Speaker
Description
Monolithic active pixel sensor (MAPS) developments have pushed the detection performance in various directions, especially relative to timing where nanosecond-level precision is now considered. This evolution calls for a simultaneous upgrade of the simulation tools. We have developed a simulation flow that covers steps from signal creation in the sensitive volume to the output of the pixel digital logic that performs the time-of-arrival and time-over-threshold (ToA/ToT) measurements.
This approach adds several new features to the traditional use the of the TCAD - Allpix Squared duo:
-
Precise location of the n-well and deep p-well implants, imported into TCAD from the pixel layout, allows to fully understand and highlight crucial aspects of the pixel behavior ie. evolution of the leakage current and potential punch-through current depending on sensor biasing and irradiation levels.
-
Doping profile map with the damages introduced by the irradiation, obtained in the TCAD and exported to Allpix Squared, allows to reduce significantly the simulation time without loss in precision.
-
Precise timing description of the current induced at the collection node, guarantee an embedded simulation of the front-end electronics with realistic signal events. Moreover coupling Monte-Carlo simulation (Allpix Squared) with high precision electrical simulations (SPICE), allow to benefit from the accuracy of both tools in a single iteration.
We applied this methodology to the MAPS developed in the context of the Belle II vertex detector upgrade [1]. The SuperKEK-B collider, located in Tsukuba, Japan, and hosting the Belle II experiment, will be upgraded in 2032 to reach a luminosity of 6.0 cm$^{-2}$.s$^{-1}$. To deal with the higher hit rate generated at this luminosity a new fully pixelated vertex detector is being developed – the VTX. All the VTX detection layers will be equipped with the same MAPS: OBELIX (Optimized BELLE II pIXel sensor). OBELIX is required to operate at room temperature after a NIEL fluence of 5$\times$10$^{14}$ 1 MeV n$_{eq}$.cm$^{-2}$ and to provide time-stamping at 50 ns as baseline or 3 ns with increased power dissipation.
The OBELIX sensor chip is derived from the TJ-Monopix2 prototype (initially developed for ATLAS ITK outer layers [2]). Detailed in-beam characterization of TJ-Monopix2 has been performed to validate key performance [1, 2, 3].
In this contribution, we detail the key features of the exhaustive simulation, presents the outcome of the comparison with the TJ-Monopix2 measurements and discuss the interest of the methodology for the development of modern MAPS.
[1] M.Babeluk, “The DMAPS upgrade of the Belle II vertex detector”, NIM A 1064 (2024), 169428, DOI: 10.1016/j.nima.2024.169428.
[2] C.Bespin et al, “Development and characterization of a DMAPS chip in TowerJazz technology for high radiation environments” NIM A 1040 (2022) 167189, DOI: 10.1016/j.nima.2022.167189.
[3] D.Auguste et al, “ Upgrade of the Belle II Vertex Detector with Depleted Monolithic Active Pixel Sensors ”, JINST 20 (2025) 10, C10013, DOI : 10.1088/1748-0221/20/10/C10013.