Pd surface alloying induced exothermic effects enable low‑temperature Cu‑Cu bonding with glycerol assistance

Simple immersion surface Pd alloying modification yields high-strength Cu-Cu bonding. • Disorder-to-order transformation offers extra driving force unlike inert Pd layer. • Lattice distortion & atomic pinning give 160 MPa shear strength at 76% bonding ratio. • Glycerol reduces oxide to nanocrystals and prevents oxidation during bonding in air. Low-temperature Cu-Cu thermal-compression bonding is essential for three-dimensional integrated circuits (3D ICs) to achieve high-density vertical interconnects. The key scientific challenge lies in achieving sufficient atomic interdiffusion at temperatures below 250 °C, where volume diffusion is sluggish and native oxides impede metal‑to‑metal contact. Here, this challenge is overcome by a simple galvanic displacement reaction that forms a disordered Cu-Pd surface alloy, followed by glycerol‑assisted bonding. This surface engineering simultaneously reduces roughness, raises surface energy, refines grains, and introduces micro-strain, while glycerol removes residual oxides in-situ and protects the interface from re-oxidation. These modifications accelerate atomic diffusion, and the exothermic defect annihilation and disorder-to-order transformation further lowers the diffusion barrier. Consequently, Pd‑displaced samples exhibit high shear strength and bonding ratio, and Kirkendall voids shrink under bonding conditions, potentially mitigating electrical concerns. The measured strength exceeds predictions from the bonding area alone due to solid‑solution hardening and lattice distortion. This work provides a simple, selective, and industrially viable route for low‑temperature Cu-Cu and hybrid bonding, offering a new surface‑alloying strategy with potential applicability to advanced packaging. Simple immersion surface alloying with glycerol reduction and protection achieves high‑strength Cu‑Cu bonding in air via abundant grain boundaries, disorder‑to‑order transformation and lattice distortion (exceeding estimated bulk Cu shear strength), while Kirkendall voids shrink with bonding durations to reduce potential electrical concerns. Download: Download high-res image (408KB) Download: Download full-size image