wafer bonding

Wafer bonding joins two wafers together to create composite structures for 3D integration, SOI substrates, or MEMS devices. Multiple bonding techniques exist with different characteristics. Direct bonding (fusion bonding) joins atomically smooth hydrophilic surfaces without intermediate layers, creating strong bonds through molecular forces, typically followed by high-temperature annealing to strengthen bonds. Anodic bonding uses electric field and heat to bond silicon to glass for MEMS packaging. Adhesive bonding uses polymer layers (BCB, polyimide) providing tolerance to surface roughness but with lower thermal conductivity. Metal bonding (copper-copper or gold-gold) provides both mechanical and electrical connection through thermocompression or diffusion bonding. Hybrid bonding simultaneously bonds dielectric (oxide-oxide) and metal (copper-copper) regions, enabling high-density interconnects without solder bumps. Wafer bonding requires careful surface preparation, particle control, and alignment (sub-micron for 3D integration). Applications include SOI wafer fabrication, 3D integrated circuits, MEMS packaging, and photonics integration. Bonding quality is verified through acoustic microscopy and mechanical testing.