High-temperature stability of Au/Pd/Cu and Au/Pd(P)/Cu surface finishes

Highlights: Thermal stability of Au/Pd/Cu and Au/Pd(P)/Cu multilayers at 180 °C. Amorphous structure was caused with minor addition of P into Pd. Amorphous Pd(P) film possesses better oxidation resistance. Significant interdiffusion among Au, Pd, and Cu occurred in the Au/Pd/Cu couple. Out diffusion of Cu through polycrystalline Pd induced void formation. Thermal reliability of Au/Pd/Cu and Au/Pd(4–6 wt.% P)/Cu trilayers in the isothermal annealing at 180 °C were investigated by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and transmission electron microscopy (TEM). The pure Pd film possessed a nanocrystalline structure with numerous grain boundaries, thereby facilitating the interdiffusion between Au and Cu. Out-diffusion of Cu through Pd and Au grain boundaries yielded a significant amount of Cu oxides (CuO and Cu₂O) over the Au surface and gave rise to void formation in the Cu film. By contrast, the Pd(P) film was amorphous and served as a good diffusion barrier against Cu diffusion. The results of this study indicated that amorphous Pd(P) possessed better oxidation resistance and thermal reliability than crystalline Pd.