Thermal stability of fluorinated amorphous carbon (a-C:F) thin films with a low dielectric constant was investigated by electron spin resonance (ESR), infrared (IR) absorption, optical absorption, and x-ray photoelectron spectroscopy (XPS) as well as measurements of film thickness and dielectric constant. IR and XPS measurements suggested that the strength of theCF3andCF2bonding configurations against annealing are weaker than that of the CF bonding configuration. ESR measurements revealed that the dangling bond density decreased by one order of magnitude after annealing at 300 °C and increased after annealing at 400 °C. Furthermore, thegvalue and the linewidth of the ESR spectrum decreased with increasing annealing temperature. Based on these results, the changes in structure and defect configuration are discussed. ©1998 American Institute of Physics.