Probing plasticity at small scales: From electromigration in interconnects to dislocation hardening processes in crystals

Investigations into the role of plasticity in the mechanical behavior of materials have great importance to the field of materials science, especially in today's nano-age where sub-micron and nanoscale devices are built near the size of their microstructural features. The creation of such small components requires a thorough understanding of the mechanical properties of materials at these small length scales. A synchrotron white-beam diffraction technique utilizing focused X-ray beam into the submicron resolution has proved to be a unique, powerful tool for the study of plasticity due to its sensitivity to local lattice rotation. This capability becomes crucial when the plastic mechanisms of crystalline materials at submicron and nanoscales are increasingly known to deviate from their bulk (classical) mechanisms leading to their unexpected mechanical behaviors. Understanding and controlling plasticity and the mechanical properties of materials on this scale could thus lead to new and more robust nanomechanical structures and devices.; This Scanning X-Ray Submicron Diffraction (muSXRD) technique developed in the Beamline 7.3.3 at the Advanced Light Source (ALS) Berkeley Lab has been used to study the microstructural evolution at granular level of Cu polycrystalline lines during electromigration. An unexpected mode of plastic deformation was observed in damascene Cu interconnect test structures during an in situ electromigration (EM) experiment and before the onset of visible microstructural damages (void, hillock formation). The deformation geometry and the extent of plasticity observed in this study lead us to conclude that this mode of deformation could have direct bearing on the final failure stages of electromigration.; When crystalline materials are mechanically deformed in small volumes, higher stresses are needed for plastic flow. This has been called the "Smaller is Stronger" phenomenon and has been widely observed. Various size-dependent strengthening mechanisms have been proposed to account for such effects, often involving strain gradients and geometrically necessary dislocations (GNDs). Since the GND density is directly related to local lattice rotation, the muSXRD technique provides the key tool to probe the plastic behavior of the materials at small scales. Here we report on the search for strain gradients and geometrically necessary dislocations as a possible source of strength for two cases of deformation of materials at small scales: nanoindented single crystal copper and uniaxially compressed single crystal submicron gold pillars. These observations suggest that plasticity in one case is indeed controlled by the GNDs (strain gradient hardening), whereas in the other, plasticity is not controlled by strain gradients or sub-structure hardening, but rather by dislocation source starvation, wherein smaller volumes are stronger because fewer sources of dislocations are available (dislocation starvation hardening).