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 th rough on-wafer measurements. The effective refractive index and attenuation were extracted from raw (uncalibrated) measurements up CX to 40Hz through the thru-reflection-line approach. The characteristic impedance was then ob tained from the propagation constant, using an accurate estimate of the in- vacuo capacitance derived from a new conformal mapping approach able to exa ctly account fbr large electrode thickness. We observed that the attenuatio n of lines with or without the oxide buffer layer consistently exhibits a d ifferent frequency behavior, thus suggesting that dielectric losses can pla y a significant role in the upper microwave range. This is confirmed by the results from a full quasi-TEM analytical model, including losses and frequ ency dispersion, which was derived by extending the model in [1]. The measu red and simulated data show good agreement both for the propagation charact eristics (attenuation and effective permittivity) and for the line impedanc e.