Reactive Phase Formation in Sputter-Deposited Ni/Al Thin Films
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INTRODUCTION
Thin-film reactions in the Ni-Al system have been extensively characterized. The studies prior to 1990, summarized in a review by Colgan[l], show that NiA13 is the first phase to form. Two more recent studies by Ma et al.[2] and Edelstein et al.[3] include detailed calorimetric measurements on Ni/Al multilayer films prepared by electron beam evaporation and by ion-beam sputter deposition, respectively. In contrast to all earlier investigations, Edelstein et al. find that the first phase in ion-beam sputtered films can be NiAl, Ni 2 AI 9 or NiA13 depending on the overall stoichiometry and the modulation period of the film. Ma et al., on the other hand, find that the first phase formed is NiA13 and observe that the nucleation and growth to coalescence of the first layer of grains is kinetically separated from the following growth stage, giving rise to two separate calorimetric peaks although only a single product phase is formed. This two-peak phase formation, which was previously observed for NbA13 in Nb/Al multilayer thin films[4], demonstrates the occurrence of nucleation barriers during the formation of the first phase and is in disagreement with the large driving force expected for the reaction. The reader should note that large driving forces imply that nucleation barriers should not be present and, therefore, only one calorimetric growth peak should be observed for any one phase. Ma et al. go on to conclude that the effective driving force for phase formation must be considerably smaller than the bulk heat of formation of the product phase from pure reactants. Three possible sources of this reduction in driving force are the mixing of the reactants by bulk interdiffusion [5], the mixing of reactants by grain-boundary interdiffusion [6], or the formation of another phase preceding nucleation of the nominal first phase. Ma et al. found that the Ni and Al composition profiles broadened in a film annealed to the beginning of the first calorimetry peak compared with an as-deposited film. However, they were not able to identify any precursor phase associated with this interdiffusion. In this paper, we report on reactive phase formation in magnetron sputter deposited Ni/Al multilayer thin films with a 3:1 molar ratio and periodicities ranging from 2.5-320 nm. Our study includes multilayer films with periodicities smaller than those investigated by Ma et al. and Edelstein et al. In addition, we report on the transformation of a codeposited film.
33 Mat. Res. Soc. Symp. Proc. Vol. 382 © 1995 Materials Research Society
2. EXPERIMENT The samples were prepared by triode-magnetron sputtering at 0.37 Pa argon (99.99999 %) pressure from pure Ni and Al targets (both 99.99%) onto a rotating substrate holder in a chamber which was evacuated to approximately 1 x 10-5 Pa prior to deposition. The substrates were aplane sapphire as well as glass slides. The slides were covered with a 200 nm-thick Cu layer prior to film deposition. Both the glass and sapphire substrates were mounted on a thick copper block, which ac
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