Comprehensive transmission electron microscopy study on Cu-Al intermetallic compound formation at wire bond interface

Comprehensive intermetallic compound phase analysis at wire bond interfaces was performed for a side-by-side comparative study between 18 mu m Pd-only coated Cu wire and 18 mu m Pd-coated Cu wire followed by Au flash coating. Scanning electron microscopy and transmission electron microscopy results combined with nanobeam electron diffraction and structure factor calculation identified the formation of metastable theta'-CuAl2 and Cu9Al4 in both of the wire bonds before and after high temperature storage test. In particular, nanobeam electron diffraction and structure factor calculation unambiguously revealed that the two intermetallic compound phases grow in size after high storage temperature test in a manner that they maintain their epitaxial relationships that minimize lattice mismatch at the Cu/Al wire bond interface. Nanobeam electron diffraction and energy dispersive x-ray spectroscopy results found no significant Au flash coating effects in terms of intermetallic compound morphology, phase, and thermal evolution.