Timing-Optimised 3D Silicon Sensor with Columnar Electrode Geometry

Among various silicon sensor technologies, 3D silicon sensors demonstrate significant potential for applications requiring exceptional radiation hardness and intrinsic high time resolutions. Silicon pixel sensors with columnar-type electrodes are already operational within the ATLAS experiment, serving in the previous Inner B-Layer (IBL) and the upcoming Inner Tracking (ITk) detectors. Concurrently, advancements driven by the next-generation LHCb VELO detector have led to the development of fast-timing 3D trench sensors within the INFN TimeSPOT project, achieving intrinsic time resolutions close to 10 ps. Remarkably, this performance is sustained even under irradiation levels far exceeding the expected limits for High Luminosity LHC operations. Despite these advantages, 3D trench sensors face challenges related to fabrication, as their production yields remain lower than those of the well-established columnar-type sensors. This highlights the necessity of designing a timing-optimized 3D sensor that leverages the robustness of a columnar electrode fabrication while achieving an intrinsic time resolution as close as possible to the trench-based designs. The design study addressed in this paper aimed to computationally compare the already designed and characterised TimeSPOT 3D trench sensor with alternative columnar electrode-based geometries, focusing particularly on configurations that approximate trench electrodes using parallel-oriented columnar designs. Different geometries and pixel sizes were designed, simulated, and compared. This work presents the entire design and selection effort as well as the preliminary layout of the selected pixel geometries, which are set to feature in FBK's upcoming production run in 2025.