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

Arya Fallahi, Sina Hashemizadeh, and Niels Kuster, IEEE Transactions on Instrumentation and Measurement, Volume 70, Article number: 3123257, online 26 October 2021; doi: 10.1109/TIM.2021.3123257

The use of an open-ended coaxial probe (OCP) and a vector network analyzer is a conventional technique for 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 paper, 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.

The scientific and technical impact of the study can be summarized as:

  • An improved method for noninvasive characterization of material layers with OCPs has been developed
  • The method includes a new algorithm to improve the accuracy of the OCP technique for noninvasive broadband permittivity measurements at high frequencies and high dielectric constants
  • The use of a lossy platform allows the OCP method to be extended to the accurate measurement of high permittivity and low loss materials 
  • A novel calibration algorithm that evaluates the effective air-gap between the probe and sample originating from surface imperfections was used to achieve reduced sensitivity to small air-gaps