Monolithic pixel development in TowerJazz 180 nm CMOS for the outer pixel layers in the ATLAS experiment

The upgrade of the ATLAS (The ATLAS Collaboration, 2008) tracking detector for the High-Luminosity Large Hadron Collider (LHC) at CERN requires novel radiation hard silicon sensor technologies. Significant effort has been put into the development of monolithic CMOS sensors but it has been a challenge to combine a low capacitance of the sensing node with full depletion of the sensitive layer. Low capacitance brings fast signal response at low noise combined with low analog power. Depletion of the sensitive layer causes the signal charge to be collected by drift sufficiently fast to separate hits from consecutive bunch crossings (25 ns at the LHC) and to avoid losing the charge by trapping after irradiation. This paper focuses on the characterization of charge collection properties and detection efficiency of prototype sensors originally designed in the framework of the ALICE Inner Tracking System (ITS) upgrade. The prototypes are fabricated both in the standard TowerJazz 1 1 Manufactured by Tower Semiconductor Ltd, Israel. 180 nm CMOS imager process (Senyukov et al., 2013) and in an innovative modification of this process developed in collaboration with the foundry, aimed to fully deplete the sensitive epitaxial layer and enhance the tolerance to non-ionizing energy loss. Sensors fabricated in standard and modified process variants were characterized using radioactive sources, X-ray beam and test beams before and after irradiation. Contrary to sensors manufactured in the standard process, sensors from the modified process remain fully functional even after 10$^{15}$n$_{eq}$ /cm$^2$ which is the expected NIEL radiation fluence for the outer pixel layers in the future ATLAS Inner Tracker (ITk).