Direct wafer bonding was performed under medium vacuum conditions. The effects of the sequence of wafer contact and vacuum application, annealing temperature, and annealing time on the bonding quality and bonding speed were investigated. From the comparison of the bonding efficiency, contacting two wafers in air rather than in vacuum produces much less bubbles. For vacuum bonding, good bonding, such as high bonding strength and high bonding efficiency, can be achieved at temperatures as low as 300 • C. From the comparison between air wafer bonding and medium vacuum wafer bonding, it is obvious that medium vacuum annealing can improve the bonding strength and accelerate the bonding process. Based on the results observed, a qualitative mechanism of medium vacuum wafer bonding at low temperatures was proposed.
Low-temperature direct wafer bonding was successfully performed under medium vacuum level. A mathematical model was developed based on the qualitative understanding of the bonding mechanisms. The model combined the diffusion-reaction model of water in SiO 2 and the diffusion theory in porous media. It is found that the model agrees well with the experimental data. This model can be applied to predict the effects of annealing time, annealing temperature, ambient vacuum, wafer orientation, and wafer dimension on the bond strength. Index Terms-Bond strength, bonding mechanisms, diffusion theory, mathematical model, wafer bonding.
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