Determination of the optical constants of thin films in the visible by static dispersive Fourier transform spectroscopy

Abstract

Conventional dispersive Fourier transform spectrometry (DFTS) is a powerful tool for determining optical constants of materials. However, the refined and intrinsically high-cost mechanically scanned interferometers that are necessary are not well suited to use in hostile environments or for time-resolved operation. We describe here a novel approach to DFTS that employs a combination of a Wollaston prism and a linear detector array. It is ideally suited to the precision characterization of thin films with physical thicknesses of up to about 1000 wavelengths or typically about 1 mm. The simplicity and optical efficiency of conventional DFTS are combined with the inherent robustness, superior time resolution, and high repeatability of spatial interferometry. The technique offers an optical throughput that is an order of magnitude higher than spectrophotometry or spectral ellipsometry while accuracies of 1 part in 104 and repeatability of 1 part in 105 are possible for the measurements of the real part of the refractive index. The imaginary component of the refractive index of thick transparent samples has been measured with an absolute error of less than 2×10−4. The technique may be readily applied from the vacuum ultraviolet to the mid infrared. We present proof-of-principle measurements of optical constants at wave numbers between 9000 and 25 000 cm−1 for a self-supporting film of Melinex and for a thin film of ZnSe grown by molecular beam epitaxy onto a glass substrate.