Electrical performance of a silicon micro-strip super-module prototype for theHigh-Luminosity LHC collider
A. Clarkc,∗, G. Barbierc, F. Cadouxc, M. Endod, Y. Favrec, D. Ferrerec, S. Gonzalez-Sevillac, K. Hanagakid, K. Harab,G. Iacobuccic, Y. Ikegamia, T. Korikia, D. La Marrac, M. Pohlc, Y. Takuboa, S. Teradaa, Y. Unnoa, M. Weberc
aKEK, High Energy Accelerator Research Organization, Oho 1-1, Tsukuba, Ibaraki 305-0801, JapanbUniversity of Tsukuba, School of Pure and Applied Sciences, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan
cDPNC, University of Geneva, CH 1211 Geneva 4, SwitzerlanddUniversity of Osaka, Kaneyama-cho 1-1, Toyonaka-shi, Osaka-fu 560-0043, Japan
Abstract
Following the Phase II upgrade of the CERN Large Hadron Collider (LHC) foreseen in 2022, the High Luminosity LHC (HL-LHC)is expected to deliver a peak luminosity in excess of 5 × 1034 cm−2s−1 and an integrated luminosity of 3000 fb−1 until 2030. TheATLAS Collaboration plans to replace the existing Inner Tracking Detector by a new tracker, with readout electronics as well assilicon pixel and strip sensor technology capable of maintaining the excellent tracking performance of the existing tracker in thesevere radiation and high collision rate environment of the HL-LHC. The promising super-module integration concept extends theproven design of the existing silicon strip tracker to the HL-LHC, with double-sided stereo silicon micro-strip modules assembledinto a low mass local support structure. The Super-Module R&D program is described, with reference to HL-LHC requirements,and key prototype results summarized.
Keywords: HL-LHC, ATLAS Upgrade, Silicon, Strip Module, Super-Module.
1. Introduction1
During machine shutdowns in 2013-4, 2017-8 and 2022-23,2
the Large Hadron Collider will be upgraded in steps to operate3
at a centre-of-mass collision energy√
s = 14 TeV and from4
2022 peak luminosities exceeding 5 × 1034 cm−2s−1, with a 255
nsec. bunch spacing and on average ∼140 inelastic collisions6
per bunch crossing [1]. In the period until 2030, the ATLAS7
experiment is expected to collect data for an integrated lumi-8
nosity of 3000 fb−1.9
The channel occupancy from the large track multiplicity ex-10
pected per bunch crossing as well as the integrated radiation11
damage to the front-end electronics and sensors from the colli-12
sion products, is beyond the performance capability of the exist-13
ing Inner Tracking Detector [2]. From 2022, ATLAS will install14
a new Inner Tracking Detector (ITK), with increased channel15
granularity and improved radiation hardness, to maintain or im-16
prove the track reconstruction efficiency and precision, as well17
as the b-tagging performance.18
Figure 1 shows a bench-mark layout for the ITK. The silicon-19
based tracker foresees at least 4 inner layers of 50 × 250 µm220
silicon pixels at inner radii followed by 5 double-sided stereo21
silicon strip layers, 3 with strips 80×2400 µm2 and 2 with strips22
80×9600 µm2 at radii up to ∼100 cm. This is complemented by23
6 pixel and 5 double-sided strip disks in each forward region.24
The layout ensures a 9-hit coverage in the psuedo-rapidity range25
∗Corresponding authorEmail address: [email protected] (A. Clark)
|η| ≤ 2.5. Detailed simulation studies including complex event26
signatures and high luminosity machine conditions are ongo-27
ing, to characterize and to identify weaknesses of this bench-28
mark layout.29
31 March 2011 5 Nigel Hessey, Nikhef Oxford Upgrade Week, PhaseII plenary
Utopia Layout
Details available in the <parameter book>
Figure 1: Bench-mark layout for the ATLAS Inner Tracker Up-grade.
