A 1 μW radiation-hard front-end in a 0.18 μm CMOS process for the MALTA2 monolithic sensor

Monolithic pixel sensors integrate the sensor matrix and readout in the same silicon die, and therefore present several advantages over the more largely used hybrid detectors in high-energy physics. In this paper, a low-power, radiation-hard frontend circuit for monolithic pixel sensors, designed to meet the requirements of low noise and low pixel-to-pixel variability, the key features to achieve high detection efficiencies, is presented. The sensor features a small collection electrode to achieve a small capacitance (< 5 fF) and allows full CMOS in-pixel circuitry. The circuit is implemented in the 180 nm CMOS imaging technology from the TowerJazz foundry and integrated in the MALTA2 chip, which is part of a development that targets the specifications of the outer pixel layer of the ATLAS Inner Tracker upgrade at the LHC. One of the main challenges for monolithic sensors is a radiation hardness up to 10¹⁵ 1 MeV n_eq/cm² Non-Ionizing Energy Loss (NIEL) and 80 Mrad Total Ionizing Dose (TID) required for this application. Radiation source and charge injection tests up to 3 ° 10¹⁵ 1 MeV n_eq/cm² and 100 Mrad were performed on the MALTA2 sensor and front-end circuit, which still show good performance even after these levels of irradiation, promising for even more demanding applications such as the future experiments at the HL-LHC.