On the Dielectric Measurement of Thin Layers Using Open-Ended Coaxial Probes

Using an open-ended coaxial probe and a vector network analyzer (VNA) is a conventional technique for the noninvasive and broadband measurement of dielectric permittivity. The method is widely applied to characterize bulk material and liquids and has recently been extended to thin sheet material samples. However, it is commonly believed that the method is not suitable in the high-frequency regime as perfect contact between the probe and the sample is required. Furthermore, the method is limited with respect to the characterization of thin sheet samples made of low-loss materials. In this article, we analyzed the main causes of these limitations, namely, low sensitivity, probe resonances, and the occurrence of air gaps between probe and samples. We also discuss and demonstrate solutions to overcome some of these limitations and improve the accuracy by introducing a lossy platform and an effective air-gap calibration algorithm. These improvements lead to a reliable and robust framework to characterize thin and flat dielectric samples, even at high frequencies and in high-permittivity regimes.