ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 579 (2007) 806–811
www.elsevier.com/locate/nimaHybrid and module designs for ATLAS silicon tracker upgrade
for super LHC
T. Kohrikia, Y. Unnoa, S. Teradaa,, Y. Ikegamia, K. Harab, K. Inoueb, A. MochizukibaHigh Energy Accelerator Research organization, KEK, Tsukuba, Ibaraki 305-0801, Japan
Institute of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japanb

Available online 31 May 2007

Abstract
Based on the ATLAS SCT experience, hybrid and module designs of a short strip silicon detector for an upgraded ATLAS innertracker for the super LHC have been pursued. At the super LHC, which provides 10 times more intense beam collisions than the LHC,
the detectors envisage a very high rate operation in a much harsher radiation environment. Immediate concern is an increase of leakage
current due to a radiation damage, which may induce a catastrophic break down of a thermal runaway. We have deduced a simple modelfor the thermal runaway, which can be conveniently utilized for the module designs. The thermal runaway model was veriﬁed bycomparing with the corresponding FEM thermal analyses.
The upgraded module is structurally similar to the present SCT barrel module. Two rectangular sensors, each of 12:5  6:4 cm2 , aresandwiched along with a thermo-mechanical baseboard to provide two-dimensional space points. A one-piece wrap-around hybrid
bridged over the sensors serves both the top and bottom sensors. Mounting and cooling schemes also follow the present barrel SCT
designs with necessary modiﬁcations and improvements.
r 2007 Elsevier B.V. All rights reserved.
PACS: 29.40.Gx; 29.40.Wx; 61.80.x; 61.82.Fk
Keywords: Silicon strip detector; Thermal runaway; SCT barrel module; ATLAS tracker upgrade; Super LHC

1. Introduction
The module and service structure designs of the present
ATLAS silicon tracker (SCT) have evolved as a result of anumber of technical and performance considerations [1]. In
pursuing a silicon strip detector design for an upgrade of
the ATLAS inner tracker, it is imperative to carefully
evaluate the technical and performance issues of thecurrent SCT to understand how the current design can be
modiﬁed to adapt to the super LHC that becomes ten-foldintense in luminosity than that of LHC [2].
Unlike starting from scratch, in order to avoid unnecessary hardship in the replacement, a new inner tracker
has to follow many constraints not only an overall tracking
volume but also cabling and cooling routes and dimensions
set for the present inner tracker. A harsher radiation
Corresponding author. Tel.: +81 29 864 5355; fax: +81 29 864 2580.

E-mail address: susumu.terada@kek.jp (S. Terada).
0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2007.05.299

environment will introduce severe radiation damage on themodule, particularly, a serious increase of leakage current.
Therefore, the module needs to be carefully evaluatedconcerning thermal properties and cooling requirements.
2. Leakage current
One of the most critical issues is how to overcome tentimes harsher radiation environment. Immediate concern
in operating a silicon detector is an increase of the leakage
current that is proportional to an accumulation ofradiation [3]. The leakage current, which is estimated bytaking a damage constant of 4  1017 A/cm after annealing of 80 min at 60 1C or equivalently 14 days at 20 1C, isplotted Fig. 1 as a function of radiation ﬂuence. The
expected ﬂuences in the inner tracker [4] are overlaid in theﬁgure for the integrated luminosity of 3000 fb1 with asafety factor of 2. As the leakage current increases, adanger of starting a thermal runaway rises. The leakage

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operating temperature for the super LHC becomes about27  C rather than 7  C for the LHC.

I (T= 0°C) [µA /mm2 ]

18
16
14
12
10

r=30 cm/SLHC
3000 fb-1xSF

8

3. Thermal runaway—simple modelr = 5 cm/SLHC
3000fb-1 xSF

r =10 cm/SLHC
3000 fb-1 xSF

An exponential rise of the leakage current as an increaseof temperature invokes a catastrophic thermal behavior
called thermal runaway, in which the module is heating up
abruptly once it goes beyond some critical temperature. In
a module design and its operation, an evaluation of thiscritical temperature is in prime importance.
Approximating the leakage current behavior,t (Si) = 300 µm

6
4
2
0

0

20

40

60

80

100

120

140

Fluence [1014 1MeV-neq /cm2 ]Fig. 1. Silicon sensor leakage current as a function of radiation ﬂuence;the expected ﬂuences with a safety factor (SF) of 2 are indicated in theﬁgure for the integrated luminosity of 3000 fb1 at various distances fromthe interaction point.

