Local inspection of refractive index and thickness of thick transparent layers using spectral reflectance measurements in low coherence scanning interferometry

Abstract

For a long time, obtaining with a great accuracy the optical and morphological properties of a transparent sample without degrading the layer has been challenging. To achieve these expectations, contactless techniques are well suitable and brought optical methods to the forefront. Over recent years white light scanning interferometry has been increasingly used for studying and characterizing transparent materials with thicknesses ranging from a few hundred nanometers to several micrometers. Then, multiple techniques have been developed to retrieve the transparent layer properties from interferometric data. The more recent techniques, based on the use of an error function which defines the best fit between the experimental and theoretical data, allow the determination of the properties of very thin films (< 1 µm). We show here that a similar method can be applied to thicker layers (> 1 µm) for simultaneously measuring their optical and morphological properties, provided that a crucial step is carefully considered during the data acquisition process. This enables the simultaneous measurements of both the thickness and the refractive index (dispersion) without any prior assumptions about one of the two parameters. We demonstrate the proposed method by accurate measurements on a few micrometers thick PMMA layer as well as on a SnO2 layer, which is a much more dispersive sample.

Characterization of Functional Materials Using Coherence Scanning Interferometry and Environmental Chambers

“…5 The results can also be used in colorimetry to determine the colors of different materials. By windowing the fringe signal, this technique has also been adapted to performing local spectroscopy on interfaces and structures buried under transparent layers, 17 for carrying out local measurements of refractive index and thickness of transparent layers, 18 and for studying the properties of particles buried in transparent and diffusing layers. 19 ■ IMPROVED LATERAL RESOLUTION USING MICROSPHERES While interferometry allows nanometric axial sensitivity for topographic measurement, the lateral resolution is several hundred nm, limited by diffraction.…”

“… 5 The results can also be used in colorimetry to determine the colors of different materials. By windowing the fringe signal, this technique has also been adapted to performing local spectroscopy on interfaces and structures buried under transparent layers, 17 for carrying out local measurements of refractive index and thickness of transparent layers, 18 and for studying the properties of particles buried in transparent and diffusing layers. 19 …”

Spatially Resolved Optical Characterization of Functional Materials Using Coherence Scanning Interferometry

“…Neglecting second‐order reflections, applying a Fourier transform to the interference signal from the transparent layer and taking the magnitude squared leads to the expression of the spectral reflectance of the layer and can be written as [ 26 ] …”

“…The results for the SnO 2 layer in Figure 7b show a comparison between the experimental reflectance spectrum (black curve) and the theoretical optimized model (blue curve) with n taken as a function of the wavelength. [ 26 ] The resulting dispersion law over a wavelength range of 600–1000 nm in Figure 7c shows the average refractive index of ten measurements (blue curve) and the standard deviation (gray area). The comparison with measurements using ellipsometry (dashed–dotted line) shows that the results are quite satisfactory even for a highly dispersive layer.…”

“…Finally in Figure 7d, the results of measuring the layer thickness at ten different points (blue circles) are given, together with the results of measurements made with a Dektak stylus profiler (black curve) and the associated measurement uncertainty, [ 26 ] showing a very good accuracy of the local measurement technique.…”