Abstract: A method for forming a self-aligned trench is disclosed. The steps of the method include providing a semiconductor substrate with a buried layer. A buffer layer and a first hard mask layer are formed sequentially on a surface of the semiconductor substrate. Parts of the first hard mask layer, the buffer layer and the semiconductor substrate are removed to form openings. A capacitor is formed in the interior of the opening of the semiconductor substrate. A second hard mask layer is formed conformally on the first hard mask layer and the capacitor. An insulator layer and a pattern photoresist layer are formed sequentially on the second hard mask layer.