10
Ratio I (T) / I (T_ref =0°C)

807IðTÞ ¼ IðT ref ÞðT=T ref Þ2 expfE g =2kB ð1=T  1=T ref Þg
to a simple exponent function IðT ref Þ2ðTT ref Þ=T half

0.01

(3)

the temperature of the module increases to T from theinitial temperature T 0 , with which the heat generation of
the sensors is zero ðQ ¼ 0Þ. As illustrated in Fig. 3, thistemperature rise,

I (T)/I (T_ref)

0.1

(2)

where T ref is an arbitrarily chosen reference temperature,
we could easily deduce a thermal runaway condition and its
scaling behavior. When the module is in operation at a bias
voltage, V, it generates a heat, Q, caused by the leakage
current, I,
Q ¼ VI1(1)

DT ¼ T  T 0

x 2 by 6.5ºC(4)is proportional to the heat generation, Q, with the thermal
resistance, R, as a coefﬁcient. Therefore,
-40

-30

-20

-10

0

10

20

30

Temperature [ °C]

Fig. 2. Silicon sensor leakage current as a function of temperature:
exponential dependence (ﬁlled diamond) and a simple exponent approximation (open square).

DT ¼ RQðTÞ
¼ RVIðTÞ ¼ RVI ðT ref Þ2ðTT ref Þ=T half
¼ RVIðT ref Þ2ðDTþT 0 T ref Þ=T half
¼ RQðT ref Þ2ðDTþT 0 T ref Þ=T half
¼ RQðT 0 Þ2DT=T half

current also affects requirements to a HV power supply
together with a front-end ampliﬁer. At high ﬂuence, inorder to keep a good signal to noise ratio, the operating
voltage may have to be raised as high as possible, e.g.
800 V, about twice that of LHC, requiring even more HV
power capacity.
By lowering the temperature, however, the leakage
current of the silicon detector can be rapidly reduced.
The temperature dependence of the leakage current insilicon is formulated as in Eq. (1) in the next section [5] andis shown in Fig. 2, with a marker of IðTÞ=IðT ref ¼ 0  CÞ.
The leakage current changes by a factor of 2 as thetemperature changes by 6.5 1C in the region around 0 1C.
The silicon detectors for the super LHC need to be farther
cooled by about 20 1C, DT ¼ 20  C, to keep the power inthe silicon detectors at the same level of those for the LHC.
Thus, by the leakage current requirement alone, theð5Þ

where we take T 0 ¼ T ref . Then,RQðT 0 Þ
DT DT=T half¼
2.
T halfT half(6)

Fig. 3. Generated heat on the sensors, Q, introduces a temperature
increase, DT, along the module which has a thermal resistance of R.

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The function in R.H.S. has a single peak at
DT ¼

4. Thermal analysis for SCT module

T halfln 2(7)

and the peak value isT half.
e ln 2(8)

Therefore, the thermal runaway condition isQðT 0 ÞRXT half.
e ln 2(9)

Using the T half is 6.5 1C, numericallyDT ¼ 9:38  C

(10)

and
QðT 0 ÞR ¼ 3:45  C

ð11ÞQðT ref ÞR ¼ 3:45  2

ðT 0 T ref Þ=T half 

C.

ð12Þ

From these we can immediately realize a kind of scaling
behavior:(1) Amount of temperature increase from the non-heating
point to the runaway point is always the sameas 9.38 1C, independent of the initial non-heating
temperature.
(2) The tolerable heat can be doubled as the initial
temperature is reduced by T half of 6.5 1C. Fig. 4
demonstrates this scaling behavior. The top curvestarted at 0 1C and rose to the runaway point of
9.38 1C at QðT ref ¼ 0  CÞR ¼ 3:45  C, while the bottom
one was able to tolerate up to twice QðT ref ¼ 0  CÞR of
6.90 1C, as it started at the temperature which was
lower by T half of 6:5  C.

12

Temperature (T) [°C]10

T0 =0ºC
T0 = -6.5ºC

8
6
4
2
0
-2
-4
-6
-8

0

2

4
Q (T_ref) * R

68

Fig. 4. Scaling behavior of the thermal runaway. The top curve (ﬁlled
diamond) started at 0 1C and rose to the runaway point of 9.38 1C at
QðT ref ¼ 0  CÞR ¼ 3:45  C, while the bottom one (open square) was
extended up to twice QðT ref ¼ 0  CÞR of 6.90 1C, since it started at thelower temperature by T half : 6:5  C.

