Fine-Pitch $30\ \mu \mathrm{m}$ Cu-Cu Bonding using Electroless Nano-Ag

The development of direct Cu-to-Cu bonding technologies has attracted increasing attention due to the increased requirement for advanced high-density packaging. For Cu-Cu bonding, the main issue is that the surface roughness of the Cu pad is not uniform, and the uneven surface induces the partial-bonding, void, and delamination during bonding process. Thus, the Cu pad is needed to use the chemical machine polishing (CMP) to vanish the above-mentioned defects. Alternatively, R. Liu's study observed that the plating nano-Ag film was coated on the surface of the Cu pad to make the interconnect area more uniform [1]. However, if the Ag concentration was too high, it would occur the dendrites on the Cu pad, and the roughness became larger. In this study, the nano-Ag layer is formed using a replacement reaction during the electroless plating process, to decrease costs. Moreover, to compare with CMP, using electroless nano-Ag can lessen the probability of damage on the surface. This study further examined the optimum Ag concentration mixed with the additive for Ag coating on the Cu pad. The function of the additive is to contribute to the flatter Ag layer deposition on Cu pad. In addition, the sintering mechanism of Ag layer is observed with annealed temperature at 190°C, 220 °C, and 250°C, respectively. From atomic-force microscopy (AFM) results, the surface roughness of Ag layer is increased at 190°C compared with the initial surface morphology which can be attributed to the Ag agglomerates to form the Ag nodule. Contrarily, the roughness is decreased as annealing treatment changed from 190°C to 250 °C, which might be attributed to the surface diffusion and grain boundary diffusion of Ag nanoparticles during sintering process [2]. The tensile test is conducted after Cu-Ag-Cu bonding samples by using blanket and patterned metal bonding wafers. There're two failure modes, one is fracture on the electroless Ag-to-Cu joint interface, and the other is fracture on the Ag layer. Furthermore, electroless Nano-Ag was applied on the chip-to-wafer metal bonding process under 220°C, 1MPa and 5min of bonding condition. In daisy-chain electrical resistance verification, Nano-Ag assisted Cu-Ag-Cu bonding has 20 % improvement of yield compared with CMP assisted Cu-Cu bonding. In summary, electroless nano-Ag with the additive cannot only enlarge the process window, but also is strongly adhesive on the Cu part well. Therefore, Cu-Ag-Cu bonding using electroless nano-Ag is the competitive application compared to direct Cu-Cu bonding for applying the fine-pitch IC package under low-temperature.