FinFET with dummy fins and methods of making the same

A method of fabricating a semiconductor device includes forming a semiconductor fin structure over a substrate, where the semiconductor fin structure includes a plurality of semiconductor fins and defines trenches among the semiconductor fins, and forming a dielectric fin structure having a plurality of dielectric fins. Forming the dielectric fin structure includes filling the trenches with a first dielectric material layer and a second dielectric material layer over the first dielectric material layer, the second dielectric material layer having a composition different from that of the first dielectric material layer, removing a portion of the second dielectric material layer to form a recess, and filling the recess with a third dielectric material layer, the third dielectric material layer having the same composition as the first dielectric material layer.