Critical Roles of Copper Surface Planarity in Achieving Reliable Cu/SiO 2 Hybrid Bonding

Fine-pitch interconnects have become essential for high-density 3D integrated circuits, with Cu/SiO₂ hybrid bonding emerging as a key enabling technology for low-resistance, high-density interconnection. However, Cu pad surface planarity following chemical mechanical polishing (CMP) critically influences bonding quality, with Cu dishing and oxide corner rounding representing a major challenge. This study investigates the relationship between Cu pad dimensions, spacing, and their effects on Cu dishing behavior and hybrid bonding performance. Cu pads with widths of 4, 6, 8, and 10 μm were designed with varying inter-pad spacings, patterned using standard photolithography, and fabricated through electroplating. CMP was applied, and dishing depths were quantitatively evaluated using atomic force microscopy (AFM). Prior to bonding, two-step Ar/N₂ plasma surface activation was conducted to remove native oxide and enhance both Cu-Cu and SiO₂-SiO₂ bonding interfaces. 4 μm pads exhibited minimal dishing and achieved void-free bonding, while 6, 8, and 10 μm pads showed progressively deeper dishing and incomplete Cu bonding. In addition to Cu CMP optimization, Cu pad geometry plays a crucial role in determining reliable hybrid bonding quality.