Development of a small collection electrode monolithic pixel sensor in a novel 180 nm process for the ATLAS ITk upgrade

Monolithic pixel sensors are currently in consideration for the outer layers of the ATLAS ITk. Implementations that are based on a small collection electrode are advantageous in terms of power consumption and timing performance but can be sensitive to bulk radiation damage effects. A large-scale demonstrator chip, called TJ-Monopix01 has been developed in a modified 180 nm process that employs an implanted n-layer to achieve full depletion and enhanced radiation tolerance. It consists of 36$\times$40 $\mu$m${}^2$ pixels arranged in a 224$\times$448 matrix and features sensor capacitance $\le$3 fF, a synchronous "column-drain'' readout architecture and total power consumption of 120 mW/cm${}^2$. Laboratory measurements demonstrate the high signal to noise ratio by means of ENC$\approx$10 e${}^{-}$ and fast timing response. These promising results encouraged the ongoing design of TJ-Monopix02, which addresses the challenges of a full-scale matrix (2$\times$2 cm${}^2$) and will include further optimization of the process modification and improvements in the front end and pixel design to withstand radiation dose up to 1$\cdot$10${}^{15}$ n${}_{eq}$/cm${}^2$ towards an ATLAS ITk compatible monolithic sensor.