Method of fabricating an air gap

A method of fabricating an air gap, comprising:

• providing a substrate, wherein a semiconductor device is disposed on the substrate, a stress layer contacts the semiconductor device, an inter-layer dielectric layer is disposed on and contacts the stress layer, an etching stop layer is disposed on and contacts the inter-layer dielectric layer, an inter-metal dielectric layer is disposed on and contacts the etching stop layer, a first metal interconnection and a second metal interconnection are respectively disposed within the inter-metal dielectric layer at two sides of the semiconductor device;
• performing a first etching to etch the inter-metal dielectric layer and the etching stop layer between the first metal interconnection and the second metal interconnection and directly on the semiconductor device to form a first trench;
• performing a second etching to form a second trench by etching the inter-layer dielectric layer without etching the stress layer and without widening the first trench, wherein the first trench and the second trench form a trench within the inter-metal dielectric layer, the etching stop layer and the inter-layer dielectric layer, and wherein a widest width of the second trench is not greater than a widest width of the first trench;
• and
• forming an insulating layer to seal the trench to form the air gap.