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 ~frac14, substantially smaller than what would be expected for bulk, lattice diffusion (1/2). We conclude from the IMC thickness time exponent that 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 Au ₄Al and Au on the Al-rich side of the failing interface