A material layer having recesses is formed on a substrate including a high pattern density area and a low pattern density area. A first dielectric layer and a second dielectric layer are sequentially formed to cover the material layer, wherein a top surface of the first dielectric layer in the high pattern density area is higher than a top surface of the first dielectric layer in the low pattern density area, thereby a thickness of the second dielectric layer in the low pattern density area being thicker than a thickness of the second dielectric layer in the high pattern density area. An etching back process is performed to remove the second dielectric layer and the first dielectric layer, wherein the etching rate of the etching back process to the second dielectric layer is lower than the etching rate of the etching back process to the first dielectric layer.