Recent measurements on MiniMALTA, a radiation hard CMOS sensor with small collection electrodes for ATLAS

The upgrade of the ATLAS tracking detector for the High-Luminosity Large Hadron Collider at CERN requires the development of novel radiation hard silicon sensor technologies.The MALTA Monolithic Active Pixel Sensor prototypes have been developed with the 180 nm TowerJazz CMOS imaging technology. This combines the engineering of high-resistivity sub- strates with on-chip high-voltage biasing to achieve a large depleted active sensor volumes, to meet the radiation hardness requirements of the outer barrel layers of the ATLAS ITK Pixel de- tector (1.5× 10$^{15}$ 1 MeV n$_{eq}$/cm$^{2}$ and 80 MRad TID). MALTA combines low noise (ENC < 20 e$^{-}$) and low power operation (1 uW / pixel) with a fast signal response (25 ns bunch crossing) in small pixel size (36.4 $\times$ 36.4 $\mu m^{2}$), and a small collection electrode (3$\mu$m), with a novel high- speed asynchronous read out architecture to cope with the high hit rates expected at HL-LHC. The latest developments, embedded in so-called Mini-MALTA chip, address the issues observed in previous designs to meet the desired radiation hardness requirements. This contribution will summarize the design and recent improvements of this technology, together with the measure- ments of analog and digital performance, as obtained in test beams and lab and radioactive source tests.