Measurement of thin films and interfacial surface roughness using SWLI

Interferometry is now an established technique for the measurement of surface topography. It has the capability of combining sub-nanometre resolution, high measurement speed with high repeatability. A very useful extension to its capability is the ability to measure thick and thin films on a local scale. For films with thicknesses in excess of 1-2 μm (depending on refractive index), the SWLI interaction with the film leads to the formation of localised fringes, each corresponding to a surface interface. It is relatively trivial to locate the positions of these envelope maxima and therefore determine the film thickness, assuming the refractive index is known. For thin films (with thicknesses ~20 nm to ~2 μm, again depending on the refractive index), the SWLI interaction leads to the formation of a single interference maxima. In this context, it is appropriate to describe the thin film structure in terms of optical admittances; it is this regime that is addressed through the introduction of a new function, the 'helical conjugate field' (HCF) function. This function may be considered as providing a 'signature' of the multilayer measured so that through optimization, the thin film multilayer may be determined on a local scale.