1.3.2 Analysis of Cu-Cu indentation morphology

To evaluate the deformation behavior of the contact surface, the indentation morphology at the atomic level is investigated. Fig. 1-4 shows atomic (010) slice of the indentation morphologies during the loading and unloading processes. The color change corresponds to the change in atomic displacement. Blue color represents atoms without movement (0 Å), and red color represents atoms that move furthest (1 Å). Atoms near the contact region of the wire and substrate change from blue to red, indicating the generation of deformations. Corresponding to the load force of the Cu wire versus the indentation displacement curve, when the indentation displacement reaches-1.0 Å, the attractive force makes the atoms on the surface bulging and bonded together. With the increase of the indentation displacement, the repulsive force increases, and the attractive force decreases; therefore, the atomic displacement decreases. For the substrate, when the indentation displacement reaches 2.6 Å, the atomic displacement of the substrate reaches approximately 0 Å, due to the equilibrium of the attractive and repulsive forces. After that, the substrate atomic displacement increases by increasing the indentation displacement. For the wire, the atomic displacement of the wire first increases because of the attractive force. Thus, it reduces because of the attraction and repulsion equilibrium. After the indentation displacement reaches 2.6 Å, dislocations in the middle area of the Cu wire slice are observed and the atomic displacement of the wire increases by increasing the indentation displacement.

Fig. 1-4 Atomic (010) slice of the indentation morphologies during the loading and unloading processes

During the unloading process, the total force acting on the substrate atoms changes from repulsion to attraction with the continuation of the unloading. The repulsive force at the indentation displacement of 4.3 Å is smaller than that of 4.6 Å. Moreover, the substrate deformation near the contact region at the indentation displacement of 4.3 Å is smaller than that of 4.6 Å. There are still dislocations in the wire at the indentation displacement from 4.3 Å to 2.9 Å. By continuing to retract the wire, the main force changes to an attractive force. The deformation of the substrate near the contact region increases by increasing the attractive force. At the indentation displacement of 2.0 Å, wire atoms start to rearrange, the strain energy is released, and the deformation of the substrate near the contact region reduces. At the indentation displacement of 1.0 Å, the strain energy in the wire fails to rearrange the atoms, and dislocations are generated in the atomic lattice. After retracting the indentation displacement to-2.3 Å, a neckband is formed between the wire and substrate. The deformation near the contact region increases by increasing the attractive force.