Low-Temperature and Room-Temperature Surface-Activated Au-Au Bonding: Surface Requirements, Preparation Methods, and Emerging Applications.

Gold-to-gold (Au-Au) bonding forms an oxide-resistant metallic interface without solder or conductive adhesive, making it attractive for heterogeneous integration, MEMS sealing, optoelectronic packaging, and flexible hybrid electronics (FHEs). This review focuses on low-temperature and room-temperature surface-activated Au-Au direct bonding and emphasizes the roles of surface roughness and activation state. Compared with thermocompression bonding (TCB), low-temperature and room-temperature bonding impose stricter surface requirements because heat and pressure in TCB can deform asperities and increase real contact area. Room-temperature bonding instead depends strongly on the surface condition before contact. Successful bonding generally requires very smooth Au surfaces, activation, and contamination control. Reported roughness ranges from below 0.5 nm for smooth sputtered or transferred Au films to tens or hundreds of nanometers for rough plated Au before smoothing. Smoothing strategies can therefore expand the direct-bonding process window. Plasma activation removes contaminants and increases surface reactivity; Ar plasma promotes strong bonding, whereas O2 plasma can form Au oxide and weaken the interface. Water-vapor plasma-assisted bonding (WVPAB) further enables bonding on rougher electrodes and flexible polymer substrates. Applications include optoelectronic integration, MEMS hermetic packaging, heterogeneous integration, and FHE. Remaining challenges include wafer-scale roughness control, activated-surface lifetime, patterned Au bonding, long-term reliability, and mechanism-based process optimization.