Precise spectrophotometric method for semitransparent metallic thin-film index determination using interference enhancement

Riley Shurvinton,Fabien Lemarchand,Julien Lumeau,Antonin Moreau

doi:10.1186/s41476-021-00172-9

Abstract

A precise spectrophotometric method to determine the refractive index of a semitransparent metallic thin film is presented. This method relies on interference enhancement of the measured spectra, employing an opaque substrate with a dielectric spacer layer beneath the absorbing layer of interest to create interference fringes.The resulting spectral oscillations of the stack are highly sensitive to the properties of the top absorbing layer, allowing precise determination of the refractive index via fitting. The performance of this method is verified using simulations in comparison to the typical method of depositing the absorbing thin film directly onto a transparent substrate. An experimental demonstration is made for titanium thin films over the visible range (370-835 nm). The refractive index of these films is extracted from experimental data using a combination of the Modified Drude and Forouhi-Bloomer models. This method showed high repeatability and precision, and is verified for Ti films between 6-70 nm thickness.

Highlights

Precise determination of the refractive index of materials is of paramount importance in the field of optical coatings
These results show that the improvement factor gained with the addition of oblique measurements depends on the stack (OS or transparent substrate (TS))
The titanium layer deposited on these samples was expected to be 28 nm thick based on initial calibration of our quartz microbalance

Summary

Introduction

Precise determination of the refractive index of materials is of paramount importance in the field of optical coatings. The refractive index dispersion of a given material when deposited as a thin-film often differs from the bulk value reported in literature, and is further influenced by the exact method and parameters of deposition [1]. Even for two nominally identical machines performing electron beam (e-beam) evaporation deposition, the parameters of deposition (surface properties, evaporation voltage and deposition rate, environment gasses) are generally not perfectly identical and will influence the final outcome of the index dispersion of the film. Different approaches are required depending on whether the metallic film in question is opaque or sufficiently thin to be semitransparent

Methods

Results

Conclusion