Selection of the silicon sensor thickness for the Phase-2 upgrade of the CMS Outer Tracker

During the operation of the CMS experiment at the High-Luminosity LHC the silicon sensors of the Phase-2 Outer Tracker will be exposed to radiation levels that could potentially deteriorate their performance. Previous studies had determined that planar float zone silicon with n-doped strips on a p-doped substrate was preferred over p-doped strips on an n-doped substrate. The last step in evaluating the optimal design for the mass production of about 200$\,$m$^2$ of silicon sensors was to compare sensors of baseline thickness (about 300$\,\mu$m) to thinned sensors (about 240$\,\mu$m), which promised several benefits at high radiation levels because of the higher electric fields at the same bias voltage. This study provides a direct comparison of these two thicknesses in terms of sensor characteristics as well as charge collection and hit efficiency for fluences up to $1.5\times 10^{15}\,$n$_{\mathrm{eq}}$/cm$^2$. The measurement results demonstrate that sensors with about 300$\,\mu$m thickness will ensure excellent tracking performance even the highest considered fluence levels expected for the Phase-2 Outer Tracker.