RD50-MPW4: a thin backside-biased High Voltage CMOS pixel chip for high radiation tolerance

The RD50-MPW prototypes are High Voltage-CMOS (HV-CMOS) pixel chips in the 150 nm technology from LFoundry S.r.l. aimed at developing monolithic silicon sensors with excellent radiation tolerance, fast timing resolution and high granularity for tracking applications in future challenging experiments in physics. RD50-MPW4, the latest prototype within this programme, implements significant improvements for a high breakdown voltage (> 400 V), and therefore an excellent radiation tolerance, through a multi-ring structure around the chip edge and substrate backside-biasing to high voltage. Fabricated samples have been irradiated with neutrons up to 10¹⁶ n_eq/cm² high fluence. Measured current-to-voltage characteristics and pixel equalisation with trimming Digital-to-Analogue Circuits (DACs), to prepare for a test beam with irradiated samples, are presented.