Application of atomic layer deposited thin films to silicon detectors

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Silicon is used in a variety of semiconductor radiation and particle detectors due to its reasonable cost, good availability and processability, and performance at room temperature. Pixel detectors made of silicon are used for particle tracking at different high-energy physics experiments, including all large experiments at the LHC. The planned upgrades of the tracking detectors of the LHC experiments, e.g. the CMS Inner Tracker, as well as experiments at future colliders, are a very challenging environment for silicon detectors due to their extreme radiation levels, combined with high occupancies, fast readout and high segmentation of the sensors. This thesis features the application of aluminium oxide thin films grown by atomic layer deposition as field insulation and surface passivation in silicon pixel detectors. This approach aims to mitigate some of the radiation-induced damage on detectors, as well as reduce high-temperature fabrication steps, by using AC-coupled sensors with Al2O3 as insulator and high-k coupling dielectric and TiN biasing resistors. Surface passivation and electrical properties of Al2O3 films deposited on high-resistivity silicon are studied on n- and p-type FZ-silicon at different post-deposition anneal temperatures. On magnetic Czochralski silicon, which is the target material for the pixel detectors discussed in this work, Al2O3 films deposited with different oxidants in ALD are studied first with the same contactless characterization methods, and later in diode and MOS capacitor devices. While traditional H2O as oxidant provides the best recombination lifetimes, as well as the best breakdown properties in diodes, the oxide charge achieved with this ALD process is comparatively low and is compensated in gamma irradiation. Films deposited using O3 exhibit higher charge and better radiation hardness, and considering all experiments, a combination of H2O and O3 is identified as the best choice of ALD oxidant. In addition, the phenomenon of blistering in AlOx films deposited with H2O is investigated in more detail, and new observations on the influence of substrate doping on blistering are shown. Processing of pixel detectors is described with an emphasis on the ALD steps, and also noting on post-anneal temperature and TiN bias resistor fabrication. For pixel detectors, Al2O3 is deposited with oxidants H2O+O3 based on the earlier results. Finally, pixel detectors are flip-chip bonded to the CMS PSI46dig readout chip and tested with radioactive sources, confirming their functionality. It is concluded that ALD-grown Al2O3 films are a promising alternative to SiO2 and insulating implants for small-pitch silicon pixel detectors, which are under development for a variety of future collider physics experiments.
Translated title of the contributionAtomikerroskasvatuksella tuotettujen ohutkalvojen soveltaminen puolijohdeilmaisimiin
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Savin, Hele, Supervising Professor
  • Luukka, Panja, Thesis Advisor
  • Härkönen, Jaakko, Thesis Advisor, External person
Publisher
Print ISBNs978-952-64-0276-5
Electronic ISBNs978-952-64-0277-2
Publication statusPublished - 2021
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • silicon
  • pixel detector
  • radiation hardness
  • atomic layer deposition
  • aluminium oxide

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