Surface Activated Bonding of 8 in. Si Wafers for MEMS and Microfluidic Packaging

The challenges for direct wafer bonding using surface activated bonding (SAB) method have been investigated. For this purpose, 4–8 in. silicon wafers with and without fine patterns were bonded in vacuum. Three challenges are identified through comprehensive investigations of the bonded interfaces using infrared (IR) transmission, tensile pulling, and high-resolution transmission electron microscopy (HRTEM) observation. First is the alignment between the wafers. While the alignment accuracy is about $5\ \mu{\rm m}$ before contacting the wafers, it becomes about $17\ \mu{\rm m}$ after bonding. Second are the air-gaps across the bonded interface due to processing induced artifacts and particles. Third is the wafer bow, which control the three-dimensional (3D) bonding. Three-bulk Si 8 inch wafers were bonded together for 3D bonding. Thin wafers with small bow are recommended for 3D bonding using the SAB method.