TEM and Electrical Analysis of Sputtered Barium Strontium Titanate (BST) Thin Films on Flexible Copper Substrates
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TEM and Electrical Analysis of Sputtered Barium Strontium Titanate (BST) Thin Films on Flexible Copper Substrates Brian Laughlin, Jon Ihlefeld, Jon-Paul Maria Institute for Electroceramic Thin Films, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, U.S.A. ABSTRACT Ba0.6Sr0.4TiO3 (BST) films were deposited on copper foils by radio frequency magnetron sputtering. These films will be an integral part of flexible capacitor sheets intended for space borne re-configurable antenna arrays. By the use of controlled pO2 high temperature anneals, the films were fully crystallized in the absence of substrate oxidation. X-ray diffraction and transmission electron microscopy (TEM) showed no existence of copper oxidation (i.e. Cu2O or CuO phases). The deposited BST films exhibit a high permittivity (625) and a low tan δ (0.020) at zero bias and room temperature. A pronounced electrical tunability ratio of 3.5:1 is observed on these devices. Devices show loss tangents as low as 0.003 in fields approaching 400 kV/cm. Electrical field calculations are based on cross-sectional atomic force microscopy (AFM) images that reveal a film thickness of 800 nm. Temperature dependent measurements show a Tmax ~ 230 K with a diffuse dielectric anomaly. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) analyses indicate a conformal film with a mixed grain morphology and an abrupt Cu/BST interface. INTRODUCTION There has been extensive research into barium strontium titanate, (Bax,Sr1-x)TiO3 (BST), for use as a charge storage material in DRAM and frequency agile circuits. Processing methods such as MOCVD [1-3], chemical solution processing [4-7], pulsed laser deposition [8], cosputtering of BaTiO3 and SrTiO3 [9], and sputtering of stoichiometric BST [10-14] have all been utilized with success. The highest quality thin films have typically been deposited by MOCVD, which includes costly instrumentation and complex processing. Sputtering from stoichiometric targets can be used to deposit BST with a similar level of quality as MOCVD material but with a significant reduction in complexity and expense. To accomplish this, careful selection of process parameters and a film-substrate combination compatible with the necessary post deposition anneals are required. Historically, the bulk of BST thin film research has used platinum electrodes and silicon substrates due to the inherent chemical inertness of Pt and the availability, quality, and uniformity of Si. Unfortunately, platinum suffers from poor adhesion to SiO2 and a significant mismatch of thermal expansion coefficient to Si. Consequently, there is a tendency for platinum delamination when films are grown above 100 nm and process temperatures exceed ~ 600 °C. As such, the ability to make low power consumption devices is severely limited by the high electrode resistance established by the relatively thin platinum. Conversely, high conductivity polycrystalline copper substrates made by electrodeposition can be used as a s
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