A FEA analysis can be used to understand a thermal
behavior of the SCT module, thus deducing an averagethermal resistance of the module. Once the thermal
resistance is known, the simple runaway model can beutilized to look for an optimum condition, such as acooling temperature and allowed heat generation, by
making use of the scaling behavior.
The present SCT barrel module is made of four sensors,each of 64  64  0:3 mm3 . Shown in Fig. 5 is a drawing of
the module. Pairs of the sensors are glued on the baseboard
on the top and bottom surfaces. The baseboard (shadedarea) is made from a highly thermal conductive TPG. One
side of its extended ends is attached to a cooling pipe. The
Cu/Polyimide ﬂex-circuit-based hybrid with a pair ofcarbon-carbon plates for backing is wrapped around thesensor-baseboard sandwich. The hybrid populated withreadout ASICs, which can be a potential heat source, isbridged over the silicon sensors and attached on the bothends of the baseboard.
A detailed three-dimensional FEA thermal analysis has
been conducted with the ANSYS program [6] to the SCT
barrel module. Results are plotted in Fig. 6 together withthe simple runaway model calculations. The simple model
calculations reproduce the thermal runaway quite well withsomewhat conservative at the runaway point.
In order to develop a silicon tracker module for the super
LHC, a two-dimensional model, which is a simple
extension of the present SCT barrel module, is investigatedby means of the ANSYS program. The module for thesuper LHC is expected to generate about 10 times moreheat on the sensors, if operated at the same temperature.
Furthermore, number of ASICs on the hybrid may become
48 (four rows of ASICs), rather than 12 (single row of
ASICs) of the present module, resulting a heat generationof 24 W/module, 4 times larger than that of the present
module, provided that the upgraded module has four
sections of short strips (about 3 cm in length) on each sideof the module for accommodating a super LHCs’ highluminosity.
Relevant features and parameters used for the twodimensional model analysis are summarized in thefollowings.
 Silicon sensor, width ðwÞ: 64 mm, thickness ðtÞ: 0.3 mm,thermal conductivity ðkÞ: 126 W/m/K.
 Baseboard (TPG1400), w: 88 mm, t: 0.4 mm, thermal
conductivity in longitudinal ðkx;z Þ: 1400 W=m=K, intransverse ðky Þ: 25 W/m/K.
 BeO facing, t: 0.25 mm, k: 280 W/m/K.
 Hybrid ﬂexible circuit layer, t: 0.27 mm, kx;z : 0.2 W/m/K, ky : 1 W/m/K.
 Hybrid Carbon–Carbon backing, t: 0.3 mm, kx;z : 650
W/m/K, ky : 30 W/m/K.
 ASICs, area ðw  tÞ: 6:55  0:5 mm2 , 12/module withheat generation of 28 or 42 W/module.

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Fig. 5. Drawing of the present SCT barrel module.

4 Single end cooling using an aluminum conduit, 7 mm
square with a 4 mm diameter hole for a coolant, k:
200 W/m/K.

Temperature [ °C]20r =30 cm / 700 fb -1
120 µW/mm2

-2
-4
-6

-14 ºCFEA-17 ºC
FEA

-8
-10
-12

0

50

100

150

200

250

300

350

400

q (Si) (T=0°C) [µW/mm2 ]Fig. 6. Comparison of the detailed FEA analysis with the simple runaway
model to the present SCT barrel module: cooling wall temperature at14  C (FEA (open square), simple model (ﬁlled diamond)) and at 17  C(FEA (open circle), simple model (ﬁlled triangle)). Simple models arenormalized to the temperatures at qðSiÞ ¼ 0 mW=mm2 .

Figs. 7 and 8 show results of the two dimensional ANSYSFEA analysis. Depicted in Fig. 7 is an isothermal contourof the module, while Fig. 8 shows a runaway behavior of
the module in comparison with the simple runaway
calculations. Three sets of curves (top, 2nd top andbottom) correspond to a different coolant temperature of10 (open square), 15 (open circle) and 30  C (opentriangle), with an ASIC heat of 28 W/module, respectively.
An extreme case of six rows of ASICs, 42 W/module (thirdfrom top; ﬁlled diamond), is also shown in the ﬁgure. The
simple model calculations (solid lines) agree quite well with
the FEA analysis. Without sensor heat generation:qðSiÞ ¼ 0, module temperatures are already about 10 1C
higher than the coolant temperature. This temperatureoffset is caused by the heat ﬂux from the ASICs which

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generate 28 W/module. After bearing a planned full term
operation of 3000 fb1 integrated luminosity, at 30 cm fromthe collision point in the super LHC, the silicon sensors’
heat generation, with a contingency factor of two, reaches
to 1000 mW=mm2 at 0 1C, provided that the operating bias
voltage is 800 V. The bottom curve in Fig. 8, which starts at
20  C (coolant: 30  C), shows the module is comfortably safe against the thermal runaway with a safety factor
of more than 5. Concerning an actual operation, however,further requirement arises from the fact that the HV cables
may not be upgradable, and we may need to keep the HV
power as low as that of the present situation. This requires
even lower operating temperature of about 27  C to thesensor, therefore 37  C to the coolant.
5. Short strip module
Fig. 7. Isothermal contour of the two-dimensional ANSYS FEA analysis.

