Semiconductor structures and fabrication methods thereof are provided. An exemplary fabrication process includes providing a base substrate; forming a carbon-containing dielectric layer over the base substrate; and performing a chemical mechanical polishing (CMP) process on the carbon-containing dielectric layer. The chemical mechanical polishing process includes performing a plurality of polishing processes on the carbon-containing dielectric layer and a weak acid solution is used to clean a polishing pad before and after each of the polishing processes.