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 mum before contacting the wafers, it becomes about 17 mum 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.