The method includes steps: supplying a substrate; forming a grating typed isolation in shallow ridges as well as multiple active regions covered by hard mask layer on the substrate; using photomask primary stage considering along X direction only to define pattern needed by the invention on photoresist; using both of the hard mask layer and the isolation in shallow ridges being as a mask to etch out multiple deep trench downwards; then, carrying out subsequent procedure to accomplish preparation of trench capacitance and memory unit.