NiAl₃ formation in Al/Ni thin-film bilayers with and without contamination

The interfacial reactions induced by vacuum furnace annealing and rapid thermal annealing in sequentially deposited Al/Ni bimetallic thin-film diffusion couples have been investigated with MeV ⁴He⁺ backscattering spectrometry, cross-sectional transmission electron microscopy, and Auger electron spectroscopy. Upon annealing, NiAl₃ is the first aluminide phase to grow. In uncontaminated samples, the NiAl₃ growth proceeds in uniform planar fashion, governed by diffusion-limited kinetics. The kinetics data fit well with those for NiAl₃ growth on large-grained Al substrates, yielding a common kinetics law of x²=kt, where x is the thickness of the NiAl₃ grown at the interface, t is the annealing duration, and k is the growth constant, which is given by k=2.24(cm ²/s) exp(-1.5±0.1 eV/k_BT), in which T is the annealing temperature and k_B is the Boltzmann constant. Microscopic examination reveals slight nonuniformity at the Al/NiAl₃ interface resulting from shallow local protrusions of NiAl₃ grains into the Al layer at grain boundaries. When either the Al film or the Al/Ni interface is purposely contaminated during sample preparation, the roughness at this Al/NiAl₃ interface becomes very pronounced, and the reaction rate is significantly reduced. Meanwhile, Ni motion becomes appreciable as NiAl ₃ grains and/or Ni severely penetrate the Al layer. In contrast, the NiAl₃/Ni interface remains sharp in all samples. The irregular morphology and nonuniform reaction cannot be attributed uniquely to the presence of grain boundaries in the Al film, but rather are a combined effect of impurities and Al grain boundaries. Short-term rapid thermal annealing at elevated temperatures appreciably alleviates the nonuniformity at the Al/NiAl₃ interface in contaminated samples.