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Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210
Results on 0.7% X0 thick pixel modules for
the ATLAS detector
P. Netchaevaa,*, R. Beccherlea, G. Darboa, K. Einsweilerb, G. Gagliardia,
C. Gemmea, M. Gilchrieseb, P. Oppizzia, J. Richardsonb, L. Rossia, E. Ruscinoa,
F. Vernocchia, G. Znizkab
Universita" di Genova and INFN, Genoa, Italy
Lawrence Berkeley National Laboratory, USA
a
b
Presented by Polina Netchaeva on behalf of the ATLAS Pixel Collaboration
Abstract
Modules are the basic building blocks of the ATLAS pixel detector system, they are made of a silicon sensor tile
containing 46 000 pixel cells of 50 mm 400 mm, 16 front-end chips connected to the sensor through bump bonding, a
kapton flex circuit and the module controller chip. The Pixel detector is the first to encounter particles emerging from
LHC interactions, minimization of radiation length of pixel modules is therefore very important. We report here on the
construction techniques and on the operation of the first ATLAS pixel modules of 0.7% radiation length thickness. We
have operated these modules with threshold of 3700 10 300 10, mean noise value of 225 10 and 0.3% dead
channels. # 2001 Published by Elsevier Science B.V.
1. Introduction
In the ATLAS Pixel detector [1] there are 2228
modules. One module (Fig. 1) consists of a silicon
sensor tile, 16 front-end readout integrated circuits
(FE chips) and a kapton flex hybrid. The flex
hybrid distributes power and control signals to the
FE chips and allows reading them out through a
module control circuit (MCC). Passive components including termination resistors, decoupling
capacitors and temperature sensor are also included. The sensitive area of a module, i.e. of a
sensor tile, is 16.4 mm 60.8 mm. The FE chips
are connected to the pixel cells through bump
*Corresponding author.
E-mail address: [email protected] (P. Netchaeva).
bonds, which are made of Indium in the modules
described in this paper. The size of one pixel is
50 mm 400 mm and each FE chip serves 18 160
pixel cells. The flex hybrid is glued to the backside
of the sensor tile; electrical connections from the
MCC and the 16 FE chips to the flex hybrid are
done through ultrasonic wedge bonding.
The prototypes we describe do not have optical
connections or flexible power connection (pig tails)
and have been mounted on printed boards for
testing.
2. Material budget
The material budget for the modules presented in
this paper is shown in Table 1. The normalization
0168-9002/01/$ - see front matter # 2001 Published by Elsevier Science B.V.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 3 9 1 - 6
P. Netchaeva et al. / Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210
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Fig. 1. Module components.
Table 1
Material budget for the produced thinned modules
Item
%X0
Sensor (280 mm thick)
Front-end electronics (150 mm thick)
MCC (560 mm thick)
Kapton flex (50 mm thick+coverlay)
Passive components on Kapton (R, C)
Wirebond potting
Wirebonds, bump bonds and glue (Kapton on
sensor, MCC on flex hybrid) are individually
negligible and they sum up to
0.34
0.21
0.04
0.05
0.03
0.03
50.01
Total
0.70
has been done to the module sensitive area
(16.4 mm 60.8 mm). The total radiation length
value meets the ATLAS Pixel Detector Technical
Design Report [1] specifications, in particular
electronics chips thinned down to 150 mm have
been used.
3. Module production
Module production begins with bump deposition on sensor and electronics wafers followed by
the spinning of a thin polyamide layer to protect
the bumps. The sensor tiles are then singled out,
while the electronics wafers are thinned by back
grinding before the FE chips are cut out. Finally
16 FE chips are flip-chipped to the sensor tile and
the bare module is ready.
The quality of this assembly is then checked
with high (2 mm) resolution X-ray radiography as
shown in Fig. 2.
To operate the module it is necessary to dress it
with a flex hybrid. This is a two-layer circuit on a
50 mm thick kapton substrate and is described in
more detail in Ref. [2]. Flying probe test of each
flex hybrid is performed before the MCC and the
passive components are mounted. The fully loaded
flex hybrid is shown in Fig. 3 and is tested by
measuring reference voltages and reading/writing
MCC registers.
The next stage of the module production is
gluing of the flex hybrid assembly to the bare
module, the various steps of the procedure are
illustrated in Fig. 4.
The flex hybrid assembly is fixed on a special
profile plate (3) not to damage the wire-bonded
MCC and the passive components. The glue
(EPOTEK 353) is deposited on the back plane of
the flex hybrid under the wire bonding pads both
of FE chips and of MCC. Optical alignment
between bare module and flex hybrid is performed
(5) under microscope with micrometric stages. The
glue polymerization process takes 12 h at 408C,
an infrared lamp maintains this temperature.
The module is glued to the support printed
board with silicon glue (Dow Corning 740) at
room temperature. The polymerization time is
24 h. The quality of the thermal contact between
support card and module is being tested with a
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Fig. 2. Microradiography test of the sensor-FE chips bump bondings. Left picture represents the ‘‘bare’’ module assembly i.e. the
sensor tile bump bonded to the 16 FE chips. On the right picture there are two fragments of the X-rays image of this assembly where
two kinds of defects are visible: shorts between bumps and small bumps.
