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Direct wafer bonding was performed under medium vacuum condition. High bonding strength (larger than 20 MPa) is achieved at the bonding temperature of only 400°C, and the annealing time for complete bonding is less than 5 hours. The bonding efficiency (percentage of the bonded area over entire wafer area) of the medium vacuum wafer bonding (MVWB) is also found to be better than the traditional wafer bonding.
Qualitative description of the mechanism of MVWB is proposed in present work. It is believed that the medium vacuum can enhance the out-diffusion of the water molecules and other trapped impurities through the bonding interface which is porous initially. This enhanced diffusion speeds up the chemical reaction for the formation of Si-O-Si, and thus more bonding sites are available before the interface close-up. As a result, we observe an increase in bonding strength, bonding efficiency and the bonding speed.
In the present work, a novel method is proposed to re-construct voids in passivated metal interconnections. In this method, the conventional SEM and EBIC systems are assembled and utilized without much modification. In principle, a constant current is applied to the metal interconnections while an electron beam is scanning and impinging upon the surface of the sample. The voltage at the terminals is monitored simultaneously during electron beam scanning. Resistance change, and hence voltage perturbation are expected when the electron beam approaches the defective area, caused by uneven electron beam heating (EBH) and heat transmission. Information on defects or voids is thus obtained by analyzing the voltage alteration. Finite element simulation showed that the recorded voltage perturbation is not dependent of the length of the interconnect, but a linear function of the void volume. Thus, the method is essentially useful as the metal length has increased tremendously in copper technology. In addition, it can provide the void size and depth, with the possibility to reconstruct the entire void shape in 3D.