A structure having air gaps between interconnects is disclosed. A first insulating material is deposited over a workpiece, and a second insulating material having a sacrificial portion is deposited over the first insulating material. Conductive lines are formed in the first and second insulating layers. The second insulating material is treated to remove the sacrificial portion, and at least a portion of the first insulating material is removed, forming air gaps between the conductive lines. The second insulating material is impermeable as deposited and permeable after treating it to remove the sacrificial portion. A first region of the workpiece may be masked during the treatment, so that the second insulating material becomes permeable in a second region of the workpiece yet remains impermeable in the first region, thus allowing the formation of the air gaps in the second region, but not the first region.