Twelve inch wafer-to-wafer bumpless direct Cu bonding was achieved at room temperature for 60 h, and followed by annealing at 300 degrees C for 30 min. Interconnections with different pad pitches from 6 mu m to 25 mu m were obtained. Low electrical resistances of 27.73 m Omega indicated the bonding was completed. High shear strength of 35.9 MPa suggested the potential application of this process in wafer thinning. Observations of interface morphology presented continuous and tight bonding interface. Grain orientation inspections indicated that two kinds of preferred orientations were found at the bonding interface, one of which was near (100) and the other was near (110). This difference for preferred orientations may be related with yielding process. Furthermore, grain boundaries were found to be respectively zigzag and straight at the bonding interface. Based on these observations, bonding mechanism is proposed by the driven force of energy minimization theory. In bonded Cu pads, strain-favorable grain (with lower free energy) growth happened at the expense of other less strain-favorable grains (with higher free energy). Besides initial grain nucleation was formed by small grain connection with the same orientation at the bonding interface and grain growth happened after further annealing. (C) 2015 Elsevier B.V. All rights reserved.