Semiconductor memory device and method of manufacturing the same

A method of manufacturing a semiconductor memory device, the method comprising:

• forming a tunnel insulating layer and a first conductive layer configured as a floating gate in an active region of a semiconductor substrate, and forming a trench in an isolation region of the semiconductor substrate;
• filling the trench with a sacrificial layer having a top surface higher than a surface of the semiconductor substrate;
• forming a first oxide layer along an entire surface of a resultant structure filled with the sacrificial layer;
• forming an air gap in the isolation region by removing the sacrificial layer while maintaining the first oxide layer;
• transforming a portion of the first oxide layer into a first nitride layer;
• forming a second nitride layer and a second oxide layer over the first oxide layer;
• and
• forming a second conductive layer configured as a control gate on the second oxide layer.