Semiconductor structures are provided. The semiconductor structure includes a substrate and a metal gate structure formed over the substrate. The semiconductor structure further includes a sealing layer comprising an inner sidewall and an outermost sidewall. In addition, the inner sidewall is in direct contact with the metal gate structure and the outermost sidewall is away from the metal gate structure. The semiconductor structure further includes a mask structure formed over the metal gate structure. In addition, the mask structure has a straight sidewall over the metal gate structure and a sloped sidewall extending from the inner sidewall of the sealing layer and passing over the outmost sidewall of the sealing layer.