With the upgrade of the Large Hadron Collider (LHC) to the High-Luminosity LHC (HL-LHC) scheduled for installation in 2024, the Inner Detector will be replaced with the new all-silicon ATLAS Inner Tracker (ITk) to maintain tracking performance in this high-occupancy environment and to cope with the increase of approximately a factor of ten in the integrated radiation dose. The outer four layers in the barrel and six disks in the end-cap region will host strip modules, built with single-sided strip sensors and glued-on hybrids carrying the front-end electronics necessary for readout.

The strip sensors are manufactured as n-in-p strip sensors from high-resistivity silicon, which allow operation even after the fluences expected towards the end of the proposed lifetime of the HL-LHC. Prototypes of different sensor designs have been extensively tested electrically as well as in testbeam setups, yielding generally satisfactory results, but also revealing ongoing challenges. During electrical sensor evaluation many prototype sensors are found to not comply with specifications for stable long-term operation. This outcome was attributed to the influence of humidity, which has been subsequently investigated in great detail and will have lasting consequences for sensor production and module assembly. Repeated measurements on continuously biased sensors revealed a decrease in tracking performance and increased charge-sharing due to the accumulation of charges in the Si-SiO$2$ interface. The dynamics of this effect and how it will affect operation of the finished ITk have been examined with a novel approach using the time-development of inter-strip quantities and trap energy level characterisation in SiO$2$ with Deep-level Transient Spectroscopy (DLTS).