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Microwave Phase Shifters Using Ferroelectric (Ba,Sr)TiO3 Films Won-Jeong Kim, Eun-Kyoung Kim, Seung-Eon Moon, Seok-Kil Han, Su-Jae Lee, and KwangYong Kang Basic Research Laboratory, Electronics and Telecommunications Research Institute, Kajung-Dong 161, Taejon, 305-350, Korea ABSTRACT The ferroelectric (Ba0.6Sr0.4)TiO3 (BST) films were prepared on (001) MgO single crystals by pulsed laser deposition. Coplanar waveguide (CPW) type phase shifters controlled by external dc bias field were fabricated on BST films using a 2 µm thick metal layer to reduce metal loss. Microwave properties of the CPW phase shifter were measured using a HP 8510C vector network analyzer from 0.1 – 20 GHz. The fabricated CPW phase shifters (8 mm long) exhibited differential phase angle of 100 o at 10 GHz with a dc bias field of less than 80 kV/cm between center and ground conductors. Furthermore, a stable differential phase angle (102 ± 3.5 o) was observed from another CPW while changing the power of incident microwave from –10 to +30 dBm. Gap size dependent dielectric constant of the BST film was observed and a simple correction method was suggested in the paper. These results demonstrate the possible application of ferroelectric tunable devices on a high power tunable wireless telecommunication. INTRODUCTION Ferroelectric thin films with electric field dependent dielectric constant are being used to develop a new class of tunable microwave devices.[1-7] An important tunable microwave device using ferroelectric film is a wideband phase shifter which is the most important component of the phased array antenna. A simple co-planar waveguide (CPW) type phase shifter has advantages over other type phase shifters; easy fabrication with one-mask, easy to measure the microwave characteristics, and easy to extract film parameters. In this paper, we report the microwave characteristics of the CPW fabricated on (Ba0.6Sr0.4)TiO3 (BST) in terms of differential phase shift, and dielectric constant of BST. The fabricated CPW phase shifters exhibited a large differential phase angle with a dc bias field of less than 80 kV/cm between center and ground conductors. Furthermore, a stable differential phase angle was observed from another CPW while changing the power of incident microwave from –10 to +30 dBm. At the end of this paper, we will discuss dielectric properties of the BST film in terms of the gap size of the device, which exhibits that the dielectric constant of the BST extracted by the conformal mapping increases with decreasing gap size. We introduced a simple empirical method to correct dielectric constant of a thin layer of high-k material. FILM GROWTH AND DEVICE FABRICATION Single phase BST films were deposited using a well-known pulsed laser deposition (PLD) method onto (001) MgO single crystal. A focused Kr:F excimer pulsed laser was used to ablate BST target. MgO substrates were heated at 750 oC and the deposition chamber was kept in the oxygen pressure of 170 mTorr. The thickness of the BST films were controlled by changing the deposition time