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Three-dimensional integrated circuit (3D IC) technology has become increasingly important due to the demand for high system performance and functionality . Many challenges arise from the third dimension to successfully achieve excellent electrical and mechanical interconnection by wafer bonding approach. In this work, face-to-face (F2F) stacking of wafer-on-wafer (WoW) is successfully demonstrated using bump-less Cu-Cu bonding on 200 mm wafers. Experimental failures are investigated and discussed in details. Optimized bonded Cu structure is found to provide sufficient mechanical strength to sustain shear force during wafer thinning.