Download N16-91 The Compact Muon Solenoid Silicon Tracker

The Compact Muon Solenoid Silicon Tracker:
Testing of Hybrids, Modules and Substructures
at Operating Temperature
M. Pöttgens, III. Physikalisches Institut B, RWTH Aachen,
on behalf of the CMS Tracker Collaboration
Abstract--The Compact Muon Solenoid (CMS) is one of two
general purpose detectors which are foreseen to operate at the
Large Hadron Collider (LHC), which is presently being built at
the European laboratory for particle physics (CERN) in
Switzerland.
The Central Tracker of CMS consists of a Pixel System, which
is located close to the interaction point and a Silicon Strip
Tracker (SST) which instruments the intermediate and outer
region. The SST is composed of 15148 Silicon Microstrip Detector
Modules which contain the read-out electronics (hybrids) and
sensors. These modules will be assembled into substructures with
control electronics and optics for transmitting data. The
substructures will be integrated into the subsystems of the SST.
The SST will be operated for up to ten years in the harsh
radiation environment of the LHC. The lifetime of the SST will be
extended by operating the detector at lowered temperature. The
sensors, which are very delicate parts in respect to radiation
damage, will be operated at a maximum temperature of –10°C.
Since the assembly of the modules as well as the mounting on
Substructures is done at room temperature, tests in a CMS-like
environment are necessary to prove the mechanical and electrical
stability.
I.
T
INTRODUCTION
Large Hadron Collider (LHC) is a future accelerator
which will be located at the European laboratory for
particle physics (CERN) in Switzerland, where the
construction of the machine has already begun. The principle
aim of the accelerator is to provide an environment where
evidences of the Higgs boson and supersymmetric particles
can be studied. When the machine is ready in 2007 protons
will be accelerated in two counter-rotating beams to a center of
mass energy of 14 TeV. The desired luminosity is 1033 cm-2 s-1
at the beginning, later during the high luminosity phase it will
be increased to 1034 cm-2 s-1.
At a later stage also heavy ions will be accelerated to 5.5 TeV
per nuclei. The collisions of these vast objects compared to
protons are expected to lead to a region where the density of
energy is high enough to create a quark-gluon plasma.
One of the two general purpose experiments at the LHC is the
Compact Muon Solenoid (CMS).
The central tracking system of CMS is formed by a pixel
detector and a silicon strip tracker (SST). The size of the active
HE
detection area of the SST is 198 m2, which makes it the largest
all silicon tracker ever built.
II. OPERATION CONSTRAINTS
The SST is planned to be operated in the harsh radiation
environment of the LHC for at least ten years. Some irradiated
parts of the SST will suffer an average fluence of 1.6x1014 1MeV-equivalent neutrons per cm2 during this period. For this
reason radiation hardness is a mayor issue for the constituents
of the SST.
To reduce the radiation induced loss of performance, the SST
is cooled down to avoid reverse annealing effects that
deteriorate the properties of the sensors. The temperature
needed to guarantee this is –10°C. Therefore the cooling
system has to act as a heat sink in a way that –10°C is the
maximum temperature of the silicon of all sensors. The ASICS
used in the tracker were specially designed to tolerate high
radiation doses and have shown to be still performing well
after being irradiated with doses expected during operation in
CMS.
III. HYBRIDS, MODULES AND SUBSTRUCTURES
The SST of CMS is divided into four sub-detectors. The inner
barrel (TIB) and the outer barrel (TOB) instrument the barrel
region of the SST while the forward region is instrumented by
the inner disks (TID) and the tracker end caps (TEC). Some
information on the basic electronics is given below.
A. The Hybrids
The hybrids house the read-out electronics for the silicon
sensors. They are composed of a four layer polyimide circuit
and bring power and control lines to the ASICS which are
mounted on them. Each hybrid houses four or six analogue
pipeline voltage (APV) chips as well as one phase locked loop
(PLL), one 2:1 multiplexer (MUX) and one detector control
unit (DCU) chip. The APVs shape, amplify, buffer and
multiplex the signals coming from it’s 128 analogue input
channels. The PLL is used for timing adjustment and trigger
and clock decoding. The differential output of two APVs is
multiplexed onto one differential line by the MUX chip.
Monitoring of some interesting physical values is provided by
the DCU. Such values are the temperatures of the silicon
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Ceramic
Support Piece
APV25S1:
Four-Layer
Kapton Circuit
MUX:
The mass production of modules was started in 2003. Presently
about 2100 good modules have been produced. The production
suffered from bad vias and weak bonds on hybrids as well as
shortance of qualified thick sensors.
amplifier,
shaper,
buffer
multiplexer
PLL:
clock distribution,
trigger reconstruction
and distribution
DCU:
measurement of hybrid
voltage supply, temperatures,
detector leakage current
Figure 1: A hybrid with read-out and support chips.
sensors, the hybrid and the DCU chip itself, the two hybrid
supply voltages and the current on the high voltage line used to
deplete the sensors. Before being mounted on modules a pitch
adapter is glued to the hybrid and bonded to the inputs of the
APVs. This is necessary to adjust the pitch on the APVs to the
pitch of the sensors. A picture of a hybrid is shown in Fig. 1.
