The disclosure relates to integrated circuit fabrication, and more particularly to an electronic device with an isolation structure having almost no divot. An exemplary method for fabricating an isolation structure, comprising: forming a pad oxide layer over a top surface of a substrate; forming an opening in the pad oxide layer, exposing a portion of the substrate; etching the exposed portion of the substrate, forming a trench in the substrate; filling the trench with an insulator; exposing a surface of the pad oxide layer and a surface of the insulator to a vapor mixture including at least an NH3 and a fluorine-containing compound; and heating the substrate at a temperature between 100° C. to 200° C.