ALICE - ITS3 — A bent, wafer-scale CMOS detector

ALICE is developing the ITS3 (inner tracker system) as upgrade of the inner layers of the presently installed ITS with the aim to improve the pointing resolution by factor of two and to lower the material budget to an unprecedented value of 0.05% $X_{0}$ per layer. Its three layers are based on Monolithic Active Pixel Sensors (MAPS) thinned down to 20–40 $\mu$m. The configuration employs half-cylinders, which are bent around the beam pipe with bending radii of 18, 24 and 30 mm, respectively. The sensors are produced using stitching techniques allowing to reach a length of 28 cm. As consequence, the detector consists of six MAPS sensors only. The material budget is kept to a minimum using carbon foam elements to hold the stitched sensors in place, as well as by employing airflow cooling. This contribution summarizes the achieved R&D; results on bending and thinning, the mechanical prototype, beam tests and the 65 nm prototype ASIC test, as well as the path towards the last remaining milestone, the implementation of a wafer-scale, stitched sensor design.