Recrystallized Cu–Cu Joints Show Superior Thermal Fatigue Resistance
Nanotwinned copper (nt-Cu) with a ?111? crystallographic orientation offers notable advantages for microelectronic interconnects. This orientation exhibits the highest surface diffusivity and lowest oxidation rate among low-index copper surfaces, a result of its high atomic packing density and minimal dangling bonds. These properties enable bonding at reduced temperatures or shorter process times. In the reported study, nt-Cu microbumps approximately 7 ?m thick were fabricated, with orientation imaging microscopy revealing about 50% ?111? orientation. To further enhance bonding quality and oxidation resistance, the surfaces were planarized via chemical mechanical polishing (CMP), achieving a root mean square roughness of roughly 4 nm.
The test vehicles included Kelvin structures and daisy chains containing 40 and 400 bumps, each microbump measuring about 30 ?m in diameter and 7 ?m in height. These assemblies were subjected to thermal cycling tests to monitor changes in electrical resistance. Initial analysis showed that as-fabricated bumps retained a visible bonding interface, while post-annealed bumps exhibited complete elimination of this interface. This transformation was driven by recrystallization and grain growth across the interface, which strengthened the Cu–Cu joints.
Electrical resistance measurements at room temperature showed little difference between as-fabricated and post-annealed joints, consistent with findings by Nitta et al., who noted that above 12 K, electron scattering from thermal lattice vibrations dominates resistivity. However, under thermal cycling, differences became pronounced. After 1000 cycles, post-annealed joints maintained nearly constant resistance, while as-fabricated joints exhibited increases of 12% and 17.4% for bonding pressures of 47 MPa and 93 MPa, respectively. This indicated that post-annealing significantly improved thermal fatigue resistance.
Focused ion beam (FIB) imaging revealed the microstructural basis for this improvement. In as-fabricated joints, only limited grain growth occurred across the interface, and cracks propagated along the original interface after cycling. In contrast, post-annealed joints showed extensive consumption of columnar grains, with new grains growing across the interface. This eliminated the weak, planar interface and replaced it with a continuous grain structure, preventing crack initiation and propagation.
Electrical resistance measurements taken at intervals during cycling showed that differences between the two joint types became evident after about 500 cycles. The as-fabricated joints experienced steady resistance increases due to crack formation, while post-annealed joints showed minimal change even after 1000 cycles. A drop in resistance after approximately 750 cycles was observed in both cases, attributed to recovery processes in copper. Recovery involves the reduction of dislocation density and internal strain energy through dislocation motion and atomic diffusion, processes accelerated by thermal stress gradients during cycling between ?55 °C and 125 °C.
Finite element analysis (FEA) provided insight into the stress environment during thermal cycling. The large mismatch in coefficients of thermal expansion between copper and the underfill material generated significant stresses at the bonding interface. At ?55 °C, tensile stress at the interface center reached 3.8 MPa, rising to 14.3 MPa at 125 °C. These stress fluctuations promote void and crack formation, particularly along the straight bonding interface in as-fabricated joints. By eliminating this interface through post-annealing, cracks were forced to propagate along more tortuous recrystallized grain boundaries, extending their paths and enhancing joint reliability.
The two-step bonding process, incorporating post-annealing, thus transformed the microstructural weak point into a robust, continuous grain network. This approach suppressed crack formation, maintained electrical performance under severe thermal cycling, and offered a pathway toward more durable interconnects in applications where thermal fatigue is a critical reliability concern.