30
Separate R&D projects address the development of radia-31
tion hard front-end readout integrated circuits (IC) in 130 nm.32
CMOS technology [3], and radiation tolerant silicon sensors33
[4], independent of the layout or the thermo-mechanical and34
readout architectures used. The Super-Module integration R&D35
project [5] extends the demonstrated merits of the existing tracker36
to the HL-LHC (mechanical and thermal stability, true stereo37
space-point reconstruction, overlaps to mitigate ambiguities of38
weak alignment modes amongst others) while addressing some39
design short-comings. A major aim is to optimise the material40
budget for both the pixel and strip detectors in the presence of41
Preprint submitted to Nuclear Instruments and Methods A January 28, 2012
vastly increased detector granularity. This dictates substantial42
material reductions for the services and power distribution, as43
well as an optimal service routing.44
This paper describes the super-module concept for the strip45
detector, where individual short-strip (SS) or long-strip (LS)46
double-sided stereo modules are attached to a light, stable carbon-47
carbon local support that is inserted into the overall tracker sup-48
port structure. The local support holds at least 12 modules de-49
pending on the layout, and allows full sensor hermiticity in φ50
and z. The concept has additional advantages of proven accu-51
racy and stability, extreme modularity for each of the assembly52
items (stucture, coolimg, modules etc.) allowing efficient qual-53
ity assurance and repairability, and parallelism of procurement54
and construction. In this paper the major components and pro-55
totype results are summarized, and future R&D requirements56
are outlined. More detailed accounts of the module and super-57
module electrical performance, the service structure and the58
data acquisition are given at this conference [6, 8] . An alterna-59
tive concept where an carbon-based foam core with integrated60
cooling pipes, and with service buses and single-sided modules61
glued to each side of the stave, is also described at this confer-62
ence [9].63
2. Double-sided silicon strip modules64
The basic individual prototype SS module is a symmetric65
double-sided module of Fig. 2 [6, 7]. Two 96 × 96 mm2 n-66
on-p silicon micro-strip sensors are glued to a central Ther-67
mal Pyrolitical Graphite (TPG) base-board, four bridged hy-68
brids (each holding two columns of ten 128-channel readout69
ABCN25 ICs) are located as pairs on both sides of the mod-70
ule and four aluminum nitride (AlN) facings are located at each71
end of the base-board. The sensors are divided into four seg-72
ments, two with axial and two with stereo strips inclined by73
40 mrad. The strip-length in each segment is ∼ 2.4 cm and74
the strip-pitch is 74.5 µm. The total number of strips per seg-75
ment is 1280. The TPG base-board provides mechanical sta-76
bility and ensures good thermal contact for heat dissipation to77
cooling blocks (Section 3). The LS module is of identical de-78
sign, but with less channels.79
HybridsAlN facing
Washers
Silicon sensor
ABCN FE chips
Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.
Figure 2: Components of the double-sided short strip module.
