Disclosed is a method of manufacturing a semiconductor device. The method includes the steps of forming gates on a substrate, forming junction areas on a surface of the substrate, forming a first BPSG layer on a resultant structure of the substrate, performing a first CVD process for the first BPSG layer, forming a second BPSG layer on the first BPSG layer, forming a landing plug contact, depositing a polysilicon layer on a resultant structure of the substrate, and performing a second CMP process for the polysilicon layer, the second BPSG layer and the nitride hard mask. The CMP processes are carried by using acid slurry having a high polishing selectivity with respect to the nitride layer, so a step difference between the cell region and the peripheral region is removed, thereby simplifying the semiconductor manufacturing process and removing a dishing phenomenon.