Abstract
For the Phase-II Upgrade of the ATLAS detector (The ATLAS Collaboration, 2008 [1]), a new, all-silicon tracker will be constructed in order to cope with the increased track density and radiation level of the High-Luminosity Large Hadron Collider. While silicon strip sensors are designed to minimise the fraction of dead material and maximise the active area of a sensor, concessions must be made to the requirements of operating a sensor in a particle physics detector. Sensor geometry features like the punch-through protection deviate from the standard sensor architecture and thereby affect the charge collection in that area. In order to study the signal collection of n+-p−-p+ silicon strip sensors over their punch-through-protection area, ATLAS silicon strip sensors were scanned with a micro-focused X-ray beam at the Diamond Light Source. Due to the highly focused X-ray beam (2×3μm2) and the short average path length of an electron after interaction with an X-ray photon (≤2μm), local signal collection in different sensor areas can be studied with high resolution. This study presents results of high resolution 2D-scans of the punch-through protection region of ATLAS silicon micro-strip sensors, showing how far the strip signal collection area extends towards the bias ring and how the region is affected by radiation damage.
Highlights
Introduction tects readout electronics from high currents induced by beam splashes [3]. While this gap is short compared to the length of a sensor strip (16.2 μm compared to about 2.4 cm strip length, i.e. less than 0.1 %, for modules with short sensor strips), it will make up a combined area of about 800 cm2 on the 18,000 detector modules foreseen to be built for the ATLAS
Inefficient charge collection in the punch-through protection region would effectively increase the amount of inactive material surrounding the active sensor area and reduce the detector performance
Reconstructed positions of aluminium bond pads on sensors (AC pads) from the coarse scan, the positions of bias ring and end of strip implant were determined with a precision of ±20 μm
Summary
Studying this sensor region requires a position resolution O(μm), which was achieved by using a micro-focused X-ray beam (2 × 3 μm ). It was important to study how far a sensor strip can collect signals beyond the end of its implant (see figure 1). Bond pads are large (200 × 56 μm2 ) compared to the punch-through protection region and can be found and mapped in a coarse scan over a large sensor area. Reconstructed positions of AC pads from the coarse scan, the positions of bias ring and end of strip implant were determined with a precision of ±20 μm. Hit maps obtained at the end of strip implants showed few hits being collected between the end of the strip implant and the bias ring, both for an unirradiated sensor (see figure 3a) and an irradiated sensor (see figure 3b). The gap between strip implant and bias ring does not provide reliable charge collection
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