Instrumentation & Measurement Magazine 24-9 - 5

Monolithic Picosecond Silicon
Pixel Sensors for Future Physics:
Experiments and Applications
Giuseppe Iacobucci, Lorenzo Paolozzi and Pierpaolo Valerio
P
article-physics, space research and several other
fields of basic and applied science necessitate the
production of large area detectors made by a new
generation of thin sensors able to provide, at the same time, excellent
position and time resolution.
In the case of high-energy physics experiments, for example,
with the advent of the very large number of collisions per
bunch crossing foreseen at the High-Luminosity Large Hadron
Collider (HL-LHC) (HL-LHC) and at the future Future Circular
Collider (FCC), the density of particles close to the interaction
point due to the pileup of events will be so high that efficient
pattern-recognition and track-reconstruction will not be possible
without precise timing information. At the HL-LHC, 4D
event reconstruction will be achieved by two different detectors,
specialised either in space or in time measurement. This
expensive and complex solution will not be sustainable beyond
the HL-LHC. Monolithic Active Pixel Sensors (MAPS)
[1], which contain the sensor in the same CMOS substrate utilized
for the electronics, might provide a viable solution. MAPS
are particularly appealing since they offer all the advantages
of an industrial standard processing, avoiding the production
complexity and high cost of the bump-bonded hybrid pixel
sensors that are commonly used in high-energy physics. MAPS
are used in STAR, Mu3e and in the ALICE tracker upgrade and
were proposed for the ATLAS HL-LHC pixel upgrade.
A research project to develop MAPS able to provide accurate
space resolution and time resolution better than 10 ps is
ongoing at the University of Geneva, Switzerland. The strategy
we followed is to produce a low-noise fast amplifier using
silicon-germanium (SiGe) BiCMOS technology combined
with a novel sensor concept.
Timing with Silicon Sensors
The time resolution of silicon detectors depends on the noise
and on the fluctuations of the signal current Iind
induced by
ionizing particles. This current is expressed by the ShockleyRamo
theorem:

I qv E
i
ind  
December 2021


i d W
(1)
where qi are the charge carriers moving inside the silicon, d
is the weighting field,
v

the drift velocity of the carriers and W
expressing the coupling of the charge qi
E

in the silicon with the
readout electrode.
The factors contributing to the time resolution, and our approach
to exploit them at best, are weighting field uniformity,
capacitance matching, electronics noise and Landau noise,
presented here.
Weighting Field Uniformity
A uniform signal induction independent of the charge carrier
position in the sensor is necessary to minimize the time jitter.
This can be achieved by implementing a readout based on a
planar geometry, with pixel electrodes much wider than the
silicon bulk thickness, small inter-pixel distance and saturated
carrier drift velocity. In this condition, the drift velocity and
the electric field are both constant scalar numbers, and the induced
current can be expressed with the simple formula:
Iq
D
ind  
i
v d
i
(2)
where D is the distance between the pixel electrode and the
backplane electrode. Such working condition has an impact
on the pixel capacitance, which will be larger than the typical
value of a tracking pixel detector with equivalent space resolution.
In our approach, we favor a geometry with large pixel
area w.r.t. drift distance.
Capacitance Matching
A larger pixel capacitance translates to stricter requirements on
the front-end electronics, in particular on the high frequency
performance of the amplifier: the transistor needs to have an
input capacitance comparable to the pixel in order to match
the detector capacitance and effectively transfer charge from
the pixel to the amplifier. To match the pixel capacitance without
disrupting the performance of the amplifier, it is necessary
to exploit the Miller effect to increase the transistor feedback
capacitance from approximately 1fF to a few tens of fF. This
technique is effective if the voltage gain of the amplifier is
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
5
is

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