Nuclear Instruments and Methods in Physics Research A 765 (2014) 91–98

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A
journal homepage: www.elsevier.com/locate/nima

Double-sided super-module R&D for the ATLAS tracker
at HL-LHC – A summary
A. Clark e,n, G. Barbier e, F. Cadoux e, M. Endo f, Y. Favre e, D. Ferrere e, S. Gonzalez-Sevilla e,K. Hanagaki f, K. Hara d, G. Iacobucci e, Y. Ikegami a, O. Jinnouchi c, D. La Marra e,
K. Nakamura a, R. Nishimura b, E. Perrin e, W. Seez e, Y. Takubo a, R. Takashima b,
S. Terada a, K. Todome c, Y. Unno a, M. Weber eaKEK, High Energy Accelerator Research Organization, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan
Department of Science Education, Kyoto University of Education, Kyoto 612-8522, Japan
Institute of Science and Engineering, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japand
Institute of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-9751, Japane
DPNC, University of Geneva, CH 1211 Geneva 4, SwitzerlandfDepartment of Physics, Osaka University, Machikaneyama-cho 1-1, Toyonaka-shi, Osaka 560-0043, Japanbc

art ic l e i nf o

a b s t r a c t

Available online 15 May 2014

Following successive upgrades of the CERN Large Hadron Collider (LHC) until approximately 2025, the Highpﬃﬃ
Luminosity LHC (HL-LHC) is expected to deliver pp collisions of centre-of-mass energy s ¼ 14 TeV with a1perlevelled peak luminosity in excess of 5  1034 cm  2 s  1 and an integrated luminosity of order 300 fb
year. The ATLAS Collaboration intends to replace the existing Inner Tracking Detector by a new tracker, withreadout electronics as well as silicon pixel and strip sensor technology capable of maintaining the excellent
mechanical and electrical performance of the existing tracker in the severe radiation and high collision rateenvironment of the HL-LHC. The super-module integration concept extends the proven design of the existing
barrel silicon strip tracker to the HL-LHC, with double-sided stereo silicon micro-strip modules assembled intoa low mass local support structure. The ﬁrst phase of the Super-Module R&D programme has been successfullycompleted, demonstrating the feasibility of the Super-Module concept. A summary is made up of the key
prototype mechanical and electrical results of the R&D, as well as a short perspective of future developments.
& 2014 Elsevier B.V. All rights reserved.

Keywords:
HL-LHCATLAS upgrade
Silicon
Strip module
Super-Module

1. Introduction
Following successive upgrades until approximately 2025, the CERN
Large Hadron Collider (HL-LHC) will deliver pp collisions at a centrepﬃﬃ
of-mass collision energy s ¼ 14 TeV, with levelled peak luminosities34
2 1
exceeding 5  10 cms[1], with a 25 ns bunch spacing andwith on average  140 inelastic collisions per bunch crossing. In
the following decade, the ATLAS experiment is expected to collect
pp collision data corresponding to an integrated luminosity of order13000 fb .
The channel occupancy from the large track multiplicity expected
per bunch crossing (more than 1000 tracks per rapidity unit), as wellas the integrated radiation damage to the front-end electronics andsensors from the collision products, is beyond the performancecapability of the existing silicon strip detector (SCT) [2]. The SCT was
designed to operate at a peak luminosity of 1034 cm  2 s  1 up to annCorresponding author.
E-mail address: Allan.Clark@cern.ch (A. Clark).

http://dx.doi.org/10.1016/j.nima.2014.05.010
0168-9002/& 2014 Elsevier B.V. All rights reserved.

equivalent 1 MeV neutron ﬂuence of 2  1014 neq cm  2 , comparedwith more than 5  1014 neq cm  2 expected at the HL-LHC. For the
HL-LHC, ATLAS intends to install a new Inner Tracking Detector (ITK)[3], with increased channel granularity and improved radiation hardness, to maintain or improve the track reconstruction efﬁciency andprecision, as well as the b-tagging performance.
The Super-Module (SM) integration R&D project [4] discussed inthis paper extends the proven merits of the existing barrel silicon
strip tracker (SCT) [5] to the HL-LHC (mechanical and thermal
stability, true stereo space-point reconstruction, sensor overlaps toprovide ofﬂine alignment constraints using reconstructed tracks [6]amongst others). It also addresses some perceived design shortcomings. In the SM concept,1 individual short-strip or long-strip
double-sided stereo (DSM) modules are screwed to a light, stablecarbon-ﬁbre local support (LS) and inserted into the overall tracker
support structure (Global Structure, GS). The LS holds at least 12

