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 lowtemperature 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 degrees C for 30 min, followed by annealing at 200 degrees 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 similar to 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.