Instrumentation & Measurement Magazine 24-9 - 10

Fig. 5. (a) Section of the PicoAD, showing at the top the N+ pixels interleaved by pstops, and the deep n-well that hosts the electronics. The PN gain junction is
placed in the opposite side of the P-drift region with respect to the pixels. Electrons drift towards the top, holes towards the bottom; (b) GEANT4 simulation of the
PicoAD with gain Ge
= 50, a threshold of 450 e−
and 75fF capacitance.
the gain layer, being far from the pixel structures, can be continuous.
Thus, small pixels can be realized without creating a
large number of inter-pixel zones of degraded time resolution,
which presently impact the overall performance of silicon timing
detectors with internal gain.
A full simulation of the detector and the of electronics was
performed, comprising GEANT4 for the generation of the
primary charge profile, TCAD for the electric-field, gain and
weighting-field calculations and CADENCE Spectre for the
electronics response.
We verified with a TCAD simulation that the sensor geometry
depicted in Fig. 5a (50×50 μm2
pixels, 5 μm drift
region and 2.5 μm inter-pixel spacing) ensures uniform
weighting and electric fields. In these conditions, the pixel
capacitance is calculated to be 75 fF, for which our SiGe HBTbased
amplifier can achieve ENC = 90e−
RMS (Fig. 1) and
200 ps rise time (from CADENCE simulation). Fig. 5b and Table
1 show the results of the simulation. The threshold values
achievable with our ultra-fast and low-noise SiGe amplifier
and the very fast rise of the current signal should provide a
time resolution below 10 ps, including the contribution of
the electronics. In the table, the third column shows the time
resolution of the sensor obtained from the Gaussian fit to
distributions of 1000 fully simulated GEANT4 events after
time-walk correction. The fourth column shows the contribution
from the present version of our SiGe electronics [17]
obtained by CADENCE Spectre transient noise simulation,
while the fifth column shows the sum in quadrature of the
two resolutions. The contribution of the 2 ps binning TDC
foreseen by the project is negligible and therefore not shown
in the table.
The implant of a radiation-hard gain layer is one of the
critical and challenging aspect of the project. Research and
development was started in the framework of the H2020
ATTRACT MonPicoAD project [16] that involves a direct participation
of IHP. A first PicoAD prototype, submitted in 2020
and being tested in the labs of the DPNC, shows I-V curves according
to expectations and gives confidence in the PicoAD
concept.
The MONOLITH project will fully exploit the high performance
SiGe BiCMOS to produce more advanced versions of
the ultra-fast and low-noise frontend and of the TDC and synchronisation
system that we already realized [6], [14], [17]. The
monolithic structure will permit to take full advantage of standard
industrial processes and produce an inexpensive and
thin detector that will enable future physics experiments as
well as applications that can profit of excellent Time-Of-Flight
(TOF) measurement.
The outstanding time resolution of the PicoAD sensor,
combined with the high granularity, the low material budget
and the reduced production complexity and cost of MAPS,
represents a ground-breaking instrument for basic research
that will open new horizons in particular for:
◗ Future high-energy physics experiments at colliders
◗ Rare-decay experiments featuring active targets or
precise tracking
Electron gain
15
50
100
10
Table 1 - Time resolution for the PicoAD sensor for different electron gains and thresholds
amplif
Threshold [e]
450
800
450
800
450
800
 [ps]
2.0
2.7
2.0
2.4
1.8
2.2
sensor
T
 [ps]
5.4
T
4.7
4.2
3.0
3.9
2.7
IEEE Instrumentation & Measurement Magazine
σ [ps]
5.8
total
T
5.4
4.7
3.8
4.3
3.5
December 2021

Instrumentation & Measurement Magazine 24-9

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