Optical nanoform characterization by imaging Mueller matrix ellipsometry

Abstract

Using conventional Mueller matrix ellipsometry, the geometries of periodic nanostructures can be easily determined if the measurement fields are not smaller than the illumination spot size. Measurements on individual nanostructures smaller than this can be accounted for by imaging ellipsometry, which allows measuring all 16 Mueller matrix elements for each pixel in the camera of the imaging system. These so-called Mueller matrix images contain additional information about the spatial distribution of the sample’s polarizing properties that are useful in the characterization of individual nanostructures. We built an imaging Mueller matrix ellipsometry system for measurements in the visible regime. Our system allows for the analysis arm, which holds the CCD camera and the polarization state analyser, to be rotated freely around the sample. By this, measurements in reflection and in transmission can be performed at arbitrary angles of incidence between 37.5° and 90°. Additionally, we implemented a reflection mode for 0° angle of incidence. Using this setup, our goal is to characterize the shape of individual nanostructures much smaller than the illumination spot using the additional information from the Mueller matrix images. Thus, we designed and fabricated a sample containing various individual nanostructures with different geometrical features. The structures are of square or circular shape, ranging in size from 5 µm to 50 nm. Additionally, the square structures feature corner rounding with different radii for a transition between circle and square. With these structures, we systematically measure the influence of the shape on the Mueller matrix elements. We also investigate in using Mueller matrix images for the characterization of subwavelength sized features significantly smaller than the resolution limit of our microscope system at about 800 nm. First results show clear distinctions between opposing edges of the nanostructures in off-diagonal Mueller matrix images.