Evaluation method of high-permittivity materials in microwave region using coplanar waveguide

Abstract

This paper proposes a novel evaluation method for dielectric characteristics of ceramic substrates made of high-permittivity material, (Ba, Sr)TiO₃ (BST), in the microwave region. The dielectric characteristics are obtained accurately and simply by measuring S-parameter and by full-wave analysis for coplanar waveguide (CPW) composed on the substrate. Only the measurement of S₂₁ response of the CPW is required for evaluation. The effective permittivity (ε_eff) and attenuation constant (α) are calculated from the S₂₁ data. Relative permittivity (ε_r) and tanδ of substrate material are estimated by relating the ε_eff and α quantitatively with their simulated values calculated by the Extended Spectral Domain Approach (ESDA) taking the thickness effect of the conductor into consideration. Three kinds of BST substrates with different Ba to Sr ratios were measured as test samples. The estimated ε_r's of Ba_0.3Sr_0.7TiO₃, Ba_0.5Sr_0.5TiO₃, and Ba_0.7Sr_0.3TiO₃ were 662, 1650, and 4450, respectively. These ε_r's decrease slightly as the frequency increases. The estimated errors for the ε_r's are estimated to be less than 0.3%. This method is useful for the evaluation even of a small variation of dielectric constant in high-permittivity materials. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(3): 17–25, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20296