1
The SM unit includes the LS fully loaded with DSM modules and theirelectrical and thermo-mechanical services.

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A. Clark et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 91–98

DSM modules2 depending on the layout, and allows full sensor
hermiticity in ϕ and z.
The aims of the R&D project have been three-fold as follows:(a) To minimise the material budget for the pixel and strip
detectors. This objective applies not only to the DSM modules,
but also to the services and power distribution, despite thevastly increased detector granularity.
(b) To demonstrate the reliable and reproducible electrical andmechanical fabrication and operation of so-called DSM250
modules using the prototype ABCN 250 nm CMOS front-end
ASIC,3 for the period of HL-LHC operation.
(c) To demonstrate a rigid but light CFRP4 structure for the DSM
modules, together with their thermo-mechanical and electrical services, that fulﬁls the HL-LHC requirements of stability
and position accuracy.
To meet the aims of the project(a) Prototype DSM250 modules have been constructed and successfully characterised thermo-mechanically, electrically, andwith respect to irradiation [4,7], see Section 2. The construction procedure is also simpliﬁed with respect to the existing
SCT modules, and is adapted for large-scale construction at
multiple sites.
(b) Eight prototype DSM250 modules have been loaded onto analuminium-based LS that accurately reproduces the positioning and electrical connectivity of the SM. The modules havebeen successfully read out simultaneously [8], see Section 3.
(c) A prototype demonstrator CFRP mechanical structure for theLS has been fabricated and successfully tested [9]. Either as
specialised pieces or as part of the LS prototype, key attachment points have been tested, and the insertion of a SM has
been fully validated. Using the proposed design for DSM130
modules, a detailed ﬁnite element (FEA)5 model has beendeveloped for thermal, thermo-mechanical and mechanicalstudies of the LS before and after loading with DSM modules
and services [9], see Section 4.
Refs. [7–9] summarise the successful completion of the ﬁrst
phase of the SM R&D project. Section 5 brieﬂy discusses further
design and prototyping work (largely associated with the electricalservices for DSM130 modules on a LS) that is still required to
optimise a SM-based solution for the ATLAS barrel silicon strip
tracker. Also in this section, the major steps foreseen for the next
preproduction SM prototype using the DSM130 design are outlined.
Separate R&D projects address the development of radiation
hard front-end readout integrated circuits (ICs) in 130 nm CMOS
technology [10], and radiation tolerant silicon sensors [11] that areused by, but are largely decoupled from, the module design andthe thermo-mechanical or readout techniques used in Refs. [7]through [9].

2. Double-sided silicon strip modules
The prototype DSM250 modules are described in Refs. [4,7].
Fig. 1 shows a prototype DSM250 short-strip (SS) module, andFig. 1. (Upper) Schematic showing the major elements of the DSM250 module.
(Lower) Upper-side photograph of a prototype DSM250 module. The active siliconsensor area is 96  96 mm2.

Fig. 2 shows the 3D CAD design of the future DSM130 module. The
design is a symmetric double-sided module with two 96  96 mm2n-on-p silicon micro-strip sensors glued to a central Thermal
Pyrolitical Graphite (TPG) base-board. The DSM250 prototype
has four bridged hybrids, each holding two columns of ten 128channel readout ABCN 250 nm ICs [10], located as pairs on bothsides of the module. Aluminium nitride (AlN) facings are locatedat each end of the base-board. The hybrid is a 4-layer copperpolymide (Cu/PI) ﬂexible circuit designed at KEK. The sensors of
the existing short-strip design are divided into four segments, twowith axial and two with 40 mrad stereo strips of length  2:4 cm
and pitch 74:5 μm. The number of strips per segment is 1280, plus
a guard strip at each end. The TPG base-board provides mechanicalstability and ensures good thermal contact for heat dissipation
to cooling blocks (Section 4). Long-strip modules are of identicaldesign, but with less channels. The DSM130 module will be ofsimilar design but with hybrids adapted for the 256-channel ABCN
130 nm CMOS IC.
The module extends and improves the existing SCT module
design:2

