Very fast biaxial texture evolution using high rate ion-beam-assisted deposition of MgO
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We examined crystalline-texture evolution during ion-beam-assisted deposition (IBAD) of MgO thin films. We have demonstrated for the first time that in-plane crystalline texturing in IBAD of MgO scales with deposition rate. At high ion currents an in-plane texture full width at half-maximum (FWHM) of 10 can be achieved in less than 1 s, and 6 in 2.2 s. MgO texture further improves with thickness of a homoepitaxial layer deposited on top. We have developed an empirical quantification of the texture evolution in both IBAD and homoepitaxial layers. The best texture attained thus far in the MgO layer on polished Hastelloy tape has an in-plane FWHM of 1.6 . The high deposition rates demonstrated here make high-throughput manufacturing of IBAD textured templates a practical and cost-effective concept. I. INTRODUCTION
Ion-beam-assisted deposition (IBAD) texturing of MgO to obtain biaxially aligned templates for growth of superconductors or other functional materials has been established over the past ten years following the breakthrough discovery at Stanford University.1 This process is sometimes called nano-IBAD texturing or ion-texturing at nucleation (ITAN) to distinguish it from the much slower IBAD texturing previously used with materials such as ytrria-stabilized zirconia (YSZ), in which texture evolves on micrometer-scale film thickness.2 Much of the follow-up work has centered on the optimal buffer layer structure for such textured templates, primarily to prevent interdiffusion from the substrate elements into the functional layer.3–6 Here we present data on how the IBAD-MgO texture evolves and how the process scales with deposition rate. From the early work, it was always assumed that the process can scale to higher rates, although the early data were not conclusive.7 Here we explicitly show that the process does scale to high rates. We also demonstrate the best texture reported to date, i.e., less than 2 mosaic spreads in the MgO layer. Concurrently, we demonstrate that a homoepitaxial layer can be deposited at high rates retaining the same high degree of crystallinity. The IBAD layer architecture we use is a simplified one, consisting of three deposited layers, slightly a)
Address all correspondence to this author. e-mail: [email protected] b) Present address: IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany DOI: 10.1557/JMR.2009.0036 J. Mater. Res., Vol. 24, No. 1, Jan 2009
modified from the original Stanford work (Fig. 1). One starts with an almost arbitrary, but smooth, substrate. This is one of the main benefits of the IBAD texturing technique, in that aligned crystalline films can be formed on any substrate, amorphous or polycrystalline, including flexible metallic tapes, glasses, and ceramics. We use an a-Y2O3 layer as a nucleation surface, as reported earlier by Arendt et al.4 Several groups reported success with other IBAD nucleation surfaces such as a-Si3N4, ion-treated glass, and Si, and spin-on-glass.8–10 The Y2O3 layer is followed by deposition of the MgO IBAD textured layer and then a thicke
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