Characterization Of MOSS For The ALICE ITS3 For The LHC Run 4 (2502.13591v2)

Abstract: ALICE, one of the four main CERN LHC experiments, studies heavy-ion collisions at ultra-relativistic energies. For LHC Run 4 (starting 2030), ALICE will replace the three innermost cylindrical layers of its inner tracking system during Long Shutdown 3 (2026-2029). The new ITS3 will enhance pointing resolution by a factor of two and improve tracking efficiency at low transverse momentum (pT < 0.3 GeV/c). It will feature ultra-thin (< 50 micrometer), stitched wafer-scale Monolithic Active Pixel Sensors (MAPS) using 65 nm CMOS imaging technology. These sensors can be bent, forming a truly cylindrical barrel with an innermost radius of 19 mm and an average material budget of 0.09 X/X0 per layer. ITS3 development involves cutting-edge R&D, including MAPS production in 65 nm CMOS, stitched wafer-scale fabrication, ultra-light mechanics, and a novel air cooling system. The 65 nm CMOS technology was validated using Multi-Layer Reticle 1 test structures. In mid-2023, the first stitched prototypes, MOnolithic Stitched Sensors (MOSS), were produced to demonstrate stitching feasibility and assess wafer-scale sensor yield and performance. Each MOSS chip (1.4 cm x 25.9 cm) contains 6.7 million pixels across 10 repeated sensor units, each with 8 pixel matrices: top matrices (256 x 256 pixels, 22.5 micrometer pitch) and bottom matrices (320 x 320 pixels, 18 micrometer pitch). These layouts help evaluate yield variations based on density and spacing. This work presents results from MOSS characterization, including power domain impedance, DAC performance, pixel readout, threshold, and fake-hit rate scans. Beam and lab tests confirmed an efficiency >99% and a Fake Hit Rate lower than 0.1 pixel-1 s-1, meeting ITS3 requirements for Run 4.