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C8.23.1

Electrode effect on microwave properties of ferroelectric (BaxSr1-x)TiO3 thin films Won-Jeong Kim*, Sang-Su Kim, and Tae-Kwon Song Dept. of Physics, Changwon National University, Changwon, 641-773, Korea Seung Eon Moon, Eun-Kyoung Kim, Min-Hwan Kwak, Seok-Kil Han, Young-Tae Kim, HanCheol Ryu, and Su-Jae Lee Electronics and Telecommunications Research Institute, Daejon, 305-350, Korea * [email protected]

ABSTRACT Microwave properties of coplanar waveguide (CPW) transmission lines fabricated on high dielectric materials, such as ferroelectric Ba1-xSrxTiO3 films, are highly sensitive on the dimension and shape of electrodes. A small change in device dimension affects the total electrical length of the CPW, which may mislead the effective dielectric constant of the dielectric layer. Furthermore, extracting dielectric constant of high-k thin films from the measured microwave properties, such as S-parameters, is very difficult. The well known a modified conformal mapping method frequently exhibits an inconsistent dielectric constant for CPW on high-k materials. CPW transmission lines were fabricated on high-k thin films, ferroelectric Ba0.6Sr0.4TiO3, which were deposited by the pulsed laser deposition with partial oxygen backgrounds. A large phase shift angle of 100o at 10 GHz was observed from the CPW (gap = 4 µm, length = 3 mm) with a 40 V of dc bias, which supports that the idea of the tunable microwave device application using ferroelectrics films. The dielectric constant of the thin ferroelectric film was extracted from the dimension of the CPW (gap, width, length) and the measured S-parameters by a modified conformal mapping. However, the dielectric constant of the ferroelectric thin film calculated by a modified conformal mapping exhibits a gap dependency; dielectric constant (990 ~ 830) decreases with increasing gap size (4 ~ 19 µm, respectively). For comparison, dielectric properties have been extracted by extensive EMsimulation using a HFSSTM (Ansoft) with observed dimensions of CPW devices. Total phase, which is closely related with the dielectric constant of the film, is strongly affected by gap size, film thickness, and slanted angle of CPW.

INTRODUCTION Electrically tunable microwave devices using ferroelectric thin films, which exhibit electric field dependent dielectric constant, have been investigated for decades.[1-7] For example, a wideband phase shifter, an important tunable microwave component for the phased array antenna, can be realized using ferroelectric film by controlling applied dc bias field. A simple design coplanar waveguide (CPW) transmission lines with ferroelectric thin film serve as a phase shifter when dc bias field applied between ground and center conductors, which has advantages over other type phase shifters; easy fabrication with a single-mask, easy to measure the microwave characteristics, and easy to extract film parameters by a conformal mapping method. In this paper, we report the microwave characteristics of the CPW fabricated on (Ba0.6Sr0.4)TiO3 (BST) in ter