A method is provided for fabricating a semiconductor device. The method includes providing a substrate having a device region and a peripheral region; and forming device structures on the substrate in the device region so as to form trenches between adjacent device structures. The method also includes forming a stop layer on the substrate and the device structures; and forming a first dielectric layer on the stop layer such that a portion of the densified first dielectric layer fills the trenches and a top surface of a portion of the first dielectric layer in the peripheral region is lower than a surface of the stop layer on the device structures by a densify high aspect ratio process. Further, the method includes forming a second dielectric layer on the densified first dielectric layer; and performing a plurality of polishing processes until the top surface of the device structures is exposed.