The use of scanning white light interferometry (SWLI) to study thin films is becoming more and more widespread as understanding of the measurement increases. Techniques for film thickness measurement of thick films of 1.5 micrometres or greater using interferometry have been available for a few years now, but measurement of thinner films is a much more recent option.

Application of a helical conjugate field (HCF) function has opened up the possibility of accurately measuring surfaces with semi-transparent thin films with thicknesses down to 20 - 30 nm. An exciting extension of the HCF function is the possibility of studying true top surface and interfacial surface roughness as well as the film thickness measurement.

Thin Film Analysis

With a transparent or semi transparent film coating on a surface it is possible to conduct film thickness measurement using SWLI if the fringe envelopes arising from the two surfaces can be resolved. This limits the thinnest films that can be measured with any confidence to between 1-2 microns depending on the refractive index of the film.

When the film is too thin for the individual fringe envelopes to be resolved a different approach has to be used. One solution is the application of a helical conjugate field (HCF) function to the raw data in order to extract information about the thin film coatings.

This technique, in combination with knowledge of some of the optical properties of the materials, is able to calculate accurate thin films thicknesses down to at least about 20 - 30 nm. The technique is capable of measuring multilayer film thickness.

Top Surface and Interfacial Surface Roughness

One of the major challenges in interferometry has been the measurement of surface roughness when a film is present; often the measured roughness is a combination of the top surface roughness and the interfacial surface roughness. Application of the HCF function to SWLI can also be used to obtain accurate surface roughness. This includes true analysis of the top surface and the interface between the thin film and substrate.