ALICE ITS3: new bent, large-scale sensors for Run 4

During the LHC Long Shutdown 3 (LS3, scheduled 2026–2030) a new Inner Tracking System (ITS) will be installed in ALICE for the future Run 4 data taking. At present, the ALICE ITS is at its second version, ITS2, formed by 7 layers of silicon CMOS Monolithic Active Pixel Sensors (MAPS). The 3 innermost layers are collectively referred to as Inner Barrel (IB), while the outermost 4 form the Outer Barrel (OB). Each layer is then divided along the azimuthal direction into staves of ALPIDE (ALICE Pixel Detectors) chips, and cooled by a water system. The future ITS upgrade to ITS3, after LS3, will see an impressive change of the IB structure, through a replacement of the ITS2 layers with innovative bent large-scale stitched sensors, produced with the TPSCo 65 nm technology. Each of the ITS2 IB layers will be replaced by 2 of these sensors, bent into truly half-cylindrical shapes. Sensor bending allows to reduce the need for mechanical frame support: this innovation, in addition to the replacement of water cooling with air cooling, is expected to reduce the material budget down from an average 0.36% X_0 to 0.09% X_0 per layer. The upgrade will also see a reduction of the minimum radial distance from the beam axis from 23 to 19 mm, an improved granularity of 20×22.5 μm^2 and a wider pseudo-rapidity range, up to |𝜂| ≤ 2.5 (ITS2 coverage is |𝜂| ≤ 1.2) for the innermost layer. Compared to ITS2, the new ITS3 will also have to withstand larger radiation loads, a maximum limit of which has been estimated as 4·10^12 1 MeV n_eq cm^−2 NIEL and 4 kGy TID. This contribution will give an overview of the ALICE ITS3 upgrade project goals and innovations, and briefly report the R&D process results and achievements: TPSCo 65 nm technology has been validated by testing the first Multi-Layer Reticle 1 (MLR1) small test devices, both in laboratory and under test beam, even at NIEL levels larger than the ITS3 expected load. For the yield and stitching assessment part of ITS3 R&D, current test results of ITS3 stitched devices from Engineering Run 1 (ER1) are also shown. Finally, an outline of the MOSAIX design is given: they will be the final ITS3 stitched sensor prototypes, and are currently awaiting submission.