A semiconductor structure and its manufacturing method are provided. The method includes sequentially forming an insulating layer and a patterned mask layer on a substrate. The patterned cover curtain layer has an opening, and the opening includes a main body portion and two extension portions located at both ends of the main body portion. The method includes sequentially forming a first sacrificial layer, a second sacrificial layer, and a third sacrificial layer on the insulating layer. The first sacrificial layer fills the extension portions and defines a recess in the main body portion. The second sacrificial layer is formed in the recess defined by the first sacrificial layer. The third sacrificial layer is formed on the first sacrificial layer located in the extension portions.