Abstract: The present invention discloses a method for manufacturing a semiconductor device. According to the method provided by the present disclosure, a dummy gate is formed on a substrate, removing the dummy gate to form an opening having side walls and a bottom gate, a dielectric material is formed on at least a portion of the sidewalls of the opening and the bottom surface of the opening, and a pre-treatment is performed to a portion of the dielectric material layer on the sidewalls of the opening, and thus the properties of the dielectric material is changed, and then the pre-treated dielectric material on the sidewalls of the opening is removed by a selective process. The semiconductor device manufactured by using the method of the present disclosure is capable of effectively reducing parasitic capacitance.