Early stage of plastic deformation in thin films undergoing electromigration

Abstract

Electromigration occurs when a high current density drives atomic motion from the cathode to the anode end of a conductor, such as a metal interconnect line in an integrated circuit. While electromigration eventually causes macroscopic damage, in the form of voids and hillocks, the earliest stage of the process when the stress in individual micron-sized grains is still building up is largely unexplored. Using synchrotron-based x-ray microdiffraction during an in-situ electromigration experiment, we have discovered an early prefailure mode of plastic deformation involving preferential dislocation generation and motion and the formation of a subgrain structure within individual grains of a passivated Al (Cu) interconnect. This behavior occurs long before macroscopic damage (hillocks and voids) is observed.

Unexpected Mode of Plastic Deformation in Cu Damascene Lines Undergoing Electromigration

“…[2][3][4] Recently, a very early stage of plastic deformation and microstructural evolution during an electromigration test was detected in Al(Cu) interconnect lines, long before any macroscopic damages become visible, by using a synchrotron technique involving white beam X-ray microdiffraction. 5 In particular it was observed that during in-situ electromigration a gradient of plastic deformation evolves along the line which results in bending and in polygonization of the largest grains between the cathode and the anode end. Smaller grains do not readily deform but do rotate as electromigration proceeds.…”

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

“…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.…”

“…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.…”

High dislocation density of tin induced by electric current

“…White light synchrotron X-ray microdiffration has been applied for the purpose of visualizing the grain morphology and estimating the strain occurring under electromigration. 11,12 In situ scanning electron microscope (SEM) investigations have revealed that dissolution phenomena occur in the second phases of 95Pb5Sn 13 and Sn0.8Cu 14,15 solders under electric current. Electrorecrystallization was observed in the second phases after the current was stopped.…”