The current ITK baseline foresees 13 modules.
Future DSM module prototypes (labelled DSM130 modules where relevant)will use the pre-production ABCN 130 nm ASIC that has recently been fabricated.
4
CFRP: Carbon Fibre Reinforced Polymer layup providing a light structure witha controlled CTE, using successive layers (plies) of different carbon ﬁbre orientation.
5
ABAQUS Software from Simulia Dassault Systems, Tool for multi physics FEA
simulation, see http://www.simulia.com/products/multiphysics.html.
3(a) The two sensors are mounted back-to-back allowing accurate
space point reconstruction with a relative sensor alignment at
the 71 μm level. The modules are then centred and aligned
precisely on the cooling plates of the LS using kinematic dowel
ﬁxations ( 7 10 μm).

A. Clark et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 91–98

93Fig. 3. Measured ENC noise for each channel of a typical hybrid for DSM250
modules.

Fig. 2. The CAD design of the planned DSM130 module using the 256-channel
ABCN 130 nm front-end ASIC, attached to the cooling plates and the LS. The active
silicon sensor area is 96  96 mm2.

(b) The hybrid and sensor thermal paths are independent. The use ofmaterials with low thermal expansion (CTE) and good thermal
conductivity minimises deformations during temperature cycling.
Detailed ﬁnite element (FEA) calculations exist (Section 4) indicating a maximum longitudinal deviation of less than 15 μm for atemperature change between room temperature and  35 1C. The
sensor temperature is sensitive to the contact with the coolingplates of the SM design. For the pessimistic case of thermal
contact at the attachment screws, the existing module prototype
can be maintained with a temperature difference of 11–13 1C withrespect to CO2 cooling tubes at  35 1C. FEA calculations indicate
an additional temperature increase of  4 1C if the hybrid is
directly glued to the sensors. For DSM130 modules with a thermal
grease contact along the length of the cooling plate, and ambient
convection cooling at 0 1C, the temperature difference is reduced
by several degrees (Section 4).
Sixteen DSM250 modules have been constructed at KEK (Japan)
and the University of Geneva (UniGe), including one half-module used
for radiation studies. Production has been industrialised at KEK. The
noise level for directly powered modules in a DSM test box is typicallybetween 575 and 600 electrons equivalent noise charge (ENC) at 1 fC
input charge, measured at 250 V bias and a module temperature of
approximately 35 1C [7], see Fig. 3. The noise uniformity is at the levels  25 ENC. The performance of the modules has been tested to bestable for bias voltages of beyond 500 V, over a range of temperatures.
Proton irradiation tests were also made at 10 1C on asingle-sensor module at the CERN PS irradiation facility [12]. The
1 MeV neutron equivalent ﬂuence recorded varied between 3 1014 neq cm  2 and 1:4  1015 neq cm  2 . Despite a leakage current
increase of 5–6 orders of magnitude, the noise increase was small.
Data from single-sided modules of an alternative ATLAS base-line
construction concept [13] conﬁrmed this result. However, no datafrom test beams or from radioactive sources are available.

3. Electrical performance of a super module prototype using
DSM250 modules
The benchmark SM concept for the barrel strip detector foresees
DSM modules attached to a light, stable carbon ﬁbre local support thatis end-inserted into an overall support structure. Depending on theﬁnal ITK layout, the local support will hold 12 or 13 modules. It isessential to verify this concept mechanically, thermo-mechanicallyand electrically.
DSM250 modules have been used for a prototype electrical SM,
constructed from aluminium, and loaded with 8 modules, as wellFig. 4. Schematic (upper) and photograph (lower) of electrical SM prototype withthe 8 DSM250 modules read via a cable bus to a SM Controller card. The DC–DC
converters are also visible. Taking account of a 1 mm active overlap between
modules, the total active length of the SM electrical prototype is 761 mm.

