Optical Thickness Determination on Structured Samples

Transparent films of several micrometers thickness are almost everywhere present in technical applications. In some applications a further treatment of the films, e.g. in lithography, leads to structured surfaces where the original layer is reduced on smaller areas. Then, the optical thickness determination of the film is rendered more difficult, because the reflection of the layer is superposed or mixed with the reflection of the substrate. In these cases a microscope can be used when measuring the film reflectance. Here, we present a numerical study on the thickness determination without microscope. We will demonstrate that it is possible to determine the film thickness exactly from the superposed signal when the portion x as percentage of the film area to the total area of the detection spot becomes larger than a threshold value. For that purpose we studied films of SiO2, Si3N4 and of a photoresist material on substrates of Si or GaAs and varied the film thickness. For thickness evaluation the Fast Fourier Transform was applied. To get results that are comparable to measured values the computed reflectance spectra were artifically made noisy.