The electrical properties of a novel, cross-linked polycarbosilane, which is thermally stable up to 450 degrees C, were investigated as a potential interlayer dielectric for advanced interconnect technology. This polycarbosilane is derived from a cyclolinear precursor polymer, which was spin-coated from xylene solution and cured at 300 degrees C to induce cross-linking. The resultant films were found to have a dielectric constant of 2.32 (nonporous) and to exhibit excellent resistance to Cu diffusion under standard bias temperature stress (BTS) test conditions. This resistance to Cu diffusion is attributed to the fact that this polymer does not contain oxygen and that Cu ionization through metal oxidation at the metal-polymer interface can thus be prevented. It was further demonstrated that an ultrathin layer of polymer with a nominal thickness of 20 nm is sufficient to block the Cu ion penetration into an ultralow-k porous methylsilsesquioxane film when subjected to BTS. This polycarbosilane holds considerable promise for application as an interlayer dielectric in the next generations of high-performance integrated circuits.