Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

Construction and performance of the ATLAS silicon
microstrip barrel modules
T. Kondoa,*, R. Apsimonb, G.A. Beckc, P. Belld, R. Brennere,
P. Bruckman de Renstromf, A.A. Carterc, J.R. Carterg, D. Charltond,
W. Dabrowskih, O. Dorholti, T. Ekelofe, L. Eklunde, M. Gibsonb, S. Gadomskif,
A. Grilloj, J. Grosse-Knetterk, C. Haberl, K. Haram, J.C. Hillg, Y. Ikegamia,
Y. Iwatan, L.G. Johanseno, T. Kohrikia, A. Macphersonk, S. McMahonb,
G. Moorheadp, J. Morinc, J. Morrisc, M. Morrisseyb, K. Nagaic, I. Nakanoq,
J. Paterr, H. Perneggerk, E. Perrins, P. Phillipsb, D. Robinsong, B. Skubict,
N. Spencerj, S. Stapnesi, B. Stuguo, R. Takashimau, S. Teradaa, M. Tyndelb,
N. Ujiiea, Y. Unnoa, M. Vosv
a

High Energy Accelerator Research Organization (KEK), Oho, 305-0801 Tsukuba-shi, Japanb
Rutherford Appleton Laboratory, Chilton, Didcot, UKc
Department of Physics, Queen Mary and Westfield College, University of London, London, UKdSchool of Physics and Astronomy, University of Birmingham, Birmingham, UKe
University of Uppsala, Uppsala, SwedenfHenryk Niewodniczanski Institute of Nuclear Physics, Cracow, PolandgDepartment of Physics, University of Cambridge, Cambridge, UKhFaculty of Physics and Nuclear Techniques of the Academy of Mining and Metallurgy, Cracow, Polandi
Department of Physics, University of Oslo, Oslo, NorwayjInstitute for Particle Physics, University of California, Santa Cruz, CA, USAkEuropean Laboratory for Particle Physics (CERN), Geneva, SwitzerlandlLawrence Berkeley Laboratory and University of California, Berkeley, CA, USAm
University of Tsukuba, Tsukuba, Japann
Faculty of Science, Hiroshima University, Hiroshima, Japano
University of Bergen, Bergen, NorwaypResearch Centre for High Energy Physics, Melbourne University, Melbourne, AustraliaqFaculty of Science, Okayama University, Okayama, Japanr
Department of Physics and Astronomy Schuster Lab, The University of Manchester, Manchester, UKs
Department de Physique Nucl!eaire et Corpusculaire (DPNC), Universit!e de Gen"eve, Gen"eve, SwitzerlandtRoyal Institute of Technology (KTH), Stockholm, SwedenuKyoto University of Education, Kyoto, Japan
v
Instituto de Fisica Corpuscular (IFIC), Centro Mixto Universidad de Valencia, CSIC, Burjassot, Valencia, Spain
ATLAS SCT Barrel Module Collaboration*Corresponding author. Tel.: +81-298-64-5336; fax: +81-298-64-2580.
E-mail address: taka.kondo@kek.jp (T. Kondo).
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 5 2 8 - 4

28

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

Abstract
The ATLAS Semiconductor Tracker (SCT) consists of four barrel cylinders and 18 end-cap disks. This paper
describes the SCT modules of the barrel region, of which more than 2000 are about to be constructed. The module
design is ﬁxed. Its design concept is given together with the electrical, thermal and mechanical speciﬁcations. The preseries production of the barrel modules is underway using mass-production procedures and jigs. The pre-series modules
have given satisfactory performances on noise, noise occupancy, electrical as well as mechanical and thermal properties.
In addition, irradiated modules were demonstrated to work successfully. Also ﬁrst results from a 10-module system test
are given. r 2002 Elsevier Science B.V. All rights reserved.

1. Introduction
ATLAS is a multi-purpose detector designed forthe new physics in the TeV mass range, especiallyfor the search of the Higgs and SUSY particles at
LHC [1]. Three sets of large aperture superconducting toroids generate toroidal magnetic
ﬁeld for the muon chambers. Inside the muon
system, there are scintillation-tile hadron calorimeters, LAr electromagnetic calorimeters andInner Detectors (ID) [2]. A central solenoid
generates magnetic ﬁeld of 2 T in the ID volume.
From inside to out, the ID consists of the pixel
detector, the Semiconductor Tracker (called SCT)of silicon microstrip sensors, and the straw
transition radiation tracker. Its overall dimension
is 2.3 m in diameter and 7 m in length. Its main
requirement is the precision tracking of charged
particles using ionising detection devices that arecapable of 40 MHz bunch crossing identiﬁcationand that can tolerate large radiation doses withminimal possible material.
The SCT [3] covers the region from 300 to
520 mm in radius from the beam line. It is divided
into the barrel part of 4 cylindrical layers in thecentral region and the forward part of nine disks ineach side. Total area of the silicon microstrip
sensors is 61 m2. The rf position resolution per
SCT layer is aimed to be around 17 mm, covering a
rapidity range of 72.5.
When LHC operate at its design luminosity of1034cm2 s1, the annual ﬂuence at the inner-mostsensors at r ¼ 30 cm is about 1.8  1013 neq/cm2
expressed in terms of equivalent 1 MeV neutron
ﬂux. Thus 10-year operation at LHC requires theradiation tolerance up to 2  1014 n/cm2 in 10 years
including 50% uncertainty.

