Intermetallic compound formation at Cu-Al wire bond interface

Intermetallic compound (IMC) formation and evolution at Cu-Al wire bond interface were studied using focused ion beam /scanning electron microscopy, transmission electron microscopy (TEM)/energy dispersive x-ray spectroscopy (EDS), nano beam electron diffraction (NBED) and structure factor (SF) calculation. It was found that discrete IMC patches were formed at the Cu/Al interface in as-packaged state and they grew toward Al pad after high temperature storage (HTS) environment at 150 °C. TEM/EDS and NBED results combined with SF calculation revealed the evidence of metastable θ′-CuAl2 IMC phase (tetragonal, space group: I4¯m2, a = 0.404 nm, c = 0.580 nm) formed at Cu/Al interfaces in both of the as-packaged and the post-HTS samples. Two feasible mechanisms for the formation of the metastable θ′-CuAl2 phase are discussed based on (1) non-equilibrium cooling of wire bond that is attributed to highly short bonding process time and (2) the epitaxial relationships between Cu and θ′-CuAl2, which can minimize lattice mismatch for θ′-CuAl2 to grow on Cu.