Shallow trench isolation structures are simultaneously fabricated such that ones in a cell region have first-type features and others in a periphery region have second-type features. The first-type features can be rounded edges or can be first depths and widths, and the second-type features can be unrounded edges or can be second depths and widths which are different from the first depths and widths. The method includes forming patterned photoresist over a hard mask over portions of a cell and a periphery region, and removing the exposed hard mask layer in the periphery region while removing a portion of the exposed hard mask layer in the cell region. A trench is then partially formed in the periphery region and more of the hard mask layer is removed in the cell region, followed by the trench in the periphery region being deepened while a trench in the cell region is formed.