ATLAS decided to use the p-in-n type of silicon
microstrip sensors for its radiation tolerance andcost. The temperature of the sensors is kept below
01C, preferentially at around 71C all year aroundincluding non-operation time to suppress the antiannealing while keeping beneﬁcial annealing ofradiation induced effective doping. Low-temperature operation also helps in reducing bulk leakage
current.

2. Design of the barrel module
In the barrel SCT system, exactly the same
modules are used for all 4 cylindrical layers. The
module is a basic detector unit carrying standalone mechanical rigidity together with electricalas well as thermal functionalities. It consists offour single-sided p-in-n silicon sensors and a
hybrid for readout. The required tracking precision is obtained by modules with an intrinsic point
resolution of 23 mm in the rf coordinate per single
side measurement. The precision is obtained forthe binary readout scheme (on-off readout) using
silicon microstrip sensors with 80 mm readout
pitch. Two sensors on one side of the module areconnected together strip-by-strip to form aneffective strip length of 126 mm. These strips arealigned at a 720 mr half-stereo angle with respect
to the module axis. Adjacent modules are overlapped longitudinally each end, requiring that they
are staggered in radius by 2.8 mm to leave a 1 mmstay clear between sensors. A tilt angle of 101
allows adjacent rows of modules to be overlapped,
providing hermetic detector coverage.
The barrel modules are mounted on four concentric carbon-ﬁbre cylinders through attachment

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

to CFRP brackets that are located by precision
inserts in the cylinders. The mean radii of thebarrel modules are 300, 370, 450 and 525 mm.
The mechanical tolerance for positioning sensors within the back-to-back pair must be B5 mm
lateral to the strip direction. In this way, the ﬁnealignment for tracking can be handled at themodule as one solid object.
Another important property of the module is athermal function. The 12 ASICs dissipate a powerof 6 W for nominal operating conditions andnominal process parameters, up to a maximumof 8.1 W for the worst case combination of processparameters, supply voltages and radiation effects
in electronics. As the dose accumulates, the heatgeneration in the sensor bulk become appreciable
due to leakage current. The maximum heatgeneration qmax per unit area of the sensor isestimated by multiplying the maximum designed
applied bias Vmax of 500 V and the leakage current
deduced from the current related damage constant
a of 4  1017 A/cm at 201C as
qmax ¼ a  2  1014 =cm  FT  Vmax
285 mm ¼ 120 mW=cm2(normalized at 01C) in which FT is the reduction
factor of B1/1000 in leakage current at 01C withrespect to 201C and 285 mm is the sensor thickness.

29

If all 4 sensors are at 01C, total bulk heat is 1.8 W.
Since the bulk leakage current grows non-linearlywith sensor temperature (approximately it doubles
every 71C), so called thermal runaway may occur if
the cooling is not adequate [4]. Including a safety
factor, the goal for thermal runaway point is set at
240 mW/cm2 (at 01C). The effective in-plane
thermal conductivity must be increased beyond
that of silicon. In practice this is achieved by theuse of high-thermal conductivity material thermalized pyrolytic graphite, referred to as VHCPG(very high-thermal conductivity pyrolytic graphite)or TPG, in a baseboard which is laminated as partof the detector sandwich.
After considering the combined electrical, thermal and mechanical requirements, the optimized
design of the barrel modules has a hybrid placednear the centre of the module, above and below thesensors, and connected to the strips near the
middle of their total length, as shown in Fig. 1.
This conﬁguration minimizes the space conﬂictwith neighbouring modules. The central signal
tapping is better electronically as it results inminimum spread resistance of metal readout strips
seen by the front-end circuits. A piar of 128 mm
long units is back-to-back aligned with a stereo
angle of 40 mrad and glued together with abaseboard in between. Two hybrid circuits with aﬂexible wrap-around connection in between makeFig. 1. 3D view of the ATLAS barrel SCT module.

30

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

Fig. 2. Cross-section of the module at the location of the hybrid. Note that the vertical direction is magniﬁed by 5. The numerals are
thicknesses in unit of mm.

a single-piece construction of the hybrid possibleusing Cu/polyimide laminates. The baseboard
extends outwards, with BeO facings epoxy-fusedto the VHCPG surfaces, on both lateral sides ofthe module. These facings are the places at which
the readout hybrids are attached. Each module isfastened by 2 points at the longer facings onto a
supporting CFRP bracket and by a third mounting point situated at the shorter facing onto theadjacent bracket. The module mounting accuracy
is achieved by precision set screws locating on theinternal bore of a circular and a slotted hole on thelonger facing of the baseboard.
Fig. 2 shows the cross-section of the modulewith magniﬁcation in vertical direction. It is notedthat the hybrid ‘bridges over’ the sensors with an
air gap of 0.4 mm. Thus the sensor surfaces areintact except bonding wires. This has the advantage of avoiding gluing to the active sensor surface,which would carry the potential for damage anduncertainties associated with long-term ageing andradiation effects. The hybrid bridge must be strong
enough to make the supersonic wedge wirebonding possible. In this conﬁguration, the thermal path for the ASIC heat is largely decoupledfrom the path of the bulk heat, keeping the sensors
as cold as possible.
The cooling of the modules is provided by athermal contact over an area of 420 mm2 between
the lower BeO facing of the module and analuminium-cooling block. Two spring clips com-

Fig. 3. Thermal proﬁle of the barrel SCT module with a ASIC
power of 6 W.

