Method of manufacturing isolation structure for semiconductor device

A method for manufacturing a semiconductor device, comprising:

• forming a first trench to define a first pair of active regions in a substrate;
• forming a second trench to define a second pair of active regions in the substrate, wherein the first and the second trenches communicate with each other, wherein the second trench is formed wider than the first trench;
• forming a silicon layer over an inner surface of the first trench and over an inner surface of the second trench;
• forming a first oxide layer over the silicon layer;
• performing an oxidation process of the silicon layer and the first oxide layer to form a second oxide layer, wherein the second oxide layer substantially completely fills the first trench;
• forming a nitride layer over the second oxide layer so that the nitride layer fills the second trench;
• planarizing the nitride layer and the second oxide layer to form an isolation structure in the first and the second trenches;
• forming a gate trench that extends across the isolation structure and the active regions;
• and
• forming a gate dielectric layer over an inner surface of the gate trench.