Elimination of hillock formation in Al interconnects using Ni or Co

The effect of using bilayer Ni/Al-1%Cu or Co/Al-1%Cu metallization stacks with regard to hillock formation is investigated in this study and compared to hillock suppression capabilities of Al-1%Si and Al-1%Cu single layer films and Ti/Al-1%Cu bilayer metallization stacks. Various heat treatments during and immediately after depositions were used to study the resistances of different metallization stack specimens to hillock formation. The densities and sizes of hillocks were characterized using scanning electron microscopy and atomic force microscopy, while interfacial reactions between the transition metal layer and Al-1%Cu in the metallization stack were examined using glancing angle x-ray diffraction. Resistivities of the bilayer metallization stacks were characterized using conventional four-point probe measurements. Our studies indicate that metallization processes involving bilayer metallization stacks of nickel and cobalt with Al-1%Cu are more robust compared to those involving titanium with Al-1%Cu. The mechanism of hillock formation is analyzed in light of intermetallic compound formation and stress-strain considerations.