An embodiment of the disclosure includes a method of forming a shallow trench isolation structure. A substrate is provided. The substrate includes a top surface. A trench is formed extending from the top surface into the substrate. The trench has sidewalls and a bottom surface. A liner oxide layer is formed on the sidewalls and the bottom surface. The liner oxide layer is treated in a plasma environment comprises at least one of NF3, F2, and BF2. The trench is filled with a dielectric layer.