Manufacturing method of semiconductor device

The invention provides a manufacturing method of a semiconductor device, which comprises the following steps: forming a plurality of grooves in a substrate, forming gates in the grooves, forming gate dielectric layers between the grooves and the gates, and extending the gate dielectric layers to cover the substrate at two sides of the grooves; performing an anisotropic first etching process to remove a part of thickness of the gate dielectric layer on the substrate; and performing an isotropic second etching process, and removing the residual gate dielectric layer on the substrate. Through the anisotropic first etching process and the isotropic second etching process, the thickness uniformity of the gate dielectric layer is maintained when the gate dielectric layer is thinned, holes at the top of the gate dielectric layer on the side wall of the groove are reduced or avoided, the threshold voltage distribution and the reverse voltage resistance of the semiconductor device are optimized, and the reliability of