Simulation of MAPS and a MAPS-based Inner Tracker for the Super Tau-Charm Facility

TL;DR

The study addresses the need for a low-mass, time-resolved inner tracker at STCF by evaluating MAPS-based designs through a combined TCAD–Monte Carlo workflow embedded in the OSCAR framework. Four MAPS processes and two strip-like geometries are analyzed, leading to the selection of an HR epi active-connect strip-like pixel as the baseline. A three-layer cylindrical ITKM model demonstrates average detection efficiency above 99%, spatial resolutions of $8.2\,\mu\mathrm{m}$ in $r\!-\,\phi$ and $44.8\,\mu\mathrm{m}$ in $z$, and an intrinsic sensor time resolution of $5.9\,\mathrm{ns}$ at $1\mathrm{GeV}/c$ muons, with a projected overall timing of $\sim16\,\mathrm{ns}$ due to clocking. The results support the feasibility of a MAPS-based inner tracker for STCF and establish a robust simulation pipeline within OSCAR for detector optimization and physics analyses, with potential sub-10 ns timing achievable through targeted front-end and architectural upgrades.

Abstract

Monolithic Active Pixel Sensors (MAPS) are a promising detector candidate for the inner tracker of the Super Tau-Charm Facility (STCF). To evaluate the performance of MAPS and the MAPS-based inner tracker, a dedicated simulation workflow has been developed, offering essential insights for detector design and optimization. The intrinsic characteristics of MAPS, designed using several fabrication processes and pixel geometries, were investigated through a combination of Technology Computer Aided Design (TCAD) and Monte Carlo simulations. Simulations were conducted with both minimum ionizing particles and $^{55}$Fe X-rays to assess critical parameters such as detection efficiency, cluster size, spatial resolution, and charge collection efficiency. Based on these evaluations, a MAPS sensor featuring a strip-like pixel and a high-resistivity epitaxial layer is selected as the baseline sensor design for the STCF inner tracker due to its excellent performance. Using this optimized MAPS design, a three-layer MAPS-based inner tracker was modeled and simulated. The simulation demonstrated an average detection efficiency exceeding 99%, spatial resolutions of 44.8$\rm{μm}$ in the $z$ direction and 8.2$\rm{μm}$ in the $r-φ$ direction, and an intrinsic sensor time resolution of 5.9ns for 1GeV/c $μ^-$ particles originating from the interaction point. These promising results suggest that the MAPS-based inner tracker fulfills the performance requirements of the STCF experiment.

Figures (20)

• Figure 1: Overview of the MAPS simulation workflow and key outcomes.
• Figure 2: Schematic cross section of four process variants in simulation: HR epi (a), N blanket (b), LR epi (c), and HR substrate (d) modified.
• Figure 3: Schematic top view of three different pixel geometry variants used in the simulation (not to scale): standard pixel (a), active-connect large pixel (b), and metal-connect large pixel (c).
• Figure 4: Single-pixel TCAD models for four process variants under an operational substrate bias of $V_{sub} = -6V$: (a) HR epi, (b) N blanket, (c) LR epi, and (d) HR substrate. For each process, a 3D map of the doping concentration plot and a 2D cross-sectional view of the electrostatic potential through the pixel center are shown. White contour lines in the 2D plots represent the boundaries of the depletion region.
• Figure 5: Sensor capacitance $C_s$ as function of substrate voltage $V_{sub}$ (with n-well voltage $V_{nw}$ fixed at 0.8V) for different sensor variants.