMD mounting after the SMD reﬂow
soldering process. The solder joints between the connector
pins and the ﬂex surface were checked where possible. The
values of all resistors as well as the resistance between alllines routed to a connector pin were measured to ensurethat no trace in the wrap-around region had been broken
during the lamination of the ﬂex to the substrate. The HVline was probed for continuity with an ohmmeter from theconnector pin to the HV ﬁngers. The measurement wasdone manually since the connections to the pads on the two
ﬁngers could not be measured with the automaticconnector tester. The HV part of the hybrid had to passa HV stability test at an applied DC voltage of 500 V. The
leakage current was measured after 1 m, and was required

to be below 100 nA. Hybrids passing the connector test,
HV continuity and leakage current tests were transferred tothe bending test before assembling the ASICs. The chip
mounting (COB) and wire bonding were carried out inindustry as well.
In order to verify the functionality of all active components assembled onto the hybrid—ABCD3TA and optical
ASICs—a quick full electrical test was run in the company
using a pseudo-optical readout mode (see Section 5.5).
It mainly checked the redundancy mechanisms both for the
ABCD3TA and the optical chips as well as the datatransmission at different digital supply voltages.
Hybrids passing these tests were delivered to the threehybrid QA centres (see Section 6.1). There they were allvisually inspected and the analogue and the digital part of
the ABCD3TAs were thoroughly tested. The test sequence
used in hybrid QA includes a conﬁrmation sequence, along-term test at elevated temperature and the ﬁnal cold
characterisation. A more detailed description of the tests
performed during the characterisation sequence can befound in Section 5.5.
4.2.5. Hybrid production
All necessary assembly steps to build the ﬁnal hybrid
were developed at Freiburg University and then transferred to industrial partners. By implementing a fast
feedback between the production companies andFreiburg University it was possible to minimise lossesof limited material (especially the ABCD3TAs) and to
supply all module assembly sites with a sufﬁcient ﬂow ofhybrids to continuously work throughout the production
period. It was also of invaluable help in detecting any
quality variations or other problems during the mass
production.
One observed problem was delaminations between
the individual ﬂex circuit layers, which occurred without

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369

Fig. 16. Hybrid delivery rate.

a change in process parameters. The root cause of thedelaminations was high ambient humidity at the production site in summer. The problem was overcome byadding extra drying steps in the processing to driveout any residual humidity. These drying steps, however,made the whole circuit more rigid, which meant thatsome cracks were observed in the wrap-around region. Thisin turn was cured by bending the circuit in that region
over a ﬁxed radius and inserting glue to stabilise thewrap-around.
Another problem was reduced bondability on a few
particularly exposed gold bond pads, which occurred again
whilst process parameters remained unchanged. This
problem was eventually traced to the slow degradation ofgalvanic baths in the ENIG process, and was overcome
once the bath was replaced.
2587 fully assembled hybrids were produced in industry.
Of these, 2489 were delivered to the module assembly sites
and 32 were declared lost due to massive electricalproblems. In total 296 hybrids required the replacementof at least one ABCD3TA and 53 hybrids were returned
twice for rework. Eleven hybrids were sent to be reworked
three or more times. Fig. 16 shows the hybrid delivery rateover the production period.
4.3. Fan-in design, testing and results
Each of the sides of the SCT end-cap module has 768
sensor channels that are connected to six ABCD3TA chips
for the signal readout. The separation of the ASICs on the
hybrid, together with the different pitch of the various
sensor types and the stereo-angle between the two sensor
planes make impossible the direct and automatic wirebonding of the sensors to the readout chips and force theuse of pitch adaptors or fan-ins for this function.

Fig. 17. (a) A fan-in showing the traces from the ASICs, bottom, to thesensors, top. (b) Fan-in glued to the hybrid and the spine spacers.

The purpose of the fan-ins is, therefore, the electricalconnection of every channel from the sensors to thereadout chips (see Fig. 17), adapting the different pad pitchand conﬁguration. They also contribute to the mechanicalsupport between the hybrid and the sensors, and maintainan effective barrier to heat ﬂow between these parts.
4.3.1. Description
A set of four fan-ins is used for each module. Each fan-in
connects three readout chips with half the channels of eachside of the module resulting in 128  3 ¼ 384 metal tracks

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370

Fig. 18. Fan-in dimensions.

Table 9Fan-in dimensions in mm

LEFT
RIGHT

Table 10
Characteristics of the materials used in the fan-in

A

B

AþB

C

D

E

DþE

Component

Material

Thickness (mm)

28.95
28.88

7.65
10.12

36.60
39.00

10.19
8.67

2.20
1.40

7.25
8.05

9.45
9.45

Glass substrate
Metal tracks

Type DESAG 263T
Sputter deposition
Al (99.5%) Cu (0.5%)
Positive Photoresist HiPR6512(Arch Chemical Inc.)

300  20

Passivationin each fan-in. There are three different types of fan-ins,
one for each type of module: outer, middle, and inner, andfor each of them, there are two kinds: left and right, foreach half of the sensor.
Mechanical dimensions are speciﬁed in Fig. 18 andTable 9. All dimensions refer to cut pieces, with a toleranceof 50 mm. All types of LEFT fan-ins have the samemechanical dimensions, and so do the RIGHT ones.
The fan-ins are made of high density metal tracks
deposited on top of an isolating glass substrate. A passivation
layer covers the tracks for both mechanical and chemical
protection.
The metal is made of an alloy of aluminium (99.5%) andcopper (0.5%), which is a standard alloy used to increase
the electro-migration hardness. The metal is deposited by
sputtering from a high purity target. This metal layer islater etched using a standard photolithographic process todeﬁne the tracks and bonding pads. The metal lines areprotected by a passivation layer with openings at the pads
for the wire bonding. This layer is made of a standard
positive photolithographic resist which is spin deposited on
the substrates. The characteristics of the different materials
used in the fan-in fabrication are speciﬁed in Table 10.
A detailed description of the technology used for the
fabrication of the fan-ins can be found in Ref. [28].
4.3.2. Quality assurance
Irradiation tests with neutrons, protons, and X-rays havebeen made on the fan-ins to ensure that they do not suffer
signiﬁcant degradation. Mechanical and electrical quality
tests have been carried out after irradiation ensuring goodradiation hardness of the technology. Track resistance,inter-strip conductivity and inter-strip capacitance havebeen measured after irradiation. No changes have been
observed in any of these electrical parameters from theirpre-irradiation values.
Nano-indentation tests have also been carried out on thepassivation layer in order to assure that there is nodegradation in its mechanical parameters. Universal
Hardness (H) and Elastic Modulus (E) have been obtained
for this layer before and after irradiation, demonstrating

1:00  0:25
2:00  0:25

Table 11
Results on radiation hardness tests
SampleNumber ofindentations

Not irradiated 9#1
Not irradiated 10#2
Not irradiated 10#3
Neutrons irrad. 20
Protons irrad. 20

H
(GPa)

sH
(rms)

E
(GPa)

sE
(rms)

