Hydrothermal Fabrication of Barium Strontium Titanate Ceramics for High-Frequency Wireless Communication Networks
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Hydrothermal Fabrication of Barium Strontium Titanate Ceramics for High-Frequency Wireless Communication Networks K. Zelonka1, M. Sayer1, H. Hammad2 and A.P. Freundorfer2 1 Department of Physics, 2 Department of Electrical Engineering, Queen’s University, Kingston, ON, K7L 3N6, CANADA. ABSTRACT Thin films of barium strontium titanate are fabricated on alumina by a hydrothermal method. Crystalline films are produced at temperatures as low as 60°C. The relative dielectric permittivity of films with the composition Ba0.15Sr0.85TiO3 lies in the range between 150 and 220 for low frequencies (100 kHz to 1 MHz), decreasing with frequency. The loss tangent for the films increases from 0.05 to 0.15 over the same frequency range. Relative concentrations of barium and strontium in the film are controllable through the hydrothermal reaction conditions. The surface microstructure of the films is examined. INTRODUCTION The demand for communication via wireless networks is necessitating the design of novel signal transfer technologies. Integration of these devices on a large scale requires that they be made inexpensively. This demands that components be small and easy to produce. On-chip fabrication of the device components is the most direct method of obtaining small, inexpensive devices. In order for on-chip fabrication to be feasible, however, each step in the process must maintain the products previously produced. In the case of devices to be fabricated on GaAs, the substrate itself limits the process. Upper process temperatures are limited to 200°C on GaAs substrates. Such a low upper temperature limit precludes use of conventional fabrication techniques to deposit ceramic structures. In this paper we examine hydrothermal processing as a novel technique that allows ceramics to be deposited at temperatures below 200°C, and is thus compatible with technologies to be fabricated on GaAs. In particular, we examine the deposition of barium strontium titanate by the hydrothermal method. Ba1-xSrxTiO3 (BST) is selected due to its high relative permittivity and low loss tangent. These characteristics are favourable for the production of dielectric grating antennae with small physical dimensions and large bandwidth. EXPERIMENTAL DETAILS Hydrothermal BST films are currently fabricated on alumina. The precursor titanium layer is deposited via RF sputtering in argon from a 10cm diameter titanium target. This metal layer is reacted chemically to form the BST ceramic. The reactant solution is prepared by dissolving Ba(OH)2·8H2O and Sr(OH)2·8H2O in deionized water. This solution is placed in a Teflon beaker, and the sample (secured in a Teflon holder) is lowered into the solution. The beaker is then placed into and sealed in an autoclave. Pressure inside the apparatus is the autogenous pressure of the solution at the reaction temperature. For the solutions used, this pressure (~ 1 MPa for processing at 180°C) is very CC12.13.1
close to the vapour pressure of water at the reaction temperature. The reaction temperature of 60oC to 200oC is con
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