Comparative annealing effect on bonded wafers in air and ultrahigh vacuum for microelectromechanical systems/microfluidics packaging

The fundamentals of room temperature bonding methods—surface activated bonding (SAB) and sequentially plasma-activated bonding (SPAB)—are reviewed with applications for packaging of microelectromechanical systems (MEMS) and microfluidic devices. The room temperature bonding strength of the silicon/silicon interface in the SAB and SPAB is as high as that of the hydrophilic bonding method, which requires annealing as high as 1000°C to achieve covalent bonding. After heating, voids are not observed and bonding strengths are not changed in the SAB. In the SPAB, interfacial voids are increased and decreased the bonding strength. Water rearrangement such as absorption and desorption across the bonded interface is found below 225°C. While voids are not significant up to 400°C, a considerable amount of thermal voids above 600°C is found due to viscous flow of oxides. Before heating, interfacial amorphous layers are observed both in the SAB (8.3 nm) and SPAB (4.8 nm), but after heating these disappear and enlarge in the SAB and SPAB, respectively. This enlarged amorphous layer is SiO2, which is due to the oxidation of silicon/silicon interface after sequential heating. The bonding strength, sealing, and chemical performances of the interfaces meet the requirements for MEMS and microfluidics applications.