XRR is a non-destructive, highly accurate method used to determine thickness and roughness of thin layers, with thicknesses ranging from a few nanometers to some hundred nanometers, as well as the optical properties of the reflecting interfaces. The technique uses the average electron density of a layer and thus cannot provide information on the elemental composition. In XRR the specular reflected intensity resulting from an incident X-ray beam is measured. Varying the incidence angle shows oscillations in the recorded intensity due to interference effects between reflections from different layer boundaries, and accordingly the period of these oscillations is directly correlated with the thickness of the layer.