A spectrophotometric method for the characterisation of thin metallic layers and validation in the example case of titanium

Abstract

Semi-absorbent metallic layers offer some unique possibilities in thin-film coating design due to their versatile dispersion properties. However, the presence of absorbance and the dependence of the properties on thickness present significant challenges for characterisation of such films. Therefore, as of today, this is no reliable and universal technique to characterize these layers. We propose here an alternative spectrophotometric method to determine the refractive index of a semitransparent metallic thin film. The method involves the preparation of a semi-reflective silicon substrate plus a thick dielectric layer several hundred nanometers thick. A thin, semitransparent metallic film is then deposited over this dielectric layer, creating an asymmetrical Fabry-Perot structure. The resulting spectrum displays oscillatory features from the dielectric layer, which are modulated by the dispersion properties of the thin metallic layer to be determined. A numerical optimization is then used to estimate the refractive index dispersion via use of an appropriate dispersion model. The sensitivity of the spectrum to the dispersion properties of the thin metallic layer allows these properties to be determined with a higher accuracy and robustness. In this paper, we detail a numerical and experimental validation of the method in the case of titanium thin films. To model the dispersion properties of these layers, we use the combined Modified Drude and Forouhi-Bloomer models. The index dispersion was determined for a range of titanium layer thicknesses from 10 nm to 70 nm. We show that the proposed method is accurate and stable and allows determining dispersion properties that can then be used for the design of multilayer structures for purposes such as colorimetry.