A CPW Resonator for Complex Dielectric Characterization of Thin Films at W-Band

A coplanar waveguide (CPW) composite resonator for dielectric characterization of thin films at ${W}$ -band is presented in this article. The sample holder is fabricated using gold evaporation on a fused silica wafer to minimize the resonator losses. The composite resonator is composed of two inductively coupled short-circuited CPW resonators to improve the quality factor of the sample holder. The technique is suitable for complex dielectric characterization of very thin materials (a few hundreds of micrometers), such as vehicular paint due to the high sensitivity of the CPW resonator to the superstrate material. An inversion algorithm is developed to calculate the real part of the sample's permittivity from the shift in the resonant frequency and the imaginary part from the change in the quality factor of the composite resonator after being loaded by a thin film of the unknown material. The CPW sample holder is fed by 1-mm end launchers to simplify the measurement process and overcome the complexities associated with probe station measurements. The proposed technique is validated through measurements of known Rogers substrates, polycarbonate, parylene-C, and SU-8 polymer. Then, it is used to measure and report few vehicular paint samples and clearcoats.