0.540

0.012

8.31

0.53

0.536

0.025

8.38

0.50

0.539

0.013

8.55

0.42

0.535
0.534

0.015
0.014

8.32
8.46

0.35
0.59

that there is no degradation in its mechanical properties.
Table 11 shows the results of these measurements. It can beseen that there are no appreciable differences in the resist
layer mechanical parameters before and after irradiation.
It is very important to ensure an excellent bondability of
the fan-in bonding pads. In order to guarantee thischaracteristic pull tests have been carried out on sample
pieces from the fabricated batches. The pull tests have been
performed using a DAGE Pull Tester model 4000 with anascending speed of 700 mm=s. An average bond force of
12 g has been obtained. The measured bond force wasalways well above the speciﬁed minimum pull force of 6 g
for the ATLAS-SCT modules, always having wire breaks
before the bond became loose.
Tape peel tests have been also performed ensuring agood adherence of both the metal traces and the passivation layer to the substrate.
Electrical tests have been performed on fabricated fanins in order to ensure good metal conductivity, good
isolation between tracks, and low inter-strip capacitance.
Resistance of the order of 30 O for 1 cm long, 16 mm wide
tracks are obtained, with bigger than 1000 MO resistance
between tracks separated by 15 mm. Inter-strip capacitances

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are of the order of 0.8 pF for those geometries and are
constant for a wide frequency range. Metal sheet resistancehas been also routinely measured through speciﬁcallydesigned test structures obtaining values of 0:04 O=sq.
All fan-ins were tested for defects before being supplied.
A thorough visual inspection was performed in order toassure continuity in the lines and absence of short-circuits.
The fan-ins supplied to the assembly centres had no
more than one shorted pair of neighbour strips or one
broken strip.
4.3.3. Production
The production of the fan-ins was carried out, after aselection process, in the Centro Nacional de Microelectrónica (CNM-CSIC), Barcelona, Spain. The fabrication
started early in September 2002 in order to assure
the availability of components for the assembly sites
qualiﬁcation, and to allow a thorough evaluation by thecollaboration.
Fig. 19 shows the production progress during the whole
period including shipments and accumulated stock. The
production extended over 32 months, 1639 glasses were
processed resulting in 2642 fan-in sets shipped to themodule production sites, together with more than 600
dummy fan-in sets.
The ﬁnal average yield was 70% during the production
selection, and a further 3% of the fan-ins were rejected by
the assembly sites. An average production rate of 103 sets/month was achieved, reaching a rate of 120 sets/month in
the last period.

considerations. This structure is known as a spine and isshown in Fig. 20. To minimise the overall material within amodule the spine must have the lowest possible mass, it
must be mechanically rigid, and provide the interface forthe module cooling contacts. The TPG backbone of thespine transports the heat from the sensors, which produce
up to 2 W after the expected radiation dose from 10 years
of ATLAS operation, to the module mounting/cooling
blocks at each end which are held at around 15  C. The
thermal performance of the TPG is vital to keep the sensors
cold for the lifetime of the experiment. The thermal
performance of the spine has been simulated using ﬁnite
element calculations [18] and compared with experimental
data [22].
To achieve precise tracking in the SCT the position andorientation of the individual sensors relative to the module
mounting point have to be known with high accuracy andwithin a tight envelope, described in Section 3. The size andshape of the module in the perpendicular direction to thesensor plane is strongly dependent on the size and shape ofthe spine, most notably on any bow in the TPG, therefore

4.4. Spines design, testing and results
The silicon sensors are glued back to back onto a
support structure built from thermalised pyrolytic graphite(TPG) [29–31], with aluminium nitride (AlN), and aluminium oxide ðAl2 O3 Þ, ceramic parts required for mechanical371

Fig. 20. Photograph of an end-cap module spine.

Fig. 19. Fan-in delivery rate.

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the TPG must be kept within a tight mechanical tolerance
during production.
4.4.1. Spine design
The design of the spine is detailed in Ref. [32]. The basic
structure is the 500  25 mm thick TPG heat spreader with
the AlN ceramic supports. The TPG functions as thethermal path between the sensors and cooling contact ateach end. In order to achieve electrical insulation andmechanical protection the TPG is coated with a 10 mm
layer of Parylene-C. Parylene-C was chosen because it has
a low tendency to penetrate into the TPG layers during
deposition. Such an effect was observed with Parylene-N
which leads to an increase of the TPG thickness at the cut
edges. Since TPG is fragile and tends to delaminate thestructure of the spine has to be reinforced using AlN
ceramic sections. AlN has a rather high thermal conductivity, of 180 W/mK at 20  C, and a similar thermal
expansion coefﬁcient to silicon, thus reducing mechanicalstress during any temperature changes. AlN plates ofthickness 225  25 mm cover the TPG at the cooling
contacts to ensure good thermal contact to the coolingpoints while protecting the soft TPG from mechanicaldamage. To maintain a spine of constant thickness the
TPG is proﬁled at each end where the ceramic sections are
glued; the glue is used to correct for any thickness
variation. The hybrid end ceramic has a v-groove to enablemechanical assembly of the ﬁnal module. The far-end
cooling point has an oval slot to enable the sliding joint.
The wings supporting the sensors and providing mechanical stiffness to the sensor-spine assembly have a thicknessof 500  25 mm. The AlN pieces at the hybrid end and thecentral wings contain metal traces to distribute the sensor
bias voltage. The bias is supplied from the hybrid to one
side of the spine and therefore one of the AlN wings has ametallised through hole to electrically connect both sides ofthe spine.
Attached to the AlN wing at the hybrid end there is an
Al2 O3 spacer required to match the thickness of the spine
to the hybrid for assembly. Al2 O3 was chosen as it has a
low thermal conductivity so as to maintain the thermal
break between the hybrid and the sensors. Attached to the
far-end cooling point is a precision washer, constructedfrom FR4 and 0.5 mm of aluminium, which deﬁnes theprecise oval slot. The distance between the v-groove andthe centre of the far-end washer’s slot deﬁnes the spine
length and ultimately the length of the module. The spine
has been designed for ease of construction as well as tosupply the required high thermal functionality.
4.4.2. Materials used in the spine
The TPG used in the spine is an anisotropic material
having both mechanical and thermal properties that, due tothe planar mosaic ordering of the carbon structures, arebasically constant within the plane of a substrate sheet
and are signiﬁcantly different in the orthogonal direction.
Table 12 summarises the properties of the TPG, AlN and
Al2 O3 used in the construction of the spine. The ceramic
data are from CERAMTEC [33], unless noted otherwise.
The TPG was glued to the thin AlN sheets at the cooling
block connections with ELASTOSIL 137-182 which waschosen as it has a thermal conductivity measured to be
1:79  0:1 W=m K. Also, pull strengths exhibit no deterioration after an irradiation up to a ﬂuence 2:7  1015
24 GeV=c protons=cm2 .
4.4.3. Construction process
The TPG for the spine was produced at NIIGraphite[34]. The TPG was delivered roughly cut into plates of
145  25  0:7 mm3 for outer and middle modules and80  25  0:7 mm3 for inner modules. These plates weremechanically lapped and polished into the required
thickness at NIITAP [35]. The TPG was cut to shape withlaser ablation, the end proﬁles of the TPG were thenfabricated via further grinding and polishing. The TPG washeated to 200  C for 30 min and kept for 45 min undervacuum to avoid later out-gassing. Finally the TPG wascoated with a 10 mm layer of Parylene-C. The bias contact
hole in the Parylene-C was made using plasma etching.
During the processing of the TPG, rolling was used tocorrect any mechanical deformation. The AlN ceramicparts were laser cut and proﬁled at NIITAP. Conductive
Al layers, which build up at the cut edge during
cutting, were removed using NaOH. Afterwards the pieces
were cleaned with distilled water. The metal traces were
fabricated using vacuum evaporation of Ti–Cu–Ni. The
total thickness of the three layers is 2 mm. The electricalresistivity was measured and kept lower than 20 O between

