The refractive index (at 632.8 nm, He-Ne laser light) of fluorocarbon films on silicon, deposited either inadvertently in the course of SiO2 contact hole reactive ion etching with CF4Xvol.%H2 (X≤40) or deliberately in a CF450vol.%H2 glow discharge, was determined by ellipsometry. The best fit of the data gives a complex refractive index of 1.48-i0.003. The calculated refractive index was used to measure film thicknesses of fluorocarbon films deposited under a great variety of experimental conditions. Complementary results were provided by helium ion backscattering measurements. The results obtained by the two methods compared favorably. Ellipsometry therefore represents a probe which can be used to determine rapidly the fluorocarbon contamination state of a silicon wafer after reactive ion etching in CF4-H2. The extension of the present method to characterize films deposited by reactive ion etching processes utilizing other etching gas combinations is discussed. © 1986.