3D silicon pixel detectors for the ATLAS Forward Physics experiment

TL;DR

This paper investigates the use of modified 3D silicon pixel sensors for the ATLAS Forward Physics experiment, demonstrating slim edges, radiation hardness, and high efficiency after irradiation, suitable for installation close to the beamline.

Contribution

It presents a simple method to produce slim edges on 3D silicon pixel sensors and evaluates their performance under non-uniform irradiation conditions.

Findings

Slim edges of 100-200 μm achieved with diamond-saw cut.

Sensors maintain high efficiency (>97%) after irradiation up to 10^15 n_eq/cm^2.

No detrimental impact on leakage current and hit efficiency observed.

Abstract

The ATLAS Forward Physics (AFP) project plans to install 3D silicon pixel detectors about 210 m away from the interaction point and very close to the beamline (2-3 mm). This implies the need of slim edges of about 100-200 $\mu$m width for the sensor side facing the beam to minimise the dead area. Another challenge is an expected non-uniform irradiation of the pixel sensors. It is studied if these requirements can be met using slightly-modified FE-I4 3D pixel sensors from the ATLAS Insertable B-Layer production. AFP-compatible slim edges are obtained with a simple diamond-saw cut. Electrical characterisations and beam tests are carried out and no detrimental impact on the leakage current and hit efficiency is observed. For devices without a 3D guard ring a remaining insensitive edge of less than 15 $\mu$m width is found. Moreover, 3D detectors are non-uniformly irradiated up to fluences of several 10$^{15}$ n$_{eq}$/cm$^2$ with either a focussed 23 GeV proton beam or a 23 MeV proton beam through holes in Al masks. The efficiency in the irradiated region is found to be similar to the one in the non-irradiated region and exceeds 97% in case of favourable chip-parameter settings. Only in a narrow transition area at the edge of the hole in the Al mask, a significantly lower efficiency is seen. A follow-up study of this effect using arrays of small pad diodes for position-resolved dosimetry via the leakage current is carried out.