Application of the causality condition to thin film spectroscopy. A method for the evaluation of the thickness and optical constants

Abstract

A method is described for evaluating the thickness and optical constants of a film using experimental data over a range of wavelengths obtained by either ellipsometry or specular reflection spectroscopy. It is based on a fitting procedure which involves the expansion of the dielectric constant of the film as a series of Lorentzian functions. Such a procedure yields a minimum in the variance at the correct film thickness. The method has been tested against real and synthetic data for both thick and thin films. The basis of this method is the requirement that the film dielectric constant satisfy causality conditions. Other methods of choosing the film thickness so as to satisfy the causality condition have been tested, and are superior for some applications.