A method of forming a semiconductor device includes a liner is conformally stacked on a semiconductor substrate before coating an SOG layer thereon, and then curing the SOG layer, preferably in an ambient of oxygen radicals formed at a temperature of 1000° C. or higher when oxygen and hydrogen are supplied. The oxygen radicals are preferably formed by irradiating ultraviolet rays to ozone or forming oxygen plasma. The SOG layer is preferably made of a polysilazane-based material that may promote a conversion of the SOG layer into a silicon oxide layer.