An integrated circuit structure and a method of forming the same are provided. The method includes providing a surface; performing an ionized oxygen treatment to the surface; forming an initial layer comprising silicon oxide using first process gases comprising a first oxygen-containing gas and tetraethoxysilane (TEOS); and forming a silicate glass over the initial layer. The method may further include forming a buffer layer using second process gases comprising a second oxygen-containing gas and TEOS, wherein the first and the second process gases have different oxygen-to-TEOS ratio.