Reliability Enhancement of Cu-Cu joints by Two-step Bonding Process

This study shows the relationship between the interfacial recrystallization and grain growth in the bonded Cu joints with the reliability on thermal fatigue lifetime of Cu joints characterized by temperature cycling test (TCT). We adopted a 2-step bonding process to fabricate Cu-Cu joints using highly <111> oriented nanotwinned Cu (nt-Cu). The 2-step process includes a short-time bonding (300°C for 10 s) and a post-annealing step (300°C for 1 h). We studied the fracture modes induced by thermal fatigue and the increase of electrical resistance via the microstructural characterization. Finite element analysis (FEA) was performed to study the stress distribution caused by the large coefficient of thermal expansion (CTE) mismatch between Cu joints and underfill during temperature cycling. We found that the electrical resistance of the post-annealed joints did not increase under thermal cycling while the samples without post-annealing showed 17% increase. Additionally, we found that the cracks caused by thermal fatigue can be further slowed down or avoided by triggering the interfacial recrystallization and grain growth. Thus, extremely small increases in electrical resistance were detected in the post-annealed samples.