The high-luminosity upgrade of the CERN LHC requires the replacement of the CMS tracking detector to cope with the increased radiation fluence while maintaining its excellent performance. An extensive R&D program, aiming at using 3D pixel silicon sensors in the innermost barrel layer of the detector, has been carried out by CMS in collaboration with the FBK (Trento, Italy) and CNM (Barcelona, Spain) foundries. The sensors will feature a pixel cell size of 25x 100 µm², with a centrally located electrode connected to the readout chip. The sensors are read out by the RD53A and CROCv1 chips, developed in 65 nm CMOS technology by the RD53 Collaboration, a joint effort between the ATLAS and CMS groups. This paper reports the results achieved in beam test experiments before and after irradiation, up to a fluence of approximately 2. 6 x 10^16 n₄ₐ/cm². Measurements of assemblies irradiated to a fluence of 1 x 10^16 n˙eq/cm² show a hit detection efficiency higher than 96% at normal incidence, with fewer than 2% of channels masked, across a bias voltage range greater than 50 V. Even after irradiation to a higher fluence of 1. 6 x 10^16 n˙eq/cm², similar performance is maintained over a bias voltage range of 30 V, remaining well within CMS requirements.
CMS Collaboration (Thu,) studied this question.