as ﬁrst-generation cable buses and prototype hybrid controller
boards. The connectivity and geometrical arrangement is close tothat foreseen for a production SM unit. In this prototype, shown in
Fig. 4, the high-voltage distribution is individual, but the digital
voltage for the ABCN 250 nm IC readout is provided using prototype DC–DC converters [14]. The analog voltage of the ABCN
250 nm IC front-end is provided from the digital voltage using
on-chip linear voltage regulators. A total of 81 920 channels areread by the data acquisition, by far the largest multi-module
system so far using the ABCN 250 nm IC.
The electrical SM prototype characterisations have been successfully completed and detailed in Ref. [8]. Selected results are noted
below:(a) The sensor bias voltage in this case has been supplied individually to each side of each module. In three modules, pin-holes
in the TPG coating resulted in coupling between the 2 sides.
Future designs will use a single bias channel for several
modules, to reduce the service material. The modules used acommon low-voltage line, and either DC–DC conversion orserial powering is foreseen for the ﬁnal DSM modules. In thecase of DC–DC converters,6 despite a 10 μm Cu shielding,

6
The DC–DC converter development is ongoing and the converters used were
early prototypes.

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A. Clark et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 91–98

1000

Input niose [ENC]

RUN-CFG-0
RUN-CFG-1-top

900

RUN-CFG-2

800
700
600
500

1

2

3

4
5
Module678

In summary, the electrical performance of the 8-DSM electrical
SM has been fully characterised, and the multi-module readout
has been successfully demonstrated. Results have been presented[8] of the ENC noise, NO and DTN. The data are extremely
satisfactory and largely according to the design expectation of
the ABCN 250 nm IC. However, in some modules when all of the
32 DC–DC converters are enabled, an increase of common-mode
noise is measured. In future SM developments using DSM130
modules (Section 5), particular attention must be paid to improve
the shielding of the DC–DC converters.

1000

Input niose [ENC]

RUN-CFG-0
RUN-CFG-1-bot

900

to the coupled effect of the top- and bottom-side DC–DC
converters.
(d) Double Trigger Noise (DTN) [15] measurements to measure
the effect of injecting 2 closely spaced triggers with read out
thresholds of successively 0.5 fC, 0.75 fC and 0.5 fC. For aspacing close to the IC pipeline length, the second trigger willrecord the occupancy of the module as the readout of the ﬁrst
trigger starts. As no charge is injected, no hits should berecorded. This is almost always the case. However, for a 0.5 fC
threshold in the CFG-2 conﬁguration, there is a large increasein the number of recorded hits, again indicating commonmode noise resulting from the coupling of the top- andbottom-side DC–DC converters.

RUN-CFG-2

800
700
600
500

1

2

3

4
5
Module678Fig. 5. The mean ENC, together with the channel-by-channel Gaussian spread of
ENC, measured for each side of each module for the cases of single module readout(CFG-0); top-side or bottom-side read-out for all 8 modules with the DC–DC
converters on the other side disabled (CFG-1 top or bottom); and all channels
measured simultaneously with all DC–DC converters enabled (CFG-2).

electromagnetic emissions (primary frequency 2 MHz) wereobserved. In future prototypes, the shielding will be an important issue.
(b) Following channel-by-channel gain and threshold calibrations,the output noise of each channel has been measured.
Data were collected at a sensor (hybrid) temperature of
approximately 20 (32) 1C. Three sets of measurements havebeen made: each side of each module measured individually(CFG-0); the top-side or bottom-side channels of the 8 modules
read out, with the DC–DC converters on the other side
disabled (CFG-1 top or bottom); and ﬁnally the case of allchannels measured simultaneously (CFG-2). The resulting
mean and spread of ENC for each side of each module areshown in Fig. 5. With the exception of module 6, the increased
ENC for full simultaneous readout was in the range 10–60 ENC.
The deteriorated performance of 2 hybrids in module 6, and adeteriorated common mode noise for module 7 as evidenced
by the noise occupancy, is not fully understood, but is related
to pickup from the DC–DC converter emissions. While theseresults are extremely promising, further shielding optimisation is needed for the future SM prototype using DSM130
modules.
(c) Noise occupancy (NO) measurements of the probability for agiven strip to record a signal due to noise have been made. For
a 100 mV threshold,7 the NO level is at most 10  8, excepting
one side of module 7 in the CFG-2 conﬁguration that showed asigniﬁcant increase in common-mode noise, due presumably

