Abstract: Low-dielectric-constant（low-k） fluorinated amorphous carbon（a-C:F） films were prepared by electron cyclotron resonance chemical vapor deposition（ECR-CVD） method with varying the gas flow ratio R（RCH₄/+C₄F₈）.Chemical compositions and optical properties were investigated by X-ray photoelectron spectroscopy（XPS） and spectral ellipsometry（SE）,respectively.a-C:F films with dielectric constant of 2.1~2.4 and thermal stability higher than 350℃ were obtained.The CF,CF₂ and CF₃ bonds decrease and the C-C bonds increase with increasing gas flow ratio,which leads to enhancement of π-π^* absorption and reduction of optical gap.Film thickness change is less than 4% due to stress release between the a-C:F films and the silicon substrates after 350℃ annealing.The film thickness change is around 30% mainly due to thermal decomposition of volatile fluorocarbon species after 450℃ heat treatment.