press two pieces together. The grease for thermal
contact is Dow Corning 340 which has demonstrated both good thermal conduction and highradiation tolerance [5]. The cooling block encapsulates a CuNi cooling tube which carries theevaporative ﬂuid of C3F8. The module and thecooling block are electrically isolated in the baseline but an option is kept open for grounding
connection.
A thermal FEA simulation has been carried out
as shown in Fig. 3 for an ASICs power of 6 W andnominal heat generation in the sensors. The proﬁleindicates that the hybrid and sensors are fairly welldecoupled thermally. Fig. 4 shows the temperature

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

of the furthest sensor corner as a function of thebulk heat generation, at three coolant temperatures. The simulation shows that the thermal
runaway occurs at 220 mW/mm2 at coolant temperature of 141C satisfying nearly the required
factor of 2 from runaway point.
The major module parameters are summarizedin Table 1. The overall weight of the module is 25 g
and the radiation length averaged over the siliconsensor area is 1.17%X0 :31

3. Module components
3.1. Silicon microstrip sensors
The silicon barrel sensor is made of four
63.96 mm  63.56 mm (cut-edge to cut-edge) single-sided p–n junction detectors with capacitively
coupled readout strips [6]. Two sensors are aligned
to form a 128 mm long unit. Strips of the two
sensors are wire-bonded to form 126 mm long
strips. The pitch of the strips is 80 mm and there are
770 strips physically with the ﬁrst and the last
being connected to the strip bias potential for theelectric ﬁeld shaping and deﬁning the strip
boundary. Table 2 summarizes the major parameters of the barrel SCT sensor. The detail
description and quality assurance of the sensors
are given in separate paper [7]. The sensor bows at
the level of 60 and 80 mm, intrinsically limiting theoverall module ﬂatness. Sensitive region to cutedge distance is 1 mm.
3.2. Readout ASICs

Fig. 4. Temperature of the furthest sensor corner as a functionof heat ﬂux with three coolant temperatures Tc for the case of
ASIC power at 8.1 W.

Table 1
Major parameters of the module
Number of sensors
Number of ASIC chips
Sensor directions
Sensor operating temperature
Hybrid power consumption
Thermal runaway heat ﬂux
Module thickness (sensor part)
Module thickness (hybrid part)
Mechanical accuracy
Back-to-back in-plane lateral
Back-to-back in-plane longitudinal
Back-to-back out-of-plane
Fixation point, in-plane
Radiation length/module

The main parameters of the readout ASICs,
ABCD3TA, are shown in Table 3 [8]. The
analogue part uses the bipolar technology, while
the digital part is made by CMOS. The front-end
circuit is optimized for 128 mm long strip sensors.
Any pulses exceeding the threshold of typically
1 fC are stored as 1-digit hits for three consecutive
clock buckets. No analogue information is kept. In
Table 2Main speciﬁcations of the sensors

2 in top, 2 in bottom
6 in top, 6 in bottom
720 mrado01C, optimum 71C
6 W nom., 8.1 W max.
>240 m W/mm2 at 01C
1.15 mm3.28 mmo5 mm
o10 mmo50 mm
o30 mm
1.17%X0Type
Thickness
Lenght  widthp implant pitch/width
Initial full depletion
Total initial leakage at RTGood readout stripsp+nn+ single sided
285715 mm
63,960725 mm  63,560725 mm
80 mm/16–20 mm
o150 Vo6 mA(150 V), o20 mA (350 V)>99%

Post-irradiation characteristics with 3  1014 p/cm2
Voltage for >90% CCE
o350 VInterstrip capacitanceo1.5 pF/cm at 350 V, 100 kHz
Total leakage current
o250 mA at 181C up to 450 V
Micro-discharge
o5% increase for 300–400 V
Good strips>99% at 350 V32

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

Table 3Major parameters of the ABCD3T-A chipType of circuit
Gain
Peaking time
No. of input channels
Equivalent noise charge
Threshold tuning
Timewalk at 1 fC threshold
Readout redundancies
Process technology

Binary readout
50 mV/fC
20 ns
128 ch/chip1500 e (un-irradiated)
1800 e (irradiated)
Individual 4-bit trimDAC
with 4 selectable ranges
o16 ns for 1.25–10 fC
By skipping a bid chip
0.8 mm BiCMOS (DMILL)

this type of binary readout, the uniformity ofthresholds is a critical parameter. The threshold isadjusted by a 4-bit trim DAC (TrimDAC)
attached to each channel. In addition, 4 selectable
ranges in the TrimDAC, 60, 120, 180 and 240 mV,
are implemented to cover the increased spread of
the threshold offsets (of about a factor of 3) causedby radiation. A hit can be generated by an internal
calibration pulse. Every fourth channel is pulsed
simultaneously with variable amplitude of 0–10 fC.
The calibration strobe time can be delayed withrespect to the clock phase.
The module has 6-ABCD chips on each side
forming a readout unit. The left-most chip is themaster chip connected to a ﬁbre–optic interface.
Upon the receipt of a level-1 trigger, the masterchip initiates a readout sequence from left to right.
In the event of the failure of one of the slave chips,
the chips can be programmed to route around thefailed chip. In the event of the failure of one of themaster chips, the other working master can control
and readout 10 slaves chips by routing around thefailed master chip.
Wafers of the ASICs from the foundry need to
be fully tested before dicing. The acceptance limits
for various parameters are deﬁned with sufﬁcientmargins allowing for chip degradation after full
irradiation. All chips are put through the full
conﬁguration tests as well as measurements ofpower consumptions, gains, offsets, noises, thresholds, trimDAC settings and ranges. The wafers
screening data are also used to monitor the
production yield by comparison against thefoundry data. The chip parameters extracted fromthe wafer screening are transferred to the database
via a java application.
3.3. VHCPG baseboards
The baseboard substrate is thermalized pyrolytic graphite VHCPG. Its thermal conductivity atroom temperature is 1450–1850 W/mK in-planewith 0.4%/C and typically 6 W/mK out of plane.
Its thickness is 380715 mm. The VHCPG material,
however, is intrinsically friable and easily delaminated. An epoxy encapsulation technique wasdeveloped to make up for such weaknesses forthis project. Four BeO facings are attached to the
VHCPG sheet. They have precision holes andGold HV contact pads. A picture of an assembled
baseboard is given in Fig. 5. The shape of thebaseboard is a result of material minimization.
The baseboard fabrication is being carried out
at CERN. After laser cutting to the shapespeciﬁed, a ﬁne matrix of 120 mm diameter holes
is made in the substrate as an integral feature of
the encapsulation. The positioning of the four BeO
facing plates is provided by a set of precision jigs
with the appropriate quantity of epoxy being

