A method for forming a semiconductor structure includes following steps. A substrate structure is provided. The substrate structure includes a semiconductor substrate, a first oxide-nitride-oxide (ONO) layer, and a second ONO layer. The semiconductor substrate has first and second surfaces opposite to each other. The first ONO layer includes a first oxide layer, a first nitride layer and a second oxide layer formed on the first surface in sequence. The second ONO layer includes a third oxide layer, a second nitride layer and a fourth oxide layer formed on the second surface in sequence. A nitride mask layer is formed on the first ONO layer. The fourth oxide layer is removed. The second nitride layer and the nitride mask layer are removed. The second oxide layer and the third oxide layer are removed. A fifth oxide layer is formed on the first nitride layer.