Small pitch pixel sensors for the CMS Phase II upgrade

The CMS experiment intends to exchange the pixel detector for the high luminosity phase of the Large Hadron Collider (HL-LHC). Therefore, a large R\&D effort has been started in order to develop sensors capable of withstanding the expected extremely high radiation damage. The targeted integrated luminosity of 3000 fb$^{-1}$, estimated for 10 years of operation at the design center-of-mass energy of 14 TeV, translates into an equivalent NIEL (Non-Ionizing Energy Loss) of 2$\times{10}^{16}$ neq cm$^{-2}$ and an IEL (Ionizing Energy Loss) dose in the SiO$_{2}$ of 5 MGy at the expected position of the innermost pixel detector layer. \\% The CMS collaboration has undertaken two sensor R\&D programs on thin n-in-p planar and 3D silicon sensor technologies. To cope with the increase in instantaneous luminosity, the pixel area has to be reduced to approximately 2500 $\mu$m$^{2}$ to keep the occupancy at the percent level. Suggested pixel cell geometries to match this requirement are {50$\times$50 }$\mu$m$^2$ or 25$\times$100 $\mu$m$^2$, leaving little space for design choices and a possible biasing scheme. Monte Carlo studies comparing the hit resolution for these two scenarios are presented. \\% CMS has initiated the production of test-sensors with the envisaged pixel geometries. Status, progress, and prospects of this effort are discussed.