Dielectric fluoropolymer films of 10-50 nm in thickness were deposited on the (100)-oriented single-crystal silicon and gallium arsenide wafers via RF plasma sputtering of a poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) target. X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, water contact angle measurement, and Fourier transform infrared spectroscopy results indicated that the chemical composition and molecular structure of the sputter-deposited polymer films depended strongly on the type of the sputtering gas used. The dielectric constant of the argon-plasma sputter-deposited fluoropolymer film is about 2.1, which is comparable to those of the pristine FEP and poly(tetrafluoroethylene) films. It was shown that the argon plasma sputter-deposited FEP (s-FEP(Ar)) film as thin as 6 nm could effectively passivate the HCl-etched GaAs(100) substrate under atmospheric conditions. The growth of the surface oxide layer was effectively hindered by the ultrathin s-FEP(Ar) barrier when the passivated GaAs(100) surface was exposed to the ambient air for a prolonged period of time. Peel adhesion test results suggested that all of the sputter-deposited films adhered strongly to the Si(100) and GaAs(100) surfaces.