Overview of the ALICE ITS3 Upgrade

Abstract: The ALICE experiment will replace its three innermost tracking layers with the Inner Tracking System 3 (ITS3) during LHC Long Shutdown 3. This upgrade introduces the first fully cylindrical, wafer-scale silicon vertex detector, utilising Monolithic Active Pixel Sensors (MAPS) fabricated in a 65nm CMOS process. By thinning sensors to 50$μ$m and bending them to radii as small as 19mm, the design achieves a self-supporting structure that eliminates traditional support material. Wafer-scale stitching enables 27cm-long seamless sensors with integrated power and signal distribution, removing the need for flexible printed circuits within the active volume. These innovations, combined with a move from water to air cooling, reduce the material budget to less than 0.09%X$_0$ per layer. The R&D program has been validated through full-scale prototypes (MOSS, MOST), which demonstrated stitching feasibility, high yield, and radiation hardness. Engineering models confirmed the feasibility of air-convection cooling, indicating effective thermal management and structural stability. This contribution summarises the key advances in stitched sensor development, mechanical integration, and the path toward the final qualification model.