7
A 1 fC input charge corresponds to a calibrated signal in the range 130–
150 mV.

4. Super module mechanics
The current SM design foresees that a row of DSM modules,
together with their thermal and electrical services, will be mounted
on a light, stable CFRP support (LS) that can be inserted longitudinally and without stress on a support cylinder. The emphasis of
the design has been to minimise thermo-mechanical stresses while
maintaining excellent build and placement precision and a minimal
material budget. As summarised in Ref. [9], R&D has concentrated
on the development of a prototype LS satisfying the ITK requirements [3] and the development of a detailed FEA model to allow an
optimisation of the LS and SM design. The design aims to maintainor exceed the stability speciﬁcations of the existing SCT [2], while
signiﬁcantly reducing the material budget.
Fig. 6 shows a 3D CAD perspective and a cross-sectional view of
the FEA model describing the LS and fully loaded SM (in this caseusing DSM130 modules).
The exploded view of Fig. 7 (upper) shows the individualcomponents of the LS. The components are glued together inprecision jigs with an expected room-temperature precision oforder 7 10 μm. A schematic of the assembled LS attached to theglobal structure (GS) cylinder is shown in Fig. 7 (lower). The
prototype LS has been fabricated using T300 ﬁbre CFRP layups.8
Photographs of the assembled LS prototype are shown in Fig. 8.
The precision assembly of DSM250 modules on aluminium
cooling plates has been fully tested [8,9]. The cooling plates arethen precision mounted on the LS backbone. In the baseline design,
the plates are made from Carbon–Carbon (C–C) with 2-dimensional
plies, CC2D,9 that are screwed onto the PEEK pads of the LS wings(the precision of this step requires validation). Other tailor-made
CFRP layups are also being considered. To minimise the stressfrom different plate and pipe CTE, the pipe is able to move freely
8
T300 ﬁbres have been used for prototype studies but M55J ﬁbres are being
considered for future pre-production prototypes. Toray Corporation, Japan.
9
CC2D is a carbon-ﬁbre-epoxy 2-directional (0–901) fabric that is ﬁred at hightemperature to form a C–C ceramic, with the mechanical strength of carbon-ﬁbre
and thermal conductivity of carbon in the plane of the fabric. JX Nippon Oil andEnergy Corporation, Japan.

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95

Fig. 6. (Upper) A 3-dimensional perspective of the SM used as input for FEA analyses. The central tube and wings of the LS are shown in grey, and the cooling plates areshown in red. The DSM130 modules are shown in grey; one DSM130 module has been removed from the LS to show details of the structure. The electrical service bus isshown in blue. (Lower) A cross-sectional view of the fully loaded SM. (For interpretation of the references to colour in this ﬁgure caption, the reader is referred to the web
version of this paper.)with a high thermal conductivity, radiation hard grease thermal
contact.10 Conductive grease is also used to ensure thermal contact
between the module facing and the cooling plate.
The electrical service bus is also attached to the LS wings (seeFig. 6). A prototype service bus has already operated in the
prototype electrical SM [8]. An optimised design for DSM130
modules is under development, see Section 5. A key issue is tolimit thermal stresses from the service bus onto the modules; thisis discussed in FEA studies below.
The LS design currently foresees 3 kinematic ﬁxation points
along the  1275 mm SM length. These are located in Fig. 7 at
z ¼0 mm (allowing for longitudinally sliding), at z  600 mm (alsolongitudinally free) and at z  1200 mm where all degrees of
freedom are constrained to limit mechanical stresses from service
connections. The longitudinal sliding of the other ﬁxation points
prevents stresses from GS movements or small CTE mismatches
between the LS and GS (both structures have minimal CTE). The
fully loaded prototype LS of Fig. 8 (lower) has been attached to a
mock-up of the GS using ﬁxation points fabricated from PEEK. This
has allowed a full validation of the end-insertion feature of the LS
design.

10
The most promising grease candidate is the non-silicone, ZnO-ﬁlled HTCP
grease, see www.electrobe.com. Adequate thermal and mechanical behaviour ismaintained beyond the irradiation requirements of the ITK, nevertheless othergreases are being evaluated.

