The upgrade of the ALICE Inner Tracking System at the CERN LHC

In 2021, for the third run of the CERN Large Hadron Collider (LHC), Pb–Pb collisions will be performed at a centre-of-mass energy per nucleon of $\sqrt{s_{NN}}$ = 5.5 TeV, with an integrated luminosity of 6 × 10$^{27}$ cm$^{-2}$ s$^{-1}$ and at an unprecedented interaction rate of 50 kHz. To fulfil the requirements of its physics program for Run 3 the ALICE experiment at LHC is preparing a major upgrade during the Long Shutdown 2 of LHC in 2019-2020. One of the key elements of the program, is the construction of a new ultra-light and high-resolution Inner Tracking System (ITS) to enhance the determination of the distance of closest approach to the primary vertex, the tracking efficiency at low transverse momenta, and the read-out rate capabilities, with respect to what can be achieved with the current detector. The ITS will consist of seven cylindrical and concentric layers equipped with silicon Monolithic Active Pixel Sensors (MAPS) with a pixel size of the order of 30×30 μm$^2$ built with the TowerJazz 0.18 μm CMOS Imaging process. The ITS upgrade collaboration has finished the R&D; of the detector components and has started the production phase.