The a-C∶Fx films with low dielectric constant has been studied,considering that they can be used for interlayer dielectric in ultra-large integrated circuits.These films were deposited by using electron cyclotron resonance plasma reactor with CHF3 and C6H6 mixture as source gas.The effects of microwave power,pressure and CHF3/C6H6 ratios on the film deposition rates have been investigated.The fluorocarbon and hydrocarbon radical species in the plasma discharges were monitored by the optical emission spectra.The structural properties of the films were characterized by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.A relationship between the radical species in the discharge plasma and bonding structure of the films is analysed.It demonstrates that CF2,CF and CH radicals play the important roles in the forming of films.