Application of coherence scanning interferometry for local spectral characterization of transparent layers

In the domain of optical metrology, white light interference microscopy is mainly known for performing micro and nano surface profilometry. This is achieved by identifying the envelope peak of the fringe signal. However, the polychromatic signal is rich in information and spectral characterization may be performed through the Fourier analysis of the signal, which gives local spectroscopic information about the sample surface. The use of CSI for studying transparent layers has also been well-developed since the analysis of the reflected light provides both structural and spectral information on the layer. Through the spectral analysis of the reflected light, it has been shown that the morphological properties of a thin film structure, namely the thickness and the refractive index, can be precisely measured. In this case, either the amplitude or the phase of the thin film total reflectance spectrum are used to recover the thickness. The technique is based on the best fit between the experimentally measured spectrum with that of the theoretical model using a non-linear least-squares algorithm. Usually, this spectral method is used to investigate thin films having a thickness that does not exceed a few hundred nanometers. In this work, we apply a similar technique, based on the magnitude of the total reflectance spectrum, to study thicker transparent layers. In this case, we show that precautions regarding the effective numerical aperture of the system need to be considered to obtain consistent values of both the refractive index and the thickness. In addition, we demonstrate the possibility of extracting the depth-resolved reflectance spectra of a buried interface independently from the spectral response of the surface. The consistency of these different spectra is demonstrated by comparing the results with those obtained using a program based on electromagnetic matrix methods for stratified media. The lateral spatial resolution of the measurements attained is a spot size of around 0.84 μm for spectrally characterizing small structures.

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