Broadband Determination of Microwave Permittivity and Loss in Tunable Dielectric Thin Film Materials
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253 Mat. Res. Soc. Symp. Proc. Vol. 603 © 2000 Materials Research Society
voltage, which defines the film tuning, can also be determined. The loss and tuning can in turn be used to obtain a single frequency-dependent figure of merit for the performance of the tunable dielectric thin film; the figure of merit is independent of specific device geometry. These experimental determinations of the thin film loss and permittivity can be used to compare the performance of different materials, or to optimize the performance of a specific material. The coplanar waveguide geometry is particularly well suited for the planar devices that can be fabricated using epitaxial thin films on single-crystal substrates. The measurements described here are performed in a cryogenic microwave probe station at arbitrary temperatures from 350 K down to 20 K. Because the properties of interest of these tunable thin-film materials typically depend strongly on temperature, the cryogenic probe station is an ideal configuration for optimizing the microwave response of these materials. Knowledge of both the frequency dependence and the temperature dependence of the relevant dielectric properties can provide important physical clues to the origin of losses in these materials. Since the material properties extracted using this technique are independent of measurement geometry and span a broad frequency range, the results of these experiments can also be used by designers to predict the performance of devices incorporating these thin-film materials in any planar geometry at arbitrary frequency. In what follows, we apply this new measurement technique to determine the dielectric properties of a 0.4 jim film of Ba 0.5Sr 0.5TiO 3 grown by pulsed laser deposition on a LaA10 3 substrate. Bao5Sro5 TiO 3 in thin-film form has been studied by a number of researchers for voltage-tunable applications[3-8]. We discuss in detail our determination of the propagation constant of our CPW transmission lines which are fabricated both on the dielectric thin film sample and on the bare substrate for a number of different CPW geometries. We also describe how we obtain the distributed circuit parameters (C(co), G(co)) of the transmission lines, using two different analytical techniques. We then extract geometry-independent measures of loss, permittivity, tuning and figure-of-merit, as a function of frequency from 0.1 to 26.5 GHz. We conclude with a discussion of the errors and limitations of this technique for determining loss, permittivity, and tuning of dielectric thin-film materials. EXPERIMENT Capacitance and Conductance in Coplanar Waveguide Transmission Lines We obtain the complex propagation constant y = a + i3, where a is the attenuation constant and 3 is the phase constant, of our CPW transmission lines as a function of frequency by performing a multiline thru-reflect-line (TRL) calibration[9]. The TRL calibration uses planar CPW calibration sets consisting of individual elements that are all fabricated using the same geometry (center conductor linewidth, gap
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