Table 12
Properties of materials used in the spine
PropertyTPG

AlN

Al2 O3In plane thermal conductivity at 20  C (W/mK)
Out of plane thermal conductivity at 20  C (W/mK)
Density ðg=cm3 Þ
Transverse pull strength ðN=cm2 Þ
Plane electrical conductivity ðO1 cm1 Þ
Thermal expansion coefﬁcient at 20  C ð C1 Þ
Thermal expansion coefﬁcient at 20  C ð C1 Þ

1550  130
8:9  0:4
2:15  0:2
56:5  14:2
1:63  104
1:17  0:15  106
26:8  0:4  106

180
n/a
3.3

20
n/a
3.75

103 –105
3:1  106n/a

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any two points of a trace. The thorough contact between
the bias line and the backside metallisation of thecontact hole was also made using Ti–Cu–Ni. The Al2 O3
spacers were cut and metallised at NIITAP using the same
techniques.
The components were shipped to IHEP [36] for assembly
and QA. After assembly the spines were visually inspected
and the thickness of each was measured at 25 pre-deﬁned
points. The ﬂatness was measured and spines which were
bowed by more than 1 mm were rolled ﬂat or rejected if
the bow could not be rectiﬁed. The electrical continuityof each spine was checked by measurement between thecontact pads on the AlN facings and the HV openings on
the TPG area.
The assembled spines were shipped to CERN where avisual reception test was performed before attaching the
far-end washer to the spine with the use of special jigs.
A ﬁnal QA test was made including the measurement of the
far-end spacer position with respect to the spine’s v-groove,the measurement of the Al2 O3 spacer position with respect
to the v-groove and electrical conduction measurements ofthe traces on the AlN ceramic sections. The spines were
ﬁnally shipped by the ATLAS SCT collaboration to themodule production sites. Any spines rejected by, or
damaged at, the module production sites were returned
to CERN for repair.
4.4.4. Production
The total number of spines shipped to CERN from thestart of 2003 until April 2005 was 2650. This represents an
extra 12% of spines over the number of modules that were
to be made during production. The ﬁnal spine assembly
and QA were performed at CERN and the spines were thensent onto the module production sites. The total number ofspines shipped from CERN was 2505 which consisted of
the spine types; outer: 1250, middle: 831, inner: 569. Fig. 21
shows the cumulative totals. The number of spines required373

at the module production site was in excess of the numberof modules produced by the sites due to the yield in spines,
and more importantly due to the fact that dummy and preproduction modules were also constructed by the module
sites. A spine on arrival at the module production site isinspected and either accepted, marked as defective andreturned to CERN for repair, or considered beyond repair
and marked as trashed in the production database. In total
219 (8.7% of) spines were returned to CERN where 199 ofthem were successfully repaired and re-sent to production
sites. The total number of trashed spines was 135 (5%).
4.5. Module descriptions in ATLAS simulation
4.5.1. Radiation length
The radiation length of SCT end-cap modules has been
calculated based on the knowledge of their composition
and measurements of their mass [37]. The masses ofsamples of the module components were consistent withexpectations from design except the hybrid, which was
0.28 g heavier. This was traced to the copper layers in theﬂex circuit being on average about 17 mm thick, comparedwith the design value of 15 mm. With this correction taken
into account, the individual component masses and themasses of production modules were consistent withexpectation. The ﬁnal radiation lengths of average production modules, estimated to be accurate to better than 1%,
are shown in Table 13 where it can be seen that for theouter and middle modules about 44% of it corresponds tosilicon, while this fraction is 26% in the case of the innermodules. Radiation lengths in Table 13 are normalised tothe sensor areas of the modules in order to show theaverage impact of a module at normal incidence.
4.5.2. Simulation
In order to simulate reliably the tracking performance of
the SCT (including material effects), the modules must be
properly deﬁned in the geometry used by the simulation.
The geometry and material descriptions of the SCT endcap modules were prepared in the Athena framework for

Table 13
Composition of modules, expressed as percentage of a radiation length at
normal incidence
Module typeFig. 21. Total number of spines shipped from CERN, detailed by typeand as the total.

Normalisation area ðmm2 ÞHybrid substrate
Hybrid ﬂex circuit
Other hybrid components
Spine and location washers
Fan-ins and wire bonds
Sensors
Assembly glue
Total average radiation length (%)

Inner

Middle

Outer

3090
0.149
0.595
0.477
0.388
0.099
0.609
0.027
2.345

7803
0.059
0.236
0.189
0.226
0.038
0.609
0.019
1.375

7893
0.058
0.233
0.187
0.243
0.039
0.609
0.020
1.388

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simulation in the standard ATLAS software environment[38]. In this description, the module consists of the siliconsensors, the support structure, and the electronics hybrid
populated with passive components and readout chips.
A compromise between a full detailed geometrical description and a simple average mass of materials was developed,with the goal of ensuring a correct representation of themodule radiation length without excessive computing time.

Fig. 22. Layout of the SCT end-cap modules in the Athena framework:(a) inner, (b) short middle, (c) middle and (d) outer.

4.5.3. Description of the geometry
All four different types of modules are described in the
Athena framework (Fig. 22) and converted to a Geant4[39] readable format for standard simulations of propagation of particles in matter. All parameters describing
modules are divided into two groups: common parameters
for all types and parameters speciﬁc for each type.
Furthermore, all speciﬁc parameters are sorted according
to module components which they describe. Finally theparameters were written to the Detector Description
database.
The deﬁnitions of the various module materials wereconstrained by the requirement to minimise number andcomplexity of components in order to reduce the simulation computational load.
To validate the material in the model, a distribution plotof energy losses was produced by ﬁring parallel muon beamthrough a simulated set-up composed of only one outermodule inside a box ﬁlled by air. Two projections of theaverage muon energy loss versus position are shown in
Fig. 23. The theoretical predictions of the energy loss areshown by the straight lines. The measured values from thesimulation correspond very well with these.
5. ATLAS end-cap module assembly and testingFig. 23. Average energy loss in the module. The ﬁgure at the top (a) shows
the y projection, while the ﬁgure at the bottom (b) shows the z projection.
The histogram is the output of a Geant4 simulation and the straight lines
are the theoretical prediction.

