Techniques and Challenges for Characterizing Metal Thin Films with Applications in Photonics

The proliferation of laser technologies has profoundly increased the demand for high-quality optical thin films whose physical properties are tunable and well defined. Such films are frequently deposited in thicknesses much shorter than the wavelengths of visible light and consequently present challenges for characterization by traditional microscopy. Metal films in particular exemplify these challenges, due to their broad range of refractive indices, optical absorption and often near-complete reflectivity in the visible spectrum. However, due to their relatively consistent crystalline structure, the bulk optical properties of metal thin films are chiefly dependent on their thickness. This review therefore presents a compendium of viable alternative characterization techniques to highlight their respective utilities, limitations and resolutions, specifically with regard to the characterization of the thickness of metal films. Furthermore, this review explicitly addresses the operating theories, methods and analyses relating to the five most predominantly utilized techniques: X-ray Reflectivity (XRR), Spectroscopic Ellipsometry (SE), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS). This work is intended as an introductory guide to thin film characterization modalities and their applicability for metal and optically-absorptive films, while also identifying AFM and SEM/EDS as being amongst the more reliable of the techniques.