Fig. 5. Encapsulated VHCPG baseboard with BeO facing
plates.

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

applied by screen printing to each side of thesubstrate board. The assembly is cured under anappropriate temperature cycle, pressure and surrounding vaccuum to achieve a bubble-free epoxy
coating and graphite–BeO intra-surface junctions.
The encapsulation is typically 20 mm thick. Foursmall spots of epoxy are removed from each side toprovide openings for electrical connection to bemade to the rear side of the silicon sensors using
conducting epoxy. This complements drilled andﬁlled holes in the upper BeO cooling facing forgold HV contact pads.
Each baseboard is subject to a quality assuranceof thermal and electrical properties. Injecting
known heat at deﬁned points around with a heatsink attached to the lower cooling facing, atemperature contour is taken with an intra-red
thermal imaging camera. The contour for eachboard is stored in a database to compare withstandard images and FEA simulations.

33

copper layers are removed. Electrical connections
among different layers are realized by either
through-holes, penetrating all layers, or laser-cut
via-holes between two adjacent layers. The top L1
and bottom L4 layers have extra copper for
plating these through-holes and via-holes. All
Cu/PI sheets are made with adhesive-less technology. As shown in the layout picture of Fig. 6, thetop layer L1 has bonding pads, branch traces, thesecond layer L2 contains most of longitudinal bus
lines between chips and the sensor HV line, L3
carries analogue and digital grounds all throughthe circuit, and L4 is the power supply plane withcopper meshes. The digital and analogue groundconnection is made on the hybrid aside of every
ASICs.
Seventeen thermal through-holes are added
underneath the analogue part of each ASIC. Thesevertical copper holes are ﬁlled with thermally aswell as electrically conducting glue. This ensures
thermal and electrical connections between eachchip and the carbon–carbon bridge material. The

3.4. Cu/polyimide flexible circuits
The technology of ﬂexible multiplayer circuits of
Cu/polyimide are now commercially used [9]. The
layer structure of the Cu/PI circuit is listed inTable 4. The starting core for build-up is a doublesided Cu/PI sheet in the middle. A single-sided Cu/
PI sheet is glued on each side of the core sheet. In
the wrap-around section the top and bottom

Table 4Layer structure of the Cu/polyimide circuits
Layer

Material

Thickness (mm)Top coverThrough-hole plating
Single-sided Cu/PI
Adhesive
Double-sided Cu/PI
Adhesive
Single-sided Cu/PI
Through-hole plating
Cover ﬁlm

Solder resist mask
Cu, 50% mesh
12 mm Cu, 1-mil PI

20
20
37
25
49
25
37
20
46

12 mm Cu, 1-mil PI
12 mm Cu, 1-mil PI
Cu, 50% mesh1/2-mil PI, 33 mm
adhesive

Total (circuit part)
Total (wrap-around part)

279
149

Fig. 6. Layout of the Cu/PI ﬂexible circuit. The ﬁrst and second
top layers L1 and L2 are used for signal circuits, L3 is for digital
and analog grounds, and L4 supplies the power plane.

34

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

effective thermal conductivity of this pillars is
40 W/mK.
3.5. Carbon–carbon bridge
The bridges reinforcing the Cu/PI ﬂexible circuit
is required to have a good thermal conductivity,
high young’s modulus, and low radiation length. It
also functions electrically as a ground plane. The
bridge is made of carbon–carbon with unidirectional ﬁbres. The properties of the carbon–carbon
material are summarized in Table 5 [10]. The
bridge is machined to a thickness of 0.3 mm withlegs on both ends 0.5 mm high. The surface of thebridge is coated with a polymer called Parylene of
10 mm thick. The coated surface is roughened witha laser where adhesion is required. A part ofcoating is removed to open windows to make athermal and electrical contact for ASICs.
The Cu/PI ﬂexible circuit and the carbon–
carbon bridges are glued together with thermallyconductive adhesive sheets, ABLEFILM 563 K.002 with Alumina ﬁller except the chip pillar
parts where an electrically conductive sheet of
ABLEFILM 5025E-.002 silver loaded is used[11]. During the curing at 1251C for 2 h, theﬂexible circuit and the bridges are pressed together
at 4 kg/cm2 by a press jig with a curved press
surface for compensating a bend due to thedifference in temperature expansion coefﬁcientsof two materials.
3.6. Glass pitch-adaptor
The basic pitch of input pads of the ASIC is
48 mm, while that of the silicon microstrip sensor isTable 5
Properites of the carbon–carbon for the bridge
Material

