Enhancement of Ag electromigration resistance by a novel encapsulation process

The Ag lines on Ti/SiO₂/Si stack structures were fabricated by using conventional photolithography and a reactive ion etch (RIE) in an O₂ plasma, and subsequently encapsulated by annealing in a flowing NH₃ ambient. A thin TiN(O) layer formed at the Ag surface and a Ti(O)/Ti₅Si₃ bilayer formed at the initial Ti/SiO₂ interface. Electromigration testing was performed on both bare and encapsulated Ag lines. A partial depletion region in the near-cathode segment and its drift towards the anode were observed at the early stage for the bare Ag test lines. Afterwards, the side-by-side distribution of clusters of voids and hillocks was also noted in other segments. After the encapsulation process, the electromigration resistance of the Ag lines was significantly improved due to the inhibition of the surface diffusion of Ag atoms by the encapsulation layer. This encapsulation process was also expected to address some other issues associated with the applications of the Ag metallization in advanced integrated circuits.