Effect of titanium addition to copper interconnect on electromigration open circuit failure

The effect on electromigration open-circuit failure of the addition of a small amount of titanium into copper interconnects is investigated. Mean time to failure (MTF) increased with increasing Ti concentration up to somewhere around 0.4 wt.% and decreased sharply by further addition of Ti, while the resistivity of Cu-Ti films increased monotonically with increasing Ti concentration. Cu-0.37wt.%Ti interconnect had a two-orders-of-magnitude larger MTF than pure Cu interconnect and had a low resistivity of 2.7 μΩ-cm, which was as low as the resistivity of pure aluminum interconnects. Grain size was drastically reduced by Ti addition and was independent of Ti concentration. When the Ti content in Cu exceeded the solubility limit (0.5 wt.% at 450°C), formation of a Cu-Ti compound was indicated by X-ray diffraction. Although the exact Ti concentration involved is uncertain, the small content of Ti in Cu interconnects improved the MTF unless the Cu-Ti compound was not formed. The formation of the Cu-Ti compound caused deterioration of the electromigration resistance