This section describes the detailed module speciﬁcations,outlines the assembly and QA steps taken to ensure
modules meet these requirements and discusses the results
for all the modules produced.
On average, the whole process of assembling and testinga module took about 100 h of which 70 h were fully
dedicated to testing the various aspects of the module. An
average of 20 man-hours were required to produce one
module. Three different sets of tests could be identiﬁed:
reception of module components, tests made duringassembly and, ﬁnally, the tests made on the module as awhole. All of them are fully described in Ref. [40] and will
be outlined in this section.
Components arrived at module assembly sites having all
passed their own QA so only reception tests, aimed atcatching any damage that could have occurred in transit
from the QA site to the assembly site, were needed
consisting, mainly, in visual inspection and conﬁrmationthat their main properties were still within the speciﬁcations. In particular, for silicon sensors an I–V characteristic
curve was measured. For spines the curvature along theARTICLE IN PRESS
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TPG, and for hybrids the digital circuitry of the ASICs as
well as the gain, noise and dead channels were measured.
During the assembly phase of the module, and before
gluing the hybrid to the sensor-spine assembly, an I–V
curve of each sensor was measured up to 500 V. At the endof this phase, and prior to the bonding process, all themetrology parameters were measured during the ﬁrst
period of the production in order to trace any mechanicaldeformation that could occur during the thermal cycle ofthe module.
Fully assembled modules were subject to thoroughtesting which included: Thermal cycling: the module was cycled 10 times
between 35 and 35  C with ramp up/down times not
smaller than 30 min and soak time of also 30 min.
 Full metrology survey.
 I–V curve.
 Long-term test and electrical and leakage current
stability: the modules were kept in a controlledenvironment and were clocked and conﬁgured during
18 h with the sensors biased at 150 V. Every 2 h aminimal performance test was carried out for the gain,
noise and dead channel determination as well as tocheck the conﬁgurability of the ASICs.
 Full electrical characterisation: in these tests all theanalogue and digital features of the module were tested.

375

sensors, determined by surveying ﬁducial marks on thesensor corners and forming the centre-of-gravity of thefour results. The module coordinate system ðX m ; Y m Þ isdeﬁned by the centre-of-gravity of the four sensor centres.
The direction of the X m axis is chosen so that it bisects theangle between and line joining C1 to C2 and a line joining
C3 to C4. The ﬁrst parameter, stereo, deﬁnes the angle
between X m and the line joining C1 to C2. The next pair ofparameters, (midxf, midyf), specify the mid-point of thefront pair of sensor centres (the mid-point of the back pairis at (midxf ,midyf ) by construction). sepf, speciﬁes theseparation between the front pair of sensors, and sepb thatof the back pair. The angular deviation of each sensor
centre line from the line joining it to its partner give thefour parameters a1–a4. The remaining four parametersdescribe the position of the main location hole (mhx, mhy)
and the far-end location slot (msx, msy). Inner and shortmiddle modules, having only two sensors, use a sub-set ofthese parameters with slightly different deﬁnitions asshown in Fig. 25.
Table 14 shows the nominal values of these parameters
for all module types and the assembly tolerances that areallowed on each parameter. The tolerances on stereo, midyf
and a1 to a4 are chosen to just satisfy the 4 mm rf
speciﬁcation. The tolerances on msy and mhy are chosen as

5.1. Mechanical assembly specifications
The module assembly speciﬁcations are derived from
Table 1, where an r.m.s. accuracy of 67 mm is required in
the z direction and 4 mm in the rf direction.
The thin ﬂat shape of modules with tight z tolerance
allows to decouple the z (out-of-plane) speciﬁcation fromthe xy (in-plane) speciﬁcation.
Fig. 24 shows the geometry description of a module in
the xy plane. Points C1 to C4 are the centres of the fourFig. 25. Nine parameters specify the geometry of an inner or short-middle
module in the xy plane.

Fig. 24. Thirteen parameters specify the geometry of an outer or middle module in the xy plane. The four spots (C1–C4) represent the sensor centres anddash lines the sensor orientations.

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376

Table 14
XY parameter values and tolerances
Parameter (unit)

mhx (mm)mhy (mm)msx (mm)msy (mm)midxf (mm)midyf (mm)sepf, sepb (mm)a1–a4 (mrad)stereo (mrad)

Tolerance

0.020
0.020
0.100
0.020
0.010
0.005
0.010
0.130
0.130

Table 15
Z tolerances

Nominal values
Outer

MiddleShort-midInner78.1360.000
62.2440.000
0.000
0.04061.6680.000
20.000

71.7080.000
66.6720.000
0.000
0.05359.9000.000
20.000

41.7640.000
96.6160.000
0.000
0.652––
20.000

45.0600.000
34.320
0.000
0.000
0.000
–
–
20.000

ObjectTolerance (mm)Front face

Sensor surface
End chips
Fan-in near end chips
Fan-in near middle

0:875  0:115
o1:893o1:751o1:751

Back face

Sensor surface
End chips
Fan-in near end chips
Fan-in near middle

0:375  0:115
4  1:249
4  1:057
4  0:857

occupancy level, the tracking efﬁciency, the timing and thepower consumption. The most important requirements the
SCT module needs to fulﬁl are discussed below.

Fig. 26. z survey points on an outer module.

small as can reasonably be achieved, to minimise the loss ofoverlap between adjacent modules in the rf direction.
The z tolerance is derived directly from the speciﬁcationof 67 mm r.m.s. deviation from the nominal value.
Assuming that the distribution of z values is ﬂat within
the allowed tolerance band, leads to a choice of 115 mm
for the tolerance. z is measured perpendicular from thesurface on which the module is mounted, with positive z
being towards the front face of the module.
Fig. 26 shows the points at which the z value of thesensor surface is measured. The open squares are at thesensor corner ﬁducials and the ﬁlled squares are on a 5  5array uniformly interpolated between the open squares. All
50 measurements (25 for short modules) on each side of themodule must be within the z tolerances deﬁned in Table 15.
In addition to the z tolerances on the sensor position
derived from physics, a number of other constraints stemfrom clearance to adjacent modules, services and thecooling block. These are shown by the circles in Fig. 26 andtheir tolerances are given in Table 15.

5.2.1. Noise performance
The total effective noise of the modules is the result of
the combination of several factors: the front-end electronics noise, the gain spread and the offset spread. The
former is the equivalent noise charge (ENC) for the frontend system including the silicon strip detector. It is
speciﬁed to be less than 1500e ENC before irradiationand 1800e ENC after the speciﬁed dose of 2:14 1014 neq =cm2 which includes a 50% safety factor on topof the anticipated dose. Assuming the non-ionising energy
loss hypothesis, this is very roughly equivalent to 3 1014 protons=cm2 at the CERN PS. The other two factors
affect the channel-to-channel threshold matching which, inturn, inﬂuences the ﬁnal noise occupancy. The noise hit
occupancy (NO) needs to be signiﬁcantly less than the real
hit occupancy to ensure that it does not affect the datatransmission rate, the pattern recognition and the trackreconstruction. The foreseen limit of NOo5  104 per
strip requires the discrimination level in the front-end
electronics to be set to 3.3 times the equivalent noise
charge. To achieve this condition with an operating
threshold of 1 fC, the total equivalent noise charge should
never be greater than 1900e ENC, including the electronics noise as well as the offset and gain spread in the chips.
If the noise is higher, the operating threshold could be
increased provided that it does not compromise the
efﬁciency.
5.2.2. Tracking efficiency
The tracking performance of a particle detector depends
on the intrinsic precision and efﬁciency of the detector
elements. In this respect we specify less than 16 deadreadout channels for each module to ensure at least 99% ofchannels working.

