Enhancing Radiation Hardness and Granularity in HV-CMOS: The RD50-MPW4 Sensor

The latest HV-CMOS pixel sensor developed by the former CERN-RD50-CMOS group, known as the , demonstrates competitive radiation tolerance, spatial granularity, and timing resolution -- key requirements for future high-energy physics experiments such as the HL-LHC and FCC. Fabricated using a {150}{nm} CMOS process by LFoundry, it introduces several improvements over its predecessor, the RD50-MPW3, including separated power domains for reduced noise, a new backside biasing scheme, and an enhanced guard ring structure, enabling operation at bias voltages up to {800}{V}. Tests with non-irradiated samples achieved hit detection efficiencies exceeding {99.9}{%} and a spatial resolution around {16}{μ m}. Neutron-irradiated sensors were characterized using IV measurements and test-beam campaigns, confirming the sensor's robustness in high-radiation environments. The results highlight the ability of HV-CMOS technology to restore hit detection efficiency post-irradiation by increasing the applied bias voltage. Details of these measurements and timing performance are presented in this paper.