Effects of Cu and Si Dopants on Electromigration Mass Transport in Al Interconnects for VLSI

We have investigated the effects of Cu and Si dopants on electromigration mass transport in Al interconnects for VLSI technology. Four Al alloys with different Cu and Si dopant concentrations (Al-1.5%Cu, Al-1.5%Cu-1.5%Si, Al-0.5%Cu-1.0%Si, and Al-1.0%Cu-1.0%Si) were sputter deposited onto Si substrates covered with 0.55 μm of SiO2 (tetraethylorthosilicate). All metal films were deposited using an MRC903 sputter deposition system to a thickness of 6800A. Deposition parameters were held constant for each metallurgy, with a base pressure of 7×10−7 torr, deposition pressure of 10−2 torr, forward power of 7 Kwatts, and substrate bias of 125 volts. Test structures designed according to National Institute of Standards design guidelines were fabricated in each metallurgy using conventional photolithography and reactive ion etching methods for line widths of 1.0, 1.8, 3.0, 5.0 and 10.0 μm. Accelerated test conditions of T=200°C and DC current density of 2×106 A/cm−2 were used. The results show that electromigration resistance increases with increasing Cu content and decreases as Si content increases. These results are explained in terms of precipitate, grain size distribution, orientation and stress by Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). Our results provide a general guideline relating Cu and Si dopant concentrations, film microstructure and the intrinsic reliability of the metallization system.