Broadband determination of microwave permittivity and loss in tunable dielectric thin film materials

We demonstrate a new method for determining the frequency-dependent dielectric properties of thin-film materials at microwave frequencies using coplanar waveguide (CPW) transmission line measurements. The technique makes use of the complex propagation constant determined from multiline thru-reflect-line (TRL) calibrations for CPW transmission lines to determine the distributed capacitance and conductance per unit length. By analyzing data from CPW transmission lines of different geometries, we are able to determine the complex permittivity of the dielectric thin film under study as a function of frequency from 1 to 40 GHz. By performing these measurements under an applied bias voltage, we are able in addition to determine the tuning and figure of merit that are of interest for voltage-tunable dielectric materials over the frequency range 1 to 26.5 GHz. We demonstrate this technique with measurements of the permittivity, loss tangent, tuning, and figure of merit for a 0.4 mu m film of Ba0.5Sr0.5TiO3 at room temperature.