Spatially Resolved Thickness Determination of Multi-Layer Thin Films by In-Situ Reflected Light Microscopy Using Fresnel Equations
F. Giannetti (TU Delft - Aerospace Engineering)
S.J. Garcia Espallargas – Mentor (TU Delft - Aerospace Engineering)
L. Satish Nair – Mentor (TU Delft - Aerospace Engineering)
A. Anisimov – Graduation committee member (TU Delft - Aerospace Engineering)
R.M. Groves – Graduation committee member (TU Delft - Aerospace Engineering)
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Abstract
Aerospace aluminium alloys such as AA2024-T3 have long relied on hexavalent chromium-based inhibitors for corrosion protection, and no alternative yet matches their performance. REACH restrictions on hexavalent chromium have therefore made the search for replacements urgent. With this comes the need for techniques that can reveal the local formation process of organic inhibitors. For this purpose, in-situ reflected light microscopy is attractive, as it offers high spatial and temporal resolution at low cost and can be combined with electrochemical measurements. This thesis applies the Fresnel multi-layer equations to reflected light microscopy under both monochromatic and white light illumination, and implements the method in an analysis application that converts measured reflectance changes into spatially resolved thickness maps with Monte-Carlo uncertainty estimates.
A confocal laser scanning microscope (Keyence VK-X1000) with monochromatic illumination, and a low-cost USB microscope (Dino-Lite) with coaxial white LED illumination were compared. Both were validated on gold-sputtered AA1050 and AA2024-T3 samples. Both setups resolved layers down to 15 nm, but focal drift made the confocal setup unsuitable for long-duration in-situ measurements. The Dino-Lite setup was therefore used to follow the growth of 2,5-dimercapto-1,3,4 thiadiazole (DMTD) films on AA1050 and AA2024-T3 in situ. Further improvements to the optical setup are needed to reduce artifacts and increase spatial resolution.