Measuring the border of the active area on silicon strip sensors

Silicon strip sensors for the ATLAS Inner Tracker (ITk) have been designed to provide reliable particle detection in the high-radiation environment of the High-Luminosity Large Hadron Collider. One important design criterion for their development is the minimization of inactive sensor areas, which affect the hermiticity of particle detection inside the detector. In previous measurements of ATLAS silicon strip sensors, the charge-collecting area of individual strip implants has been mapped and found to agree with the sensor strip pitch and strip length. For strip implants next to the sensor bias ring, the extent of their charge-collecting area towards the inactive sensor area was previously unknown, which limited the accuracy of both overall detector hermiticity estimates and the position resolution for particle detection at the sensor edge. Therefore, measurements were conducted to map the area of charge collection for sensor strips at the edge of the active sensor area using a micro-focused X-ray beam. This publication presents measurements showing the extent of charge collection in the edge strips of silicon strip sensors for two generations of ATLAS ITk strip sensor modules. The measurements confirmed that charge deposited in a strip implant that is neither connected nor grounded leads to capacitive coupling to the adjacent strip, where it is indistinguishable from a hit in that strip