TEM study on interface of palladium coated copper wire bonding on aluminum metallization

Wire bonding using bare Cu wire and Pd coated Cu (PdCu) wire has been adopted rapidly as a mainstream packaging technology as an alternative to Au wire bonding. The interfacial characteristics of both Au and Cu wire bonds are well understood as a result of extensive research. However, the interfacial feature and its evaluation in connection with Pd coated Cu wire bonds have not been investigated in detail. This paper is aimed to study the Pd coated Cu wire bonds made with forming gas using wavelength dispersive spectroscopy (WDS) and transmission electron microscopy (TEM). The WDS results show that up to 2.5 at.% Pd is inside the ball and 0 at.% Pd at the bond interface. It is also shown that many nano-voids are formed at the bottom of PdCu wire bond made with N₂, but they are absent in the bonds made with forming gas. Further, a TEM study reveals that the PdCu wire bond interface in the as-bonded state consists of two types of morphologies: (1) an interface containing a uniform layer of amorphous aluminum oxide with a thickness of approximately 4 nm with intermetallics being absent in these regions; and (2) island-like CuAl₂ particles of 20-30 nm thick between Cu and Al without any uniform layer of aluminum oxide. The uniform aluminum oxide layer remains after aging for 192 hours which is the longest aging time in this study. The oxide layer acts as a diffusion barrier, so no intermetallics form in those regions. However, the intermetallics grow and phases change during aging in the areas where the oxide layer is broken. Cu₉Al₄ appears between the initial CuAl₂ compound and copper at the early stage of aging. CuAl₂ and Cu₉Al₄ grow simultaneously, and CuAl₂ is the dominant phase until the Al pad is depleted. Some CuAl particles possibly form during aging between CuAl₂ and Cu₉Al₄, but they are much thinner than CuAl₂ and Cu9Al₄. Pd is not detected at the interface of the PdCu wire bond made with forming gas either in the as-bonded state or after aging, which is different than our previous report on PdCu wire bonds made with N₂, which shows that Pd congregates at the peripheral interface after aging.