Stacking Fault and Plastic Deformation Mechanism in Nano-twinned Cu Pillar under Ultrahigh Stress

Nano-twinned Cu pillar is fabricated on an as-electroplated nano-twinned Cu film, which preferentially grew in the {111} plane family. With a nano-indentation device (Picoindenter), a vertical ultrahigh pressure of 5100 MPa is applied to the nano-twinned Cu pillar. The twin lamellar grain is greatly deformed under that ultrahigh external stress. Consequently, the top surface plane is pressed into the lattice beneath to continuously form edge dislocations. Those dislocations are driven along the possible slip systems toward the outer surface of the first matrix lamella, corresponding to the plastic deformation. A dislocation-mediated mechanism is proposed to explain the plastic deformation of the nano-twinned Cu pillar. Three types of slip systems are identified for the created dislocations, each resulting in either the displacement on the first matrix lamellar grain or cross-slip on the twin boundary. At the ultrahigh external pressure, as the dislocations slide through the twin boundary, they would dissociate to Shockley partial dislocations and alter the stacking sequence in the twin boundary, creating stacking faults. The resultant stacking faults in the twin boundary are observed in the transmission electro microscopy (TEM) lattice images.