Ever growing device integration now requires low dielectric constant materials for inter metal dielectrics to improve speed performance of ULSIs. SiO:F (fluorinated silicon oxide) and a-C:F (amorphous fluoro carbon) films produced by PECVD are the major candidates for this purpose. In case of SiO:F film deposition from SiH@sub 4@/N@sub 2@O/CF@sub 4@, we found that fluorine addition to SiO@sub 2@ reduces Si-OH bonds which are the major dielectric component in SiO@sub 2@ films by PECVD. Fluorine addition also reduces growth rate and improved step coverage. We have examined the gas flow rate dependency of the growth rate by keeping the other process parameters constant. The growth rate of SiO:F film increased and the step coverage profile became poor by increasing gas flow rate. We have extracted suitable reaction kinetics to explain these phenomena. The reaction mechanism, which contains two major species in film deposition, obtained from this experimental approach well explained the step coverage behavior. In case of a-C:F film deposition from C@sub 2@F@sub 4@, the film growth rate decreased by increasing the gas flow rate. This is due to the gas phase reactions that produce main film precursors, like as CF@sub 3@@sup +@ ion and CF@sub 2@ radical. We also evaluated the contribution of ionic species and neutral radical species using step coverage into micron scale features. We found that radical species contributes to have uniform step coverage, but most of the film forming species are ionic ones and they contribute to make thermally stable films.