Method for preparing ONO medium layer

The invention relates to the field of semiconductor preparation, in particular to a method for preparing an ONO medium layer. The method is suitable for a nonvolatile NOR flash memorizer. The method includes the following steps of firstly, providing a silicon wafer, wherein a surface tunneling oxide layer and a floating gate layer formed at a preset position are sequentially arranged on a substrate of the silicon wafer; secondly, inputting first preset reaction gas through an in-situ vapor generation process, and forming bottom layer silicon oxide on the upper face of the floating gate layer; thirdly, inputting second preset reaction gas through a porous quartz tube, and forming silicon nitride on the upper side of the bottom layer silicon oxide through deposition; fourthly, inputting the first preset reaction gas through the in-situ vapor generation process, and oxidizing the surface of the silicon nitride to form top layer silicon oxide. The method has the advantages that the thickness uniformity of a thin film depositing on the surface of the silicon wafer is effectively improved, shells produced by the silicon wafer is remarkably reduced, N type dopes are not prone to being formed on contact faces of the silicon oxide and the silicon wafer, realizability is high, and the method can be widely applied to various deposition processes.