Preparation method of STI (shallow trench isolation) structure

The invention relates to the technical field of semiconductors and discloses a preparation method of an STI structure. The preparation method comprises the steps as follows: providing a semiconductor substrate covered with a silicon dioxide layer on the surface; forming a first trench in the semiconductor substrate by etching; forming a silicon dioxide film covering the surface of the first trench by oxidizing the bottom and side walls of the first trench; forming a linear silicon nitride layer on the structure surface of the trench; forming a linear silicon dioxide layer on the surface of the linear silicon nitride layer; filling the first trench by coating of polysilazane; curing the first trench at a high temperature; flattening the first trench; forming a second trench by etching and washing; depositing a silicon dioxide filling layer and filling the second trench; and annealing. According to the preparation method, the polysilazane is used for compactly filling and curing deep positions of the first tren