A thickness value is only meaningful when the measurement method and model are suitable for the film. A step-height measurement, an optical fit and an angle-resolved surface analysis may each report thickness, but they do not observe the same physical signal.
Profilometry needs a usable step
A stylus profilometer follows the surface and measures height change. It is direct and widely understood, but the sample needs a boundary between coated and uncoated regions or another reference step. Soft films, rough surfaces and stylus force require attention.
Optical methods depend on a model
Reflectometry and ellipsometry can be fast and non-contact. Their accuracy depends on optical constants, layer structure, roughness and the quality of the fitted model. A convincing fit is not enough if the model does not represent the real stack.
Surface analysis is useful for very thin layers
Angle-resolved XPS can provide thickness and composition information for ultra-thin films. Other depth-profiling methods may be useful when chemistry changes through the layer. These techniques are more specialized than routine production thickness checks.
Use more than one method when risk is high
During process development, a second technique can reveal model or sample-preparation errors. Once the relationship is understood, production may use a faster proxy with periodic cross-checks. Always record location, sample condition, calibration and uncertainty with the reported number.
