This work investigates the modification in structure, composition, and electrical properties of a porous methyl-silsesquioxane (P-MSQ) ultralow-k dielectric film when exposed to different gas plasmas that are typically used in a copper dual damascene process. Wafers with spin-coated P-MSQ films with dielectric constant (k approximate to 2.2) were treated with different plasma chemistries: O-2, H-2/N-2, C4F8, and H-2/He. The changes in film properties such as surface roughness, hydrophobicity, dielectric constant, chemical bonding, and elemental composition after the plasma treatments were evaluated. It was found that both leakage current density and breakdown voltages deteriorated after the plasma treatment. The worst case was that of a combination of fluorocarbon etching and H-2/He stripping plasmas wherein the leakage current density increased by five orders of magnitude and the breakdown field decreased by about 20%. The leakage current was partially reduced by adding an ultrathin dielectric capping layer to the plasma-treated film. The Ta barrier film microstructure deposited on the plasma-treated P-MSQ correlated with the surface roughness of the film. The Ta films deposited on fluorocarbon plasma-treated samples had the largest surface roughness and relatively less crystalline microstructure compared to Ta film deposited on nonfluorocarbon-based, plasma-treated samples. (c) 2006 The Electrochemical Society. [DOI: 10.1149/ 1.2180707] All rights reserved.