Impact of Cu microstructure on electromigration reliability

The effect of Cu microstructure on electromigration (EM) has been investigated. A variation in the Cu grain size distributions between wafers was achieved by adjusting the wafer annealing process step after Cu electroplating and before Cu chemical mechanical polishing. Void growth morphology was observed by in-situ and ex-situ scanning electron microscope (SEM) techniques. The Cu lifetime and mass flow in samples with bamboo, near bamboo, bamboo-polycrystalline mixture, and polycrystalline grain structures were measured. The introduction of polycrystalline Cu line grain structure in fine lines for the 65 nm node technology and beyond markedly reduced the Cu EM reliability. The smaller Cu grain size distribution resulted in a shorter EM lifetime and a faster mass flow. The EM activation energies for Cu along Cu/amorphous a-SiCxNyHz interface and grain boundary were found to be 0.95 and 0.79 eV, respectively.