Development of the monolithic "MALTA" CMOS sensor for the ATLAS ITk outer pixel layer

Depleted Monolithic Active Pixel Sensors (DMAPS) are an option for the outermost layer of the upgraded ATLAS ITk Pixel Detector at the CERN LHC. Two large size DMAPS named TJ MALTA and TJ Monopix were produced in a 180nm CMOS imaging process in a small collection electrode design. The TJ MALTA chip combines a low power front end with a novel matrix readout design to achieve a low power consumption of < 80mW/cm². Threshold values of ≈ 250e⁻ with a dispersion of ≈ 30e⁻ and an ENC of < 10e⁻ can be achieved before irradiation which is consistent with the results from TJ Monopix. Test beam measurements indicate an average efficiency of 96% before irradiation, with the inefficiency mainly due to a non fully functional masking scheme, forcing operation at relatively high thresholds. After neutron irradiation to 1 ×10¹⁵ n_eq/cm² the efficiency in pixel centres is retained, but it is reduced in pixel corners. A proposal to improve charge collection in the corners is backed up by TCAD simulations and promises an improved performance with small modifications.