Copper-to-copper (Cu-to-Cu) direct bonding is a promising approach to replace traditional solder joints in three-dimensional integrated circuits (3D ICs) packaging. It has been commonly conducted at a temperature over 300 °C, which is detrimental to integrated electronic devices. In this study, highly (111)-oriented nanotwinned (nt) Cu films were fabricated and polished using chemical mechanical planarization (CMP) and electropolishing. We successfully bonded and remained columnar nt-Cu microstructure at a low temperature of 150 °C thanks to the rapid diffusion of Cu on (111) surface. We employed a new microstructural method to characterize quantitatively the interfacial bonding quality using cross-sectional and plan-view microstructural analyses. We discovered that CMP nt-Cu bonding quality was greater than that of electropolished nt-Cu ones. The CMP nt-Cu films possessed extremely low surface roughness and were virtually free of pre-existing interface voids. Thus, the bonding time of such CMP nt-Cu films could be significantly shortened to 10 min. We expect that these findings may offer a pathway to reduce the thermal budget and manufacturing cost of the current 3D ICs packaging technology.
In this study, nanotwinned copper (nt-Cu) foils were fabricated using a rotary electroplating system. Their microstructures (orientation, grain size, and twin spacing) and correlated tensile properties were substantially tuned by manipulating the electroplating current density. The nt-Cu foils were further heat-treated at 150°C for 1 h and tensile-tested to characterize their thermal stability. We found that the mechanical properties of the nt-Cu foils are directly associated with the electroplating current density. The ultimate tensile strength (UTS) increases with increasing in the current density, reaching the maximum values at 25 ASD. The great enhancement can be attributed to the decrease in twin spacing and grain size. We also discovered that the UTS and ductility remain approximately intact under a high temperature, indicating the excellent thermal stability of such nt-Cu foils.
Copper joints have replaced solder interconnects in integrated
circuits due to their great electrical properties and lower-temperature
processing. To isolate Cu from oxidizing during bonding processes,
a (111)-oriented nanotwinned Ag (NT-Ag) thin layer was electroless-deposited
on a (111)-oriented NT-Cu film. Such a method outperforms the sputtering
approach in terms of expenditure, environmental impact, and deposition
rate. The microstructures of the Ag films were then analyzed. Results
show that columnar NT-Ag grains epitaxially grew along the columnar
NT-Cu grains. Additionally, two types of joints (Cu–Ag and
Ag–Ag) were fabricated and characterized. We found that the
bonding strength of the Cu–Ag joints was higher than that of
the Ag–Ag joints. This could be attributed to the greater diffusion
rate of Ag atoms in Cu than the self-diffusion of Ag.
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