Abstract: A method for preparing a deep trench first forms a trench in a semiconductor substrate and a stacked structure in the trench, wherein the stacked structure includes at least one nitrogen-containing layer. A phosphorous oxide layer is then formed on the surface of the nitrogen-containing layer. The phosphorous oxide is then transformed into an etchant in a steam atmosphere to remove the nitrogen-containing layer in the trench. The phosphorous oxide layer in the trench is then removed, and the nitrogen-containing layer can be effectively removed. The method further comprises forming a plurality of crystallites on a portion of the nitrogen-containing layer before the phosphorous oxide layer is formed on the surface of the nitrogen-containing layer, which allows the formation of a deep trench with a rough inner sidewall.