Towards MightyPix, an HV-MAPS for the LHCb Mighty Tracker upgrade

The Mighty Tracker is a proposed upgrade to the downstream tracking system of LHCb for operations at luminosities of up to 1.5 × 10$^{34}$ cm$^{−2}$ s$^{−1}$ starting with the LHC Run 5 data taking period. It foresees the replacement of the most central area of the current scintillating fibre tracker with High Voltage CMOS (HV-CMOS) pixel sensors. Due to the increased luminosity, occupancy will be too high for track reconstruction in the fibre tracker and the fibres would no longer withstand the radiation damage. HV-CMOS sensors have demonstrated a significant radiation tolerance and good performance making them an ideal choice of technology for the LHCb experiment. Monolithic Active Pixel Sensors (MAPS) fabricated in HV-CMOS processes provide fast charge collection via drift and allow the implementation of the readout on the same die as the sensitive volume. Due to the use of commercial processes, these sensors can be fabricated at low cost as no hybridisation with bump bonds is required. Since they are not fully depleted, the inactive volume can be reduced by thinning to achieve a total die thickness of 50 to 100 microns. A dedicated sensor called the MightyPix is developed for this programme. It is based on the HV-MAPS families MuPix and ATLASPix and tailored to the requirements of LHCb. To demonstrate the feasibility of this technology for the LHCb environment, prototypes have been irradiated. These sensors are tested in terms of efficiency, time resolution and power dissipation in temperature controlled environments.