80
The 128-channel ABCN25 front-end readout IC [3] is fab-81
ricated in IBM CMOS6 technology. A new 256-channel IC82
(ABCN-13) and a new hybrid controller IC (HCC) are under83
design in 130 nm technology. Planned evolutions of the sensor,84
hybrid and module designs towards a possible ATLAS imple-85
mentation are considered in Section 486
The advantages of this module design include:87
a) The two sensors are mounted back-to-back allowing accu-88
rate space point reconstruction with a relative sensor align-89
ment at the 1 µm level. The modules are then centred and90
aligned precisely on a local structure.91
b) The hybrid and sensor thermal paths are independent and92
the use of low thermal expansion materials with good ther-93
mal conductivity minimises deformations during tempera-94
ture cycling. Detailed finite element calculations indicate a95
maximum longitudinal deviation of only 1.4 µm for a tem-96
perature change between room temperature and −35◦C. For97
a local support loaded with cooling tubes and cooling blocks98
as in Section 3, the sensor temperatures on the existing mod-99
ule prototype can be maintained with a temperature differ-100
ence of 11 − 13◦C for CO2 cooling at −35◦C . FEA calcula-101
tions indicate an addition temperature increase of 3◦C if the102
hybrid is directly glued to the sensors.103
c) The modules are optimized for easy handling during proto-104
typing and quality assurance (QA) studies, this being impor-105
tant for large-scale production.106
A total of 6 double sided modules have been constructed107
and additional modules are under industrialization in Japan.108
The electrical performance is described in more detail in Ref. [6].109
The noise level of individual modules using the ABCN25 IC is110
excellent, with good uniformity and a noise value of between111
564 and 590 e− ENC, as measured at 250 V bias and with a 1 fC112
input charge. The performance of the modules has been tested113
to be stable for bias voltages of beyond 500 V. Proton irradia-114
tion tests were also made at −10◦C on a single-sensor module115
at the CERN PS irradiation facility [10]. The fluence recorded116
varied between 3 × 1014 neqcm−2 and 1.4 × 1015 neqcm−2. De-117
spite a leakage current increase of 5-6 orders of magnitude, the118
noise increase was small. Two single sided modules (sensor +119
hybrid) of the stave concept have since been irradiated to an120
even larger dose, with similarly satisfactory results [9].121
3. Prototype Super Module Design Features122
The benchmark super-module concept for the strip detec-123
tor foresees double-sided SS or LS modules attached to a light,124
stable carbon-carbon local support that is end-inserted into an125
overall support structure. Depending on the final ITK layout,126
the local support will hold at least 12 modules. Details of the127
super-module will evolve prior to production (Section 4), but128
it is crucial to verify the concept mechanically, thermally and129
electrically. This section summarizes prototype super-module130
studies that aim to demonstrate the suitability of super-modules131
for the ITK at the HL-LHC.132
2
The super-module design allows significant advantages over133
more integrated concepts, during prototyping, construction, in-134
tegration and commissioning. Figure 3 shows a schematic of135
the super-module prototype layout. Modules are attached to a136
light but stiff carbon fibre support, consisting of a thin tube sup-137
porting cross beams. On each side of the support, high conduc-138
tivity cooling blocks provide mechanical attachments for the139
modules and a thermal path to cooling tubes connected with140
high conductivity grease to minimize thermal stress. On one141
side of the support, also in thermal contact to the cooling, a142
multilayer cable bus transfers signals from each module to a143
super-module controller (SMB).144
a) All components of the super-module support; the cooling,145
the service bus, the SMC, are decoupled from the module146
design and can be separately tested and assembled. Proto-147
typing is also simplified.148
b) The mechanical connection of modules is precise and repro-149
ducible, and minimizes thermal deformations. It allows for150
module overlap in z to provide full hermiticity if chosen.151
c) The cable bus and cooling pipes are thermo-mechanically152
disconnected from the modules, minimizing stress. The choice153
of connector (or wire bonding) can be made at a late stage.154
d) Although only 0.18 X 0 averaged over the active area, the155
local support is stiff and designed to be end-insertable and156
to lock into position without stress, greatly simplifying com-157
missioning.158
e) Parallelism of the procurement, construction and QA steps159
is natural and well matched to the infrastructure of univer-160
sity departments.161
f) Full repair and rework is straight forward up to the commis-162
siong step following integration.163
g) Handling is safe.164
165
Figure 4 shows an eight-module mechanical prototype. Us-166
ing an aluminium prototype, the end insertion principle and 3-167
point locking mechanism has been fully validated. Key com-168
ponents such as the support cross-beams have been fabricated169
in carbon fibre 1 and qualified. The assembly is stiff; load and170
distortion measurements are ongoing to compare with FEA cal-171
culations prior to a design iteration and the results are very en-172
couraging.173
174
Cooling plates for the prototype have so far been prototyped175
in aluminium. FEA studies, cross-checked using measurements176
of the thermal conductivity, indicate a temperature difference177
between the cooling pipe and module sensors for the final de-178
sign of 11 − 13◦C. A key issue is the high thermal conductivity179
and the radiation hard grease joint between the cooling tubes180
and the cooling blocks. The most promising grease identified181
is a non-silicon zinc oxide loaded grease, Electrolube HTCP 2,182
as used for the ATLAS Insertable B-Layer (IBL) [11]. Samples183
have been irradiated to fluences of 1.2×1016 neqcm−2}, 10 times184
1Composite Design, ch. de la Volice 11, CH 1023 Crissier2Electrolube France, 13 rue Vladimir Jankelevitch, F-77184 Emerainville
Cooling pipe Cover
Modules (TPG, Si, Hybrid,…)
Central tube – joints in plas3c
End inser:on guiding pipes
Cooling Plates x6 (prototyped in aluminum)
Cross beam (or « wings »)
Figure 3: Schematic of the existing supermodule prototypeshowing the main components after assembly.
