Effect of the Annealing Process on Cu Bonding Quality Using Ag Nanolayer

One of the key requirements for mass production of die-to-wafer Cu bonding is preventing copper surface oxidation. In this study, Cu bonding utilizing an Ag nanolayer was examined to achieve both Cu surface oxidation prevention and low-temperature bonding. The primary focus was to evaluate the impact of the annealing process on the Cu bonding quality. The Cu surface was coated with a 15-nm-thick layer of Ag using an evaporation technique. Cu wafer-to-wafer bonding was performed at a temperature of 180 °C for 30 min, followed by annealing at 200 °C for 60 min. The annealing process resulted in the complete diffusion of Cu into the bonding interface, leading to a uniform and pure Cu-to-Cu bonding. However, the Ag nanolayer did not completely dissolve into the Cu thin film and formed a thin Ag band. The average shear strength of the specimens subjected to the annealing process was $\sim $ 6.5 MPa, which was relatively low. Nonetheless, the annealing process has proven to be a very effective way to create a homogeneous bonding interface and achieve pure Cu-Cu bonding when using Ag nanolayers in Cu bonding.