Probing Cu Diffusion Barrier Layers on Porous Low-Dielectric-Constant Films by Posireonium Annihilation Lifetime Spectroscopy

Two kinds of Cu diffusion barrier layers, sealed films and capped films, on nanoporous low-dielectric constant films are investigated by positronium annihilation lifetime spectroscopy (PALS). We have found that the minimum thickness of Ta to form an effective diffusion barrier is affected by the pore size. The films with large pores require thick barrier layers to form effective diffusion barriers. In addition, a possible ultra-thin diffusion barrier, i.e. a plasma-induced densification layer, has also been investigated. The PALS data confirm that a porous low-dielectric-constant thin film can be shrunk by exposure to plasma. This shrinkage is confined to a surface layer of collapsed pores and forms a dense layer. The dense layer tends to behave as Ps (positronium) diffusion barriers. Indeed, the controlled thin ``skin'' layer could prevent Cu diffusion into the underlying dielectrics.