Method of manufacturing trench type semiconductor device

A method of manufacturing a trench type semiconductor device, comprising:

• forming an epitaxial layer on a substrate;
• forming a trench in the epitaxial layer;
• forming a gate structure in the trench, wherein the gate structure comprises an upper gate, a lower gate and an intermediate insulating portion, and the intermediate insulating portion is located in the upper gate;
• depositing a first oxide layer in the trench;
• depositing a first polysilicon layer on the first oxide layer and in the trench;
• etching back the first polysilicon layer until lower than an upper surface of the first oxide layer;
• oxidizing a surface of the first polysilicon layer to form a second oxide layer, and enclosing the lower gate by the second oxide layer and the first oxide layer;
• depositing a silicon nitride layer in the trench;
• etching back the silicon nitride layer to form the intermediate insulating portion;
• etching back the first oxide layer to remove a portion of the first oxide layer to form a first dielectric layer and expose a partial surface of the epitaxial layer, and the first dielectric layer and the second oxide layer enclose the lower gate;
• oxidizing the partial surface of the epitaxial layer to form a gate oxide layer;
• depositing a second polysilicon layer to fill up the trench;
• and
• etching back the second polysilicon layer to expose the intermediate insulating portion.