Ion-beam mixing of Ni/Pd layers: II. Thermally assisted regime (>500K)
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I. INTRODUCTION Atomic mixing by ion bombardment with 120 keV A r + o r 145keVKr + ionsatadoserateof5.5x 1012ions cm" 2 s " ' over the temperature range 40-473 K of thin Ni/Pd bilayers and thin Pd markers sandwiched between Ni layers was reported in paper I.' The amount of mixing varied linearly with the square root of the ion dose and with the elastic energy deposited in the collision cascade. In this paper, ion-beam mixing in the thermally assisted regime at temperatures ranging from 523-673 K is reported. II. EXPERIMENT AND ANALYSIS Two kinds of Ni/Pd bilayers were prepared to examine interdiffusion by thermal anneals and thermally assisted ion-beam mixing. In the first kind, ~ 100 nm Ni was evaporated onto eletropolished polycrystalline nickel or single-crystal sapphire substrates followed by evaporation of a 40 nm Pd overlayer. The combined film thickness of ~ 140 nm is greater than the range of the bombarding ions (Rp + 2 ARp~90 nm); consequently, intermixing is investigated for small-grained Ni and Pd layers. Intermixing between large grain Ni and small grain Pd was studied in the second kind of couple formed by direct evaporation of a Pd layer ~ 4 0 nm thick on electropolished polycrystalline or single-crystal nickel substrates. Bilay er films, prepared by evaporation of ~ 30 nm Ni followed by ~ 30 nm Pd on NaCl crystals that had been cleaved in air shortly before use, were utilized to study the influence of ion bombardment J. Mater. Res. 3 (6), Nov/Dec 1988
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on the film microstructure using transmission electron microscopy (TEM). The mixing experiments were carried out as described in our previous paper using 120 keV A r + ions with a current density of 0.9 fiA cm" 2 (Ref. 1). The composition profiles in the samples were analyzed at 300 K using Rutherford backscattering (RBS) with 2 MeV 4 H e + ions at a scattering angle of 160° and an energy calibration of 4.6 keV/channel. A maximum beam current of 2 nA was used to ensure that the detector dead time was < 5% and that beam-heating effects were negligible. In several cases, overlap in the RBS spectra for Ni and Pd was observed; accordingly, profiles at 673 K were also determined by subsequent sputter profiling using Auger electron spectroscopy. Interdiffusion in the Pd/Ni couples can result from thermal anneals and ion bombardment. Meaningful interdiffusion information is obtained by considering either the number of atoms transported beyond the original interface or the solution of diffusion equations. The number of Pd atoms cm" 2 mixed beyond the original interface into Ni, or vice versa, was estimated as a function of time for both irradiated and unirradiated samples as illustrated schematically in Fig. 1. Here, the RBS spectra are shown for a bilayer film before and after irradiation. Ideally, the front edges of the Ni and Pd signals and the back edge of the Pd signal are step functions for the as-prepared sample. In reality these edges are smeared due to the finite resolution of the detection system, energy straggling of t
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