Method of manufacturing semiconductor device

Reliability of a semiconductor device is improved. A field plate electrode is formed on an insulating film inside a trench. Next, by an isotropic etching process to the insulating film, the insulating film is thinned, and an upper portion of the field plate electrode is exposed from the insulating film. Next, an isotropic etching process (chemical dry etching process) is performed to the field plate electrode exposed from the insulating film. In this manner, a corner of the upper portion of the field plate electrode is chamfered or rounded, and therefore, a concentration of electric field at the upper portion of the field plate electrode can be moderated.