dc.title: Quantitative evaluation of diffusion in thin filmsvia ToF-SIMS depth profiling dc.description.abstract: Thin films are essential components of modern functional devices, yet their long-term performance and reliability are often controlled by nanoscale mass transport. Secondary Ion Mass Spectrometry (SIMS) is one of the primary techniques for probing diffusion processes with nanoscale depth resolution. However, the quantitative extraction of transport parameters is currently hindered by unvalidated data processing methods,oversimplified analytical models, and ill-posed inverse fitting problems. This dissertation contributes to addressing these challenges by establishing a methodological framework for quantifying mass transport in thin films via SIMS. First, a critical evaluation of widely used concentration proxies (normalized secondary ion intensities) demonstrates that their use can introduce severe artifacts into diffusion analyses. As the validity of these proxies depends on the spatial and temporal stability of reference ions, which are rarely guaranteed in chemically evolving matrices, this work demonstrates the necessity of modeling raw ion-intensity data and their derivatives to prevent misleading diffusion gradients. Furthermore, a systematic benchmarking of approximate analytical diffusion models against an exact closed-form solution for finite bilayers reveals that widely adopted semi-infinite approximations are accurate only within narrow parametric subdomains. To ensure the reliability of extracted diffusion coefficients, the broader adoption of exact finite-domain solutions is recommended. This methodological framework was then applied to quantify mass transport in a technologically relevant system: water diffusion in molybdenum disulfide (MoS2) solid lubricant coatings. Using time-resolved ToF-SIMS with in-situ heavy water isotopic tracing, the diffusivity of water in MoS2 coatings grown under varying deposition conditions was successfully extracted. A robust inverse modeling approach was used to address solution non-uniqueness, which provided evidence that water diffusivity in MoS2varies by orders of magnitude depending on the MoS2 cross-sectional morphology. These findings link nanoscale water transport to the macroscopic tribological degradation known as the dwell-time effect. Altogether, this dissertation provides a broadly applicable methodological framework for quantitatively evaluating diffusion in thin layers. This outcome is broadly applicable to a range of applications in which the functional behavior of thin films is controlled by their mass transport properties.