Beam Test Measurements on Planar Pixel Sensors for the CMS Phase 2 Upgrade

The requirements for the CMS Inner Tracker for the high luminosity upgrade of the Large Hadron Collider (LHC) are driven by the high expected instantaneous luminosity of up to $7.5\times10^{34}$\,cm$^{-2}$s$^{-1}$. The detector will have to withstand a 1\,MeV neutron equivalent fluence of up to $2\times10^{16}$\,cm$^{-2}$ at 2.8\,cm distance from the beam. To improve the spatial resolution and limit cluster merging in this environment the current pixel size of $100\times150$\,$\mu$m$^{2}$ will be reduced by a factor of 6. Planar pixel sensors are the baseline technology for the outer 3 of the 4 pixel layers, where the irradiation level will reach a 1\,MeV neutron equivalent fluence of $5\times10^{15}$\,cm$^{-2}$. For the innermost layer 3D sensors are also considered. Several variants of new n$^{+}$p-planar pixel sensors with pixel sizes of $50\times50$\,$\mu$m$^{2}$ and $100\times25$\,$\mu$m$^{2}$ and an active thickness of 150\,$\mu$m have been designed, produced and bump bonded to readout chips. Apart from the pixel size, the design variants differ with respect to the implantation and metalization layout as well as the pixel isolation and biasing scheme. To select the most promising design for the future CMS Inner Tracker, these sensors were irradiated with protons and neutrons up to 1\,MeV neutron equivalent fluences of $14\times10^{15}$\,cm$^{-2}$ and characterized at the DESY II test beam facility. The key requirement, a hit efficiency above 99\,\% at bias voltages below 800\,V, is met for sensors proton irradiated to 1\,MeV neutron equivalent fluences above $5\times10^{15}$\,cm$^{-2}$, thus demonstrating the suitability of planar sensors for the pixel layers 2 to 4. In addition, the hit efficiency, signal to noise ratio and spatial resolution are determined as a function of the bias voltage, neutron equivalent fluence and angle, for proton and neutron irradiation and for different design variants, providing input for the final choice of the sensor design.