Novel method to assess the electromagnetic radiation damage using HGCAL silicon sensors

Abstract Studies of the silicon dioxide (SiO2) passivation layer in n-on-p silicon sensors, which will be used in the CMS High Granularity Calorimeter (HGCAL), are important for improving their performance not only at the High-Luminosity LHC, but also at future high-energy physics experiments. This work explores a novel method for estimating the effect of surface radiation damage on inter-pad isolation in HGCAL sensors by measuring the threshold voltage for inter-electrode isolation Vth,iso. Test diodes with different SiO2 variants are irradiated with X-rays, and their characteristics are measured at a controlled temperature of -20°C. The results are then compared with previous measurements with MOS and strip-like sensors. Three measurement techniques — CV, IV, and interstrip like resistance measurements — are employed to extract Vth,iso. Among these, the CV-based extraction method is found to be the most robust. The results show an unexpected dependence of Vth,iso on the ionising dose, which has not been observed before. Possible explanations for the observed behaviour are also discussed.