Structural and Microwave Characterization of Magnetron Sputtered Ba 0.5 Sr 0.5 TiO 3 Films on c -plane Sapphire Substrat
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0928-GG09-17
Structural and Microwave Characterization of Magnetron Sputtered Ba0.5Sr0.5TiO3 Films on c-plane Sapphire Substrates E. A. Fardin1, A. S. Holland1, K. Ghorbani1, and B. F. Usher2 1 School of Electrical and Computer Engineering, RMIT University, GPO Box 2476V, Melbourne, Victoria, 3001, Australia 2 Department of Electronic Engineering, La Trobe University, Bundoora, Victoria, 3086, Australia
ABSTRACT Many studies of barium strontium titanate (BST) thin films for RF / microwave applications have employed MgO, LaAlO3 or Pt/Si as the substrate material for BST deposition. However, there have been relatively few reports of BST films grown on sapphire, despite the excellent microwave properties of this material. In this investigation, BST thin films were deposited by RF magnetron sputtering on (001) single crystal c-plane sapphire substrates. Interdigitated capacitors (IDCs) patterned on the film surface were used to measure the dielectric tunability and loss tangent at microwave frequencies. Thick Au conductors were electroplated to minimize conductor losses. Post deposition annealing in air was found to significantly improve the tunability of the sputtered films. INTRODUCTION Barium strontium titanate (BST) thin films are currently being investigated for application in electronically tunable microwave circuits [1-3]. The parameters that need to be optimized for microwave devices are the change in dielectric constant with applied bias field (tunability) and the dielectric loss at microwave frequencies. Typically, BST-based voltage tunable capacitors, or varactors, are fabricated using either parallel-plate electrodes or an interdigitated electrode structure on top of the BST film. Because the electrode separation in the parallel-plate approach is equal to the film thickness, of the order of 100 nm, the required tuning voltages are lower than the interdigitated case, where finger separation is of the order of microns. However, it has proven difficult to fabricate bottom electrodes with low conductor loss which are also stable under the high deposition temperatures required for BST films [4]. BST varactors with interdigitated electrodes are commonly realized by depositing BST films on cubic substrates such as MgO and LaAlO3 [1,5]. Potential limitations of these substrates include their high cost and surface deliquescence [6]. Despite the relatively low cost and very low microwave loss of sapphire, there are few reports of BST films deposited on this material by sputtering. In this paper, the structural and microwave properties of Ba0.5Sr0.5TiO3 films deposited on c-plane (001) sapphire substrates are presented. The epitaxial relationship between the film and substrate is also investigated. EXPERIMENTAL DETAILS RF magnetron sputtering was used to deposit BST thin films from a commercial 100 mm diameter stoichiometric Ba0.5Sr0.5TiO3 target onto single crystal c-plane sapphire substrates (Techno Chemics, Ichikawa City, Japan) with dimensions 10 × 10 × 0.5 mm. An Ar/O2 9:1 gas mixture was used in the experiments
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