Development of Novel Pixel Sensors for the CMS Upgrade for HL-LHC

The LHC is planning an upgrade program which will bring the luminosity up to about 7.5$\times10^{34}\textrm{ cm}^{-2}\textrm{s}^{-1}$ in 2027, with the goal of an integrated luminosity of 3000 fb$^{-1}$ by the end of 2037. This High Luminosity scenario, HL-LHC, will present new challenges of higher data rates and unprecedented radiation levels for the CMS Inner Tracker (IT): $\Phi_{\textrm eq}=2\times 10^{16}$~cm$^{-2}$, and 10 MGy, are expected at the inner layer of the pixel detector for 3000 fb$^{-1}$ integrated luminosity. To maintain or even improve the performance of the present vertex detector, new technologies have to be fully exploited for the so-called Phase-2 upgrade. Among them is the future version of front-end chips in 65-nm CMOS by the CERN RD53 Collaboration which supports small pixel sizes of 50$\times$50 or 25$\times$100 $\mu$m$^2$} and lower charge thresholds ($\approx1000~\textrm{e}^-$).\\ Thin planar n-in-p type silicon sensors (of thickness 100-150 $\mu$m), segmented into pixel sizes of 25$\times$100 $\mu$m$^2$ or 50$\times$50 $\mu$m$^2$ are expected to allow for a good detector resolution that is expected to be more robust with respect to radiation damage compared to the Phase-1 detector. For the innermost detector layer, the option to use 3D silicon sensors is pursued, offering intrinsically higher radiation resistance because of the combination of a short charge collection distance with relatively thick sensors.\\ CMS has launched several R\&D submissions for the development of suitable silicon sensors at HPK photonics and FBK Trento (planar), and FBK Trento and CNM (3D sensors). We will present results for measurements on such prototype sensors bump bonded to the ROC4Sens R\&D readout chip from PSI, Switzerland, and to the RD53A prototype chip developed by the RD53 collaboration at CERN. Different pixel cell designs are compared and evaluated in testbeams at DESY and CERN for charge collection, efficiency and spatial resolution before and after irradiation.