5.2. Electrical performance specifications
The LHC operating conditions demand challenging
electrical performance speciﬁcations for the SCT modules
and the limitations [41] mainly concern the accepted noise

5.2.3. Timing requirements
For a correct track reconstruction, every hit has to be
associated to a speciﬁc bunch crossing. For ATLAS
operation, the fraction of output signals shifted to the

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wrong beam crossing is required to be less than 1%. Thisrequires a time-walk of less than 16 ns, where the time-walkis deﬁned as the maximum time variation in the crossing of
the discriminator threshold at 1 fC over a signal range of
1.25–10 fC.
5.3. Assembly procedures
The collaboration approach has been to build modules
to meet a well-deﬁned QA speciﬁcation, rather than to
specify in detail the procedures and jigs that the module
assembly sites should use. This has allowed pre-existing
equipment to be used, as long as each site has been able todemonstrate a consecutive run of at least ﬁve modules
produced to all the required speciﬁcations and yields.
However, the early exchange of experience when setting
up—mostly with dummy components—and of tacklingcommon problems has led to almost the same module
assembly procedure, with just two main variants. Similarly,
the production sites have converged on two designs of jig
sets, which have a lot in common [42] so no distinction will
be made between them in this section.
The ﬁrst step is to align the pairs of sensors that will
make up the front and back faces of the module. A pair ofsensors is placed on a pair of vacuum chucks, each of which
is mounted on a compact xyy stage integrated with ameasuring microscope. The sensor positions were measured using ﬁducial marks in the aluminium layer, near thesensor corners. Recognition and measurement of ﬁducials
and movement of the stages and microscope were
controlled by software, being able to place a pair ofsensors at the required position within 1 mm after a small
number of iterations, taking a few minutes.
After alignment, a pair of sensors is moved to anothervacuum chuck which will transfer them to their ﬁnal
positions on the module. The move is done by placing
the transfer chuck so that it rests on or just above
the pair of sensors, then switching the vacuum off at
the alignment chucks and on at the transfer chuck.
Viewing holes above the sensor ﬁducials in the transfer
chuck allow measurement of the sensor positions after thevacuum switch; if the sensors are now misaligned by morethan 2 mm the alignment and transfer steps have to berepeated, but this is not usually necessary. The xy positionof the transfer chuck in this and later assembly steps
is deﬁned by plain contact bearings of hardened steelpressed together with a spring, giving reproducibility better
than 1 mm.
Glue2 is then distributed onto both sides of the spine,
either in the form of a few lines or an array of dots
using a simple xyz robot and a volumetric or pressuretime dispenser. Either way, the aim is to achieve maximum
coverage of glue between sensors and spine, while
avoiding an excess that can spill out onto the front faceof the sensors. A number of small spots of electrically2

Araldite 2011.

377

conducting glue3 are added to distribute the sensor bias
voltage from a trace on one wing of the spine to the backplanes of all four sensors.
With the spine in a frame, the two pairs of sensors ontransfer chucks are sandwiched around it and left clamped
there overnight for the glue to cure. Part of the frame isshaped like the cooling blocks that will eventually support
the module. The spine is clamped onto these blocks,
deﬁning its position in the z direction and ensuring that it
will make good contact with the real blocks. The xy
position of the spine within the frame is controlled at the
hybrid end by the V shaped groove pressed against a
3.2 mm pin and at the other end by the slot washer ﬁtting
over a 2 mm precision pin. The position of the transfer
chucks in the z direction is set such that glue layer will be
90 mm thick if the spine and sensors have their nominal
thickness. Component tolerances mean that the actual glue
thickness can vary by 50 mm.
At this point in one variant of the assembly procedurethe spine-plus-sensors assembly was tested for alignmentand sensor leakage current. If it failed either test themodule was aborted, to avoid wasting a hybrid. The othervariant was to run the two gluing steps together and have
only one curing step, giving a higher rate of production
from each jig set but wasting a few percent of hybrids that
could have been saved by the intermediate test.
The ﬁnal step is to glue on the hybrid, fan-ins and main
location washer. The hybrid is clamped against the cooling
block template, to ensure that it will be coplanar with thematching surface of the spine. Its xy location is deﬁned bya U shaped notch ﬁtting against the same pin as the V
tooth of the spine. A spot of conducting glue makes thesensor bias contact from a tab on the hybrid to a trace on
the spine. The fan-ins are held on vacuum chucks, similar
to the transfer chuck but with less precision. The module isthen removed from its jigs and placed in a transport frame
for testing, wire-bonding and thermal cycling. The ﬁrst
thermal cycle could be considered as the last part of theassembly procedure because it is speciﬁed to go up to 40  C
and stay there for 30 min to post-cure the Araldite.
5.4. Mechanical assembly and testing
During the various assembly steps the modules movefrom one working place to a test bench or storage place forqueueing into the pipe line. The internal organisation ofthis was left to the individual institutes which put the basic
information for every module onto a traveller documentthat was signed off at each stage. Important information
like identity of components and details of wire bonding andtests were put into the SCT production database. The main
tests performed during the assembly were: Visual inspection after each assembly step to check forany damage, like scratches in the silicon sensor surface
3

Tra-duct.

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or fan-ins, cracks in the ceramic cooling contact area,
deformed wirebons or glue overﬂow on the edge of thesilicon sensors or the fan-ins towards the bonding pads.
 I–V measurements of the individual sensors after
assembly and then of the whole module after wirebonding. The main reasons for an excess of current weremechanical stress induced by the sensor gluing, charge
trapping on the surface or conductive debris as small asa few tens of micrometers.
 XYZ-metrology survey after thermal cycling. This wasdone using different methods in the collaboration,
resulting in the measurement of the parameters described in Section 5.1 together with the determination of
the module envelope.
5.5. Electrical test equipment and procedure
The tests described in this section, aim to verify thehybrid and sensor functionality after the module assemblyand characterise the electrical performance of the completed module. The standard data acquisition system used
to perform the electrical tests of the modules is also
described.
5.5.1. Data acquisition system (DAQ)
A system was developed, based on VME modules, whichreads out up to six modules and up to 12 hybrids using theoptical ASICs electrically while also testing the functionality of the module redundancy links. This system contains
the following VME modules:

 CLOAC (CLOck And Control) [43]: CLOAC generates
the clock, fast trigger and reset commands for the SCT
modules in the absence of the timing, trigger and control
system.
 SLOG (SLOw command Generator) [44]: SLOG generates slow commands for the control and conﬁgurationof SCT front-end chips for up to six modules. It fans out
clock and fast commands from an external source(CLOAC). Alternatively an internal clock may be
selected, allowing SLOG to generate clock and commands in stand-alone mode. When the SLOG runs instand-alone mode, the CLOAC is not used in the set-up.
 AERO (ATLAS End-cap Read-Out) [45]: One AERO
card provides an electrical interface for up to six end-cap
modules. Data communication to and from the modules
is via their onboard optical ASICs—DORIC4A and
VDC (data receiver and transmitter, respectively).
AERO encodes the module Clock and Commandsignals onto a single bi-phase mark (BPM) carrier signal
for transmission to DORIC4A. The two module data
links are transmitted back to AERO via the VDC andthen routed to MuSTARD. Conﬁguration of thechannel allows the module to be read out using either
the primary (optical) or redundant data routes.
 MuSTARD (Multichannel Semiconductor Tracker
ABCD Readout Device) [46]: MuSTARD receives,
stores and decodes the data from multiple SCT modules.
Up to 12 data streams (six modules) can be read out
from one MuSTARD card.
 SCTHV [47]: A prototype high voltage unit providing
sensor bias to four SCT modules.
 SCTLV [48]: A custom-designed low voltage power
supply for two SCT modules.

