Simulation of Mechanical Cu-Pad Expansion Mechanism and Measures to Increase Expansion

In recent years, the demand for higher performance and enhanced functionality of semiconductor devices has increased, driven by advancements in the Internet of Things, machine learning, and digital technologies. As the performance gains from the miniaturization of semiconductors approach their limits, performance enhancement through the three-dimensional (3D) stacking structure of integrated circuits is attracting attention. One of the key technologies in the manufacturing of 3D stacked structures is hybrid bonding. In plasma-activated hybrid bonding, Cu pads are expanded during the annealing process after bonding to make contact and achieve conduction. As the miniaturization of Cu pads progresses, the amount of expansion decreases, leading to critical issues of poor conduction. In this study, we investigated the mechanism underlying the mechanical expansion of Cu pads through simulations and measurements. We found that the thermal expansion of Cu pads is constrained by the insulating layer on the pad sidewalls—particularly when the pad is narrow. According to the mechanism, we propose that a lower yield stress and elastic modulus of Cu and higher thermal expansion coefficient correspond to a larger amount of expansion.