400 angstroms as shown in fig5 a photoresist layer 42 is formed on the semiconductor substrate 10 to fill the sti trenches 40 the photoresist layer 42 is then dry cured or hardened then a dry etching process is performed to etch back the photoresist layer 42 to expose the silicon oxide layer 34 after the dry etching the top surface of the photoresist layer 42 is lower than the silicon oxide layer 34 and is inside the sti trenches 40 to form a recessed area 44 as shown in fig6 an etching process such as a wet etching process is performed to remove the silicon oxide layer 34 and the trench top oxide layer 18 after removing the silicon oxide layer 34 the polysilicon layer 32 is exposed as shown in fig7 a wet etching process involving the use of hot phosphoric acid solution is performed to laterally etch the pad silicon nitride layer 14 that is not covered by the photoresist layer 42 according to the preferred embodiment of the present invention the lateral etching distance of the pad silicon nitride layer 14 is about 550 angstroms and the pad silicon nitride layer 14 of about 450 angstroms remains the remnant pad silicon nitride layer.