The Coherent Limitation of the Specular X-ray and Neutron Reflectivity on the Characterization of the Physical Vapor Deposition Thin Films

The problem of non-uniformity in film thickness in the measurement of X-ray and neutron reflectivity is studied in this work. This problem becomes severe when the evaporator source is small and the source to substrate distance is very small. The non-uniformity of the film thickness which excesses the beam coherent length on the film surface, contributes an additional amount of smearing out the reflectivity data resulting in an inaccuracy in determining the film density and interface roughness. Narrowing down the incident slit size will alleviate this problem with the sacrifice of the dynamic range of reflectivity measurements. In addition, the non-uniformity of the thin film can also be found in the system of island growth of the thin film when the island size is well beyond the surface coherent length of incidence X-rays. In this case, the incidence X-ray coherent length should be improved to avoid the problem.