A module patch card (see Fig. 27) is also required tointerface a single AERO channel to a module. The
connections to AERO are made via two standard ethernet
category 5(E) screened cables, allowing separation of the
primary and redundant data routes onto the individual
balanced cables. Using screened twisted pair cables anddifferential signals results in a system with low electromagnetic interference and good immunity to external noise.

Fig. 27. Photograph of a patch card linked to an end-cap module.

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379

Fig. 28. Module readout modes.

Fig. 29. Schematic diagram of the SCTDAQ system.

The AERO card provides three readout modes, sketchedin Fig. 28, for testing a module or hybrid, as follows: Redundancy mode: The module is conﬁgured via the
Redundant CLK1/COM1 provided from SLOG and themodule data is read out via the Master ABCD3TAs.

 Primary mode: The module is conﬁgured and read out
via the Opto Chips DORIC4A and VDC.

 VDC Bypass test mode: The module is conﬁgured via
the DORIC4A outputs CLK/COM and the module
data is read out via the Master ABCD3TAs. This
scheme is used so that the VDC is bypassed for datatransmission from the module whilst retaining the
DORIC4A for module conﬁguration.

The SCTDAQ [49] software package has been developed
for testing both the bare hybrids and the modules using the
VME units described above. SCTDAQ consists of a C þ þdynamically linked library and a set of ROOT [50] macros
which analyse the raw data obtained in each test andstore the results in a database. A schematic diagram of
SCTDAQ is shown in Fig. 29.

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5.5.2. Electrical tests
After the thermal cycling, the module was placed inside alight-tight aluminium test box where it was supported at
the two cooling blocks of the spine. The test box provided
dry-air ﬂow and cooling through a channel connected to an
adjustable liquid coolant system (Fig. 30). Up to six
modules in their test boxes could be placed inside acontrolled environment and tested simultaneously.
Every module was electrically characterised with atemperature of 10  5  C on the hybrid, as measured byan integrated thermistor. Front-end parameters such as
gain, noise and channel-to-channel threshold spread were
determined by using the internal calibration circuit of the
ABCD3TA chips to inject charge of adjustable amplitudein the preampliﬁer of each channel. The characterisation
sequence [51] included the following steps:
Fig. 30. Photo of an outer end-cap module supported by its transport
frame inside its aluminium test box. The dry-air and coolant inlets arevisible, as well as the support card.

 Digital tests were executed to identify chip or hybrid
malfunction. These included tests of the redundancy
links, the chip by-pass functionality and the 128-cell
pipeline circuit.

Fig. 31. Typical set of plots obtained with the Response Curve procedure before trimming for one data steam, corresponding to six chips (768 channels).
From top to the bottom the vt50 value, the gain, the offset and the input noise are shown for each channel. The empty channels are the small number ofdead channels.

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381

 The optimisation of the delay between calibration signal

 A threshold scan without any charge injection wasand clock (strobe delay) was performed on a chip-tochip basis.
 The gain and offset are deduced from the correlationof the output voltage, in mV, versus the injected charge
in fC. The input noise is then given by dividing theoutput noise by the measured gain. The voltage output
and the output noise were measured by analysing
S-curves obtained through repeated threshold scans
performed for 10 different values of injected charge,
ranging from 0.5 to 8 fC (see Fig. 31). For each injectedcharge the threshold was scanned and the occupancy
was ﬁtted with a complementary error function. The
output voltage corresponds to the threshold giving an
occupancy of 50%, the vt50 parameter, and is given by
the mean value of the error function. The output noisecorresponds to the sigma of that error function.
 To minimise the impact of the threshold non-uniformity
across the channels on the noise occupancy, the ABCD3TA allows to adjust the discriminator offset using a
digital-to-analogue converter (Trim DAC) per channel
with four selectable ranges (common for each chip). This
trimming procedure is important to achieve the desired
4% threshold spread due to the poor matching of the
DMILL transistors and, in particular, due to the increaseof the offset spread with radiation dose. The effect oftrimming on the threshold uniformity is evident bycomparing the ﬁrst plot of Fig. 31 with Fig. 32.

performed to obtain a direct measurement of the noise
occupancy (NO) at a threshold of 1 fC. NO is theprobability for a channel to produce a hit for a certain
event due to noise (Fig. 33). Trimmed discriminator
offsets are applied to ensure a uniform measurement
across the channels. It is expected that the SCT modules
will operate at a threshold set to 1 fC, or slightly
higher after heavy irradiation. This value is chosen tominimise noise occupancy while retaining a high signal
sensitivity.
 The time-walk was calculated by a dedicated scan of
the strobe delay for different values of the injectedcharge, ranging from 1.25 to 10 fC, with a ﬁxed thresholdof 1 fC. The efﬁciency curve obtained that way is 100%
only during a 25 ns interval whose width was used tocalibrate the strobe delay DAC. The discriminatorresponse delay is given by the falling edge of that
distribution. Finally, the time-walk was computed as thedifference between delays calculated for a 1.25 fC and fora 10 fC injected pulse.
A long-term test with electrical readout was performed to
conﬁrm each module’s long term electrical and leakage
current stability at low temperature. The ASICs werepowered, clocked and triggered during at least 18 h while
the sensor bias voltage was set to 150 V and thetemperature measured by the hybrid thermistor wasFig. 32. The vt50 value after trimming for the same module as in Fig. 31.

Fig. 33. Noise occupancy, at 1 fC threshold, plot for one data stream: occupancy vs. channel number and vs. threshold (left); average occupancy for the
stream vs. threshold (right). The threshold is expressed with respect to the 1 fC point (0 mV) as determined during the trimming procedure.

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Fig. 34. Long-term test results for six modules showing from top to bottom: hybrid temperature; analog ðIcc Þ and digital ðIdd Þ current; sensor leakage
current (Idet ); and noise occupancy as a function of time.