15

FEA (-10 ºC, 28 W)
FEA (-15 ºC, 28W)
FEA (-30 ºC, 42 W)
FEA (-30 ºC, 28 W)-15 ºC, 28W-30 ºC, 42W-30 ºC, 28W

Temperature [ °C]

10
5
0
-5

SLHC

-10
-15
-20
-25

0

1

2
3
4
5
q (Si) (T=0°C) [mW/mm2 ]6

7

Fig. 8. Runaway behavior of the module analyzed with a two-dimensional
ANSYS FEA in comparison with the simple runaway calculations: FEA
analyses (dashed lines with markers) and simple calculations (solid lines).
The nominal heat ﬂux of the super LHC (1 mW/mm2) is overlaid in theﬁgure.

The silicon microstrip sensors for the ATLAS tracker at
the super LHC can be fabricated from 6-in wafers. From asingle 6-in wafer, one rectangular sensor of 12:47 
6:37 cm2 is laid out. Four sections of 3 cm strip elements
are embedded in the single sensor. Two sensors are glued
back-to-back on a thermo-mechanical baseboard, similar
to the present SCT barrel modules.
A single wrap-around hybrid serves both the top andbottom sides. Since there are four sections of short strips ineach sensor, the readout ASICs of four rows have to be
populated on a single large area hybrid. The design of thehybrid with the four rows of ASICs is one of R&D issues,
together with the engineering of cooling contacts, module
ﬁxation points, thermal managements with the baseboard
design, and electrical connectivities.
Fig. 9 illustrates a basic concept of the four row ASICs
hybrid with the wrap-around conﬁguration. Although theavailable width for each row is narrower than the present one(also shown in Fig. 9), a redundancy scheme, which allows forskipping a failed chip and reading all healthy chips even with
reduced number of master chips, should be accommodated.
As illustrated in Fig. 10, the module is attached to the support

Fig. 9. Conceptual layout of the hybrid (bottom illust.) in comparison with the present one (top one).

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Fig. 10. Layout of the upgraded modules with a tilt angle of 161.

cylinder by using a bracket with a tilt angle of 161 in order tomatch the Hall angle to gather electrons for the upgraded
SCT, rather than holes for the present SCT.
6. Conclusions
We have proposed a silicon detector for the upgraded
ATLAS central tracker for the super-LHC based on thepresent ATLAS SCT barrel module. While studying
thermal properties of the module, we have come up witha simple runaway model which help us to understandthermal runaway, and thus to handle it rather conveniently. The two-dimensional ANSYS model, that representsa ‘‘full baseboard’’ module with a single-side cooling,shows a large safety margin against thermal runaway at a
coolant temperature of 30  C (sensor about 19  C). It
turns out, however, that the most imposing requirement to
the module thermally is to keep the sensor power as low as
the present SCT module. To this end, the temperature of
the silicon sensors has to be kept at 27  C or lower(coolant: less than 37  C).
The size of 12:47  6:37 cm2 sensor is obtained from a 6in wafer. It can have four sections of 3 cm strips, about aquarter of the present SCT strip length. Such a short strips

are needed to handle a high hit rate at the super LHC. Two
sensors are glued back-to-back on a thermo-mechanicalbaseboard as just like the present SCT module. A large
area hybrid with four rows of readout chips wraps aroundthe sensor-baseboard sandwich. A single end cooling is
sufﬁcient with a comfortably large safety margin against
thermal runaway.

References[1] ATLAS Inner Detector Technical Design Report, vol. I, ATLAS
TDR4, CERN/LHCC 97–16, and vol. II, ATLAS TDR5, CERN/LHCC 97–17; T. Kondo, et al., Nucl. Instr. and Meth. A 485 (2002)
27–42;
A. Abdesselam, et al., Nucl. Instr. and Meth. A 568 (2006) 642–671.
[2] O. Bruning, et al., LHC luminosity and energy upgrade: a feasibility
study, LHC Project Report 626, CERN 2002; other latest references
can be reached in the URL: hhttp://care-hhh.web.cern.ch/care-hhh/publications.htmi.
[3] M. Moll, Ph.D. Thesis, University of Hamburg, 1999, p. 99.
[4] S. Baranov, et al., ATL-GEN-2005-001, CERN, and the URL: hhttp://
dawson.web.cern.ch/dawson/ﬂuka/index.htmli.
[5] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New
York, p. 91 and associated equations.
[6] ANSYS Multiphysics, ANSYS, Inc.