Curing early breakdown in silicon strip sensors with radiation

Abstract

In preparation for the forthcoming High-Luminosity phase of the Large Hadron Collider, the ATLAS experiment is working on major upgrades to its detector systems to effectively accommodate the increase in radiation levels and track density. The foremost among these upgrades entails the replacement of the current inner tracking detector with an advanced all-silicon Inner Tracker (ITk). In the outer region of the ITk apparatus is the Strip Detector.Central to the new strip tracking system are the ITk Strip modules, comprising silicon sensors and hybrid Printed Circuit Boards housing the integral read-out Application-Specific Integrated Circuits (ASICs) as well as power distribution services. Thorough characterisation of the electrical characteristics of the silicon strip modules at various stages of the assembly procedure holds paramount significance in evaluating module performance. This rigorous evaluation ensures timely identification of any anomalies, thereby enabling proactive remedial measures. Notably, during the course of these electrical assessments, certain modules manifested breakdown phenomena occurring below the prescribed threshold of 500 V, mandated by the ITk Quality Control protocols.Based on these observations, controlled exposure to low levels of radiation was suspected to elevate the breakdown voltage in susceptible sensors. This contribution presents results from two irradiation campaigns to investigate potentially beneficial effects of irradiation. One study investigates the effects of gamma irradiation on modules showing an early breakdown after gluing. For this study, modules were exposed to an ionising dose of 11 krad (corresponding to the dose accumulated after several days of operation in the HL-LHC) utilising a 60Co source. The second campaign focuses on silicon sensor test structures with low breakdown voltage due to intentionally caused mechanical defects and their development after exposure to reactor neutrons. In both cases, preliminary findings suggest a discernible improvement in the breakdown voltage.