Grain growth and void formation in dielectric-encapsulated Cu thin films

Grain growth in 40-nm-thick Cu films encapsulated by over- and under-layers of SiO₂, Al₂O₃, Si₃N₄, and MgO was investigated. The films were magnetron sputter deposited onto cooled SiO₂/Si substrates in an ultrahigh vacuum purity environment. Ex situ annealing was performed at 400 and 800 °C in 1 atm reducing gas. Films deposited at −120 °C exhibited more extensive grain growth after annealing than films deposited at −40 °C. Films annealed at room temperature had grain sizes less than 35 nm. All films exhibited some void formation after annealing at 400 and 800 °C, but the films encapsulated in Al₂O₃ exhibited the lowest area fraction of voids. The mean grain sizes of the Al₂O₃-encapsulated films, as measured by the linear intercept method, were 86 and 134 nm after annealing at 400 and 800 °C, respectively.