Wafer direct bonding is an attractive approach to manufacture future micro-electro-mechanical system (MEMS) and microelectronic and optoelectronic devices. In this paper, a combined hydrophilic activated Si/Si wafer direct bonding process based on wet chemical activation and O₂ plasma activation is explored. Additionally, the effect on bonding interface characteristics is comprehensively investigated. The mechanism is proposed to better understand the nature of hydrophilic bonding. The water molecule management is controlled by O₂ plasma activation process. According to the contact angle measurement and FTIR spectrum analysis, it can be concluded that water molecules play an important role in the type and density of chemical bonds at the bonding interface, which influence both bonding strength and voids’ characteristics. When annealed at 350 °C, a high bonding strength of more than 18.58 MPa is obtained by tensile pulling test. Cross sectional SEM and TEM images show a defect-free and tightly bonded interface with an amorphous SiO_x layer of 3.58 nm. This amorphous SiO_x layer will induce an additional energy state, resulting in a lager resistance. These results can facilitate a better understanding of low-temperature hydrophilicity wafer direct bonding and provide possible guidance for achieving good performance of homogenous and heterogenous wafer direct bonding.

This paper presents a detection method for characterizing the bonded interface of O₂ plasma activated silicon wafer direct bonding. The images, obtained by infrared imaging system, were analyzed by the software based on spatial domain and morphology methods. The spatial domain processing methods, including median filtering and Laplace operator, were applied to achieve de-noising and contrast enhancement. With optimized parameters of sharpening operator patterns, disk size, binarization threshold, morphological parameter A and B, the void contours were clear and convenient for segmentation, and the bonding rate was accurately calculated. Furthermore, the void characteristics with different sizes and distributions were also analyzed, and the detailed statistics of the void’s number and size are given. Moreover, the orthogonal experiment was designed and analyzed, indicating that O₂ flow has the greatest influence on the bonding rate in comparison with activated time and power. With the optimized process parameters of activated power of 150 W, O₂ flow of 100 sccm and time of 120 s, the testing results show that the bonding rate can reach 94.51% and the bonding strength is 12.32 MPa.