Microstructural Transformations in Solid-State Annealed Al/Ag/Al Diffusion Couples Examined via High-Voltage Electron Microscopy (HVEM)

Abstract

This study focuses on the practical relevance of the Al-Ag bonding interface in electronic device fabrication, particularly in wire bonding, which is crucial for enhancing component reliability and performance. Experiments involved Al/Ag/Al diffusion couples, annealed at 703 K, revealing two stable intermediate phases, μ and δ. Characterizing the intermediate phases’ compositions and concentration profiles exposed a vital transition at the δ-Al interface. We used high-voltage electron microscopy (HVEM) to examine crystal structure evolution, identifying a (hexagonal close-packed) hcp structure in the intermediate phase between δ and Al, matching the δ phase. Notably, a substantial microstructural transformation occurred within the Ag-Al diffusion couple, as nano-sized precipitates transitioned from spherical to plate-like, along specific {111} planes, reflecting the evolution from off-stoichiometric, disordered phases to ordered ones. Mapping the concentrations of intermediate phases on the Al-Ag phase diagram revealed shifted and narrower solubility ranges compared to the calculations. This study provides insight into the crystal structure and microstructure changes during diffusion in Al/Ag/Al diffusion couples, holding implications for electronic device fabrication. Understanding intermediate phase behavior and evolution is vital in this context, potentially influencing materials development and process optimization in the electronic components industry, and thus, enhancing device performance and reliability.

Keywords

• Ag-Al system
• intermediate phase
• HVEM
• Ag₂Al
• metastable phase
• EPMA