Semiconductor device including a planarized surface and method thereof

A method of planarizing the surface of a semiconductor substrate to reduce the occurrence of a dishing phenomenon. A patterned etch stop layer defining a trench region is formed on a substrate. The substrate is etched to form a trench region, and a medium material layer and an oxide layer are subsequently formed on the substrate, filling the trench region. Chemical mechanical polishing (CMP) is performed on the oxide layer until the medium material layer is exposed. CMP is then performed until the patterned etch stop layer is exposed and a planarized oxide layer is formed. Because the medium material layer has a higher removal rate during CMP than the oxide layer, occurrences of the dishing phenomenon are reduced. A slurry including an anionic surfactant is used to increase the CMP removal ratio of the medium material layer to the oxide layer.