Characterisation of planar sensors for the inner tracker of the CMS experiment

The Compact Muon Solenoid (CMS) experiment is expected to collect an integrated luminosity of $3000 \, \text{fb}^{-1}$ during the High Luminosity phase of the Large Hadron Collider (HL-LHC). This scenario comes with a high number of collisions per bunch crossing, and in turn, a high level of radiation for the innermost layer of the CMS tracker. Simulations estimate a $1 \, \text{MeV}$ neutron equivalent fluence, $\Phi_\text{eq}$, of $2.3\times 10^{16} \, \text{cm}^{-2}$ at a distance of $2.8 \, \text{cm}$ from the collision point. The inner tracker of the CMS detector is required to withstand this range of fluence and maintain tracking performance. Planar pixel sensors with an active thickness of $150 \, \mu \text{m}$ and pixel sizes of $25 \times 100 \, \mu \text{m}^2$ or $50 \times 50 \, \mu \text{m}^2$ have been produced by Hamamatsu Photonics (HPK) and Fondazione Bruno Kessler (FBK). The sensors were bumped bonded to the RD53A readout chip prototype. The sensor-chip modules were irradiated with $23 \, \text{MeV}$ protons to the $1 \, \text{MeV}$ neutron equivalent fluence of up to $2.4 \times 10^{16} \, \text{cm}^{-2}$ at the Zyklotron AG (ZAG). Non-irradiated and irradiated modules were tested in the DESY II beam test facility. The spatial resolution as a function of the incidence angle and hit efficiency as a function of the bias voltage of the sensors were determined from these measurements. It is shown that for the highest fluence, the planar modules still reach $98 \, \%$ hit efficiency at bias voltages below $800 \, \text{V}$.