Microwave modeling and characterization of thick coplanar waveguides on oxide-coated lithium niobate substrates for electrooptical applications

A set of thick coplanar waveguides on lithium niobate substrates with and without a thin SiO₂ buffer layer, has been experimentally characterized through on-wafer measurements. The effective refractive index and attenuation were extracted from raw (uncalibrated) measurements up to 40 GHz through the thru-reflection-line approach. The characteristic impedance was then obtained from the propagation constant, using an accurate estimate of the in-vacuo capacitance from a new conformal mapping approach able to account for large electrode thickness. We observed that the attenuation of lines with or without the oxide buffer layer consistently exhibits a different frequency behavior, thus suggesting that dielectric losses can play a significant role in the upper microwave range. This is confirmed by the results from a full quasi-TEM analytical model, including losses and frequency dispersion. The measured and simulated data show good agreement both for the propagation characteristics (attenuation and effective permittivity) and for the line impedance