Low-K dielectric adhesion problems were observed at M1 and M2 levels during thermal cycling of a flip chip product. Nano-indentation of simple BEOL test structures was used to determine the relative strength of the various interfaces in the BEOL stack. It is observed that the weakest adhesion is associated with the initial stages of the SiCOH low-K dielectric deposition. Adhesion loss related to the SiCN etch stop deposition is not observed.

Coating of the Cu bond wire with Pd has been a rather widely accepted method in semiconductor packaging to improve the wire bonding reliability. Based on comparison of a Cu bond wire and a Pd-coated Cu bond wire on AlCu pads that had passed HAST, new insight into the mechanism of the reliability improvement is gained. Our analysis showed the dominant Cu-rich intermetallics (IMC) were Cu3Al2 for the Cu wire, and (CuPdx)Al for the Pd-coated wire. The results have verified the Cu-rich IMC being suppressed by the Pd-coating, which has been extensively reported in literature. Binary phase diagrams of Al, Cu, and Pd indicate that the addition of Pd elevates the melting point and bond strength of (CuPdx)Al compared with CuAl that formed with the bare Cu wire. The improvements are expected to decrease the kinetics of phase transformation toward the more Cu-rich IMC. With the suppression of the Cu-rich IMC, the corrosion resistance of the wire bonding is enhanced and the wire bonding reliability improved. We find that Ni behaves thermodynamically quite similar to Pd in the ternary system of Cu wire bonding, and therefore possesses the potential to improve the corrosion resistance.

Cu wires were bonded to AlSi (1%) pads, subsequently encapsulated and subjected to uHAST (un-biased Highly Accelerated Stress Test, 130 °C and 85% relative humidity). After the test, a pair of bonding interfaces associated with a failing contact resistance and a passing contact resistance were analyzed and compared, with transmission electron microscopy (TEM), electron diffraction, and energy-dispersive spectroscopy (EDS). The data suggested the corrosion rates were higher for the more Cu-rich Cu-Al intermetallics (IMC) in the failing sample. The corrosion was investigated with factors including electromotive force (EMF), self-passivation of Al, thickness and homogeneity of the Al-oxide on the IMC, ratio of the Cu-to-Al surface areas exposed to the electrolyte for an IMC taken into account. The preferential corrosion observed for the Cu-rich IMC is attributed to the high ratios of the surface areas of the cathode and anode that were exposed to the electrolyte, and the passivation oxide of Al with the lower homogeneity. The corrosion of the Cu-Al IMC is just a manifestation of the well-known phenomenon of dealloying. With the understanding of the corrosion mechanisms, prohibiting the formation of Cu-rich IMCs is expected be an approach to improve the corrosion resistance of the wire bonding.