Wafer bonding is an emerging and promising technology for the manufacture of complex three-dimensional structures as well as achieving vacuum sealing. Photolithography, which enables the mass production and batch fabrication of MEMS devices, is subject to the limited manufacturability of three-dimensional structures, resulting in a narrow design space and deterioration in the device performance. In wafer bonding, device wafers are patterned individually to create semi-three-dimensional structures by microfabrication and, subsequently, bonded together to form complex three-dimensional structures at the wafer level. In addition, wafer bonding can be conducted in a vacuum ambient and achieve vacuum sealing, which is crucial in many MEMS sensors and actuators since damping by air predominates at the microscale. Wafer bonding techniques utilized in the MEMS field are categorized into four types, namely: direct bonding, anodic bonding, intermediate-layer bonding, and surface activated bonding. This paper gives a brief review of recent work relevant to these bonding techniques and their applications with the emphasis on silicon direct bonding.
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