White light scanning interferometry for thickness measurement of thin film layers

White light scanning interferometry generates short coherence interferograms whose fringe visibility is narrowly localized along the scanning dimension so that the optical path difference between the test and reference beams can be scaled without 2 pi-ambiguity. Much attention has been paid during last two decades to the three-dimensional surface mapping using white light scanning interferometry, and as results, quite a few noble different techniques are available in the published literature. The techniques differ from each other in the intricate way of fringe data processing, but just about all of them are effective in measuring top profiles of opaque surfaces with nanometer precision. However, when the surface is coated with transparent thin film layers, multiple reflections occur from the boundaries of film layers to produce complicate overlapped interferograms which are not readily treated by the existing techniques. The thickness measurement technique presented in this paper is as an extended version of the frequency domain analysis of white light interferometry. Special attempt is made to provide a means of interpreting interferograms of multiple reflection so that physical properties of thin film layers are precisely identified utilizing sampled phase distribution in the spectral frequency domain. In conclusion, the technique is capable of not only probing thickness point by point but also providing complete volumetric film profiles digitized in three dimensions.