80 mm. In order to make a simple parallel wirebonding, a pitch-adaptor is used in front of the
ASICs. Since a ﬁne pitch of 48 mm is required, thepads and traces are fabricated on a separate piecewith aluminium deposition of 1–1.5 mm thick on a
glass substrate. A tape-peel test of the A1 traces
and a wire-bond pull test are performed forsamples. The wire-bond pull strength must be
greater than 6 gr.
3.7. Passive components and mounting onto hybrid
All chip capacitors are of type X7R which has acapacitance change of o5% between 20 and+401C. The resistors are of metal ﬁlm precision
type. A thermistor is mounted one each side tomonitor the hybrid temperature. Each surfacemount part is temporally ﬁxed on the Cu/PI
surface with a dot of epoxy and then solderedmanually. Solder re-ﬂow technique is not used to
avoid temperature application of higher than
601C.
After soldering all passive components, twoglass pitch adaptors are glued onto the hybrid witha room temperature curing epoxy Araldite-2011.
Its low viscosity makes the glue layer thin andbubble free. The pitch-adaptor is positioned undera microscope to align to the ﬁducial marks within
750 mm.
All the hybrids, after thermal-cycling between
30 and +601C for 5 times, are electricallychecked via the I/O connector for bus line
terminations as well as capacitances between
power and ground. The hybrids with all passive
components mounted but without ASICs are thendelivered to the module assembly sites. Fig. 7 is aphotograph of the hybrid with all components
mounted on.

Unidirectional carbon ﬁbres

4. Module assembly
Thermal conductivity (8 ﬁbre)
700720 W/m/K(> ﬁbre)3575 W/m/K
Density
1.9 g/cm3
Young’s modulus (8 ﬁbre)
294 Gpa
Tensile strength (8 ﬁbre)
294 Mpa
Thermal exp. coeff. (8 ﬁbre)
0.8 ppm/C(> ﬁbre)10 ppm/C
Resistivity (along ﬁbre direction) 2.5  106 O m

The barrel module assembly is carried out in thefour assembly cluster in Japan, UK, Scandinavia
and US. Assembly jigs are slightly different insome technical details, but each assembly clusters
is required to assemble the modules to meet theexactly same speciﬁcations.

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

35

Fig. 7. Picture of the hybrid with chips and components mounted on.

4.1. Sensor-baseboard assembly
A great amount of effort was devoted todeveloping procedures and ﬁxtures since the mostdemanding mechanical precision is required at thisstep. Requirements are to align not only each pairof sensors in the plane but also the top-bottom
pairs. In addition sensors must be aligned to themodule reference point, the dowel holes of the
BeO facings. It is also important to minimize
module distortion out of the module plane.
(1) Sensor alignment: Two silicon sensors areplaced strip-side up vacuum chucked on a pairof precision x2y-rotation stages under amicroscope. Two sensors may be pre-aligned
in a separate jig and transferred using avacuum chucking. Fig. 8 shows an example
of such precision x2y-rotation stages which isequipped with 6 piezo actuators for submicron adjustment. An automatic patternrecognition program may be used to identify
and precisely locate the ﬁducial marks on thesensors. The surfaces of silicon sensors areprotected at all stages, when in contact withjigs by a new piece of disposable pre-cut clean
room paper.
(2) Sensor pickup: A pair of sensors thus
individually and precisely aligned under the
microscope is transferred to a pick-up jig withvaccum-chucking. A deﬁnite sequence of
vacuum-chucking and releasing is needed inorder to keep the alignment precision during
the sensor transfer. A precision alignment
between the stage and the transfer jig is
realized with a set of linear bearings. One
can see these linear bearings in Fig. 8.
(3) Sensor-baseboard gluing: After positioning of
the baseboard on a precision glue ﬁxture, a 3axis dispensing workstation is used to dispense the high viscosity adhesive mix ofFig. 8. Typical assembly microscope unit for sensor alignment.

Araldite-2011 and boron nitride ﬁller on the
baseboard. Drops of the adhesive are applied
to one side of the baseboard according as a
pre-deﬁned pattern. Then two sensors areplaced with appropriate pressure and gap
spacing. The silver loaded conducting epoxy
Eotite p-102 is applied at two spots eachper sensor manually for electrical connection
between the baseboard and the sensor backside.
(4) After curing, another pair of sensors is glued
on the second side of the baseboard in thesame way.
(5) After the detectors are glued to the baseboard,
the I2V curve is recorded for each sensor
individually up to 500 V. If any current at
500 V bias differs by more than 1 mA fromthat last recorded, the assembly is put aside
for checks.
4.2. ASICs stuffing
The chips are glued on the hybrid withelectrically conductive epoxy, Eotite p-102. Four
ﬁducial marks on the hybrids are used for aligning

