Nitrogen Plasma Activation of SiO2 Surface for Low-Temperature Wafer-to-Wafer Bonding: Experiments and Multiscale Simulations.

Wafer-level hermetic bonding technology is crucial for encapsulating micro-electro-mechanical systems (MEMS), protecting them from environmental threats and ensuring long-term reliability. Surface activation before wafer-to-wafer (W2W) bonding is a critical step in this process, which can enhance the bonding strength of oxide interfaces in W2W integration. In this study, nitrogen (N2) plasma was employed to activate silicon dioxide (SiO2) films on silicon wafers, combining experiments with multiscale simulations: ion impact energy and flux extracted from finite element analysis (FEA) of a capacitively coupled plasma chamber were incorporated into a molecular dynamics (MD) framework to analyze atomic-scale surface evolution. The optimized plasma power produced the lowest mean contact angle and the highest bonding strength. The simulation results further indicate that moderate plasma activation yields a more favorable balance among surface reactivity, wettability, and near-surface structural modification, whereas excessive plasma exposure promotes surface reconstruction and the formation of surface states that are less conducive to bonding. Notably, the near-surface low-density region that develops under the optimized plasma power facilitates sub-surface water storage, which likely contributes to subsequent interfacial strengthening during annealing. This study provides in-depth insights into enhancing SiO2 bonding strength via N2 plasma activation and offers an efficient computational strategy for guiding surface modification processes.