Depleted CMOS Sensors for the ATLAS Tracker Upgrade and Future Collider Experiments

Duration: 1.7.2018 - 30.06.2021
Project type: ARRS research project

Project leader: Marko Mikuž
Coworkers: Vladimir Cindro, Andrej Gorišek, Bojan Hiti, Borut Paul Kerševan, Boštjan Maček, Gregor Kramberger, Igor Mandić, Marko Mikuž, Andrej Studen, Marko Zavrtanik
Partners: Jozef Stefan Institute, University of Ljubljana, Faculty of Mathematics and Physics

In this research project we propose studies of charge collection properties and radiation hardness of detectors fabricated in CMOS technology. The detectors are being developed for charged particle tracking in the very harsh radiation environment in experiments at the upgraded high luminosity hadron collider HL-LHC. This technology opens the possibility to produce particle detectors with superb spatial resolution in a standard processes in foundries producing commercial electronic chips. The technology offers the potential to produce monolithic detectors with sensor and readout electronics integrated onto the same chip with sufficient speed and radiation hardness for the HL-LHC environment. Successful development of monolithic detectors in CMOS technology would bring detectors with excellent tracking performance, lower mass, large savings in cost and much faster detector production. CMOS detector technology is also a viable candidate for application in future particle physics facilities like CLIC and FCC planned beyond the HL-LHC.

CMOS detectors have already been successfully used for charge particle tracking but they were not suitable for the HL-LHC environment due to their low speed and poor radiation hardness. The reason was that diffusion was the main charge collection mechanism in these detectors. Designs of CMOS detectors for HL-LHC allow for usage of higher bias voltages. Therefore significant depletion depths can be achieved and charge collected by drift to assure sufficient speed and radiation hardness. Another possibility for HL-LHC is CMOS on a thin layer of high resistivity epitaxial material that gets depleted at relatively low bias voltages. This design employs a charge-collecting electrode of a small area to reduce capacitance, and thus minimize noise and power. One of the main open questions in CMOS detector development is the efficiency of charge collection after irradiation to high hadron fluences. A parameter closely related to this issue is the initial resistivity of detector material. This can span across a wide range in CMOS detectors so radiation induced initial acceptor removal plays an important role and will be studied within this project.

Charge collection and its dependence on irradiation fluence will be studied with Edge-TCT, with measurements of signals from 90Sr source and in test beam experiments. Measurements will be done with passive test structures and with segmented active pixel arrays. The test structures will be designed by partner groups participating in collaborations formed at CERN to study this detector technology and the samples shall be acquired through these collaborations. Samples will be irradiated with neutrons in the reactor in Ljubljana and with charged hadrons at other irradiation facilities for example at the CERN PS.

The outcome of this project will significantly contribute to knowledge of CMOS detector technology for particle trackers and will have a decisive impact on the technology choice for the pixel detector in the upgraded ATLAS experiment. It shall also provide guidelines for further research oriented towards tracking detectors for experiments at future colliders.