Amorphous and Crystalline Phase Formation in Ni/Al Multilayer Thin Films
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ABSTRACT We have investigated reactive phase formation in magnetron sputter-deposited Ni/Al multilayer films with a 1:3 molar ratio and various periodicities ranging from 320 nm to a codeposited film with an effective periodicity of zero. The films were studied by x-ray diffraction, differential scanning calorimetry, electrical resistance measurements, and transmission electron microscopy. We find that a reaction which results in the formation of an amorphous phase has taken place during the multilayer deposition process. This reaction substantially reduces the driving force for subsequent reactions and explains why nucleation kinetics become important for these reactions. The mode of transformation for a film with 10 nm periodicity was investigated, in detail, by applying the Johnson-Mehl-Avrami analysis to data obtained from isothermal and constant heating rate differential scanning calorimetry, in combination with electron microscopy studies of the transformation microstructure. INTRODUCTION Thin film reactions in the Ni-Al system have been extensively characterized. The studies prior to 1990 are summarized in a review by Colgan and show that NiAI3 is the first phase to form [1]. More recent studies, however, have yielded new and intriguing information concerning these reactions [2-5]. For example, Ma et al.[2] find that while NiA13 is the first phase to form in evaporated Ni/Al multilayers, the nucleation and growth to coalescence of the first layer of grains of this phase is kinetically separated from a following growth stage. Edelstein et al. [3], on the other hand, report that the first phase formed during annealing of ion-beam sputtered films is NiAl, AI2Ni 9 , or NiA13 depending on overall stoichiometry and modulation period of the films. In this paper, we present the results of our investigations of reactive phase formation in Ni/Al multilayers deposited by triode magnetron sputtering, a different deposition method than those used in the above two studies. We find both similarities and differences in the reaction behavior of our multilayers to those reported by Ma et al. and Edelstein et al. Our microstructural studies of the 10 nm period film and the correlation of these studies to JohnsonMehl-Avrami analysis of calorimetric results will also be discussed. EXPERIMENT Details of the deposition, x-ray diffraction (XRD), differential scanning calorimetry (DSC) and electrical resistance experiments are reported elsewhere [4,5]. The microstructure of the films was studied by transmission electron microscopy (TEM) in a Philips EM400T microscope. Films deposited on sapphire were annealed in the calorimeter furnace at 20 K/min to different stages of the reaction and electron transparent plan-view and cross-sectional specimens were prepared by grinding, polishing and cold stage ion milling to electron transparency. 227 Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
RESULTS As reported in our previous publications [4,5], combination of XRD, TEM, DSC and electrical resistance measurements ha
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