Sputtered YBCO films on metal substrates
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Magnetron sputtering was used to deposit superconducting YBCO thin films on four polycrystalline metal substrates. Yttrium-stabilized zirconia (YSZ) and silver were used as buffer layers. The thermal expansion coefficient of the substrate was apparently correlated to the properties of the superconductor. Auger depth profiles showed that the YSZ buffer blocked diffusion of most metal elements from the substrate. Nevertheless, contamination of the films depressed Tc to 81 K on Hastelloy X from 87 K on single-crystal strontium titanate. The films on metal substrates were c-axis oriented, but with random rotation in the a-b plane and smaller grain size than epitaxial films on single crystals. The weak links from these factors restrict Jc to 104 A/cm 2 at 40 K.
I. INTRODUCTION
TABLE I. Deposition parameters.
Epitaxial films of the high-temperature superconductors have been deposited by a variety of techniques on several types of single-crystal substrates. Singlecrystal substrates would not be practical for the long tapes or wires needed for electric power applications. Polycrystalline metal tapes are considered to be good candidates, especially for magnet windings where high tensile strength is needed. Fibers of yttriumstabilized zirconia (YSZ) or other ceramics might serve for simple power transmission. Laser ablation of YBa 2 Cu 3 0 7 _ x (YBCO) onto various buffered metal substrates has produced superconducting films with zeroresistance temperatures between 83 and 86 K.1^ Threetarget cosputtering5 produced films with Tc = 84 K on Hastelloy X with an amorphous MgO buffer layer. Single-target magnetron sputtering 56 produced films with Tc = 11A K as-deposited and Tc = 82 K after postdeposition annealing.7 In this paper we describe our technique for sputtering films on metal substrates with high Tc as-deposited. We compare the properties of these films on several metal substrates and buffer layers, exploring the effects of structure and composition. II. EXPERIMENTAL The YBCO, YSZ, and Ag films were deposited by magnetron sputtering under the conditions summarized in Table I. In the conventional sputtering mode, with planar target directly facing the substrate, YBCO films suffer damage and resputtering from high-energy oxygen ions as well as charge-exchanged and reflected neutrals. The most common solution to this problem is to sputter off the axis of the target8 at relatively high pressure in order to slow and avoid the particles. This successful offaxis technique is adopted for the purpose of this study. We find that we cannot simply duplicate the original conditions used in an apparently very similar system. 1636 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 7, Jul 1992 Downloaded: 11 Mar 2015
Target diameter thickness purity Target-substrate distance Power Working gas Total pressure Total gas flow Substrate temperature
YBa 2 Cu 3 0 7 ^ 48 mm 2 mm 99.9%
(ZrO2)0.9 (Y203)o.i 38 mm 3 mm 99.9%
Ag 38 mm 3 mm 99.99%
15 mm 120 W RF 25% Ar:75% O 2 100 mTorr 360 seem
25 mm 100 W RF 50% Ar: 50% O 2 20 mTor
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