A semiconductor structure includes semiconductor fins protruding out of a substrate, dielectric fins protruding out of the substrate and disposed among the semiconductor fins, and gate stacks disposed over the semiconductor fins and the dielectric fins. The dielectric fins include a first dielectric material layer, a second dielectric material layer disposed over the first dielectric material layer, and a third dielectric material layer disposed over the second dielectric material layer, where the first and second dielectric material layers have different compositions and the first and the third dielectric material layers have the same compositions.