A detailed FEA model of a fully loaded SM, using DSM130 modules
and an optimised LS design, has been used to quantify the expectedmechanical and thermo-mechanical deformations. This FEA model has
been used to generate the SM perspective of Fig. 6. The DSM130
modules have a detailed model deﬁnition: silicon wafers on each sideof a TPG element with appropriate glue layers, a detailed hybrid
deﬁnition based on the ABCN 130 nm IC, and an AlN base plate with athermal grease contact to the cooling plate. The same thermal grease
is applied between the cooling pipes and the cooling plates. The
thermal grease is modelled using a conservative Young Modulus of
230 MPa, an over-estimate of stresses in the LS following an integrated
radiation dose expected for the HL-LHC ATLAS silicon strip tracker. A
conservative solution, assuming no thermal grease between the DSM
module and the cooling plate, is also studied. The kinematic links
between the modules and cooling plate, and between the LS and theglobal structure, are included. The service bus assumes the material
properties as used for the IBL service bus. Details of the assumed
material and thermal properties, and the different material options
considered, are described in Ref. [9].
The FEA models assumes an initial steady-state temperature ofT¼ 20 1C for all components. The ﬁnal steady-state temperatures
assume a coolant temperature of T¼  35 1C, powered sensor andon-detector electronics, and dry air circulation at T¼ 0 1C. Detailsof the assumed material and thermal properties, and the different
material options considered, are described in Ref. [9].
A simpliﬁed model using material properties of the loaded
prototype SM has been compared with deﬂection measurements

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A. Clark et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 91–98

Fig. 7. (Upper) Exploded view of the LS CFRP components, including the 7 module andservice attachment wings, the central support tube, and the LS attachments to the global
structure. (Lower) View of 2 assembled LS units on a barrel with attachment points at
3 points along the LS length (nominally at z¼ 0 mm, z  600 mm and z  1200 mmÞ.
The attachment at z¼ 0 mm is designed to capture the LS units inserted from each endof the GS. The CC2D cooling plates and titanium alloy cooling tubes are added for clarity,
but the DSM modules and the service bus are not shown.

Several options are quoted in Ref. [9]: the results quoted in thisreport concern base-line steady state operational conditions witha gravitational loading perpendicular to the plane of sensor surface(conﬁguration 2 in Ref. [9]). Taking into account the effect of
the service bus, but ignoring its own larger deformation, themaximum out-of-plane deﬂection due to the combined mechanical and thermo-mechanical stresses is  55 μm (largely gravitational), with maximum in-plane distortions (largely thermomechanical) of 15 μm. The deﬂection is sensitive to the numberof attachment links, and to the details of the CFRP backbone tube
(a 5-ply CFRP layup of M55J ﬁbre is assumed). For a 3-ply layup,
the out-of-plane deformation increases to  70 μm. Detailed
optimisations of the material budget with respect to the maximum
deformation are in progress. A mapping of the deﬂection is shownin Fig. 9. As expected, the maximum deﬂection is midway between
the global support links.
While the temperature uniformity of the sensors during
operation is small (ΔT less than  0:5 1C for a loaded SM as shownin Fig. 10), the operational temperature is sensitive to the thermal
barriers between the sensor and the coolant. For a perfect 50 μm
grease contact between the module and cooling plate, the mean
sensor temperature is expected to be T ¼  31 1C. In the case of adirect contact between the module and the cooling plate only at
the attachment screws, the mean sensor temperature is increased
by ΔT  5 1C. If the module hybrid is glued directly to the sensor,
the mean sensor temperature is also expected to increase by
ΔT  4 1C. Even if both extremes are applied, stable operation isassured for the full operational period with a sensor temperatureof less than T ¼ 20 1C.

5. Future super module R&D perspective
Both mechanically and electrically, the ﬁrst phase of the SM
R&D project has been completed. Future developments will use
the 256-channel ABCN 130 nm CMOS ASIC that has recently been
fabricated, as well as a new Hybrid Control Chip (HCC) in 130 nm
CMOS technology that is now under design and will be submitted
for fabrication in 2014.
The next phase of this project should aim to demonstrate amulti-module pre-production SM:

Fig. 8. Photograph of the assembled prototype LS on its jig, before being equipped withthe cooling plates, service bus and DSM modules (upper). Photograph after assembly of
the cooling plates, cooling tubes and dummy DSM250 modules (lower).

using a 1 kg force perpendicular to the sensor surface. The
expected FEA deﬂection of 0.5–0.6 mm can be compared with amaximum measured deﬂection of 0.65 mm.

