Patent attributes
A method includes providing a structure having transistors, an isolation structure over the transistors, metal plugs through the isolation structure and connecting to the transistors, and a trench with the isolation structure and the metal plugs as sidewalls. The method further includes forming a dielectric liner on the sidewalls of the trench and over the isolation structure and the metal plugs. The dielectric liner is thicker at an opening portion of the trench than at another portion of the trench so that an air gap is formed inside the trench and the air gap is surrounded by the dielectric liner. The method further includes depositing a sacrificial layer over the dielectric liner and over the air gap and performing CMP to remove the sacrificial layer and to recess the dielectric liner until the isolation structure and the metal plugs are exposed. The air gap remains inside the trench.