Any real layered structures show surfaces and interfaces that are not atomically sharp. The interfacial roughness and its correlation from interface to interface is of critical importance for many applications. Therefore, the knowledge and control of the quality of interfaces is of both practical and fundamental interest.

X-ray scattering offers a non-destructive method of gaining information of buried interfaces. X-ray reflectometry is sensitive to film thickness, density of the materials, and to interfacial roughness. However, from specular reflection alone it is not possible to distinguish density gradient from physical roughness. The diffuse incoherent part from a rough surface/interface can be best measured by X-ray off-specular scattering. In one of the possible scans the sample is rotating at a fixed detector angle.

The diffuse scattering data reveal the typical features of roughness. These wings (so-called Yoneda peaks) at the lower and higher side of the specular part indicate the presence of roughness. The range from the wings to the specular part is affected by the scattering length density. The analysis is carried out by fitting a sample model describing the rough interfaces in the distorted wave Born approximation.