Electromigration in Short Al Lines Studied by High-Resolution Resistance Measurement

Abstract

ABSTRACTElectromigration 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. The value of this product is close to the value of the constant critical current density - sample length product characterizing the occurrence of drift in a drift velocity experiment. 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 magnitude of the non-recoverable resistance changes can be very well estimated from the dimensions of the void or hillock. The rate of change of the non-recoverable resistance changes (or the growth rate of a void or hillock) scales linearly with the current density.The characteristic time of the relaxation process of the recoverable resistance scales with the sample length squared. The temperature dependence of the relaxation time shows a good Arrhenius behaviour with an activation energy typical for grain boundary diffusion in Al. The maximum magnitude of the recoverable resistance changes is ΔR/R∼5×10∼-4. The time and temperature dependence of the recoverable resistance changes can be very well explained by the mechanical stress evolution model of Korhonen et al. The evolution of the stress is reflected in changes of the electrical resistance by the piezoresistance effect. A good agreement is found between maximum observed recoverable resistance change and the maximum stress in the line. The good understanding of the resistance changes induced by electromigration in short lines makes this geometry very attractive for studies of fundamental aspects of electromigration in thin film conductors.