The first larger amount of hybrids was delivered for the
“milestone 200” at the beginning of 2002. At that time a
ceramic substrate was used for the hybrid circuits which
caused severe problems mainly because of the high density of
vias and difficulties soldering the read-out cable to the
substrate. An R&D program had to be started to identify a
more suitable technology for the substrate. At the beginning of
2003 a four layer kapton flex circuit laminated onto a ceramic
substrate for mechanical stability was found to be the best
solution. In mid 2004 the sequence of glue and kapton layers
was modified to reduce problems during the metallization step
for the vias.
B. The Modules
The modules are the smallest detector units. They consist of a
hybrid, one or two sensors, a capton circuit which brings the
high voltage to the sensors and a carbon fiber and/or graphite
frame which serves as mechanical support. A typical module is
depicted in Fig. 2. High mechanical precision and good
reproducibility are achieved by assembling the modules on
gantry robots.
HV
Pitch
Connector Adapter
Sensors
Kapton Circuit
Hybrid
Wire Bonds
C. The Substructures
A substructure for the TEC can be seen in fig. 3. The
substructures have to provide the modules with cooling, clock,
trigger, control signals and power. The cooling is done via
titanium pipes through which a coolant is running. Small
aluminium blocks connect these pipes to the modules. Since all
control and read-out signals are transmitted to and from the
SST via optical links, the substructures have to be equipped
with optical to electrical converters. The digital opto-hybrid
module (DOHM) is used to bring digital information like
clock, trigger and slow control from the outside of the tracker
to the communication and control units (CCU). The CCUs are
organised in a token ring like architecture which implies that a
number of CCUs share one DOHM. The analogue data is just
transmitted unidirectional. Opto-hybrids convert the analogue
output of the modules and couple them into optical fibers for
off detector digitization.
Digital Opto-Hybrid
Module
Support
Cables
Cooling
Pipes
The construction of a module is finished when the electrical
connections between the strips of the sensors and the inputs of
the read-out chips are done.
Carbon Fiber
Support Plate
Inter-Connect
Board
Analogue
Opto-Hybrid
Figure 3: Picture of a fully equipped back petal. The digital
opto-hybrid module is specific for this type of petal. The back
side is also equipped with modules so that just the small area at
the edges of the petal is insensitive to ionizing particles.
The basic built up of the four substructures for the subdetectors is identical but their shapes and thus the number of
modules per substructure varies a lot. The inner barrel uses
shells on which the modules are organised in strings, the outer
barrel uses rods which will be inserted into a carbon fiber
structure, the inner disks are arranged in three rings per disk
and the end caps will be equipped with petals.
The assembly of the substructures has just started. Very
promising results concerning the performance were obtained.
Carbon Fiber Frame
Figure 2: A wedge shaped CMS module which will be used for
ring 6 in the end caps. A total number of 15148 modules will
instrument the SST.
Optical
Fibers
IV. TEST PROCEDURES
The experience gained during the pre-production series of
hybrids and modules was used to specify an optimum set of
tests which is suitable to discover potential problems. To
ensure a good quality of the test objects and to identify critical
steps in the chain of production it is crucial to record results in
a centralized database. Visual inspections are as important as
alignment checks and electrical and functional tests. The
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careful handling of the fragile objects is obligatory. In this
article just functional tests can be mentioned.
A. Tests on Hybrids
Functional tests on hybrids and modules are done with a hardand software that was developed especially for testing
purposes. The APV read-out controller (ARC) system provides
full hybrid support like power, trigger, clock and slow control
and is distributed among all institutes involved in module
production.
The first test at low temperature is done after the hybrids are
equipped with and bonded to a pitch adapter. A standard test
includes nine cycles between 20°C and –20°C and one
between –25°C and 35°C. The tests are done to prove the
mechanical and electrical stability of the objects in a CMS-like
environment. The tests are done in a special teststation where
four hybrids can be thermal cycled and read-out at a time. A
few of these test stations are operable at CERN, UCSB and
Mexico.
