Spatially resolved thickness determination of rough thin films by reflectometry with polychromatic light

This article introduces a simple metrology set-up based on white light interference. The aim is to determine the thickness of thin transparent films by their reflection color under white illumination. Therefore, one digital color image of a large field of view is taken in nontelecentric geometry. To obtain reflected light at all angles of view without having to move the camera the substrate has to be sufficiently rough. Additionally, interference occurs at the thin transparent film covering the substrate surface from which a two-dimensional lateral thickness distribution can be calculated. This method has been used as industrial inline process control for a CdS semiconductor layer on top of a rough Cu(In,Ga)(Se,S)₂ thin film solar absorber and yielded 74 nm film thickness with only a 1 nm standard deviation. Options to further reduce the standard deviation with different illuminants and other cameras are proposed.