Thesis

Developing optical coatings for laser optics using novel high energy ion beam sputter deposition technique

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Awarding institution
  • University of Strathclyde
Date of award
  • 2024
Thesis identifier
  • T17052
Person Identifier (Local)
  • 201869068
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • This thesis outlines the fundamental aspects of various coating fabrication methods as well as the fundamentals of thin films and coatings that are typically studied and utilised to achieve high laser induced damage threshold (LIDT) properties. An overview of the two different novel high energy electron cyclotron resonance ion beam sputter deposition (ECR-IBSD) setups used to fabricate the coatings are described, along with the fundamental theory and setups of all the characterisation methods used for the films discussed. The materials of interest were hafnium oxide (HfO2), scandium oxide (Sc2O3), and different mixture percentages of these two materials with silica (SiO2). The effects of the reactive and sputtering oxygen partial pressures on the structure, stoichiometry, and optical properties of the HfO2 and Sc2O3 thin films were systematically investigated. The amorphous structures of both films were determined using X-ray Diffraction. Energy-dispersive X-ray Spectroscopy and Rutherford Backscattering Spectrometry were carried out for the composition and stoichiometry analysis, which suggested the formation of over-stoichiometric films. The transmission and reflectance spectra of the films were measured using a spectrophotometer, where the spectra were analysed by an optical fitting software SCOUT, which utilises the model modified by O'Leary, Johnson and Lim, to extract the optical properties of the films. In addition to this study, by utilising a novel 24-beam ECR-IBSD system, mixed films of HfO2:SiO2 and Sc2O3:SiO2 with different mixture percentages were investigated. The as-deposited mixed films were also found to possess an amorphous structure. The optical constants of the mixed films were extracted in the same manner as those of pure HfO2 and Sc2O3 films. In addition to studying the films at as-deposited, the effects of post-deposition heat treatment on the structure and optical properties of all the films were also investigated. From this work, it was established that the influence of oxygen incorporation in pure materials from an optical and structural perspective shows that ECR-IBSD provides over-stoichiometric HfO2 and Sc2O3 films. From the HfO2 study, it was found that both the refractive index and extinction coefficient decreased with the increase of the oxygen content, whereas the bandgap energy increased. For Sc2O3 films, there was no real correlation with changing the oxygen content during deposition on the optical properties, except for the OJL bandgap energy, which increased as the oxygen percentage increased. The effects of annealing are as follows: the HfO2 films remained amorphous after annealing to 500°C and became crystalline with a monoclinic structure after annealing to 700°C, whereas Sc2O3 films remained amorphous even after annealing to 900°C. For mixed materials, this study focused on the influence of different mixture percentages on the optical, structural, and LIDT properties of the films. The mixture materials filled the values for the refractive index between the two pure materials, indicating that the refractive index can be tuned by changing the mixture percentage, which is the case for both HfO2:SiO2 and Sc2O3:SiO2 mixed films. The same results were also found for both the extinction coefficient values and OJL bandgap energies of the mixed films, leading to the ability to easily fabricate films with tunable optical properties. The structure of the films in this study is as follows: in both cases, the films remained amorphous when heat treated up to 700°C. As the annealing temperature reached 900°C, the mixed films with less than 10% SiO2 became more crystalline. In addition to the heat treatment study, LIDT testing was also carried out for these films, which provided unexpected results, where the main damage morphologies suggest that the main types of damage observed were absorption induced damage and pits due to inclusions. Laser damage is very sensitive to contamination within the nanosecond regime, and a large amount of discharge occurs inside the chamber during deposition, which leads to contamination of the films, which can lower the LIDT values. For future studies, working towards different methods to mitigate the discharges during deposition is important, as this will provide a better understanding of the causes laser induced damage of the films fabricated by ECR within the nanosecond regime. Further discussion of future work and experiment utilising ECR-IBSD have also been presented in details.
Advisor / supervisor
  • Reid, Stuart
Resource Type
DOI

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