Influence on the Electro-Migration Resistance by Line Width and Average Grain Size along the Longitudinal Direction of Very Narrow Cu Wires

The influence of microstructures of Cu wires on electromigration (EM) resistance has been investigated using Cu wires with various line widths from 50 to 280 nm and line heights of 300 nm or 500 nm. A strong line width dependence of median time to failure was observed as the EM resistance decreases substantially with narrowing of line widths from 280 nm to 50 nm. The activation energies for 50, 80, 100 and 140 nm widths were 0.61, 0.64, 0.68 and 0.71 eV. The EM resistance and activation energy of Cu wires could be represented as a function of the ratio of line length to average grain size, which corresponds to the number of grains along the longitudinal direction, and they both increased as this ratio decreased. The influence of this ratio was particularly significant when the line width was below 100 nm. These results indicate that coarsening of grain sizes in the current flow direction is mandatory to enhance EM resistance and lower resistivity for the very narrow Cu wires.