Mapping the in-plane electric field inside irradiated diodes

Abstract

A significant aspect of the Phase-II Upgrade of the ATLAS detector is the replacement of the current Inner Detector with the ATLAS Inner Tracker (ITk). The ATLAS ITk is an all-silicon detector consisting of a pixel tracker and a strip tracker. Sensors for the ITk strip tracker have been developed to withstand the high radiation environment in the ATLAS detector after the High Luminosity Upgrade of the Large Hadron Collider at CERN, which will significantly increase the rate of particle collisions and resulting particle tracks. During their operation in the ATLAS detector, sensors for the ITk strip tracker are expected to accumulate fluences up to 1.6 • 10 15 n eq /cm 2 (including a safety factor of 1.5), which will significantly affect their performance. One characteristic of interest for highly irradiated sensors is the shape and homogeneity of the electric field inside its active area. For the results presented here, diodes with edge structures similar to full size ATLAS sensors were irradiated up to fluences comparable to those in the ATLAS ITk strip tracker and their electric fields mapped using a micro-focused X-ray beam (beam diameter 2 × 3 µm 2 ). This study shows the extension and shape of the electric field inside highly irradiated diodes over a range of applied bias voltages. Additionally, measurements of the outline of the depleted sensor areas allow a comparison of the measured leakage current for different fluences with expectations for the corresponding active areas.

Active Region Extent Assessment with X-rays (AREA-X)

“…For these diodes, the shape of the depleted volume based on the applied bias voltage was studied. As a follow-up measurement, the same study was performed for the same diodes, irradiated with protons up to different fluence levels to investigate the signal-to-noise-ratio required to map the depleted sensor volume [6].…”

“…Since the measured current (consisting of leakage current and induced photo current) contains contributions from all layers of the sensor, the beam energy can additionally be chosen to lead to a mostly flat absorption profile for similar contributions from all sensor depths. Measurements for silicon sensors with a thickness of 300 μm have been performed with 15 keV photons [2][3][4][5][6].…”

“…Using the photo current to map the depleted sensor volume has successfully been used for sensors after proton irradiation up to 2 • 10 15 n eq /cm 2 [6] and can also be used for scans along the sensor edge, if the beam energy allows a sufficient number of photons to traverse the sensor edge. An aluminium plate holding the test PCB is positioned on a cooling jig consisting of a cooling plate cooled to 10 • C and peltier elements, which reduce the aluminium holder's temperature to −20 • C. Nitrogen is fed into the box through a tube to reduce the humidity level and avoid condensation.…”