10  5  C. The bias voltage, chip currents, hybrid temperature and the leakage current were recorded every 15 min.
Every 2 h a test was performed to verify module
functionality and measure the noise occupancy (Fig. 34).
A ﬁnal I–V scan was performed at 20  C and the current
values at 150, 350 and 500 V were recorded and comparedwith measurements before and after the module assembly.
All the results were uploaded to the SCT production
database.
5.6. Overview of performance
5.6.1. Result from tests during construction
A total of 2380 modules were assembled and their main
performance parameters measured and compared to thenominal values in order to ascertain the usability of each
module. As already mentioned, mechanical and electricalproperties were the main issues checked during the
production process.
Fig. 35 top shows the deviation of the xy metrology
parameters from their nominal values normalised totheir tolerances. The vast majority of the modules were
within mechanical speciﬁcations. From the ﬁgure, one
can see that one of the most critical parameters was
midyf but, still, only a small fraction of the modules had
to be rejected because of that parameter being out ofspeciﬁcations. Fig. 35 bottom shows the z metrology
parameters of all the modules. The picture shows thatfor some modules there were some regions (the max,
min and rms parameters) out of speciﬁcations but, ingeneral, the average values were well centred in thenominal values.
Of the total module production, about 3% are out of xy
tolerance and about 2.6% are out of z tolerance. However,some of these fall into a PASS category, and are still
usable, as detailed in Section 6.4.
Similarly, Fig. 36 shows the leakage current distributionof all the modules, regardless of the strip length. Only
about 1.4% of the modules failed the I–V test irrecoverably
and had to be rejected.
Fig. 37 shows both the noise occupancy and theelectronics noise as measured in the electric tests of themodules. The noise occupancy is well below the upper limit
in the speciﬁcations and the noise ﬁgure shows how themodules cluster according to the sensor length.
Fig. 38 shows the number of bad channels. On average,
after module assembly, one ﬁnds about three more deadchannels than were found when measuring the bare hybrid.
This excess contains both the sensor defects and thechannels lost during the assembly process. Further
information on electrical tests results of production
modules is available in Ref. [52].

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383

Fig. 35. Deviation from nominal values, normalised to the tolerance, ofthe xy (a) and z (b) metrology parameters. The black dots show theaverage value and the colour scale the number of modules with a given
deviation. The thick horizontal lines represent the tolerances.

Table 16 summarises the failure modes of the production
modules. Overall, only 7% of the production modules were
rejected. Some modules failed in more than one way.
5.6.2. Tests in particle beams
In addition to the tests made in the construction phase, a
set of prototype modules were used in beam tests at the
CERN SPS and KEK accelerators. Unirradiated andirradiated modules were tested in a beam of high energy
particles.
The hit detection efﬁciency as a function of threshold set
is displayed at Fig. 39.
From this plot the median of the charge collection
distribution can be determined. It corresponds to the 50%
efﬁciency point as shown in the plot. The average mediancharge of unirradiated end-cap modules is 3:5  0:1 fC.
The main parameters driving the performance of abinary system are the noise occupancy, which should
be low, and the efﬁciency, which must be as high as
possible. Unfortunately, they are correlated and the
optimal settings are a trade-off between the two. This isshown in the expanded plot around the nominal threshold

Fig. 36. Leakage current in mA for all the modules biased with (1) 150 V
and (b) 350 V.

of 1 fC shown in Fig. 40 for unirradiated and irradiatedend-cap modules.
Before irradiation the efﬁciency is higher than the nominal
value, shown by a dashed line, over a wide range of operating
thresholds while the occupancy is within speciﬁcations. In thecase of irradiated modules, however, the range of thresholds
for which both the efﬁciency and the noise occupancy arewithin the speciﬁcation is narrower and operation with athreshold greater than 1 fC might be needed.
A detailed description of the testing methods and results
achieved can be found in Refs. [53,54].
6. Overview of ATLAS end-cap module production
organisation
6.1. Distribution of tasks and flow of components
The basic components for the module were the ASIC
and fully equipped hybrids, the fan-ins, the spines and thesensors. Other items were supplied by one institution to all,ARTICLE IN PRESS
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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 353–389
Table 16
Failure modes of rejected modules
Test

Modules rejected (%)

Visual inspection
I2V
Electrical
XY survey
Z survey

2.5
1.4
3.9
1.2
0.6

Fig. 39. S-curve, measured with particle tracks.

Fig. 37. (a) Noise occupancy at 1 fC threshold. (b) Noise of the modules.

Fig. 38. Number of bad channels.

such as the glue and the electrical test boxes (Valencia), thetest read-out kaptons (Geneva), the washers (Manchester),the module boxes (Liverpool) and the transport boxes(Prague, Charles). The fan-ins were produced by CNM
Barcelona and also supplied via CERN to all module
assembly sites. The spines were produced under the control
of IHEP Protvino and provided to CERN where washer
mounting, spine testing and distribution were organised by
Glasgow and Protvino personnel. Hybrids produced inindustry under the control of Freiburg, were distributed toFreiburg, Krakow and RAL for testing before being
forwarded to the module assembly sites.
The end-cap module production organised itself into
three assembly lines to optimise the use of the available
personnel and expertise. The diagram of component andmodule ﬂow is displayed in Fig. 41. Hybrids tested by
Krakow were forwarded to Geneva and Melbourne. Thoseat Geneva were assembled into outer modules, wire bonded
and tested for mechanical accuracy (both before and afterthermal cycling) and for basic electrical functionality. They
were then forwarded to CERN for full electrical testing
including the long-term tests. Those at Melbourne underwent full assembly into outer modules and in-house
testing. The modules were ﬁnally packaged and sent to thedisk assembly sites at Liverpool and NIKHEF. The
Hamamatsu sensors for these modules were nearly all
tested at Prague AS CR but with some, for extra modules,
tested at Lancaster.
Hybrids tested at Freiburg were forwarded to NIKHEF
and MPI Munich. In the former case they were assembled

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385

ﬁnal metrology before assembly to disk. Valencia usedmostly Hamamatsu sensors tested by themselves to makecomplete and tested outer and middle modules. They also
constructed additional middle modules using sensors tested
by Lancaster.
6.2. QA organisation
ATLAS SCT is required to deliver high performance in avery harsh environment with effectively no access after
installation. A QA plan [40] was therefore agreed at an
early stage of the project.
QA was realised in three stages of the project: design, sitequaliﬁcation and production quality control.
6.2.1. Design
All components were extensively prototyped and theirperformance was evaluated before and after irradiation to
the expected dose [6,9,21]. Module prototypes were shownto have the required electrical [21] mechanical and thermal[18,22] properties, and to work with the required precisionand efﬁciency in particle beams [54] even when fully
irradiated.
6.2.2. Site qualification
All sites taking part in module production were subject
to a thorough qualiﬁcation procedure, where all aspects ofproduction and testing were reviewed, including documentation, cleanliness, ESD safety, component traceability andaccountability, etc. Each production line was required to
produce at least ﬁve modules fully within speciﬁcations.
Modules were exchanged between sites to ensure uniform
quality and comparability of measurements.

