A method of fabricating an integrated circuit includes forming a first opening having a first width and a second opening having a second width in a first dielectric layer over a silicon substrate. The openings expose the silicon substrate and the exposed silicon substrate is oxidized to form first and second LOCOS structures having a first thickness. A polysilicon layer is formed over the silicon substrate, so that the polysilicon layer fills the first and second openings. A blanket etch of the polysilicon layer is performed to remove at least a portion of the polysilicon layer over the second LOCOS structure while leaving the first LOCOS structure protected by the polysilicon layer. The silicon substrate under the second LOCOS structure is further oxidized such that the second LOCOS structure has a second thickness greater than the first thickness.