Semiconductor device and method of manufacturing the device

A method of manufacturing a semiconductor device comprising the steps of:

• (a) preparing a semiconductor substrate having a surface and on said surface, forming a mask material with a first opening having a first width and a second opening having a second width greater than said first width;
• (b) performing etching using said mask material as a mask to form a first trench having a first depth in an exposed portion of said semiconductor substrate at said first opening and a second trench having said first depth in an exposed portion of said semiconductor substrate at said second opening;
• (c) depositing a first insulating film isotropically over the entire surface of said semiconductor substrate of said step (b) to completely fill up said first trench and to leave a space in a central portion of said second trench;
• (d) etching back said first insulating film to expose said semiconductor substrate at said central portion of said second trench and to leave part of said first insulating film as sidewalls of said second trench;
• (e) etching said semiconductor substrate using said sidewalls as a mask to form a third trench having a third width smaller than said second width and a second depth greater than said first depth in said second trench;
• (f) depositing a second insulating film that is superior in step-covering properties to said first insulating film over the entire surface of said semiconductor substrate of said step (e) to fill said third trench;
• and
• (g) performing planalization processing on said semiconductor substrate of said step (f) to remove said second insulating film which is deposited farther from said semiconductor substrate than said mask material thereby to leave said first and second insulating films only in said first through third trenches.