Surface damage characterization of FBK devices for High Luminosity LHC (HL-LHC) operations

The very high fluences (e.g. up to $2 \times 10^{16} 1$ 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 (e.g. C–V and I–V). In particular, we have studied the properties of the SiO$_{2}$ layer and of the SiO$_{2}$/Si interface, using MOS capacitors, gated diodes (GD) and MOSFETs manufactured by FBK on high-resistivity n-type and p-type silicon, before and after irradiation with X-rays in the range from 50 krad(SiO$_{2}$) to 20 Mrad(SiO$_{2}$). Relevant parameters have been determined for all the tested devices, converging in the oxide charge density NOX, the surface generation velocity s0 and the integrated interface-trap density NIT dose-dependent values. These parameters have been extracted to both characterize the technology as a function of the dose and to be used in TCAD simulations for the surface damage effect modeling and the analysis and optimization of different classes of detectors for the next HEP experiments.