Method for fabricating semiconductor device

A method for fabricating a semiconductor device, the method comprising:

• forming an insulating layer over a substrate, the substrate including a cell region and a peripheral region;
• forming an opening by selectively etching the insulating layer over the substrate of the cell region;
• forming a plug conductive layer to fill the opening and cover the insulating layer;
• etching the plug conductive layer and the insulating layer over the substrate of the peripheral region by using a peri-open mask covering the cell region;
• trimming the peri-open mask to expose the plug conductive layer over the substrate of a boundary region where the cell region and the peripheral region contact each other;
• etching the plug conductive layer over the substrate of the boundary region by using the trimmed peri-open mask;
• forming a peri-gate conductive layer over an entire surface of the substrate;
• and
• etching the peri-gate conductive layer by using a cell open mask.