The Inner Tracker (IT) of the Compact Muon Solenoid (CMS) experiment of the Large Hadron Collider at CERN will be upgraded for the High-Luminosity LHC (HL-LHC).
The expected integrated luminosity at the end of the HL-LHC running phase is at least 3000 fb$^{-1}$, corresponding to a 1 MeV neutron equivalent fluence of
$\Phi_{\rm eq} = 2.6 \times 10^{16}$ cm$^{-2}$ and a total ionizing dose (TID) of 13 MGy at the innermost layer of the IT.
All the layers of the IT (except for the innermost barrel layer) will be equipped with planar n$^+$-p pixel sensors with an active thickness of 150 $\mu$m and pixel sizes of 25 $\mu$m $\times$ 100 $\mu$m.
The innermost barrel layer will feature 3D silicon sensors owing to their excellent radiation hardness and lower power consumption; it is foreseen to be exchanged at least once during HL-LHC operation.
Planar and 3D prototype sensors were bump bonded to the CMS prototype chip (CROCv1), implemented in 65 nm CMOS technology.
In an extensive qualification campaign, sensor-chip assemblies were tested in the lab and at the CERN and DESY testbeam facilities before and after proton irradiation up to $\Phi_{\rm{eq}} = 2 \times 10^{16}$ cm$^{-2}$, exceeding the expected maximum fluences.
In this paper, more recent measurements of the hit efficiency, spatial resolution, and noise studies are presented after proton irradiation at the CERN Proton Synchrotron (PS)
up to $\Phi_{\rm{eq}} = 1 \times 10^{16}$ cm$^{-2}$.
For all parameters investigated, the results meet or exceed the CMS specifications. Based on the results of these measurements and on tracking and thermal simulations, sensor designs were chosen for the IT Upgrade and CMS has started to prepare for the (pre)-production phase of pixel sensors and modules. The main lessons learned on the path to the choice of a radiation hard sensor are summarized.