A method includes forming a fin protruding over a substrate; forming a conformal oxide layer over an upper surface and along sidewalls of the fin; performing an anisotropic oxide deposition or an anisotropic plasma treatment to form a non-conformal oxide layer over the upper surface and along the sidewalls of the fin; and forming a gate electrode over the fin, the conformal oxide layer and the non-conformal oxide layer being between the fin and the gate electrode.