A semiconductor structure includes a substrate, an imaging pixel array disposed on a first region of the substrate, a first isolation disposed in the first region, a periphery circuitry disposed on a second region of the substrate, and a second isolation disposed in the second region. The imaging pixel array has a plurality of imaging pixels configured to capture image data. The periphery circuitry has a transistor configured to receive and process the image data. The first isolation has a first depth and a first protrusion projected from a surface of the substrate. The second isolation has a second depth and a second protrusion projected from the surface of the substrate. The first protrusion has a substantially same height as the second protrusion. The first depth is different from the second depth.