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Porous interlayer dielectric films with interconnected pores pose a serious challenge for their integration into next-generation microchips. The opening of interconnected pores in the surface region needs to be sealed to prevent intrusion of atomic layer deposition precursors used to create metal diffusion barriers. In this paper, we report the formation of a thin, nonporous surface layer on a porous methyl-silsesquioxane-based dielectric film by NH3plasma treatment. Depth-profiled beam positronium annihilation lifetime spectroscopy was applied to conveniently examine the formation of the dense layer. A nonporous surface layer was readily identified by the curtailment of positronium escape into vacuum through the surface. Among plasma treatments at temperatures ranging from 25 to 300°Cfor duration of 3–600s, the best result was achieved at 300°Cfor 10s. A very thin skin layer, ∼10nm, could be formed with little damage to the bulk of the low-κ film. This thin skin layer further proved to improve the performance of Ta barriers for Cu diffusion. Chemical analysis, infrared spectroscopy, and sputtering secondary ion mass spectroscopy were also performed to examine how the plasma treatment altered the dielectric film.