The invention relates to integrated circuit fabrication, and more particularly to an electronic device with an isolation structure made having almost no void. An exemplary method for fabricating an isolation structure, comprising: providing a substrate; forming a trench in the substrate; partially filling the trench with a first silicon oxide; exposing a surface of the first silicon oxide to a vapor mixture comprising NH3 and a fluorine-containing compound; heating the substrate to a temperature between 100° C. to 200° C.; and filling the trench with a second silicon oxide, whereby the isolation structure made has almost no void.