Fig. 40. Efﬁciency and noise occupancy for: (a) unirradiated modules and(b) irradiated modules.

into inner modules with CiS sensor measured by MPI andfully electrically tested at NIKHEF. In the latter case, they
were assembled into middle modules with CiS sensors at
Munich and checked for metrology and basic electricalfunctionality there. The modules went through theircomplete electrical QA in Prague at both the Charles andCzech Technical Universities. Freiburg also assembled
completely tested inner modules using both CiS and
Hamamatsu sensors, mostly tested at MPI but with some,for additional modules, from Lancaster.
The RAL hybrids went to Manchester and Valencia. At
Manchester they were mechanically assembled and checked
for metrology using sensor sets tested by both Lancasterand Shefﬁeld for all three module types. Only Hamamatsu
sensors were used at Manchester. The wire bonding andelectrical testing were carried out at Glasgow and Liverpool, with Liverpool completing the thermal cycling and6.2.3. Production quality control
The tests each module was required to pass are describedin Section 5. Each module and its components were tracked
through their history by means of the SCT production
database (DB). This meant that details of tests, component
trees, movements between sites and overall status ofmodules were available to the whole collaboration, andfostered a culture of transparency. The DB also allowedeasy monitoring of site production statistics and component supplies.
Additionally, the full raw test data is archived at eachtest site, together with traveller documents, check sheets
and high resolution optical scans of assembled modules.
6.3. SCT production database
The complexity of the SCT project prompted thedevelopment of a relational database (DB) [55–57] tomanage it. The DB and most of the features needed
were already working when the module production
started. The architecture for the ATLAS-SCT database isbased on the client-server model, with a main Oracle(kernel 9i) application server at the University of

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 353–389

Fig. 41. Diagram of SCT end-cap production organisation.

those objects is a data structure containing the list of tests,
together with their results, performed on each item of theassembly. The database allowed one to also store thelocation of any of the items, together with the so-calledshipments, so that one could always trace back the origin ofany module or module component, as well as component
availability.
The database has proved to be an excellent tool tomonitor the location, the assembly and QA progress of themodule production. In particular it allowed to pinpoint
possible problems in the supply of components, monitor
and to optimise the production.
6.4. Acceptance criteria and selection for assembly to disks
Fig. 42. User access scheme for recording the data into the SCT
production DB.

Geneva. Access is granted from client machines communicating over the network, either by means of dedicatedprograms to monitor particular aspects of the production, through specialised applications for massive data
upload or with the help of a WWW portal developedin Geneva [58] (see Fig. 42). The access is secure andan institute-based authentication mechanism has been
implemented.
The data in the database is organised hierarchicallystarting from the building blocks, items, that group into
the so-called assemblies, which in turn can be assembled
to form bigger assemblies. Every object has associateda unique identiﬁer and some speciﬁc ﬁelds. Associated with

Modules were selected for assembly to disks according toan agreed set of criteria [59] based on test results stored in
the production database. Each grouping of production
sites was responsible for evaluating the quality of its
modules and placing them into four categories:

 Good: Pass all acceptance tests.
 Pass: Fails one test, but within pass tolerance.
 Hold/Rework: Outside pass tolerance. May be usable if
reworked.

 Fail: Too bad to use, but stored safely.
The mechanical tolerances (Tables 14 and 15) wereoriginally set more tightly than required by spatial
resolution requirements alone. In the light of experience
we created a set of pass tolerances for some parameterswhich were 50% wider. Modules with one parameter

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 353–389(or two Z parameters) in this pass band are assigned to the
Pass category. The RMS of the detector alignment
distributions are still well within the spatial resolution
requirements even when Pass modules are included, so
there will be no effect on physics performance. For Good þ
Pass modules, the distribution of midyf, the most sensitivealignment parameter, has an RMS of 2:0 mm, well within
the target of 4 mm.
Good and pass modules could be used anywhere in theend cap; there was no selection for more or less demanding
locations. Disk assembly went on in parallel with module
production, and some ﬁne tuning was done to allow for
projected module yield while always preserving quality.
7. Summary
The production of the ATLAS SCT modules has been a
successful, long and complex process that needed about 24
months for completion. The production was planned witha contingency of 20% allowing for losses of 15% during
module assembly and a further 5% when mounting
modules on disks. More than 2380 SCT end-cap modules
have been built with a yield of 93%, despite the complexityof the design and the tight mechanical speciﬁcations.
Fourteen institutes with a wide geographical spread haveparticipated in the process in a remarkably collaborative
way which has been key to this success.

Table 17
Statistics of the SCT end-cap module production
Category

Inner Middle ðS þ LÞ Outer All

Modules assembled
Good
Pass
Hold/rework
Fail
Good þ pass ðG þ PÞG þ P required to equip end-caps

495
394
39
46
16
433
400

772
665
60
42
5
725
640

1113
995
61
23
34
1056
936

2380
2054
160
111
55
2214
1976

387

The production had two different phases. In a ﬁrst
phase, the different sites had to undergo a pre-qualiﬁcation
process in order to ensure that production tooling andprocedures were in place to produce modules reliably
within speciﬁcations. In order to exercise and test theprocedures the production sites were supplied with second
grade components. The full qualiﬁcation process, however,was made with production grade components and was also
intended as a short ramp-up to production. The delivery ofcomponents for the qualiﬁcation also followed as close as
possible the production procedures in order to exercise andﬁnd the weaknesses of the component distribution.
Table 17 shows a breakdown of module production
statistics according to the module type and module quality.
Fig. 43, on the other hand, shows the cumulative numberof modules built as a function of time together with theyield both in terms of strictly GOOD modules andGOOD þ PASS modules. The ﬁrst part, where theproduction rate is smaller, corresponds to the period inwhich the different sites went through the qualiﬁcation
process. That process took almost one third of the total
time. After that period a production rate between 40 and
50 modules per week was achieved almost immediately
with a constant yield above 90%.
The ﬂow of components and a very tight schedule havebeen the most notable problems during the module
production. Although the delivery of the module components has not been constant, the production was neverstopped for that reason, mainly because of the close
cooperation of all groups participating in the project.
However, it was a complex task to get all the different
component manufacturers into production mode and thiswas the main reason for a late start for all the assembly
sites to achieve full production rate.
The production was organised in a ﬂexible, cooperative
collaboration of the different laboratories. Some of them
carried out all the steps of the module fabrication in house,
while others were specialised only in some aspects of it, likeassembly or testing. Also, some of the institutes haveproduced only one of the four different module types while

Fig. 43. Module production performance. The ﬁlled histograms show the cumulative number of all, GOOD and GOOD þ PASS modules built as afunction of time. Also shown is the production yield (right scale) both considering just GOOD modules and also GOOD þ PASS modules.

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A. Abdesselam et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 353–389

others were able to produce more than one type. This
structure allowed overcoming the problems that appeared
during the production.
Acknowledgements
We are greatly indebted to all the technical staff who
worked on the end-cap module project from the ATLASSCT 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 andthe Canton of Geneva; the Slovenian Research Agency andthe Ministry of Higher Education, Science and Technologyof 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;
the Australian Department of Education, Science andTraining; Dutch Foundation for Fundamental Researchon Matter (FOM); the Ministry of Education, Youth andSports of the Czech Republic; the National Science
Council, Taiwan; the Swedish Research Council.
The Research was supported in part by the EU under
RTN Contract HPRN-CT-2002-00292 Probe for New
Physics within the Sixth European Community Framework
Programme.
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