A Cavity in Transmission for Extracting Electric Parameters of Thin Layers

A cavity in transmission, used for complex permittivity measurements for thin layers (inferior to 525 mum for example) is presented in this paper. This broad-band technique allows the extraction of the material relative permittivity by using a Pi electrical network. Compared to coaxial probes in reflection, this new method is not limited by resonances and discontinuities when extracting electric parameters in a larger frequency range in which we work. Beside, the cavity in transmission permits to characterize material having low-loss tangent. In the theoretical derivations, it is assumed that only the fundamental TEM mode propagates inside the coaxial line because of the probe dimensions and higher modes are vanishing. Sample dimensions are so little that we work below the first cut-off frequency. The wavelength lambda is shorter than the cut-off wavelength lambda_c, so we use the distributed elements theory to extract electric parameters. To validate the procedure, we have used the silica case and the results were obtained with a total relative error around 1.6% on the relative permittivity. We testify that this technique might be used for multilayer electric characterization, only if the material has symmetric layers.