Spectroscopic measurement of material properties using an improved millimeter-wave ellipsometer based on metallic substrates

In this paper, we introduce an experimental method that improves the measurement accuracy of millimeterwave (mmW) ellipsometry for characterization of homogeneous polymers and dielectric composite materials. Our approach is based on using a metal substrate on which the samples are placed. This enhances the amount of Fabry-Perot reflections within the material under investigation, and therefore reinforces a stronger phase rotation of the electromagnetic field. We discuss the achieved measurement accuracy with respect to the critical measurands, the ellipsometer angles (psi) and (Dreieck), and show that it is significantly improved by the presented method. In order to overcome ambiguities in the material parameters, we measure the ellipsometer angles over a frequency range from 80 to 110 GHz. In addition, we present an approximation method that reduces the measurement time of mmW ellipsometry in the case of lowloss samples. We demonstrate that it is sufficient to determine the maximum and minimum power of the measured ellipsometer curve and discuss the limits of this approximation. We performed numerous test series on different plastic materials over a frequency range of 20 GHz, and will discuss the measurement accuracy with respect to the refractive index and the thickness of our samples.