3D Pixel Sensors for the High Luminosity LHC ATLAS Detector Upgrade

The Large Hadron Collider (LHC) located in Geneva is the largest accelerator ever constructed. It produces proton-proton collisions in the center of the ATLAS detector, which collects the information of the collisions. The LHC started operations in 2008 at a center of mass energy of 7 TeV and an instantaneous luminosity of 10$^{34}$ cm$^{-2}$s$^{-1}$. In 2011 the energy was increased to 8 TeV. The starting luminosity was upgraded in 2015 to 2$\times$10$^{34}$ cm$^{-2}$s$^{-1}$ and the center of mass energy to 13 TeV. The extensive program of the LHC includes an accelerator upgrade in 2026 to an energy of 14 TeV and a luminosity of 7$\times$10$^{34}$ cm$^{-2}$s$^{-1}$. This is known as the High Luminosity LHC (HL-LHC) phase. \par Following the LHC upgrades, the ATLAS detector also has an upgrade program. The original ATLAS detector was first upgraded in 2015, when a new layer of pixel detectors (IBL) was mounted directly on the beam pipe to improve its detection capabilities. To cope with the conditions of the HL-LHC, the innermost subsystem of the ATLAS detector (Inner Detector - ID) will be completely replaced by the the new Inner Tracker (ITk). This new fully silicon-based subsystem is formed by layers of pixel detectors and layers of strip detectors. The innermost layer of the new ITk pixel detector is specially important: it plays a critical role in the determination of the track impact parameter and thus it is fundamental for b-tagging. At the same time, it is the layer exposed to the highest particle rates and radiation damage. \par The 3D pixel sensors, which are the topic of this thesis, are the strongest candidates to be used in the innermost layer of ITk thanks to the advantage that they offer over the planar pixel sensors. Since in 3D sensors the electrodes are columns that penetrate the silicon bulk, instead of implants on the surface (like in planar sensors), the distance between electrodes is disentangled from the thickness of the device. This allows to reduce the electrode distance (hence increase radiation hardness) while keeping the signal amplitude (which is proportional to the thickness). Furthermore, with the reduced electrode distance, the depletion voltage is lower hence reducing the power dissipation. \par The 3D pixel sensors studied in this thesis were coupled to the chip used in IBL, the FE-I4, since the first prototype of chip to be used in ITk (RD53A) was only available in 2018. They were tested in beam tests before and after irradiation up to a fluence of 2.8$\times$10$^{16}$ n$_{eq}$/cm$^2$ comfortably exceeding the ITk requirements of 1.4$\times$10$^{16}$ n$_{eq}$/cm$^2$. An efficiency of 97\% was achieved for the highest fluence at 150 V. Also, for the benchmark fluences of 5$\times$10$^{15}$ n$_{eq}$/cm$^2$ a voltage of 40 V is needed for 97\% efficiency with a power dissipation of 1.5 mW/cm$^2$ and for 1$\times$10$^{16}$ n$_{eq}$/cm$^2$ a voltage of 100 V is needed for 97\% efficiency with a power dissipation of 8 mW/cm$^2$. These results show that 3D sensors largely outperformed the planar technology in terms of radiation hardness and power dissipation. The original work presented in this thesis resulted in the choice of 3D pixel sensors as the baseline technology for the innermost layer of ITk.