Imaging Ellipsometry

Abstract

Ellipsometry is a very sensitive measurement technique that uses polarized light to characterize thin films, sur-faces, and material microstructure. An ellipsometric experiment measures changes in the state of polarization which occur upon reflection on a film cov-ered support. The measurement yields two quantities, Ψ and ∆, which are related to the ratio of the complex re-flectivity coefficients for s and p polarization by the basic equation of ellipsometry: tan Ψe i∆ = r p r s (1) The principle of an ellipsometer is sketched in the following figure. Polarizer and retarder are used to produce ellipti-cally polarized light. The state of polarisation is changed upon reflection and the resulting linear polarized light is then cancelled by the analyzer. The null setting of the optical components gives the ellipsometric angles. The reflected light can be completely cancelled if the sam-ple is uniform. This is not the case if the sample possesses inhomogeneity on a micrometer scale. In this case the lateral inhomogeneity is transfered in the state of polar-ization as sketched in the following figure: Each region of the surface corresponds to a different ∆ and Ψ. A microscopic image of the surface can be formed with the aid of a proper imaging optics. The lateral res-olution is determined by the numerical aperture of the microscope objective ≈ 2µm and the vertical resolution is in the sub-nm range due to the ellipsometric contrast. The measurements are non-destructive and many sample work, even liquid-liquid interfaces are accessible. Imaging ellipsometry can be used for an assessment of two dimen-sional morphologies and surface pattern. It combines the high vertical resolution of conventional ellipsometry which is in the sub-nm rang with a lateral resolution in the Mi-crometer range. Hence structures within a monolayer can be visualized. Typical application