Resistance changes induced by the formation of a single void/hillock during electromigration

Abstract

Electromigration induced resistance changes in short Al lines have been studied by high-resolution AC bridge measurements. The samples were pure, unpassivated Al lines having a length of 3, 5, 8, 12, 17 or 100 μm, a width of 2 μm and a film thickness of ∼100 nm. Depending on current density and sample length the induced resistance changes fully recover or do not recover after removal of the DC stressing current. The transition from recoverable to non-recoverable behavior is clear-cut and is characterized by a constant critical current density—sample length product. Inspection of the lines after current stressing by atomic force microscopy revealed that non-recoverable resistance changes are caused by the growth of a single void, hillock or hillock/void pair. Negative resistance changes correspond to the growth of a hillock and positive resistance changes to the growth of a void. The characteristic time of the relaxation process of the recoverable resistance changes scales with the sample length squared. The recoverable resistance changes are caused by the evolution of mechanical stress in the line during electromigration. A very good agreement is obtained between the observations and solutions of the stress evolution equation of Korhonen et al. for a one-dimensional model consisting of a concatenation of grain boundary segments. The evolution of the stress is reflected in changes of the resistance by the piezoresistance effect. It is shown that the diffusion coefficient of the prevailing transport process can be determined from the characteristic time of the relaxation process.