a low - k dielectric sacrificial material is formed within a microelectronic structure covered with a layer defining an exhaust vent . at an appropriate time , the underlying sacrificial material is decomposed and exhausted away through the exhaust vent . residue from the exhausted sacrificial material accumulates at the vent location during exhaustion until the vent is substantially occluded . as a result , an air gap is created having desirable characteristics as a dielectric .