Voiding induced stress redistribution and its reliability implications in metal interconnects

Abstract

The evolution of stress affected by the formation of voids in metal interconnects is studied numerically. The objective is to examine how thermal stresses redistribute in response to voiding, rather than how voids form in response to stress, for gaining insights into the physics of reliability in integrated circuit devices. The finite element method is employed to model the thermal and voiding histories of the interconnect structure. Voiding is simulated by removing relevant material elements in the metal. Bulky and slit-like voids of various sizes are considered. The stress relaxation due to voiding is found to be a local phenomenon, bearing no direct relation with the global thermomechanical conditions of the interconnect. The concept of the saturation void fraction thus needs to be revisited. The stress gradient along the line on both sides of the void is found to be essentially constant for the various void shapes and sizes considered. This has implications on the resolution limit of micro-diffraction techniques needed for sensing the existence of voids. The resulting stress field is also used as the initial condition in modeling the electromigration flux divergence by employing the finite difference method. A mechanistic understanding of electromigration voiding due to the presence of locally debonded slits is established. The debond-induced stress gradient is found to cause back atomic flow, leading to local flux divergence. The flux divergence, and thus the voiding propensity, increases with an increasing size of the debond. The same approach is also used to study if a pre-existing stress-void can grow into an electromigration void. A simple correlation, which appears to rationalize experimental observations, is identified: a large stress-void is more prone to growth during subsequent electromigration. The validity of applying the critical stress concept in characterizing electromigration failure is discussed.