Preparation of Sputtered (Ba x ,Sr 1Àx )TiO 3 Thin Films Directly on Copper Brian Laughlin, w Jon Ihlefeld, and Jon-Paul Maria Department of Materials Science and Engineering, The Electroceramic Thin Films Group, North Carolina State University, Raleigh, North Carolina 27695 (Ba 0.6 ,Sr 0.4 )TiO 3 (BST) films were deposited on copper foils by radio frequency magnetron sputtering. By the use of controlled pO 2 high-temperature anneals, the films were completely crys- tallized in the absence of substrate oxidation. X-ray diffraction and transmission electron microscopy (TEM) revealed an abrupt Cu/BST interface. The deposited BST films exhibit a zero bias permittivity and loss tangent values of 600 and 0.018, respec- tively. An electrical tunability ratio of 3.5:1 is observed on these metal–insulator–metal devices. Devices show leakage currents of 10 À8 A/cm 2 at 710 V/lm, and loss tangents as low as 0.003 in fields approaching 40 V/lm. I. Introduction T HERE has been extensive research into barium strontium ti- tanate, (Ba x ,Sr 1Àx )TiO 3 (BST), for use as a charge storage material in integrated memory, and frequency-agile circuits. Al- though it is generally accepted that the highest quality films are grown by chemical vapor deposition, 1–3 sputter deposition can be used to prepare materials with comparable properties. 4–7 The advantage of physical vapor deposition lies in the substan- tially simplified instrumentation. Regardless of the synthesis technique, the ability to fabricate high-frequency, low-power consumption devices is routinely limited by electrode resistance as opposed to dielectric loss. This high resistance results from the resistivity of Pt, the most common electrode metal, and the layer thickness, which are practically limited to sub-micrometer dimensions. This thickness limitation stems from the thermo- physical instability associated with the accumulation of a ther- mal expansion mismatch-induced strain upon cooling. This issue has been quantitatively discussed in the open literature. 8 Ideally, thick and high conductivity base metal layers should be used as electrodes in oxide devices, but obstacles associated with chemical reactivity create a technological barrier. If, how- ever, the concepts of base-metal processing well known to the multilayer capacitor and coated conductor industries are tran- sitioned to thin-film techniques, integration of ferroelectric oxides with numerous base metals can be realized. 9–12 Conse- quently, a set of electrode materials with significantly lower resistivities would be available, and the dramatically reduced expense (as compared with noble metals) would enable attrac- tive new substrate possibilities—for instance, flexible metal foils. The key to success in this area lies in an understanding of the temperature- and pressure-dependent thermodynamics of metal oxidation. These data clearly show that direct compatibility be- tween complex oxides like BST and base metals like copper can be achieved in the absence of chemical barrier layers. 13–15 In this report, we discuss how this compatibility can be achieved using radio frequency (RF) magnetron-sputtered BST—the model ferroelectric oxide, and untreated copper foil—the model base metal. II. Experimental Procedures Polycrystalline electrodeposited copper foils were obtained from Oak-Mitsui (Hoosick Falls, NY), and used as substrates in this study. The RMS roughness of these foils is B5 nm on a 1 mm  1 mm sampling area, and increases and saturates to B20 nm at a 20 mm  20 mm area. These substrates are 18 mm thick and represent an industry-standard metal foil used for lam- ination into printed wiring boards or flexible electronics. The foils were used ‘‘as- received’’; prior to deposition, 2 in.  1 in. foil sections were clamped into a metal frame and blown dry with N 2 gas to remove any particulates. BST thin films with a 60:40 Ba:Sr ratio were prepared by RF magnetron sputtering from a 4 in. diameter stoichiometric BST target. The sputter gun was oriented approximately 301 off-axis, and an argon/oxygen gas mixture (Ar:O 2 5 5:1) was used to optimize film stoichiometry and thickness uniformity. Depositions were performed at tem- peratures B1001C. A 90 min deposition at 5 mTorr sputtering pressure and 300 W target power resulted in an 800 nm thick film. After deposition, the BST films were heat treated in an at- mospheric furnace in which oxygen partial pressure (pO 2 ) could be controlled and measured in situ with a doped zirconia ceramic oxygen probe. Oxygen partial pressure control was achieved by mixing 400 sccm N 2 gas supplied from a liquid nitrogen source with 5 sccm 1% H 2 , balance UHP N 2 , forming gas. This gas mixture was passed through a 251CH 2 O bubbler, and the N 2 – H 2 –water vapor mixture was placed in a sealed tube furnace at one atmosphere of pressure. Figure 1 shows a pO 2 –T phase di- agram for Ba, Ti, Sr, and Cu metals and their respective oxides. This diagram clearly identifies a portion of processing space in which equilibrium between (Ba 1Àx Sr x )TiO 3 ,O 2 (g), and Cu metal occurs. Provided that process conditions can be main- tained in this zone, an abrupt Cu/BST interface is expected. The samples in this study were annealed at temperatures up to 9001C, while mantaining the maximum pO 2 just below the value predicted for equilibrium formation of Cu 2 O (area ‘‘a’’ of Fig. 1). A subsequent lower temperature, higher pO 2 anneal was used to annihilate oxygen vacancies created by previous processing. As predicted by thermodynamics, the equilibrium concentration of these defects scales exponentially with temperature and can be quenched into a material after the high-temperature annealing cycle. Anneals were conducted between 4501 and 7501C in a vacuum furnace, where total pressures of oxygen from 5  10 À6 to 1  10 À3 Torr were used to significantly reduce the number of oxygen vacancies (area ‘‘b’’ of Fig. 1). This step is necessary since the presence of large oxygen vacancy concentrations in perovskite oxides is directly linked to reduced insulation resist- ance. The conditions of this annealing step are favorable for Cu 2 O formation, but with a fully dense dielectric, the diffusion of O 2 to the copper interface is sufficiently slow such that J ournal J. Am. Ceram. Soc., 88 [9] 2652–2654 (2005) DOI: 10.1111/j.1551-2916.2005.00488.x r 2005 The American Ceramic Society 2652 D. W. Johnson—contributing editor Supported by the National Aeronautics and Space Administration Goddard Space Flight Center under Grant No. FAS5-03014 and contract #NAS5-03014. w Author to whom correspondence should be addressed. e-mail: bjlaughl@ncsu.edu Manuscript No. 10937. Received March 31, 2004; approved July 9, 2004.