Grain-boundary voiding in self-passivated Cu-1 at.% Al alloy films on Si substrates

The new generation of semiconductor metallization is based on Cu-film technology. In this paper we report on detrimental changes in surface morphology of self-passivated Cu-1 at.% Al films which were magnetron-sputtered on Si substrates. Large voids appeared at grain boundaries after annealing, oxidation at temperatures at or above 500 degreesC, and cooling to room temperature under vacuum. Grain-orientation imaging with electron backscatter diffraction revealed that preferentially high-energy grain boundaries were voided. Contrary to reports in the literature, void growth was not prevented by the extremely clean ultrahigh-vacuum conditions during film fabrication nor by the strong and very sharp grain texture. Instead, it was clearly correlated with the stable surface oxide on the self-passivated films. Void growth was not found after annealing passivation-free films such as pure Cu or unoxidized Cu-1 at.% Al films.