Hydrogen silsesquioxane, a material with low dielectric constant, can successfully suppress Cu diffusion without using a barrier metal by implementing a NH3 plasma treatment. Lower leakage current and better barrier capability can be achieved by hydrogen silsesquioxane film after NH3 plasma treatment. Having been treated with different plasma exposure times, this film can still maintain its original dielectric constant with few changes. The decrease in leakage current with increasing exposure time can be attributed to the following mechanisms: dielectric film becomes denser, dangling bonds are passivated, nitride film is formed on the hydrogen silsesquioxane, and the bulk damage of hydrogen silsesquioxane is annealed out. A thin layer of nitride formed on the dielectric is the cause for having better capability. The thickness of the nitride layer on hydrogen silsesquioxane is about 35 nm, and it can prevent the Cu diffusion/migration into the underlying dielectric. The role of our nitride film is to act as a passive diffusion barrier.