This thesis presents the development and characterisation of monolithic silicon pixel detectors for current and future high-energy physics experiments at the LHC, with emphasis on the design, production, and commissioning of a new high-granularity tungsten–silicon (W–Si) Pre-Shower detector for the FASER experiment, aimed at enhancing sensitivity to long-lived particles decaying into di-photon final states.
The research demonstrates the feasibility of 130 nm SiGe BiCMOS monolithic detectors with heterojunction bipolar transistors (HBTs), providing fast, low-noise front-end electronics. Prototypes with 100 µm hexagonal pixels were fabricated and tested at the CERN SPS with a 120 GeV pion beam. The first achieved >99.9% detection efficiency and 36 ps time resolution at 1.8 W/cm²; a second, with refined front-end design, reached similar timing at 0.36 W/cm², while 20 ps resolution was obtained at 2.7 W/cm². After proton irradiation up to 10¹⁶ neq/cm², efficiencies above 99.6% and ~45 ps timing were maintained by tuning bias voltages, despite the absence of dedicated radiation-hard design. These results establish SiGe HBT-based monolithic detectors as a strong candidate for future 4D tracking systems at high-luminosity colliders, with applications beyond particle physics.
Building on these results, a large-scale ASIC was developed for FASER’s W–Si Pre-Shower. It features a 208×128 matrix of 100 µm hexagonal pixels on a 200 Ωcm epitaxial wafer, organised into 13 Super Columns of 8 Super Pixels (16×16 pixels each). Independent biasing and charge readout are performed per Super Column via a frame-based analogue memory architecture with 4-bit charge digitisation (0.5–65 fC).
Two prototype ASICs validated the design, leading to production of 1152 chips in July 2024. All were individually qualified, with the best assembled into 54 detector modules (>90% yield). Threshold, noise, and charge calibrations were performed; >90% of ASICs required masking of <2% of pixels. Noise patterns were traced to production mismatches and a high-voltage routing issue affecting one ASIC position in some modules. Test-pulse charge calibration revealed significant response variations before correction, motivating per-ASIC calibration.
The 48 best modules were assembled into four detector planes, ensuring >94% active area coverage for four-plane EM shower reconstruction. Two 2024 testbeams validated integration, DAQ, and subsystem operation. A dedicated alignment procedure for the hexagonal geometry achieved sub-pixel precision, with vertical residuals reflecting the hexagonal arrangement. Preliminary plane efficiencies with 50–250 GeV electrons increased with momentum; the first plane, optimised for charge sensitivity, reached (98.5 ± 0.01)% at highest energy, with thresholds consistent with design specifications.
Installed in February 2025, the Pre-Shower is operational, significantly enhancing FASER’s sensitivity to long-lived particle decays into photons.