Figure 4: Photograph of mechanical super-module prototypewith an aluminium frame replacing a carbon fibre support cylin-der. The modules and cooling plates are mounted onto theCRFP wings.
that expected for the strip detector. Even at those fluences, mea-185
3
surements of the Young Modulus and the thermal conductivity186
are within the required specifications.187
A separate 8-module electrical prototype super module has188
been constructed from aluminium, and loaded so far with four189
modules, as well as first-generation cable buses and prototype190
hybrid controller (BCC) boards. In this prototype, shown in191
Fig.5, the high-voltage distribution is individual, but the digi-192
tal voltage is provided using prototype DC-DC converters [12].193
The analog voltage for the front-end chips is provided from194
the digital voltage using on-chip linear voltage regulators. The195
stave and super module R&D projects have used the same data196
acquisition, and have collaborated closely on the electrical mea-197
surements. The electrical super module components as well198
as first prototype results are discussed in detail by Gonzalez199
[6]. The noise is uniform across all 4 modules, and less than200
∼ 600 e− ENC, see Fig.5 (lower) . This is 10 − 15 e− higher201
than for individual modules and work is ongoing to understand202
extraneous noise sources in this setup. These results will be203
confirmed using DC-DC and possibly serial powering options,204
with 8 double-sided modules installed.205
206
Figure 5: (upper) Photograph of electrical super-module proto-type with the 4 double-sided modules read via a cable bus tothe SMC card. The DC-DC converters are also visible. (lower)Average measured noise in the vertical axis (e− ENC) for eachhybrid of the 4 modules .
4. Design Evolution of the Super Module207
Using prototype n-on-p silicon strip sensors, and the pro-208
totype ABCN-25 front-end IC, the Super-Module R&D project209
is close to demonstrating the advantages of using double-sided210
modules with a light carbon fibre end-insertable support, for the211
ATLAS ITK. In this Section, the expected evolution of the con-212
cept towards a pre-production prototype during 2013 and 2014213
is briefly summarized.214
a) Independently of the supermodule, there is a design evolu-215
tion of the silicon sensor, and the development of the 256-216
channel ABCN-13 IC and a Hybrid Control Chip (HCC) in217
130 nm CMOS technology. All three items will be available218
in early 2013. The double-sided hybrid and module design219
will evolve to match these items, as shown in Fig. 6 to be220
compared with Fig. 2, with attention to optimizing the ma-221
terial budget.222
b) Subject to functionality of the above components, at least223
12 double-sided modules (depending on the layout at that224
time) of the new pre-production design will be fabricated225
and tested before and after irradiation. At the same time at226
least 12 non-electrical thermal modules (depending on the227
layout at that time) will be fabricated.228
c) Several second-generation local support structures will be229
fabricated, together with their services. A schematic of the230
expected changes is shown is Fig. 7. One structure will231
be loaded with non-electrical modules for extended thermal232
studies. A second structure will be used for integration stud-233
ies. At least one structure will be fully loaded with qualified234
double-sided modules for extended electrical and mechani-235
cal tests (see below).236
Power Card
Cooling plates and tubes
Flexible pigtail with connector and s7ffener HCC
CC + hybrid flex “U” bridge
S7ffener linking the 2 sides possibly free from
the local support
HCC
Input Filtering
GND AC coupling with HVret
1
Figure 6: Schematic showing the module design evolution, in-cluding the 256-channel ABCN-13 front-end and HCC hybridcontroller IC’s, as well as the power distribution hybrid for DC-DC or serial powering.
