Silicon oxycarbide (SiOC) films have been investigated as nitrogen (or amine)-free dielectric antireflection layer for a dual damascene structure at 193 nm. Films were deposited by plasma enhanced chemical vapor deposition using trimethylsilane and oxygen. As the oxygen flow rate increased, both the refractive index and extinction coefficient decreased. It was found that reflectance was less than 10% for optimum SiOC, which is comparable to silicon oxynitride. Oxygen plasma treatment converted the top surface of SiOC to silicon oxide. The thickness of the oxide layer increased with the treatment time, however it saturated around 10 nm with treatment times above 90 s. The oxide layer was found to act as a passivation layer stabilizing the SiOC film. It showed reflectance of less than 5% and was successfully utilized as the antireflection layer for a dual damascene structure with no photoresist poisoning.