Semiconductor device and preparation method thereof

The invention provides a preparation method of a semiconductor device, which comprises the following steps: firstly, forming a first groove with a trapezoidal structure in an epitaxial layer, forming a hard mask layer above the epitaxial layer, exposing the bottom surface of the first groove by the hard mask layer, and finally etching the epitaxial layer by the hard mask layer, and forming a second groove extending from the side wall of the first groove to the interior of the epitaxial layer. The length of the bottom edge of the first groove is smaller than that of the top edge of the first groove, so that filling of polycrystalline silicon is facilitated, defects such as gaps or holes in the polycrystalline silicon are avoided, and the quality and performance of the device are improved; besides, in the process of etching to form the second groove, the hard mask layer can protect the first groove, so that the first groove is prevented from being damaged, and the reliability of the device is further improved;