Shallow trench isolation etch process

A method for fabricating a plurality of shallow trench isolation (STI) structures, comprising:

• providing a substrate having a patterned mask layer disposed thereon to define a plurality of STI structures grouped into a first region having a first density of one or more first STI structures and a second region having a second density of one or more second STI structures, wherein the second density is greater than the first density;
• etching the substrate using a plasma formed from a process gas mixture to form the plurality of STI structures on the substrate, wherein the process gas mixture comprises a fluorine-containing gas and either a fluorocarbon-containing gas or a hydrofluorocarbon-containing gas;
• and
• controlling a ratio of the etch rate of the first STI structures in the first region to the etch rate of the second STI structures in the second region by adjusting a gas flow rate ratio of the fluorine-containing gas to the fluorocarbon-containing gas or the hydrofluorocarbon-containing gas.