The ATLAS Pixel Detector Upgrade at the HL-LHC

In 2025 the Large Hadron Collider will be shut down to allow upgrades to the accelerator and the experiments. After this ``Phase-II'' shutdown the LHC is expected to reach unprecedented values of instantaneous luminosity, with hundreds of interactions in each bunch crossing. This means much higher data rates and occupancies and increased radiation damage for the experiments. During the Phase-II shutdown the entire ATLAS Inner Detector will be replaced by an all-silicon system called the Inner Tracker (ITk). The innermost part of the ITk will be a state-of-the-art pixel detector with about 13 m$^2$ of active silicon, which will provide precision tracking capability up to $\mid\eta\mid=4$. The outermost layers of the ITk pixel detector are being designed to last the lifetime of the HL-LHC, collecting up to 4000fb$^{-1}$ of Integrated Luminosity; the innermost two layers will be replaced once, after about 2000fb$^{-1}$.\\ The ITk pixel detector will be instrumented with new sensors and readout electronics to provide improved tracking performance and radiation hardness compared to the current detector. The sensor type will be dependent upon location in the detector: most of the detector will be populated with thin planar silicon sensors, but 3D-silicon sensors will be used in the innermost layer, due to their higher radiation tolerance and lower power consumption which eases demands on the support structures. The sensors will be read out by new ASICs, based on the one currently being developed by the RD53 Collaboration, which will be thinned to 150$\mu$m or less to save material.\\ Support structures will be made of carbon-based materials, chosen for low mass, high stability and high thermal conductivity. They will be cooled by evaporative carbon dioxide flowing in thin-walled titanium pipes. Servicing the detector reliably within the limited space available, and without introducing excessive amounts of material, is a significant challenge. Data will be transported electrically inside the detector, on cables carrying 1.28 Gb/s; conversion to optical signals will take place at larger radii where the radiation background is less intense. Serial powering has been chosen as the baseline for the system as it minimises service cable mass.\\ The ITk pixel detector is currently in the final stages of R\&D, with production scheduled to start in 2021.