The set of tests performed in this test station is a pedestal and
noise run, a calibration run using the internal calibration circuit
of the APV and a test where signals are injected into the pitch
adapter lines. These tests allow to spot open connections
(missing bonds or open pitch adapter lines), short circuited and
bad APV channels. Short circuited channels have to be
removed because they can reduce the gain of the APV, opens
can be rebonded although this time consuming operation is just
done if many opens are found and bad APV channels cannot
be repaired. A hybrid with four APV is of grade A if less than
three channels are open or noisy. It fails if channel are shorted
or more then four are open or noisy. In any other case it is
graded B. For six APV hybrids less than four noisy of open
strips are tolerated for grade A, grade B is applied if less than
six channels are problematic and any number of shorted or
more than six noisy or open channels lead to grade C.
The tests at low temperature revealed no mechanical problems
but one functional problem was observed on the hybrids.
When the hybrid power is switched on the PLL starts
automatically an auto calibration cycle. When the cycle has
finished successfully clock and trigger signals are output. It
was found out that at low temperature the chip does not seem
to finish the auto calibration cycle which causes no clock and
trigger to be present on the hybrid. Since one can restart the
PLL by assessing the control registers this problem is not
considered to be severe.
At present thermal tests on hybrids are done in a much wider
range of temperature. They are extended to even destructive
tests to check the robustness of the interlayer connections since
the vias of some batches of hybrids were found to be badly
metallised. This caused a stop in hybrid production which will
be restarted when the quality of the vias has improved.
B. Tests on Modules
During the pre-series of module production a lot of experience
was gained on the behaviour of faults on modules. It was
shown that the various kinds of defects can be identified by
certain signatures in the data.
The official read-out used for single module qualification is the
ARC system. Compared to hybrid tests a high voltage power
supply for sensor depletion and a LED pulser for signal
injection is added to identify typical failures on modules. Tests
on modules are done completely automated with the ARC
software (ARCS) which also interfaces an especially designed
high voltage power supply and LED pulser.
The most frequent problems are strips that are not connected to
the read-out because of missing bonds (opens), short circuited
channels (shorts) and short circuited read-out capacitors
(pinholes). A module fails the qualification test if more than
2% of the channel are considered to be bad or the leakage
current is higher than 10 µA per sensor. The grade is B if the
number of bad channel is less than 2% but higher than 1%. If
the leakage current measured on the sensor level is exceeded
by a factor of five on a module, the grade A and B changes
into AF or BF respectively.
When module assembly and bonding is completed each
module is intensively tested in an rf-shielded box to exclude
pick up of noise from external sources which could falsify the
data. These tests are done in the bonding centers which can
repair all bonding related failures. The test of the modules at
low temperature includes typically ten cooling cycles and lasts
up to three days. Typically up to ten modules are mounted at a
time and read out electrically using a multiplexer that feeds the
date into the CMS-like system which is more similar to the
final read-out. No LED pulser is placed inside the test-box for
space and feed through reasons.
Because of their duration these tests are called long-term tests.
The decision on a standardized procedure is obliged to the
single sub-detector groups. The aim of this long-term test is to
assure the mechanical and electrical stability of the modules.
For the TEC community for example this test on single
modules is done as long as there are not enough fully equipped
petals.
C. Test on Substructures
The substructures will be installed into the final mechanics of
the tracker. Thus they are very advanced in respect to the final
system where the read-out is also done via optical fibers. Thus
they do not only allow to check the performance of the
electronics but also the cooling capability of the system as it
will be in CMS. Since the slow control is organized in a token
ring architecture a certain amount of substructures is grouped
together. The test of a single substructure is therefore done
without a digital optical link. This will only be done in system
tests, beam tests and during the final integration.
Up to now most experience on the read-out of substructures
was gained during test beams and in the system test.
Procedures for the qualification of all types of substructures
are almost completely defined. The preparation of test stands
for mass production has been finished in all centers as for
example in UCSB (Fig. 4).
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the gain also increases so that the overall performance is not
reduced.
The same investigation was done on modules to check the
influence of the sensors. These noise distributions are shown in
Fig. 6.
Figure 4: The most advanced test tool for substructures is the
multi rod burn in test stand at the University of California,
Santa Barbara, that can house up to eight rods at a time.
Entries per bin
Noise Histogram @ 20°C
V. RESULTS
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Noise Histo
Entries
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Mean
1.411
RMS
0.09274
10
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From the great amount of data just some plots can be shown
here. The focus is put on the performance of the test objects for
operation at cold.
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Noise Histogram @ -20°C
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Noise Histogram @ 20°C
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RMS
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Noise Histogram @ -20°C
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Noise in ADC Counts
Figure 5: Noise distributions of hybrid channels at cold vs.
room temperature taken in deconvolution mode. The mean
noise expressed in ADC counts rises by 0.05 while the
distribution broadens at the same time.
No additional failures of hybrids were observed at cold
compared to room temperature apart from the PLL behaviour
which turned out to be harmless. The power dissipation of the
hybrid rised by about 10% at cold when operated with
identical register settings of the APVs. It was shown that the
power dissipation does not change if the registers are tuned.