The above studies are essential but are technical develop-237
ments of the existing prototypes. No special difficulties are an-238
ticipated. However, two issues require further R&D.239
a) The most important open issue is the optimization both elec-240
trically and mechanically of the cable bus, together with241
a choice of DC-DC or serial power options for the digital242
low-voltage distribution, and the study of mutiplexing for243
the high-voltage distribution. This is expected to involve244
several cable bus design iterations. An enormous advantage245
of the super-module option is that this development can be246
factorised from other pre-production prototyping, and the247
decisions on powering made relatively late.248
4
Cooling'In'
Super.Module'Design'Evolu5on'
Service'bus'
TTC,$Data$&$DCS$$
PS$cable$
SMB'
Module$#1$ Module$#2$ Module$#8$
HV$cable$
DCDC
$
Extrapolated'version'with'ABCN130'&'HCC'
DCDC
$
DCDC
$
DCDC
$
DCDC
$
DCDC
$
BCC$board$ BCC$board$ BCC$board$
Cooling'In'
Cooling'In'
Service'bus'
TTC,$Data$&$DCS$fibers$
PS$cable$
DCS,$Interlock$
Cooling'In'
Opto$
SMC$
SMC'Hybrid'
Cooling'Out'
HV$cable$
Module$#1$ Module$#2$ Module$#12$
GBT$SCA$ PS'
HCC' HCC'
PS'
HCC' HCC'
PS'
HCC' HCC'
1'D.#Ferrère,#AUW#Nov.2011##
Figure 7: Evolution of the local support and super-module de-signs. The most important evolutions concern the low-massand very compact service bus, and the SMC super-module con-troller at the end of each local support.
b) The second important design issue is to minimize the total249
material budget per layer ( all items, including the modules,250
the local support, the mechanical and electrical services, and251
the global support structure), averaged over the active area,252
while maintaining the mechanical stability required for the253
support structure . The current estimate is between 0.022254
X0 and 0.024 X0 depending on the detailed design choices255
made.256
5. Conclusions257
The Super-Module R&D prototype program is working to-258
wards a silicon micro-strip tracker satisfying the HL-LHC elec-259
trical, mechanical, thermal and robustness requirements. The260
feasibility of constructing double-sided micro-strip modules sup-261
ported on light but very stable carbon-fibre supports is close to262
being established.263
Using silicon micro-strip sensors, and the ABCN-25 front-264
end readout IC from parallel R&D projects, the noise perfor-265
mance of double-sided modules has been consistently measured266
to be less than ∼ 600 e− ENC, using prototype DC-DC convert-267
ers. A support structure prototype has so far demonstrated ex-268
cellent thermo-mechanical behaviour. The extreme modularity269
of the design simplifies the development, construction, integra-270
tion and commissioning steps of the planned silicon micro-strip271
tracker.272
A realistic ”pre-production” design has started, taking into273
account expected evolutions of the sensor and front-end IC de-274
sign. The main outstanding issue is the optimization of the ca-275
ble bus and power distribution for the supermodule, taking into276
account the material budget, electrical integrity and long-term277
operational robustness.278
6. Acknowledgements279
The authors acknowledge the support of the funding author-280
ities of the collaborating institutes, including the Japan Grant-281
in-Aid for Scientific Research (A) [Grant 20244038], Japan Pri-282
ority Area [Grant 20025007] and the Swiss National Science283
Foundation and the Canton of Geneva.284
References285
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