Local spectroscopic characterization of materials using coherence scanning interferometry

Abstract

The large spot size of a few mm2 with spectrometers and a few thousand µm2 with ellipsometers means that classical spectroscopic characterization is limited to that of bulk materials. In the development of more recent heterogeneous materials in which there exists local variations between materials on a microscopic scale, a much smaller spot size is required for optical characterization. Several new techniques have been developed for performing local spectroscopy, such as by color camera microscopy, hyperspectral imaging microscopy, scattering type scanning near field optical microscopy (s-SNOM) or spectroscopic optical coherence tomography (s-OCT). Concerning the latter, the related technique of coherence scanning interferometry (CSI) also allows local spectroscopy by applying Fourier Transform processing to the local polychromatic interference fringe signal. This technique offers the advantages of not requiring an external spectrometer since an interferometer is incorporated in the microscope objective, but challenges remain in order to correctly adjust and calibrate the system. In this paper we present some of our latest results of using CSI to accurately measure the local spectra at a microscopic scale with a spot size a little larger than that defined by the diffraction limit, of around 1 µm. Results demonstrate measurements of local reflectance spectra at the surface of a heterogeneous sample and on small structures buried within or under a transparent layer. Other theory has been developed to allow the measurement of local transparent layer thickness and refractive index. As well as performing local point measurements, we show how with a single scan over the optical axis, 2D cartographic maps can be made of reflectance spectra together with the topographic height map of the same area. Any nanometric height errors present due to phase on reflection errors linked to the presence of complex refractive indices can then be corrected using the spectroscopic information.