Analysis and Optimization of Broadband Measurement Cells for the Characterization of Dielectric Polymer Films

The current techniques and methodologies used in the field of material characterization are well documented and widely accepted as reliable and accurate. However, literature describing these techniques focuses on the algorithms used during material characterization; few studies have reposted on the design of, and the selection criteria for, the test fixtures themselves. This research focuses on the measurement cell with the goal of determining the sensitivity of the measurement cell to the addition of a thin film material.\nMicrostrip and coplanar waveguide were chosen for the analysis, which included three configurations of each transmission line geometry: a reference with no additional thin film material, one with the thin film on top of the conductors and one with the thin film beneath the conductors but on top of the transmission line substrate. The scattering parameters for the reference cell are compared to the scattering parameters of the test cell with the thin film material. The additional thin film material changes the effective dielectric constant of the reference cell; this change is evident in the phase and amplitude of S21.\nThe optimum measurement cell is the one that experiences the greatest change to the effective dielectric constant with the addition of the thin film. Thus the greatest difference in S21 between the reference cell and the test cell is indicative of the reference cell's sensitivity. The figure of merit (FOM) to determine the structure's sensitivity is the integration over frequency of the magnitude of the vector difference of S21. The analysis shows that the double-layered CPW measurement cell was the most sensitive.\nOnce the optimum structure was determined an analysis of the sensitivity of the FOM to changes in the physical and electrical properties of the reference structure was conducted. The most important factors in the selection of the reference cell as evident by the FOM's sensitivity are the substrate to thin film dielectric constant ratio and the CPW conductor aspect ratio to the thin film thickness. In particular, thinner films require a smaller conductor gap while wider gaps are preferable for thicker films.\nMeasurement of four different CPW geometries, each covered in a 300 micron Polydimethylsiloxane thick film, validate the analysis process. The measurement cells differ in the conductor aspect ratio. The values of the measured FOMs trend as predicted by the simulation analysis.