Development, Characterization, and Analysis of Silicon Microstrip Detector Modules for the CBM Silicon Tracking System

The future Facility for Antiproton and Ion Research (FAIR) at GSI, Germany, will enable
scientists to create tiny droplets of cosmic matter in the laboratory—matter subject
to extreme conditions usually found in the interior of stars or during stellar
collisions. The Compressed Baryonic Matter (CBM) experiment at FAIR aims to explore the
quantum chromodynamics (QCD) phase diagram at high densities and moderate temperatures.
By colliding heavy ions at relativistic beam energies, the conditions inside
these supermassive objects can be recreated for an exceptionally short amount of time.
The CBM detector is a fixed-target multi-purpose detector designed for measuring hadrons, electrons
and muons in elementary nucleon and heavy-ion collisions over the full FAIR beam energy
range delivered by the SIS100 synchrotron.

One of the core detectors of CBM is the Silicon Tracking System (STS), responsible for
measuring the momentum and tracks of up to 700 charged particles produced in a
central nucleus-nucleus collisions. Due to the required momentum resolution, the material budget
of the STS must be minimized. Therefore, the readout electronics and the cooling and mechanical
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infrastructure are placed out of the detector acceptance. The double-sided silicon microstrip sensors
are connected to the self-triggering frontend electronics using low-mass flexible microcables with a
length of up to 50 cm.

The main goal of this thesis was to develop a high-density interconnection
technology based on copper microcables. We developed a low-mass double-layered copper
microcable at the edge of modern fabrication technology.
Based on the copper microcable, we developed a novel high-density interconnection
technology, comprising fine-grain solder paste printing on the microcable and gold stud
bumping on the die. The gold stud--solder technology combines a high automation capability
with good mechanical and electrical properties, making it an interesting technology also for
future detector systems. Building on the gold stud--solder technology, a fully customized
bonder machine was developed and constructed in hardware and software. Its main purpose
is the realization of the challenging interconnection between the microcable and the sensor.
Key components of the machine are four step motors with a sub-micron step resolution,
a dual-camera pattern recognition system, a heatable, temperature-controlled bond head and
sensor plate, as well as tailor-made mechanical supports for the STS detector modules. With the
help of this bonder machine, a full-scale STS detector module in the copper technology was built.
The noise performance of the copper module was evaluated in a bias voltage scan. Very low noise
levels were observed. Measurements of the absolute value of the signal with a radioactive source allowed
us to estimate the signal-to-noise ratio of the module. The results of these measurements give us
confidence that STS modules based on the copper technology can achieve a satisfying performance
comparable to the modules built in the aluminium technology.

Another essential component of the STS detector module is the frontend electronics chip.
During this work, the version 2.1 of the STS-XYTER readout ASIC was extensively
characterized. Noise discrepancies between odd and even channels and increasingly higher
noise towards the higher channel numbers had been observed in the predecessor chip. Our
measurements of the STS-XYTER2.1 verified that both issues were successfully resolved. Furthermore,
the noise behavior of the ASIC with respect to input load capacitance was studied. This is essential to
parametrize expected noise levels for the many kinds of detector modules employed in the STS, to which
the measured noise levels can then be compared. Measurements of the noise levels as a function of shaping
time showed that the overall noise level is practically independent of shaper peaking time. Radiation tests with
50 MeV protons were performed with copper microcables connected to the ASIC in a non-powered state.
No indications of damage to the chip and interconnects could be observed.

Finally, a complete STS detector module in aluminium technology was subjected to a
pencil-like monochromatic beam of 2.7 GeV/c protons at the Cooling Synchrotron
at the research center Jülich. Several essential performance criteria of the detector module
were evaluated. The best coincidence between the STS and the reference fiber hodoscopes
was established based on time information. An excellent time resolution of a few nanoseconds
could be demonstrated. Based on the best coincidence, the spatial resolution of the full system was
determined to be a few hundred microns. This is in line with expectations, as the resolution is limited
by the fiber hodoscope resolution. Charge distributions of 1-strip clusters showed a clear separation
between the noise and the proton signal peak, with a signal-to-noise ratio above 20 for the p-- and n-side.
The charge collection efficiency of the module was estimated to be $96 \%$. The COSY beamtime enabled
a first-time evaluation of the full analysis software chain with real data and the evaluation of the full
electronic readout chain of STS. The experience gained at COSY is immensely helpful for commissioning
and data analysis in more complex beam environments such as mCBM, where a subsample of the
CBM detectors is exposed to the particles created in a heavy-ion collision in run-time scenarios closely
resembling the final CBM environment.