Crystal plasticity in Cu damascene interconnect lines undergoing electromigration as revealed by synchrotron x-ray microdiffraction

Abstract

Plastic deformation was observed in damascene Cu interconnect test structures during an in situ electromigration experiment and before the onset of visible microstructural damage (voiding, hillock formation). We show here, using a synchrotron technique of white beam x-ray microdiffraction, that the extent of this electromigration-induced plasticity is dependent on the linewidth. In wide lines, plastic deformation manifests itself as grain bending and the formation of subgrain structures, while only grain rotation is observed in the narrower lines. The deformation geometry leads us to conclude that dislocations introduced by plastic flow lie predominantly in the direction of electron flow and may provide additional easy paths for the transport of point defects. Since these findings occur long before any observable voids or hillocks are formed, they may have direct bearing on the final failure stages of electromigration.

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

“…For instance, our own previous study of Cu interconnect lines made by Intel Corporation 13 reported a much lower level of plastic damage (q % 10 13 /m 2 ); our quick calculation following a methodology similar to that described in Ref. 8 suggests that such a low level of dislocation density would only generate diffusivity two orders of magnitude lower than that of the Cu/dielectric interface diffusion of the samples at the elevated temperature.…”

“…Kirchheim and Kaeber 2 attributed this extra depression of MTF in the high j range to a Joule heating effect. Our earlier studies [12][13][14] have also suggested electromigration-induced plasticity that can lead to new paths for EM transport which could also be responsible for this deviation. 8 It is certainly within the range of possibilities that either one of these effects dominates or that both effects contribute to various different extents to the MTF deviation from the n = 1 line at the high j range.…”

“…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.…”

“…Joule heating is widely believed to be the source of this deviation, as suggested by Kirchheim and Kaeber, 2 especially in the case of Al interconnects. 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.…”

“…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.…”