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

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.

💡 Research Summary

The paper presents a comprehensive simulation study aimed at evaluating and optimizing Monolithic Active Pixel Sensors (MAPS) for the inner tracking system of the proposed Super Tau‑Charm Facility (STCF), a next‑generation e⁺e⁻ collider operating in the 2–7 GeV energy range with a peak luminosity exceeding 5 × 10³⁴ cm⁻² s⁻¹. The authors develop a dedicated workflow that couples Technology Computer‑Aided Design (TCAD) with full Monte‑Carlo (MC) simulations within the OSCAR framework, allowing them to model sensor physics from the semiconductor level up to the full detector response.

Four CMOS processes are investigated: (1) a high‑resistivity epitaxial (HR‑epi) process based on 180 nm TowerJazz technology, (2) an N‑blanket variant of the same process that adds a low‑dose n‑type implant to enlarge the depletion region, (3) a low‑resistivity epitaxial (LR‑epi) process in 90 nm BCIS technology, and (4) a high‑resistivity substrate (HR‑substrate) process in 130 nm GSMC technology. For each process, a 33 µm × 33 µm pixel layout is built in TCAD, and both static (Poisson‑continuity, C‑V) and transient (charge drift/diffusion, recombination) simulations are performed. The static simulations provide electric field maps, capacitance, and depletion depth, while the transient simulations inject a uniform line of electron‑hole pairs (60 e⁻/µm) to emulate a minimum ionizing particle (MIP) or a ⁵⁵Fe X‑ray.

The MC chain uses Geant4 to simulate particle passage, energy loss, multiple scattering, and δ‑ray production. Energy deposits are converted to electron‑hole pairs (3.64 eV per pair) and fed into a custom carrier transport model adapted from Allpix‑2, which accounts for drift in the local electric field, thermal diffusion, and Shockley‑Read‑Hall/Auger recombination. The induced current on the collection n‑well is calculated via the Ramo‑Shockley theorem using weighting potentials pre‑computed from TCAD. Clustering and charge‑weighted centroid reconstruction are then applied to obtain hit positions.

A key design innovation explored is a strip‑like pixel geometry: one dimension is elongated to 170 µm while the orthogonal dimension remains 33 µm. This geometry aligns the long pitch with the beam (z) direction, reducing the number of readout channels and power consumption (target < 50 mW cm⁻²) while preserving the required r‑φ spatial resolution (< 100 µm).

Simulation results show that the combination of the HR‑epi process with the N‑blanket implant yields the best overall performance: charge collection efficiency above 99 %, fast collection times (< 6 ns), and robust operation under the anticipated radiation levels (1 Mrad yr⁻¹, 1 × 10¹¹ n_eq cm⁻² yr⁻¹). Using this optimized sensor, a three‑layer inner tracker (ITKM) is built, each layer contributing roughly 0.3 % X₀ to the material budget. Full detector simulations demonstrate an average detection efficiency of 99.2 %, spatial resolutions of 44.8 µm in the z direction and 8.2 µm in r‑φ, and an intrinsic time resolution of 5.9 ns for 1 GeV/c muons originating from the interaction point. These figures comfortably satisfy the STCF requirements of < 100 µm hit position resolution, ~20 ns timing, > 1 MHz cm⁻² hit rate capability, and low power consumption.

The authors also discuss the impact of radiation damage on the various processes. The N‑blanket variant maintains a large depleted volume even after substantial bulk damage, preserving charge collection efficiency above 95 % at the end of a ten‑year operation scenario.

In conclusion, the paper delivers a validated, end‑to‑end simulation framework that can guide MAPS sensor selection and inner tracker design for high‑luminosity, low‑material experiments. The chosen strip‑like MAPS with a high‑resistivity epitaxial layer and N‑blanket implant emerges as the baseline for the STCF inner tracker, offering > 99 % detection efficiency, sub‑10 µm spatial precision, and sub‑6 ns timing while staying within stringent power and material constraints. The work sets the stage for prototype fabrication and beam‑test verification, and the methodology is readily transferable to other future collider experiments requiring ultra‑thin, fast silicon pixel detectors.