Highly crosslinked a-F:C films can undergo a significant change after thermal annealing, where the film expands by ∼3%, the density reduces by ∼10% and the internal stress changes from compressive to tensile. The loss of fluorine concentration and the reduction of CF. are accompanied by the transition of (C-C, sp
3) to (C=C, sp2) groups. After annealing, the dielectric constant is reduced and the leakage current increases slightly. Most importantly, these changes occur only at the initial stage of annealing. After the initial annealing, the a-F:C film tends to be thermally stable and retains reasonably good electrical properties as a low-k interlayer dielectric. The profound impact of these results on Cu/a-F:C integration will be briefly discussed.