(a) Using the most recent prototype n-in-p sensors and the 256channel ABCN 130 nm IC, several DSM130 modules following
the design of Fig. 2 will be fabricated. The DSM130 prototypes
will initially be compatible with DC–DC powering (serial
powering remains an alternative) and with a multi-module
sensor bias line. The DSM130 hybrid is now under design at
KEK. Special attention will be given to ﬁnal material optimisation, and to the DC–DC converters. Averaged over the SM
active area, the total module material is expected to be 1.59%
X0. The modules will be individually characterised in a modiﬁed version of the existing single-module test box.
(b) Subject to satisfactory performance, at least 8 DSM130 modules will be fabricated in industry. The existing electrical SM
structure will be modiﬁed to support the DSM130 prototypes.
The major change will concern the design and fabrication of a
new very compact and low mass service bus, and the implementation of the HCC hybrid controller, as well as a prototype
end-of-SM controller board (SMC), not necessarily of the ﬁnal
geometric layout. The service bus design will be based on thatof the IBL stave ﬂex [16]. The current design assumes a 20 mm
width, except in the module connector region (22 mm) toallow for low-voltage bias and interconnections between the
Al and Cu substrates. The two 50 μm thick Al layers will betapered between 11 mm and 20 mm in width. The Cu layers

A. Clark et al. / Nuclear Instruments and Methods in Physics Research A 765 (2014) 91–98

97

Fig. 9. Mapping of the total deformation due to the combined effects of gravitational load and thermal operational conditions for 4 modules of the SM. As expected, themaximum deﬂection is midway between the global support attachments. The displacement scale is in the range 0 μm (minimum, blue) to 57:7 μm (maximum, red), notconsidering the larger cable-bus displacement. (For interpretation of the references to colour in this ﬁgure caption, the reader is referred to the web version of this paper.)Fig. 10. Mapping of the temperature on the SM, assuming steady-state operation with a CO2 coolant temperature of  35 1C. The temperature scale is in the range T ¼  35 1C(blue) and T ¼ 0 1C (red). (For interpretation of the references to colour in this ﬁgure caption, the reader is referred to the web version of this paper.)will consist of two 15 μm thick layers to carry the LVDS, HV
and DCS signals and one 5 μm thick Cu mesh to control the
LVDS impedance.
(c) On the basis of the service bus performance, the bus design
will be extended to be attached on a CFRP LS (averaged over
the SM active area, the total service material is expected to be
0.11% X0). As discussed in Section 4, an FEA model will be used
for a material and design optimisation of the LS components,
prior to the fabrication of a second-generation LS (expectedmaterial budget 0.18% X0 and 0.26% X0 including the cylinderinterface brackets and inserts). Averaged over the active
DSM area, the total material is 2.17% X0 (1.91% X0) including(excluding) the LS and cylinder interface. The nominal speciﬁcation for the maximum material budget is 2% X0, excluding
the support structure but including the service bus.
(d) Ideally, as a ﬁnal step, a multi-module electrical test, as well as
test beam studies, will be made for a fully assembled SM using
this LS.

The most challenging issue is the electrical and mechanicaloptimisation of the service bus. Fortunately, an advantage of the
SM design is that this optimisation can be factorised from otherpre-production prototyping.

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6. Conclusions
A Super Module integration R&D program for the upgraded
ATLAS Inner Tracker at the proposed HL-LHC [4] has completed theﬁrst phase of a feasibility study. The results detailed in Refs. [7–9]and summarised in this presentation demonstrate the feasibility of
the Super Module concept. Work towards a realistic preproduction
prototype is well advanced, taking into account prototype evolutions of the sensor and front-end IC design. A key issue is the
optimisation and characterisation of the service bus and power
distribution for the super-module, aiming to minimise the material budget while maintaining the mechanical and electricalintegrity and the long-term operational robustness.
Acknowledgments
The authors acknowledge the support of the funding authorities of
the collaborating institutes, including the JSPS Grant-in-Aid for Scientiﬁc Research (A) [Grant 20244038], the MEXT Grant-in-Aid for
Scientiﬁc Research in Priority Area [No. 20025007] and on Innovative
Areas [No. 23104002], Japan, as well as grants from the Swiss State
Secretariat for Education, Research and Innovation, the Swiss National
Science Foundation and the Canton of Geneva.
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