Sequence of Phase Formation in Ni/Al Contrasted with Ni/Si
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E. G. COLGAN and J. W. MAYER Department of Materials Science and Engineering 14853 Cornell University, Ithaca, NY
ABSTRACT thin We have investigated the interactions of Al/Ni, NiAl 3 /Ni, and Ni 2 Al 3 /Ni films between 350 and 500'C. Sequential layer deposition and coevaporation were used to prepare the films which were then vacuum annealed and analyzed by Rutherford backscattering and X-ray diffraction. Initially, NiAl 3 is formed, followed by the more Ni-rich phases. All four intermetallic phases were formed.
INTRODUCTION Aluminum is frequently used in metallizations for integrated circuits; consequently the reactions between Al and metals have been increasingly studied. The studies have been mainly concerned with interdiffusion and initial intermetallic phase formation. Two reviews of metal-metal reactions have been published 1,2. However, in comparison with our understanding of silicide formation , that of aluminide formation is in an early stage. The present work focuses on phase formation in the Ni-aluminide system, comparing that to the Ni-silicide system where there is a considerable experimental base. Metal-silicon reactions have been extensively studied '-', a systematic pattern of silicide formation has been found, and rules to predict the first silicide phase 6-8 and the next phase 9 proposed. For many cases, the activation energy for growth and the dominant diffusing species are known. Of the rules for predicting initial and subsequent silicide phase formation, most are largely based on information from phase diagrams, such as the lowest eutectics, congruently and noncongruently melting phases, and temperature differences. Two of these rules however, also include dominant diffusing species and epitaxial growth 7 or an effective heat of formation 8. They all do rather well at predicting the initial phase though they are at variance about the underlying mechanisms. There has only been one rule suggested for metal-metal reactions 10; it is also based on phase diagram information. There have been a number of studies of Ni/Si reactions 3,11-15. The initial phase to form is Ni 2 Si, which is not the most Ni rich phase but is the one predicted by the initial phase rules 6Ni2Si formation is followed sequentially by NiSi and NiSi 2 , when the overall stiochiometery is Si rich. Several phases are not found in this phase sequence; for example the phase Ni Si is skipped 13. We note that in lateral silicide structures 14,15 Ni 2Si is formed initially, and at T>600°C , the five remaining phases are present; this is similar to bulk results. Marker experiments have shown that Ni is the dominant diffusing species in Ni Si formation 11,12. There have been previous investigations of both bulk and thin film couples for the Ni/Al system. The phase diagram shows only four intermetallic phases. Bulk studies 16-18 have found all four intermetallic compounds in the interdiffusion zone at 600'C, with only thin layers of NiAl and Ni 3 A1 present. The dominant diffusing species has been determined for
Mat. Res. Soc. Symp. Pro
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