Semiconductor devices are provided which have MIM (metal-insulator-metal) capacitor structures that are integrated within air gaps of on-chip interconnect structures, as well as methods for integrating MIM capacitor formation as part of an air gap process flow for fabricating on-chip interconnect structures. For example, a semiconductor device includes a dielectric layer with a first pattern of metal lines and second pattern of metal lines. Air gaps are disposed in spaces between the metal lines. Portions of the spaces between the metal lines of the first pattern of metal lines include a conformal layer of insulating material disposed on sidewalls of the metal lines and metallic material that fills the spaces between the metal lines. The first pattern of metal lines comprises a first capacitor electrode, the metallic fill material comprises a second capacitor electrode, and the conformal layer of insulating material comprises an insulating layer of a MIM capacitor structure.