Methods for fabricating semiconductor devices are provided. An exemplary method includes forming fins in a dense region and in an isolated region of a semiconductor substrate; performing a plasma dry etch process to remove a portion of at least one selected fin to form a first trench in the dense region and to remove a portion of at least one selected fin in the isolated region to form a second trench in the isolated region, wherein the plasma dry etch process includes: performing a passivation-oriented process and an etchant-oriented process; and controlling the passivation-oriented process and the etchant-oriented process to form the first trench with a desired first critical dimension and first depth and to form the second trench with a desired second critical dimension and second depth.