Semiconductor structure and forming method thereof

A semiconductor structure and a forming method thereof are provided. In one form, a forming method includes: providing a base, including a device region and a zero mark region; forming a zero mark trench inside the base in the zero mark region; filling the zero mark trench, to form a dielectric layer; forming a fin mask material layer covering the base and the dielectric layer; forming a mandrel layer on the fin mask material layer above the dielectric layer and the base in the device region, where the mandrel layer covers a top portion of the dielectric layer; forming a mask spacer on a side wall of the mandrel layer; removing the mandrel layer; etching the fin mask material layer by using the mask spacer as a mask after the mandrel layer is removed, to form a fin mask layer; and etching a partial thickness of the base using the fin mask layer as a mask, where the remaining base after etching is used as a substrate, and a protrusion located over the substrate in the device region is used as a fin, and etching a partial thickness of the dielectric layer during the etching of the base. In the present disclosure, after a fin is formed by filling a zero mark trench with a dielectric layer, a probability that a residue defect or a peeling defect occurs is relatively low.