A method for forming a semiconductor structure includes following steps. A substrate is provided, and a semiconductor layer is formed on the substrate. Next, a SiN-rich pre-oxide layer is formed on the semiconductor layer. After forming the SiN-rich pre-oxide layer, an anneal treatment is performed to partially transfer the SiN-rich pre-oxide layer to form a SiN layer and a SiO layer. And the SiO layer is formed the on the SiN layer. Subsequently, a planarization process is performed to remove a portion of the SiO layer to expose the SiN layer.