A semiconductor structure comprising a first layer, a metal layer and a second layer is disclosed. The first layer comprises a recessed surface. The metal layer is above a portion of the recessed surface. The second layer is above the metal layer and confined by the recessed surface. The second layer comprises a top surface, a first lateral side and a second lateral side. The etch rate of an etchant with respect to the metal layer is greater than the etch rate of the etchant with respect to the second layer. The thickness of the second layer in the middle of the second layer is less than the thickness of the second layer at the first lateral side or the second lateral side. A method of forming a semiconductor structure is disclosed.