A semiconductor structure and a method of forming the same are provided. According to an aspect of the disclosure, a semiconductor structure includes a first layer having a bottom portion and a sidewall connected to the bottom portion, a metal layer disposed above the bottom portion of the first layer, and a second layer disposed above the metal layer and laterally surrounded by the sidewall of the first layer. The metal layer includes a periphery and a middle portion surrounded by the periphery, the middle portion being thicker than the periphery, and a first etch rate of an etchant with respect to the metal layer is uniform throughout the metal layer and is greater than a second etch rate of the etchant with respect to the second layer.