Low dielectric constant CF/SiOF composite films are deposited using tri-ethoxy-fluorosilane (FTES) and O-2 mixture in a helicon plasma reactor without intentional healing or biasing the substrate. Optical emission spectroscopy (OES) is used to study the relation between the relative densities of the radicals and the film properties. The OES data imply that the FTES and O-2 gases are greatly dissociated above the RF power of 900 W. Consequently, the deposition process of helicon plasma CVD where the source gases dissociated highly form CF/SiOF composite film is different from thermal CVD where the gases react chemically on the substrate and make the SiOF film. FTIR and XPS spectra show that the film has Si-F, Si-O, and C-F bonds. The Si-F and C-F bonds may lower the dielectric constant greatly. As the O-2/FTES ratio decreases, the fluorine concentration increases and the dielectric constant decreases. (C) 1999 Elsevier Science S.A. All rights reserved.