Semiconductor structure and forming method thereof

The invention provides a semiconductor structure and a forming method thereof, and the method comprises the steps: forming an active layer on a substrate which is provided with a word line preset region and a selection gate preset region which are adjacent to each other; forming a hard mask stack comprising a third hard mask layer on the active layer; patterning the third hard mask layer to form third hard masks, wherein a first interval exists between two adjacent third hard masks, closest to the selection grid electrode preset area, in the word line preset area, and the first interval is smaller than a second interval between any two adjacent third hard masks; forming spacers on the side walls of the third hard masks, wherein two spacers on the opposite side walls of two adjacent third hard masks are combined into a combined spacer; forming a patterned mask structure in the select gate predetermined region; and transferring the spacer and the pattern of the patterned mask structure to the active layer to fo