The noise distribution of the hybrids at cold and at room
temperature is shown in Fig. 5. The APVs were operated in
deconvolution mode which will be used during the high
luminosity phase of CMS. The mean noise expressed in ADC
counts is slightly higher. At the same time it was shown that
Figure 6: Noise distributions of module channels at cold vs.
room temperature taken in deconvolution mode. The mean of
the noise expressed in ADC counts rises by 0.12.
A slight increase of noise can also be seen on modules. The
channels with higher or lower noise compared to the central
distribution can be identified as bad for certain reasons.
Problems at low temperature would result in a higher number
of bad channels at cold compared to room temperature. The
change of noise versus temperature is rather small compared to
the change of signal height which is shown in Fig. 7. The
change of signal height is not to be seen as a precision
measurement since the change of temperature on the APV
internal calibration circuit is not exactly known. A precise
measurement of the signal to noise ratio can only be derived
from the signals of minimal ionizing particles on the sensors.
The change of signal height with temperature is about 12 ADC
counts which corresponds to an increase in the gain of about
16% if changes in the injection circuit is neglected.
High voltage stability of the modules is of great importance for
the operation of the tracker. Bad electrical contacts to the
sensors would result in the loss of a full module. Even worse
are modules that draw too much current because in the design
of the tracker it is not foreseen to switch off single but groups
of modules. A typical measurement of the detector leakage
current versus temperature is shown in Fig. 8. As expected the
current increases exponentially with increasing temperature.
This property of the sensors shows the importance of a
sufficient heat sink so that thermal runaway is improbable.
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Pulse Height vs. Temperature
89
85
81
Peak Mode
73
-20
-15
-10
-5
0
5
10
15
Temperature [°C]
Figure 7: Maximum pulse height of test pulses, which were
generated by the APV internal signal injection circuit, versus
temperature. The ICAL register of the APVs was set to 58
which corresponds to a injection charge of 36250 electrons.
A lot of data was taken in beam tests in 2004. A partly
equipped shell of the tracker inner barrel was tested, as well as
fully equipped petals and rods.
The data shown here is derived from two fully equipped petals.
A front petal and a back petal were cooled down and tested in
May and June 2004. This configuration corresponds to a
complete TEC control ring. In total 124 read-out channels of
51 modules were fed into two final front end digitizers (VME
FED9U) which provide 96 input channels each, final multi
service cables were used and the cooling was done in the same
way as it will be done in the experiment. The control ring was
also implemented optically with a prototype of the digital optohybrid module (DOHM). The control and read-out system was
therefore very similar to the final one. As an example the
signal
to noise ratio of modules of each TEC geometry is shown in
Fig. 9.
Signal / Noise
77
45
40
35
30
25
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15
10
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1
320 µm sensors
2
500 µm sensors
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Ring number
Deconvolution Mode
30
Signal / Noise
Pulse Height in
ADC Counts
93
all modules in deconvolution mode and higher than 25 for
peak mode. These preliminary results were gained using a
cluster finding algorithm asking for a signal to noise ratio
higher than five for the seed strip. Neighbouring strips are
added to the cluster if their ratio is higher than three. The noise
of the cluster is identified by the seed noise. A final analysis
with the official analysis tool ORCA is foreseen to be done in
future.
25
20
15
10
5
0
1
2
Ring number
Figure 9: Signal to noise ratio of TEC modules with different
geometry. The modules of rings five, six and seven are
assembled with sensors of 500 µm silicon while the other
geometries are equipped with sensors of 320 µm thickness.
The modules with thicker silicon sensors show a higher signal
to noise ratio although two sensors are mounted in series. Ring
one to four modules are equipped with only one sensor of 320
µm thickness which has a positive effect on the occupancy, the
radiation damage and the noise. The disadvantage of smaller
signal is partly compensated by the reduction of noise. The
length of the silicon sensors increases from ring 1 to ring 4 as
well as for ring 5 to ring 7. The decrease of the signal to noise
ratio from ring 5 to ring 7 shows this expected behaviour. The
thinner sensors do not show the expected behaviour which can
be explained by not optimum timing settings during this run.
VI. CONCLUSIONS
Figure 8: A typical measurement showing the influence of the
temperature on the leakage current.
The signal to noise ratio was determined in a 120 GeV pion
beam. The temperature range of the silicon lay between -6.5°C
and -16.5°C. The performance of the petal from the noise point
of view is good since the signal to noise is higher than 15 for
The tests done on hybrids, modules and substructures are very
convincing that the performance of the SST at operating
temperature will be good. No severe problems of any
components were observed due to low temperature.
A reasonable fraction of modules has been produced up to
now. The problems observed on the vias of the hybrid circuit
result in a delay of production which tightens the production
schedule.
Latest beam test results have shown that the SST is also well
advanced concerning the read-out.
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