Electromigration-Induced Plasticity: Texture Correlation and Implications for Reliability Assessment

Abstract

Plastic behavior has previously been observed in metallic interconnects undergoing high-current-density electromigration (EM) loading. In this study of Cu interconnects, using the synchrotron technique of white-beam x-ray microdiffraction, we have further found preliminary evidence of a texture correlation. In lines with strong (111) textures, the extent of plastic deformation is found to be relatively large compared with that of weaker textures. We suggest that this strong (111) texture may lead to an extra path of mass transport in addition to the dominant interface diffusion in Cu EM. When this extra mass transport begins to affect the overall transport process, the effective diffusivity, D eff , of the EM process is expected to deviate from that of interface diffusion only. This would have fundamental implications. We have some preliminary observations that this might be the case, and report its implications for EM lifetime assessment herein.

Electromigration-Induced Plastic Deformation in Cu Interconnects: Effects on Current Density Exponent, n, and Implications for EM Reliability Assessment

“…Based on our earlier observations on similar Cu interconnect samples 8,13 such predamaging would result in the microstructures of Cu grains having dislocations lining up along the length of the interconnect lines with a density (q) of up to 10 15 /m 2 . Evidence of such effects on Cu microstructures has been documented on Cu interconnect samples from various manufacturers 8,13 using the synchrotron x-ray microdiffraction technique. Reference 8 is most relevant to our present study as it specifically describes the dislocation density and configuration, as shown in Fig.…”

“…The Cu lines in Ref. 8 were fabricated by the Submicron Development Center (SDC) when it was still officially part of AMD in 2006, while the Cu lines of our present study were fabricated by SDC as it is currently part of Spansion, Inc. Both of them were fabricated using the same 65-nm technology node of SDC.…”

“…While this equation is widely used, the value for the current density exponent and its implications for EM lifetime prediction are still much debated. [2][3][4][5][6][7][8] Under the common atomistic description of electrotransport, 9 the electromigration flux in a metallic line is proportional to the current density and the product of flux and time (i.e., MTF) and corresponds to the removal of a certain volume of matter per unit length (i.e., the cross-sectional area) of the interconnect line, which is a necessary condition for failure. Thus MTF is proportional to j À1 (i.e., n = 1).…”

“…However the recent discovery of EM-induced plasticity, [12][13][14] wherein plastic deformation leads to new paths for EM transport, could also cause this deviation. 8 Plastic deformation behavior in metallic interconnect lines during EM experiments has been observed in both Al 12,14 and Cu interconnects, 8,13 using a synchrotron technique involving white-beam x-ray microdiffraction that can function as a local probe of plastic deformation in crystals that compose the interconnect. [15][16][17][18] The extent and configuration of dislocations induced during accelerated EM testing in a set of Cu interconnect lines have been suggested to lead to another, competing EM diffusion path, in addition to interface diffusion in Cu.…”

“…[15][16][17][18] The extent and configuration of dislocations induced during accelerated EM testing in a set of Cu interconnect lines have been suggested to lead to another, competing EM diffusion path, in addition to interface diffusion in Cu. 8 An increase in the current density is then suggested to lead to an increase in the dislocation density, q, which should lead to an increase in D eff (the overall/ effective diffusivity of the EM process). Consequently, there would be an extra EM flux, and thus an extra reduction in the time to failure of the device with increasing j.…”