Gamma irradiation of ATLAS18 ITk strip sensors affected by static charge

Construction of the new all-silicon Inner Tracker (ITk), developed by the ATLAS collaboration to be able to track charged particles produced at the High-Luminosity LHC, started in 2021 and is expected to continue until 2028. The ITk detector will include ~20,000 highly segmented and radiation hard n+-in-p silicon strip sensors, which are being manufactured by Hamamatsu Photonics. The ATLAS ITk strip sensor collaboration performs detailed measurements of individual sensor characteristics to monitor the quality of all fabricated devices. Upon delivery, several institutes involved in the complex testing program (the Quality Control or QC) conduct mechanical and electrical measurements of the sensors to ensure that their characteristics are within the specifications defined by the collaboration. QC electrical tests include current-voltage (IV) and capacitance-voltage (CV) tests, full strip tests, and a measurement of long-term stability of the sensor leakage current. Unfortunately, we have observed that a number of sensors in several production batches failed the electrical QC tests, showing an early breakdown in IV measurements, areas with low interstrip isolation in full strip tests, as well as current instabilities during the long-term leakage current stability studies. Accumulated data indicates a strong correlation between observed electrical failures and a high electrostatic charge measured on the surface of sensors and plastic sheets (mechanical protection of these sensors) during the initial reception test of these sensors. The electrostatic charge facilitates the occurrence of ESD events during handling procedures, manufacturing steps, and shipping, resulting in failed electrical QC tests. To mitigate the above-described issues, the QC testing institutes modified the sensor handling procedures and introduced sensor recovery techniques based on irradiation of the sensor surface with UV light, application of the intensive flow of ionized gas, or subjecting the sensors to high-temperature exposure. Despite implementation of the aforementioned recovery techniques, it is still possible that some affected sensors won’t be identified by the sensor QC testing or that the ESD events could occur in later manipulation of the sensor. In the presented study, we have investigated whether the total ionizing dose (TID) expected in the real experiment can effectively resolve early breakdown or low interstrip isolation caused by the electrostatic charge. We irradiated the sensors affected by this charge to the gamma rays from the 60Co source for a number of TID doses. The results of this study indicate that the negative effects of the static charge on the sensors completely disappear after a very small amount of accumulated TID, which actually corresponds to about one day in the experiment. The finding gives us confidence in mitigating the issue of static charge during the operation of the ITK strip sensors in the real experiment.