Polishing and the polishing mechanism are studied for low dielectric constant SiOC layer deposited by plasma-enhanced chemical vapor deposition (PECVD). Hardness changed from 4.6 to 13.4 GPa with varying carbon content from 0.35 to 0.50. Good barrier performance was attained for the copper diffusion in layers with carbon contents above 0.35 when annealed at 400 degreesC. Removal rate in mechanical polishing decreased, inversely proportional to the hardness as given theoretically. However, removal rate was weakly dependent an the hardness in the chemical polishing by the MnO2 slurry. Higher removal rate is fulfilled by this chemical polishing.