Jia-Hong Ke

A recent experimental study on a spent uranium dioxide (UO2) fuel sample from Belgium Reactor 3 identified a unique pair structure formed by the noble metal phase (NMP) and fission gas [xenon (Xe)] precipitate. However, the fundamental mechanism behind this structure remains unclear. The present study aims to provide an understanding of the interaction between five different metal precipitates [molybdenum (Mo), ruthenium (Ru), palladium (Pd), technetium (Tc), and rhodium (Rh)] and the Xe fission gas atoms in UO2, by using density functional theory (DFT) in combination with the Hubbard U correction to compute the formation energies involved. All DFT + U calculations were performed with occupation matrix control to ensure antiferromagnetic ordering of UO2. The calculated formation and binding energies of the Xe and solid fission products in the NMP reveal that these metal precipitates form stable pair structures with Xe. Notably, the formation energy of Xe–metal pairs is lower than that of the isolated single defects in all instances, with Pd and Mo showing the most favorable binding energy, likely accounting for the observed pair structure formation.

Excessive cathode dissolution due to high current densities is investigated. Such excessive dissolution is one of the key electromigration-induced degradation processes in micro systems, and exhibits a distinctive serrated morphology. In this study, Cu cathode and Cu anode connected with Sn is stressed at a 4.5 × 104 A/cm2 current density for time as long as 1500 h. Careful sequential micro polishing is able to establish for the first time that the serrated cathode interface in fact is the expression of rod-like indentations in three-dimensional morphology. This unique morphology supports the proposition that fast Cu diffusion through Cu6Sn5 grain boundaries is the root cause for this excessive dissolution.

Electromigration in flip chip solder joints under extra high current density (4.5×104 A/cm2) is studied. At such a high current density level, due to Joule heating, the chip temperature is strongly coupled to the applied current density. Accordingly, it is highly desirable to have the capability to decouple the chip temperature and the current density. Two experimental setups were used in this study, one with a cooling module to keep the chip temperature constant and the other one without a cooling module. Without the cooling module, the temperature increased rapidly with the applied current. When the current density reached 4.5×104 A/cm2, a rapid failure caused by excessive Joule heating was observed only after 10 min of current stressing. With the cooling module attached, the joint exhibited a much longer life (935 h) under 4.5×104 A/cm2. It was successfully demonstrated that the cooling module was able to decouple the applied current density and the chip temperature.