Increased radiation tolerance of CMOS sensors with small collection electrodes through accelerated charge collection

Mini-MALTA is a Monolithic Active Pixel Sensor prototype developed in the TowerJazz 180 nm CMOS imaging process, with a small collection electrode design (3μm$^2$ ), and a small pixel size (36.4 μm), on high resistivity substrates and large voltage bias. It targets the outermost layer of the ATLAS ITK Pixel detector for the HL-LHC. This design addresses the pixel inefficiencies observed in previous MALTA and TJ-Monopix prototypes. The implementation of a mask change in the fabrication or an additional implant allows to improve the charge collection performance at the edges of the pixel thus improving the radiation hardness properties. This contribution will present the results from characterisation in particle beam tests that show full efficiency up to 1 · 10$^{15}$ $n_{eq}$ /cm$^2$ .