Voiding induced stress redistribution and its reliability implications in metal interconnects

The evolution of stress affected by the formation of voids in metal interco nnects is studied numerically. The objective is to examine how thermal stre sses redistribute in response to voiding, rather than how voids form in res ponse to stress, for gaining insights into the physics of reliability in in tegrated circuit devices. The finite element method is employed to model th e thermal and voiding histories of the interconnect structure. Voiding is s imulated by removing relevant material elements in the metal. Bulky and sli t-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 gradien t along the line on both sides of the void is found to be essentially const ant 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 ini tial condition in modeling the electromigration flux divergence by employin g the finite difference method. A mechanistic understanding of electromigra tion voiding due to the presence of locally debonded slits is established. The debond-induced stress gradient is found to cause back atomic Row, leadi ng to local Aux divergence. The flux divergence, and thus the voiding prope nsity, increases with an increasing size of the debond. The same approach i s also used to study if a pre-existing stress-void can grow into an electro migration void. A simple correlation, which appears to rationalize experime ntal observations, is identified: a large stress-void is more prone to grow th during subsequent electromigration. The validity of applying the critica l stress concept in characterizing electromigration failure is discussed. ( C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd All rights reserved.