Multi-stack silicon-direct wafer bonding for 3D MEMS manufacturing

Multi-stack wafer bonding is one of the most promising fabrication techniques for creating three-dimensional (3D) microstructures. However, there are several bonding issues that have to be faced and overcome to build multilayered structures successfully. Among these are: (1) chemical residues on surfaces to be bonded originating from the fabrication processes prior to bonding; (2) increased stiffness due to multiple bonded wafers and/or thick wafers; (3) bonding tool effects; (4) defect propagation to other wafer-levels after high-temperature annealing cycles. The problems and the solutions presented here are readily applicable to any microelectromechanical systems project involving the fabrication of multi-stack structures of two or more wafers containing intricate geometries and large etched areas.