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Description
The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC), foreseen for 2030, requires the replacement of the ATLAS Inner Detector with a new all-silicon Inner Tracker (ITk). Radiation-hard $n^+$-in-$p$ micro-strip sensors developed for use in the ITk will be exposed to a total radiation fluence of up to $\Phi_{eq} = 1.6 \times 10^{15}$ 1 MeV $n_{eq}/cm^2$ and a total ionizing dose (TID) of 66 Mrad, both including a safety factor of 1.5. Under such extreme radiation conditions, maintaining high interstrip isolation is essential in ATLAS ITk silicon strip sensors to minimize charge sharing and suppress readout noise, thereby ensuring optimal spatial resolution in particle detectors.
During quality control (QC) and quality assurance (QA) procedures performed on ATLAS18 production sensors and test chips, ATLAS ITk sensor institutes identified instances of insufficient interstrip isolation in certain production batches. To investigate the origin of these issues and to try some technological improvements, the manufacturer, Hamamatsu Photonics K. K. (HPK), produced two dedicated sample batches containing main sensors and test structures with four variations of the p-stop fabrication process. These process types (1–4) were systematically evaluated by the ATLAS ITk strip sensor community to determine their effectiveness and uniformity. In total, 56 full-size sensors, along with numerous miniature sensors and test chips, were tested. A number of samples were used in the irradiation campaign reaching the maximum expected levels for the HL-LHC operation after 10 years - with neutrons up to $1.6 \times 10^{15}$ $n_{eq}/cm^2$, and with gamma rays from a $^{60}$Co source up to 66 Mrad.
The goal of this evaluation program is to assess the impact of fabrication process variations on interstrip isolation performance and provide feedback to HPK for future sensor development. The results show good sensor quality for all p-stop types, with no observed impact on early breakdown behavior in IV tests. Although differences in punch-through protection behavior were noted, all samples remained within QC specifications before and after irradiation. Type 4 sensors showed the most uniform punch-through voltage ($V_{PT}$) distribution pre-irradiation and the highest $V_{PT}$ values post-irradiation, suggesting stronger effective p-stop doping. Interstrip resistance ($R_{int}$) decreased with radiation dose but remained above specification for all types, with type 2 showing the highest post-irradiation $R_{int}$. These findings confirm the reliability of the evaluated p-stop designs and indicate specific advantages that can guide future sensor optimization.
