The very high fluences (eg up to 2× 10 16 1 MeV n eq/cm 2) and total ionising doses (TID) of the order of 1 Grad, expected at the High Luminosity LHC (HL-LHC), impose new challenges for the design of effective, radiation resistant detectors. Ionising energy loss is the dominant effect for what concerns SiO 2 and SiO 2/Si interface radiation damage. In particular, surface damage can create a positive charge layer near the SiO 2/Si interface and interface traps along the SiO 2/Si interface, which strongly influence the breakdown voltage, the inter-electrode isolation and capacitance, and might also impact the charge collection properties of silicon sensors. To better understand in a comprehensive framework the complex and articulated phenomena related to surface damage at these very high doses, measurements on test structures have been carried out in this work (eg C–V and I–V). In …