The molecular arrangement and packing density of molecules in a thin film differs from the bulk sample. As a consequence, film thickness differs from from expectations based on the properties of the bulk. The density, thickness and surface roughness of a thin film sample can be measured non-destructively and simultaneously using the total X-ray reflection technique. If the sample surface is flat (a requirement for reflectivity), the absolute film thickness can be measured, even on metal thin films, which cannot be evaluated using an ellipsometer.
The figure below, collected using Rigaku's SmartLab multipurpose diffractometer, shows the result of the reflectivity measurement performed on a film of silicon nitride film (Si3N4). Based on profile fitting analysis, the film thickness is 99.8 nm, the film density is 2.88 g/cm3 and the surface roughness: 0.65 nm.