Influence of deposition conditions on ALD based Ru passivation for Cu-Cu hybrid bonding

Cu-Cu hybrid bonding is a key technology for fine-pitch interconnections in advanced semiconductor packaging. However, native Cu oxidation and interface instability hinder reliable low-temperature bonding. While noble metal passivation can mitigate oxidation, conventional sputtering-based methods pose challenges for Cu/SiO₂ hybrid bonding. Atomic layer deposition (ALD) offers a promising alternative for area-selective metal passivation, enabling precise deposition on Cu surfaces without affecting surrounding dielectrics. In this study, we evaluate the bonding characteristics of ruthenium (Ru) passivation layers deposited using plasma-enhanced ALD (PEALD) and compare them with sputtered Ru passivation layers. While PEALD Ru exhibited comparable thin-film properties, bonding performance was significantly lower, as confirmed by scanning acoustic tomography (SAT) and shear strength measurements. Further analysis revealed that lower crystallinity in PEALD Ru and changes in Cu surface properties due to thermal pre-treatment contributed to reduced bonding strength. These findings provide insights for optimizing ALD-based metal passivation, enabling improved Cu/SiO₂ hybrid bonding for advanced semiconductor packaging.