A: Depleted Monolithic Active Pixel Sensor (DMAPS) prototypes developed in the TowerJazz 180 nm CMOS imaging process have been designed in the context of the ATLAS upgrade Phase-II at the HL-LHC. The pixel sensors are characterized by a small collection electrode (3 µm) to minimize capacitance, a small pixel size (36.4 × 36.4 µm), and are produced on high resistivity epitaxial p-type silicon. The design targets a radiation hardness of 1 × 10 15 1 MeV n eq /cm 2 , compatible with the outermost layer of the ATLAS ITK Pixel detector. This paper presents the results from characterization in particle beam tests of the Mini-MALTA prototype that implements a mask change or an additional implant to address the inefficiencies on the pixel edges. Results show full efficiency after a dose of 1 × 10 15 1 MeV n eq /cm 2 .

The ATLAS collaboration is currently investigating CMOS monolithic pixel sensors for the outermost layer of the upgrade of its Inner Tracker (ITk). For this application, two large scale prototypes featuring small collection electrode have been produced in a radiation-hard process modification of a standard 0.18 μm CMOS imaging technology: the MALTA, with a novel asynchronous readout, and the TJ MONOPIX, based on the well established “column-drain” architecture. The MALTA chip is the first full-scale prototype suitable for the development of a monolithic module for the ITk. It features a fast and low-power front-end, an architecture designed to cope with an hit-rate up to 2 MHz/mm2 without clock distribution over the matrix, hence reducing total power consumption, and LVDS drivers. Laboratory tests confirmed the performance of the asynchronous architecture expected from simulations. Extensive testbeam measurements have proved an average detection efficiency of 96% before irradiation at a threshold of ∼230 e− with dispersion of ∼36 e− and ENC lower than 10 e−. A non fully functional pixel masking scheme, forces operation at relatively high thresholds, causing inefficiency. A severe degradation of efficiency has been measured after neutron irradiation at a fluence 1 × 1015 1 MeV neq/cm2. Consistent results have been produced with the TJ MONOPIX. A correlation with inefficiency plots and pixel layout has triggered TCAD simulations, ending up to two possible solutions, implemented in a new prototype, the MiniMALTA.

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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 180 nm 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 < 80 mW/cm 2 . Threshold values of ≈ 250 e − with a dispersion of ≈ 30 e − and an ENC of < 10 e − 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 × 10 15 n eq /cm 2 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.