Gold–Aluminum Intermetallic Formation Kinetics

Au-Al intermetallic compounds (IMC) grow laterally (Al-rich phases) in a Fickian fashion with an activation energy of 1.0 eV, but vertical IMC thickness (Au-rich phases) grows functionally as a power law on time with a sub-Fickian exponent of 1/4, which is substantially smaller than what would be expected for bulk lattice diffusion (1/2). We conclude from the IMC thickness time exponent that an Au-rich IMC growth process is limited by grain boundary diffusion. The best bond lifetime was seen for an intermediate-thickness Al film. The activation energy and lifetime for Au-rich phase growth are each a strong function of wire impurity concentrations. We find that bond lifetime varies roughly as the square root of Pd, Cu, Pt, and As concentrations, but lifetime is not a function of Be, Ca, Fe, or Cr concentration. We find a mixture of and Au on the Al-rich side of the failing interface.