X-Ray Reflectivity for the characterisation of thin films

In the first part of this chapter the theoretical principles of X-ray reflectivity (XRR) techniques are reviewed: in particular it is possible to distinguish between specular reflectivity, when X-ray intensity is measured at the reflecting angle equal to the incidence angle, and diffuse reflectivity, that involves off-specular scattering. Structural information can be extracted from XRR patterns, as thickness, density and roughness of each layer. The reflectivity signal depends on the electron density as a function of depth. The mass density and the atomic number control the amount of signal, that can be reduced by the roughness. In the second part of the chapter, examples of XRR applications on thin films and multilayer are reviewed, with particular attention to recent developments. Major advances in X-ray optics during the past years, in particular, monochromator crystals and multilayer X-ray optics, make XRR experiments widely accessible in laboratory, and not only to the synchrotron beamlines. In recent years, in-situ XRR measurements under controlled atmosphere, humidity and temperature were also performed. In the third part of the chapter, a comparison between structural and morphological results, obtained by XRR and other standard techniques, is discussed. Finally, an overview regarding the improvements that are expected in the next future is also given.