Fig. 3. Flex-hybrid circuit.
Fig. 4. Gluing the flex hybrid to the bare module.
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207
Fig. 5. First thin module (LBNL) threshold and noise value distributions (see text for details).
thermocamera taking images of the module in
operation.
The final step is the wire bonding of all FE chips
to the flex hybrid.
of dead pixels and the stability of all these values
during the operation.
To check the operation and define the characteristics of each pixel the following laboratory
tests are performed:
4. Module tests
*
The main quality factors of the modules are:
threshold and noise, both their mean values and
their dispersion over the entire matrix, the number
*
Digital test: digital signals are injected after
each pixel discriminator, this allows to verify
the proper functioning of the readout chain.
Analog test: analog signals are injected at each
preamplifier input, this allows to measure the
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*
P. Netchaeva et al. / Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210
threshold and noise values. After this test the
tuning of threshold values for every FE chip is
done.
Test with source: 109Cd (22 KeV g), 241Am
(60 KeV g), 90Sr (b) are used to verify the whole
chain (sensor-readout) quality. The self-triggering capability of the electronics is used in this
case.
5. Results
Up to now three 0.7% X0 thick pixel modules
have been produced. In all the three modules the
thinned FE-B electronics [3] is used.
The first thin module has been made in LBNL in
November 1998. The electronics had been thinned
to 150 mm by GDSI after bump deposition. The
bump deposition and flip-chipping had been done
by Boeing. The whole module worked well
according to the digital test, one FE chip did not
respond to analog charge injection. There have
been some bump-bonding problems, including
chips with regions of merged bumps and too large
bump resistance.
The mean threshold value for this module was
4800 10 The noise distribution had the maximum at 200 10 but a tail extending up to 1000
electrons (Fig. 5). Three FE chips were particularly
noisy, by switching them off the high noise tail
Fig. 6. Second thin module (INFN Genoa) threshold and noise value distributions. To find the maximum in the noise distribution the
Gaussian fit has been done only for the first 20 channels. The tail toward large noise values is mostly due to the larger capacitance
pixels at the periphery of each FE chip.
P. Netchaeva et al. / Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210
(>500 10) disappeared and the mean noise of
the remaining chips is of 270 10.
Boeing had since then stopped bumping for
outside customers, this delayed thin modules
production for some time.
The second thin module has been produced
in INFN Genoa in April 2000. The electronics
had been thinned to 156 mm by Okamoto.
Bump deposition and flip-chipping was done
by Alenia Marconi Systems (AMS). Digital
test had shown good results. Only about
0.3% cells did not respond to the analog
charge injection (dead channels): one FE chip
had 30 dead channels, five FE chips had about 8–
10 dead channels, five FE chips had 1–2 dead
channels and five FE chips did not have dead
channels at all.
The module works with a threshold value
3700 10 300 10 and a mean noise value of
225 10, the operating characteristics are stable
over days. The distributions are shown in Fig. 6.
The noise distribution has a maximum at 170 10
and a tail up to 600 10.
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The source (109Cd) scan plot is shown in Fig. 7.
The number of g-rays converted in the various
pixel cells is represented according to the color
scale. Since the source first encounters the flex
hybrid the shadows of electronic components and
MCC (the big square in the center) are visible.
The third thin module has been produced
in INFN Genoa in cooperation with AMS
in May 2000. It works well digitally and analog
scan shows very few dead channels (again about
0.3%) for all the FE chips except one FE chip
which did not work properly and was therefore
excluded.
We operated this module with a threshold value
of 4200 10 330 10 and a mean noise value of
225 10.
The last two thin modules were recently
operated on the H8 test beam at CERN, laboratory results have been confirmed including good
stability. As an example, Fig. 8 shows the correlation between the hits measured by the pixel
module and the hits measured by the microstrip
telescope used to reconstruct the beam tracks.
Fig. 7. Source (109Cd) scan plot of the second thin module. The shadows of electronics components and MCC (the big square in the
center) are visible. Because of a design error only 10 out of 18 columns of each FE chips are operational.
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Fig. 8. Test beam online correlation plot shows the beam coordinate as measured by the microstrip telescope (horizontal axis) versus
the same quantity as measured by a pixel module (vertical axis). The left (right) plot refers to the long (short) size of the pixel. Two
different reference systems were used for the microstrips and the module measurements, therefore the correlation angle on the online
plots is inverted.
6. Conclusions
References
Three modules of 0.7% X0 have been built and
characterized. The most recent results indicate that
we can fabricate modules with thinned electronics
which obtain high yield (0.3% of dead channels)
and operate them at low threshold (3700 10 300 10) and noise (225 10).
[1] ATLAS collaboration, ATLAS Pixel Detector TDR,
CERN/LHCC/98-13 (1998).
[2] P. Skubic, Nucl. Instr. and Meth. A 465 (2001) 219, these
proceedings.
[3] K. Einsweiler et al, LBNL, FE-B Front-end Guide V1.0,
Feb12 1999, http: //wwwphysics.lbl.gov/einsweil/Rev
Feb99/FEBManual.pdf.