A pad oxide layer and a silicon nitride (SiN) layer are sequentially formed on a silicon substrate. An etching process is then performed to form a trench in the silicon substrate. A sub-atmospheric chemical vapor deposition (SACVD) process is performed to selectively form a first dielectric layer on exposed portions of the silicon substrate within the trench to fill portions of the trench thereafter. Finally, a high density plasma chemical vapor deposition (HDPCVD) process is performed to form a second dielectric layer to fill the remaining space of the trench and cover the silicon substrate.