A method for manufacturing a semiconductor device includes forming two isolation structures in a substrate to define a fin structure between the two isolation structures in the substrate. A dummy gate and spacers are formed bridging the two isolation structures and over the fin structure. The two isolation structures are etched with the dummy gate and the spacers as a mask to form a plurality of slopes under the spacers in the two isolation structures. A gate etch stop layer is formed overlying the plurality of slopes. The dummy gate and the two isolation structures beneath the dummy gate are removed to create a cavity confined by the spacers and the gate etch stop layer. A gate is then formed in the cavity.