36

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

the chip. The epoxy is cured at 501C for 2 h. All
nearby bonding pads are masked during gluing.
After all (but for input channels) wire bonds are
made, an electrical test is done to ensure that all
ASICs perform properly and that all wire bonds
are functional. Bad chips can be replaced using asimple jig with local heat application. In addition,a longer duration test at elevated temperature of
451C with nominal L1A trigger frequency of
100 kHz is performed for 100 h in order to catch
infant mortality problems in the ASICs. It is notedthat experience to-date has shown no such longterm failures. The temperature and duration will
be adjusted during production phase in the light ofexperiences gained.
4.3. Mounting the hybrid on the sensor-baseboard
assembly
This process is comparatively simpler using a setof special jigs which carry wrapping-aroundmotions over the sensor-baseboard assembly.
Great care must be paid not to touch the surfacesof both objects. The required accuracy of positioning is mild and 750 mm. The long pot life of theadhesive allows ample time for repositioning.
After curing and visual inspection, the assembly
is mounted into a special protection box forstorage, interim performance checks (including
HV application) as well as for full wire-bonding.
4.4. Full wire-bonding
Firstly, connections for the HV line and groundbetween sensors and hybrid are made via bonding
wires. Then the high-density wire-bonds of 768
channels are made with dedicated automatic wirebonder between the chip input and the pitch
adaptor, between two sensors, and ﬁnally between
the pitch adaptor and the sensor. There are in total
4608 of these wire-bonds per module. Particularcare is required in wire-bonding between pitch
adaptor and sensors for 1 mm difference in height.
Typically one module can be bonded and inspected within 4 h. Fig. 9 is a picture of the module
completed.

Fig. 9. A picture of the ATLAS barrel SCT module.

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

37

5. Quality assurance of the module
midxf, midyf(mid-point of front pair)

F1

The following QA steps are performed on fully
assembled modules. The criteria for accepting a
module as ‘good’ are for initial use, and may
evolve with experience on mass production.

a1

half-stereo

Xm
sep

f

With the ASICs unpowered, the detector I2V
curve is recorded up to 500 V. The total leakage
current of good modules at 500 V should not be
more than 4 mA than the sum in the database.
Then all modules will be tested for long-termleakage current stability at 150 V over 24 h in acold inert atmosphere. This test is performed inparallel with the long-term electrical test on
modules with ASICs powered on. The current
drawn is monitored periodically. The maximum
increase in leakage current should be o4 mA per
module, after an initial settling time of 5 min.
5.2. Metrology
Each completed module is surveyed for mechanical precision with a 3D measuring machine. For
the measurement, a module is placed on a frame
and is held at three points. The frame has anumber of transparent ﬁducials so that themeasurements of the front and back sides can becorrelated.
(1) In-plane survey. The in-plane survey characterizes the relative positions of the four
sensors and the dowel hole/slot of the baseboard. The locations of a sensor are obtainedby measuring eight ﬁducial marks on sensors.
The centres of the dowel hole and slot are
obtained by measuring their perimeters. From
34 x2y measurements, the module coordinate
and a reduced parameter set are obtained asshown in Fig. 10. In this coordinate system,
the reduced parameter set is listed in Table 6
with the design values and the tolerances
speciﬁed. Some typical deviations from thedesign values are plotted in Fig. 11 for sevenmodules constructed by the Japanese cluster.
For all the parameters, the main parts are wellB4

Ym

5.1. Sensor test

a4

b

sep

F2

B3

a2

a3
mhx, mhy(dowel hole)msx, mxy(dowel slot)

Fig. 10. Thirteen parameters to describe the module geometry.
F1, F2 and B3, B4 are the front and back sensors respectively.
The coordinates origin is the centres of four sensors. The values(midxf, midyf) are the coordinates of the front sensor pair,(sepf, sepb) are the sensor separations, (mhx, mhy) and (msx,msy) are the coordinates of the dowel hole and slot of thebaseboard respectively, a1, a2, a3, a4 are the sensor angles
stereo the half-stereo angle.

Table 6Module in-plane geometry parameters
Parameter

Design value Tolerance

Dowel hole, mhx (mm)Dowel hole, mhy (mm)Dowel slot, msx (mm)Dowel slot, msy (mm)Mid-point of front pair, midxf (mm)Mid-point of front pair, midyf (mm)Separation of front pair, sepf (mm)Separation of back pair, sepb (mm)Sensor1 angle, a1 (mrad)
Sensor2 angle, a2 (mrad)
Sensor3 angle, a3 (mrad)
Sensor4 angle, a4 (mrad)
Half-stereo angle, half-stereo (mrad)

6500
37,000
38,50037,000
00
64,09064,090
0
0
0
0
20

30
30
100
30
10
5
10
10
0.13
0.13
0.13
0.13
0.13

within the tolerances. However, occasionally,
there is a measurement at the limit of thetolerance. Similar results are accomplished formodules made by the UK-B cluster.
(2) Out-of-plane survey. The z-height of eachsensor in the module is measured at a matrixof 5  5 points. In addition, three points aremeasured on the surfaces of the cooling
contact and the far-end facings on the back38

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

Fig. 11. An example of in-plane survey: deviations from thenominal of midyf for 7 KEK prototype modules.

side of the module, deﬁning the module plane
as mounted on the supporting CFRP brackets. The out-of-plane tolerances are constrained by two factors; the maximum heightof a sensor surface from the nominal should
be o200 mm to keep at least a 1 mm ‘stay
clear’ distance between modules and theresiduals in z-ﬂatness should be o50 mm for
global track ﬁtting. Fig. 12(a) plots the maximum deviation of the sensor surface from thenominal z position with respect to the module
plane. The z-proﬁle measurement includes anintrinsic and systematic bowing in the sensorsof about 50 mm. The intrinsic bowing can be
expressed by a universal shape function. In
addition, the left-right mid-planes (deﬁned by
the average of top and bottom sensor heights)
are not on a ﬂat single plane. The latter can beﬁtted with z ¼ ax þ by þ c separately in theleft and the right sides, requiring 6 parameters. Fig. 12(b) plots the maximum
deviation of ray sensor from the ‘predicted’z-proﬁle with these two corrections of sensor
bowing and left-right asymmetry. All modules
satisfy the speciﬁcation of o50 mm for deviation.
(3) Thermal cycling and long-term tests. The samein-plane and out-of-plane metrology is repeated after thermal cycles (30B+601C,
repeated 5 times) and long-term test (24 h withhybrid power on, hybrid temperature at
B301C). Though the in-plane surveys show
no change, a change was observed in the outof-plane proﬁle, majority being within 20 mm,

