Mapping the depleted area of silicon diodes using a micro-focused X-ray beam

TL;DR

This study uses a micro-focused X-ray beam to map the electric field inside silicon diodes for the ATLAS detector upgrade, revealing how the electric field extends to the edges, which is crucial for sensor performance.

Contribution

It introduces a method to map the electric field in high-resistivity silicon sensors using micro-focused X-ray beams, providing insights into edge effects relevant for detector design.

Findings

Electric field extends beyond bias ring to the dicing edge.

Electric field dependence on bias voltage was characterized.

Mapping technique can identify potential leakage current regions.

Abstract

For the Phase-II Upgrade of the ATLAS detector at CERN, the current ATLAS Inner Detector will be replaced with the ATLAS Inner Tracker. The ATLAS Inner Tracker will be an all-silicon detector, consisting of a pixel tracker and a strip tracker. Sensors for the ITk strip tracker are required to have a low leakage current up to bias voltages of -700 V to maintain a low noise and power dissipation. In order to minimise sensor leakage currents, particularly in the high-radiation environment inside the ATLAS detector, sensors are foreseen to be operated at low temperatures and to be manufactured from wafers with a high bulk resistivity of several k{\Omega} cm. Simulations showed the electric field inside sensors with high bulk resistivity to extend towards the sensor edge, which could lead to increased surface currents for narrow dicing edges. In order to map the electric field inside biased silicon sensors with high bulk resistivity, three diodes from ATLAS silicon strip sensor prototype wafers were studied with a monochromatic, micro-focused X-ray beam at the Diamond Light Source. For all devices under investigation, the electric field inside the diode was mapped and its dependence on the applied bias voltage was studied. The findings showed that the electric field in each diode under investigation extended beyond its bias ring and reached the dicing edge.