Transmission electron microscopy of diffusion-soldered Ni/Al/Ni interconnections

Transmission electron microscopy (TEM) was used to study cross-sections of diffusion-soldered Ni/Al/Ni interconnections. Precipitation of coherent Ni-rich AlNi particles (3–10 nm) was observed within the stoichiometric NiAl compound. A new phase was detected in the peninsulas of Ni-rich AlNi embedded in AlNi3. This phase is described as enriched with Ni and with the structure of NiAl L1o (CuAu I type). It is suggested that the Ni-rich NiAl phase with cubic B2 (CsCl type) structure transforms martensitically into a close-packed tetragonal L1o (CuAu I type) structure upon cooling. It is an unexpected result because previous electron probe microanalysis and X-ray diffraction analysis of this region revealed it as a NiAl one-phase layer enriched with Ni.