Fig. 12. (a) z-Deviation of the sensor surface height from thenominal z position, and (b) z-deviation after the corrections of
sensor bowing and left-right asymmetry.

except the one showing a change from 83 mm
before to 123 mm after the thermal cycles.
5.3. Electrical tests
An extensive suite of both hardware and software has been developed to facilitate module
testing. The readout system is based around the
VME modules CLOAC, MuSTARD and SLOG,
together with low voltage and high voltage
modules SCTLV and SCTHV. Two largely automated series of tests have been devised. The
Characterization Sequence is to perform fullmeasurements of a module and they are(a) power-on/veriﬁcation of response to hard
reset,(b) clock and command reception test,(c) bypass functionality test,(d) pipeline efﬁciency test,(e) strobe delay scan,(f) three point estimation of gain, noise andoffset,(g) trim-range scan,(h) determination of the response curve,(i) noise occupancy scan,(j) time-walk scan,
whereas the shorter Conﬁrmation Sequence of (b),(c), (e) and (f) provides a minimal set to ensure thedigital functioning and analogue performance.

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

The analogue performance is measured withrespect to the internal calibration circuitry of the
ABCD chip with a correction for the variation ofcalibration capacitance between batches of wafers.
A summary of the results of each test will berecorded in the database. A module is classed as
good if at least 99% of its readout strips operate
efﬁciently with low noise occupancy at 1 fC
threshold.
A very small sample of the completed barrel
modules will be uniformly irradiated at the CERN
PS to a ﬂuence of 3  1014 p/cm2 24 GeV/cprotons. After annealing for 7 days at room
temperature, they will be checked including
mechanical integrity, noise performance, full ASIC
functionality and detector leakage current. Severalof these modules will also be tested in the beam for
S=N and efﬁciency performance.

6. Thermal performance
Over a period of several years during development of the module design, several thermal
modules were built for detailed thermal study, inparticular to check the FEA calculations regarding
thermal runaway of irradiated detectors. Agreement has been good.
The temperature proﬁle has been measured for asample of un-irradiated modules by using a infrared thermo viewer. The measurement is only
useful, due to reﬂection by sensor surfaces, forestimating the temperature of the hybrids and thefacings. Thermo viewing is not a technique thatwill be used for every module during production,
but can be a useful diagnostic tool. A typicalexample of the measured temperature proﬁleshowed that the highest temperature on the hybrid
was 11–121C at the location of the ASICs withrespect to that of the baseboard. The thermal FEA
simulation is consistent with these measurements.
A thermal module was built using irradiatedsilicon sensors in which the ASIC heat generation
was simulated with dummy heaters. This module
was cooled via a cooling pipe thermally attachedto the BeO cooling facing as planned in the ﬁnal
setup. With the hybrid power of 8.1 W and at a
coolant temperature of around 101C, no thermal

39

runaway was observed. The power dissipation in
the sensor bulk was about 130 mW/mm2 normalized to 01C. The result is in agreement with FEA
simulation.

7. Electrical performances
The number of electrical barrel modules islimited by the supply of ASICs. So far 26 modules
were constructed including modules with previous
ASIC versions of ABCD2Ts and ABCD3Ts.
7.1. Noise and noise occupancy of un-irradiatedmodulesAt room temperature, the measured noise is in
the region of 1400–1700 ENC. At the operating
temperature of B01C on the hybrid, the noise isreduced to typically about 1350 ENC. This
corresponds to an expected S=N value of better
than 14. The noise of each individual readout
channels of an ABCD3TA module, operated
warm, is shown in Fig. 13. A uniform noise
distribution is seen, apart from a single channelon the module that is bonded to only a 6 cm lengthof sensor.
Fig. 14 shows the hit occupancy dependence ofthe threshold, summed over all channels of amodule with un-irradiated sensors and ABCD3TA chips. Its smooth Gaussian-like shape is a strong
evidence of the electrical stability of the readout
system. The target threshold is set at 1 fC to ensure

Fig. 13. Measured noise values (ENC) on all channels of
ABCD3T-A module 20220170100018.

40

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

Fig. 15. Trim DAC setting (upper graph) and trimmed threshold (lower) versus channel number for a module with ABCD3TA after irradiation to 3  1014 p/cm2.
Fig. 14. Mean noise occupancy of all channels of ABCD3T-A
module 20220170100018, measured warm.

