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 2×10¹⁶ neq cm^-2 and an IEL (Ionizing Energy Loss) dose in the SiO₂ 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 μm² to keep the occupancy at the percent level. Suggested pixel cell geometries to match this requirement are 50 × 50 μm² or 25 × 100 μm², 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.