Development of planar pixel sensors for the CMS Inner Tracker at the High-Luminosity LHC

The LHC is planning an upgrade program which will bring the luminosity up to about $7.5\times10^{34}\,$cm$^{-2}$s$^{-1}$ in 2027, with the goal of delivering an integrated luminosity of 3000 or even 4000~fb$^{-1}$ by the end of 2037. This High Luminosity phase, HL-LHC, will present new challenges of higher data rates and unprecedented radiation levels for the pixel detector. A fluence of $2.3\times10^{16}\,$n$_{\mathrm{eq}}$/cm$^2$, or equivalently 12~MGy, is expected for the inner layer of the CMS Inner Tracker (IT) for 3000~fb$^{-1}$ of integrated luminosity. To maintain or even improve the performance of the present system, new technologies have to be exploited for the so-called Phase-2 upgrade. Among them is the future version of front-end chips in 65 nm CMOS technology by the CERN RD53 Collaboration, which supports small pixel sizes of $50\times50$ or $100\times25\,\mu$m$^2$ and low pixel charge thresholds ($\approx$1000 e$^-$). Thin planar n-in-p type silicon sensors with a thickness of the active layer of 150~$\mu$m, segmented into pixel sizes of $100\times25\,\mu$m$^2$ or $50\times50\,\mu$m$^2$, will be used throughout most of the IT. They have been shown to allow for a good detector resolution that is much more stable with respect to radiation damage compared to the Phase-1 detector. CMS has launched several R\&D submissions for the development of suitable planar silicon sensors at Hamamatsu Photonics K.K. and FBK Trento. We present results for measurements on such prototype sensors bump bonded to the RD53A prototype chip developed by the RD53 Collaboration at CERN. Different pixel cell designs are compared and evaluated in beam tests at CERN, DESY and FNAL for spatial resolution and hit efficiency at various track angles before and after irradiation. As an example, hit efficiencies of 99\% at vertical incidence were reached after irradiation to $5\times10^{15}\,$n$_{\mathrm{eq}}$/cm$^2$, which corresponds to the layer 2 lifetime fluence of the CMS IT.