The electromigration effect revisited: non-uniform local tensile stress-driven diffusion

Shih-Kang Lin,Hsin-Yi Lee,Shang-Jui Chiu,Yu-Chen Liu,Yen-Ting Liu

doi:10.1038/s41598-017-03324-5

Abstract

The electromigration (EM) effect involves atomic diffusion of metals under current stressing. Recent theories of EM are based on the unbalanced electrostatic and electron-wind forces exerted on metal ions. However, none of these models have coupled the EM effect and lattice stability. Here, we performed in situ current-stressing experiments for pure Cu strips using synchrotron X-ray diffractometry and scanning electron microscopy and ab initio calculations based on density functional theory. An intrinsic and non-uniform lattice expansion – larger at the cathode and smaller at the anode, is identified induced by the flow of electrons. If this electron flow-induced strain is small, it causes an elastic deformation; while if it is larger than the yield point, diffusion as local stress relaxation will cause the formation of hillocks and voids as well as EM-induced failure. The fundamental driving force for the electromigration effect is elucidated and validated with experiments.

Highlights

The electromigration (EM) effect describes atomic diffusion in conductors driven by electric currents, which may lead to the formation of voids and hillocks at the cathode and the anode, respectively[1]
The semi-empirical model successfully rationalized the non-directional EM-induced supersaturation of alloys, which leads to a fundamental question: What are the effects of electric currents upon materials lattice stability, which is independent of the direction of the electron flows?
A combinatorial approach of ab initio calculations based on density functional theory (DFT) and systematical in situ current-stressing experiments with synchrotron X-ray diffraction (XRD) and scanning electron microscopy (SEM) was performed to study the effects of electrical currents upon the lattice stability of pure Cu as a model material

Summary

Introduction

The electromigration (EM) effect describes atomic diffusion in conductors driven by electric currents, which may lead to the formation of voids and hillocks at the cathode and the anode, respectively[1]. The semi-empirical model successfully rationalized the non-directional EM-induced supersaturation of alloys, which leads to a fundamental question: What are the effects of electric currents upon materials lattice stability, which is independent of the direction of the electron flows?. A combinatorial approach of ab initio calculations based on density functional theory (DFT) and systematical in situ current-stressing experiments with synchrotron X-ray diffraction (XRD) and scanning electron microscopy (SEM) was performed to study the effects of electrical currents upon the lattice stability of pure Cu as a model material. The theoretical calculations agree closely with the experiments, opening the door for new understandings of the peculiar non-directional phenomena under electric current stressing, the analogy of materials responses under electric current stressing and conventional mechanical stress, and the fundamental driving force of EM

Methods

Results

Conclusion