Instrumentation & Measurement Magazine 24-9 - 7

Production of Very Fast MAPS in SiGe
BiCMOS Technology
Since 2013, the silicon research group of the Department of Particle
Physics (DPNC) of the University of Geneva, Switzerland
is active in the development of MAPS. After collaborating in
the development of radiation-hard MAPS for the ATLAS pixel
detector upgrade of the HL-LHC, the group concentrated on
the production of monolithic silicon pixel sensors for timing in
SiGe BiCMOS technology. Our strategy foresees the application
of this MAPS technology to produce:
◗ Small-animal PET scanners to enable ultra-high resolution
molecular imaging (TT-PET [5] and 100μPET [11]
projects);
◗ A W-Si pre-shower to enable searches for long-lived dark
matter candidate particles decaying into photons in the
FASER experiment at the LHC [12];
◗ The PicoAD detector, in which a gain layer is implemented
to achieve time resolutions below 10 ps [2].
These three projects are briefly described in the following
sections.
TT-PET and 100μPET Projects
An ultra-fast and low-noise amplifier in SiGe Heterojunction
Bipolar Transistor (HBT) technology was designed for timing
purposes. The amplifier was integrated in the prototypes
of the monolithic chip of the TT-PET project [5], a small-animal
PET scanner entirely based on monolithic pixel sensors.
With an outer diameter of 8.4 cm and a length of 5 cm, the scanner
is made of 1920 chips
for a total of ~1.5 Mpixels.
The scanner is designed
to have an unprecedented
volumetric resolution of
0.6×0.6×0.6 mm3
FWHM.
The main chal lenge
of the TT-PET sensor
is achieving 100 ps time
resolution for Minimum
Ionizing Particles (MIPs)
while keeping very low
power consumption. The
chip was designed with
pixels of 500×500 μm2
and
a fully depleted active area
thickness of 80 μm. Fig. 2a
shows the equivalent circuit
for the pixel and the
preamplifier. The bias was
applied to the pixel using
a 200 kΩ, high-resistivity
polysilicon resistor placed
in the vicinity of the pixel
well. The typical MIP signal,
generated in parallel
to a 750 fF capacitor, had a
charge of approximately
December 2021
5000 electrons and a duration of 800 ps. The requirement for
the SiGe HBT amplifier was a pulse rise time of 1 ns with an
ENC of 350 electrons RMS. The sensor was produced in the
130nm BiCMOS technology SG13S by IHP.
The demonstrator chip (Fig. 2b) containing 3×10 pixels and
a TDC with 50 ps time binning with a patented synchronization
logic [13] was thinned to 100 μm and backside metallized.
The results of a testbeam measurement at CERN are excellent
[14], with full efficiency even in the inter-pixel region and
time resolution for MIPs of 115 ps at a power consumption of
365 μW/ch. This result established the feasibility of a TOF-PET
with 30 ps time resolution with silicon sensors as the sensing
device. According to our simulations, because of the larger
charge deposition of electrons from the conversion of the
511 keV photons of a PET scanner, a time resolution of 100 ps
for MIPs in silicon corresponds to the target of 30 ps time resolution
of the TT-PET project. This relation was verified by a
measurement with a 22
Na source.
The TT-PET project recently evolved into the 100μPET
project [11] that further improves the scanner resolution with
pixels of 100×100 μm2
7μA.
area readout by a frontend operated at
High-resolution Pre-shower for the FASER
Experiment
The FASER experiment at CERN searches for dark matter at
the LHC with an alternative approach to the large LHC experiments,
since it will measure the decay of potential dark matter
Fig. 2. (a) Schematic circuit of the TT-PET pixel and preamplifier; (b) The TT-PET demonstrator chip with 3×10 pixels of
0.5×0.5 mm2
area and full readout, from [14].
IEEE Instrumentation & Measurement Magazine
7

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