Kinetics of Solid-State Reactive Diffusion between Au and Al

Abstract

In the wire bonding technique, a thin Au wire is interconnected with an Al layer on a Si chip. During energization heating at solid-state temperatures, however, brittle Au-Al compounds with high electrical resistivities are formed at the interconnection by the reactive diffusion between Au and Al. In order to examine the growth behavior of the Au-Al compounds, the kinetics of the reactive diffusion was experimentally observed using sandwich Al/Au/Al diffusion couples. The diffusion couple was prepared by a diffusion bonding technique and then isothermally annealed at temperatures of 623–723 K for various times up to 103 h. Under such annealing conditions, the diffusion couple is practically considered semi-infinite. Owing to annealing, compound layers consisting of AuAl₂, AuAl and Au₈Al₃ are produced at the initial Au/Al interface in the diffusion couple. The volume fraction in the compound layers is larger for Au₈Al₃ than for AuAl and AuAl₂ in the early stages but becomes smaller for Au₈Al₃ than for AuAl and AuAl₂ in the late stages. However, there are no systematic dependencies of the volume fraction on the annealing time and temperature. The total thickness of the compound layers is proportional to a power function of the annealing time. The exponent of the power function is smaller than 0.5 at 673–723 K. The exponent smaller than 0.5 means that the interdiffusion across the compound layers governs the layer growth and boundary diffusion predominantly contributes to the interdiffusion. On the other hand, at 623 K, the exponent is equal to unity for annealing times shorter than 8 h but smaller than 0.5 for those longer than 8 h. The exponent of unity indicates that the layer growth is controlled by the interface reaction at the migrating interface. Thus, at T=623 K, the transition of the rate-controlling process occurs at a critical annealing time of 8 h.