The 2025 International Workshop on the High Energy Circular Electron Positron Collider

This work presents the first irradiation assessment of a pixel sensor prototype (COFFEE2) fabricated in the 55nm HV-CMOS process, targeted for the high-radiation environment of the LHCb Upgrade II. The study focuses on the KIT array within COFFEE2, which features a complete readout system. Protons were used to irradiate chips to fluences up to 1×10¹⁴ n_eq/cm² at room temperature and 7×10¹⁴ n_eq/cm² at -28°C. Post-irradiation measurements show a manageable increase in leakage current. The chip remained functional with high hit efficiency even at 10¹⁴ n_eq/cm², though a reduction in pixel response uniformity was observed. Higher depletion bias was found to mitigate performance loss. The results demonstrate the strong radiation tolerance of the 55nm HV-CMOS technology, validating its promise for future particle tracking detectors requiring micrometer spatial resolution and nanosecond timing。