In most coating processes thin films are deposited atom by atom on a substrate. It is evident that the physics and functionality of the film will depend sensitively on the relative arrangement of atoms, that is their structure and morphology. The structure-forming processes of thin films can be devided into (1) surface-related processes and (2) bulk-related ones. Both processes cause pronounced preferred orientation of crystallites in polycrystalline films and lead to growth-related stress fields, the both of which may significantly influence the intended film functionality. The wavelengths of x-rays are of the same order of magnitude as typical interatomic distances, i.e. in the 0.1 nm range. Hence, x-rays are used as an effective probe to determine the local structure within thin films and to study
- chemical phases and composition,
- preferred orientation (texture),
- grain sizes and microstrains,
- macrostrain associated with residual stress,
- thin film density and voids or void network.
With these data the thin film technologist disposes of important information to elucidate the mechanisms that make the film grow and to tailor the film growth in order to achieve the intended properties.
The calculation of crystal properties often makes use of a spherical modeling of atoms on their crystal lattice sites. Strictly speaking, the approach is only justified for atoms and ions on positions of high symmetry like in the fcc lattice or the NaCl structure. In most crystal structures, however, atoms reside on less symmetric positions, and it may generally be stated that atoms on these positions represent a key issue in understanding anisotropic crystal properties.