The application provides a
semiconductor device manufacturing method and a
semiconductor device. The method comprises the following steps: providing a
semiconductor substrate, a first trench is formed in the semiconductor substrate, a
field oxide layer is formed on the side wall and the bottom wall of the first trench, so that a space outside the surface of the
field oxide layer in the first trench forms a second trench, and the first conductive material is filled in the second trench. The
field oxide layer of the side wall of the first trench is removed to a first set depth to form a third trench. A
barrier layer is formed on the bottom wall of the third trench, a
gate oxide layer is formed on the side wall of the third trench, at least two separated gates are formed in the space outside the surface of the
gate oxide layer in the third trench, the
barrier layer is removed, and the gap between the separated gates is filled. Then, the space between the bottom of the separated gate and the upper surface of the field
oxide layer forms an
air layer. Due to the introduction of the
air layer, the gate leakage
capacitance and the gate-source
capacitance are reduced, the switching loss and the
conduction loss are reduced, and the opening rate of the device is improved.