Imaging Mueller matrix ellipsometry for characterization of non-periodic nanostructures

Ellipsometry is one of the most versatile methods for optical nanostructure characterization. Especially Mueller matrix ellipsometry allows the measurement of optical or geometrical parameters with uncertainties down to the sub-nanometer range. In conventional ellipsometry, this involves averaging over the complete illumination spot size. If the structured area on the sample is smaller than the illumination spot, or the structure has no periodicity, the measurement result may contain reflected signals from the surrounding areas. This is particularly noticeable for free-standing nanostructures with diameters smaller than the illumination spot size. In such cases, imaging ellipsometry can be used. Here, a Mueller matrix is measured for each pixel in a camera image, allowing the polarization influence of the sample to be investigated locally. In this paper, we provide approaches to determine concrete relationships between geometric properties of nanostructures on off-diagonal elements of the Mueller matrix. To this end, we have implemented a setup for measuring Mueller matrix images at different angles of incidence in transmission and reflection, and fabricated a sample with which we systematically measure geometric structural properties in Mueller matrix images. We present measurements as well as numerical simulations to compare the results. Furthermore, we discuss thermal influences on measurement results and present an algorithm for their treatment.