In the wire bonding technique, a thin Au wire was 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 were 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 623 to 723K for various times up to 103 h. Under such annealing conditions, the diffusion couple was practically considered semi-infinite. Owing to annealing, compound layers consisting of AuAl2, AuAl and Au8Al3 were produced at the initial Au/Al interface in the diffusion couple. The volume fraction in the compound layers was 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 were no systematic dependencies of the volume fraction on the annealing time and temperature. The total thickness of the compound layers was proportional to a power function of the annealing time. The exponent of the power function was smaller than 0.5 at 673–723K. 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 623K, the exponent was equal to unity for annealing times shorter than 8h but smaller than 0.5 for those longer than 8h. The exponent of unity indicates that the layer growth was controlled by the interface reaction at the migrating interface. Thus, at T = 623K, the transition of the rate-controlling process occurred at a critical annealing time of 8h.

Kinetics of Solid-State Reactive Diffusion between Au and Al. M.O, M.Kajihara: Materials Transactions, 2011, 52[4], 677-84