Evaluation of HPK $n^+$-$p$ planar pixel sensors for the CMS Phase-2 upgrade

To cope with the challenging environment of the planned high luminosity upgrade of the Large Hadron Collider (HL-LHC), scheduled to start operation in 2029, CMS will replace its entire tracking system. The requirements for the tracker are largely determined by the long operation time of 10~years with an instantaneous peak luminosity of up to $7.5\times 10^{34}$~cm$^{-2}$s$^{-1}$ in the ultimate performance scenario. Depending on the radial distance from the interaction point, the silicon sensors will receive a particle fluence corresponding to a non-ionizing energy loss of up to $\Phi_{\text{eq}} = 3.5\times 10^{16}$~cm$^{-2}$. This paper focuses on planar pixel sensor design and qualification up to a fluence of $\Phi_{\text{eq}} = 1.4\times 10^{16}$~cm$^{-2}$. For the development of appropriate planar pixel sensors an R\&D program was initiated, which includes $n^+$-$p$ sensors on 150 mm (6'') wafers with an active thickness of 150~$\mu$m with pixel sizes of $100\times 25$~$\mu$m$^2$ and $50\times 50$~$\mu$m$^2$ manufactured by Hamamatsu Photonics K.K.\ (HPK). Single chip modules with ROC4Sens and RD53A readout chips were made. Irradiation with protons and neutrons, as well was an extensive test beam campaign at DESY were carried out. This paper presents the investigation of various assemblies mainly with ROC4Sens readout chips. It demonstrates that multiple designs fulfill the requirements in terms of breakdown voltage, leakage current and efficiency. The single point resolution for $50\times 50$~$\mu$m$^2$ pixels is measured as 4.0~$\mu$m for non-irradiated samples, and 6.3~$\mu$m after irradiation to $\Phi_{\text{eq}} = 7.2\times 10^{15}$~cm$^{-2}$.