high tracking efﬁciency for particles traversing thesilicon at inclined angles. As shown in the plot, thenoise occupancy at 1 fC threshold is an order of
105 satisfying well the requirement of 104, atleast at the stand-alone module level.
7.2. Timewalk
The timewalk should be o16 ns where thetimewalk is deﬁned as the difference in time stamp
threshold over a signal range from 1.25 to 10 fC,
with the comparator threshold set to 1 fC. It canbe measured by changing the pulse height andstrobe delay of the internal calibration pulses. All
modules showed the timewalk of about 5 ns well
below the limit [12].
7.3. Noise and threshold uniformity of irradiatedmodulesFour modules with chips of ABCD3T or
ABCD3T-A (with 4 trim ranges) have been
irradiated to the full dose of 3  1014 pcm2. The
trim circuitries after the exposure was fully
functional, as Fig. 15 shows, all channels beingaligned using trim range #2 (0–180 mV) withmaximum channel-to-channel threshold variation
within 12 mV. Corresponding trim DAC settings
are also plotted, indicating fairly large thresholdchanges due to radiation. The measured ASIC
noise value when operated cold, with the hybrid
around 01C, averages about 2050 ENC, resulting

the S=N ratio of 10. Though this ratio is lower
than the target value, the noise occupancy at 1 fC
threshold is about 3  104 as measured in the testbeam.
7.4. Test beam results
Various modules have been tested at CERN andalso at KEK. Both irradiated and non-irradiatedmodules have been measured in the beam tests.
During these tests, the modules are all kept cold,
with the hybrids operating at about 01C.
(1) Charge collection and S=N ratio. The mediancharge of three modules (two unirradiated,
one irradiated), obtained from the 50%
efﬁciency point in threshold scans, as afunction of detector bias voltage is shown in
Fig. 16a. The detectors of the two un-irradiated modules have depletion voltages of
B80 V, and are normally operated at B150 V
bias. The irradiated module is operated at
B400 V bias, at which point it is seen that thecollected charge on a single strip is similar tothat for unirradiated modules. Fig. 16b plots
the S=N values in which any uncertainties in
the calibration circuitry are cancelled out.
(2) Efficiency at 1 fC threshold. The efﬁciency ofthe modules at the nominal operating threshold is typically measured to be >98% at theoperational bias voltages over the full range ofparticle incident angles, with or without amagnetic ﬁeld. This is illustrated by data fromthe CERN beam test. In Fig. 17, the efﬁciency

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

41

Fig. 17. Measured efﬁciency of the fully irradiated module at
1 fC threshold as a funciton of the beam incident angle. The
triangles are with the magnetic ﬁled of 1.56 T, the circles withzero ﬁeld. The open symbols are with a bias voltage of 300 V,
the full symbols for 450 V.

Fig. 16. Median charge in fC (top) and the S=N ratio (bottom)
versus bias voltage for two un-irradiated modules and one
module irradiated up to 3  1014 pcm2.

for an irradiated module is shown as afunction of angle. The maximum efﬁciency
recorded is a function of tracking cuts, andplateau values in excess of 99% can beobtained.
(3) Space resolution. The measured resolutions
are consistent with the expectation of the
80 mm binary strip pitch (i.e. 23 mm) for tracks
at normal incidence with the magnetic ﬁeld
on. The resolution for inclined tracks isslightly better because two strip clusters occur
with greater frequency.
7.5. System test
The goal of the system test is to run as many
modules as possible in a physical conﬁguration
which is as close as possible to the planned
ATLAS conﬁguration, thereby testing the performance of the modules in such a system andcomparing it to their stand-alone performance.
Modules are mounted on a sector of the carbon-ﬁbre-corex cylinder with dimensions very near to
those of the innermost ATLAS barrel. Modules
are powered and read out via prototype barrel
opto-harnesses, which transmit power to themodules, decode the optical clock and commands,
transmit these signals electrically to the modules,
and transform the data from the modules into
optical signals for transfer to the readout electronics. Each opto-harness serves up to six modules,
as in the ﬁnal system. Almost all patch panels andpower tapes used are true to the planned ﬁnal
ATLAS design. A photograph of the barrel sector
can be seen in Fig. 18.
For the measurement reported here, there were
ten modules in the system test. Eight of these areconsidered ‘good’ modules. With the exception of
one module, all the modules showed approximately the same noise values (within about 100
ENC) when running alone on the sector as they
did on the bench. A typical multi-module test is tomeasure the gain and noise with many modules
running in parallel. This has been performed, usinga three-point gain calculation, with the ten
modules on the sector. For this test there were
two harnesses on the left-hand half of the sector,
mounted immediately adjacent to one another,with each harness holding ﬁve modules. The
measured noise values of all ASICs on themodules, with all 10 modules in operation at a
hybrid temperature of 26–301C, are shown in42

T. Kondo et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 27–42

Fig. 18. Photograph of 10 barrel modules mounted on the
carbon-ﬁbre structure for system test.

10 years under the LHC environment are satisﬁedby the present module design.
Many new technologies are for the ﬁrst time
employed such as VHCPG baseboards, Cu/Polyimide ﬂexible hybrids, carbon–carbon bridges. The
pre-series modules produced at various assemblyclusters showed satisfactory mechanical, thermaland electrical performances. High precision assembly was well demonstrated. Various beam tests
with irradiated sensors proved the functionality ofthe modules up to the highest dose expected. The
trimADCs to achieve uniform thresholds aredemonstrated to work and to be crucial for
irradiated modules.
The system test with 10 barrel modules withclose-to-ﬁnal conﬁguration started to show goodelectrical stability.

References

Fig. 19. The measured average noises for each ASIC with 10
modules operating together on the barrel system test sector.

Fig. 19. With the exception of module 0008, thenoise values lie in the anticipated range. Module
0008 showed B200 ENC more noise on the sector.
This may be of signiﬁcance, since this is the only
module made with the ASICs attached to thehybrid via non-conducting glue. The ﬁrst resultspresented here are clearly encouraging.

8. Summary
The ﬁnal module design was successfully
reached at the ﬁnal one by devoted efforts of the
ATLAS SCT group and institutes involved. So far
26 modules were fabricated and tested. Almost all
stringent mechanical, thermal and electrical requirements on the barrel SCT modules for
precision tracking as well as for the survival over

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