In situ multiple internal reflection Fourier transform infrared spectroscopy and spectroscopic ellipsometry are used to study the surfaces of Si and SiO2 films during etching with CF4/H2 plasmas. At sufficiently low H2 concentration, a thin fluorocarbon film forms on both Si and SiO2 surfaces during etching, but Si and SiO2 removal continues despite the existence of such a layer. The structure of this film depends on the H2 concentration in the feed gas. Above a critical H2 concentration, the fluorocarbon film becomes more crosslinked, fluorine deficient, and amorphous carbonlike. Formation and subsequent growth of this fluorinated amorphous carbon (a-C:F) film stops etching of both Si and SiO2. In the absence of energetic ion bombardment, the critical concentration at which etching is arrested and a-C:F growth begins is the same for both Si and SiO2 films indicating that whether this film forms or not is determined by the fluxes of reactive species arriving at the surface from the gas phase rather than by the nature of the surface. In particular, H abstraction of F from the fluorocarbon film is shown to be responsible for the formation of fluorine deficient amorphous carbonlike film. In the presence of energetic ion bombardment this critical H2 concentration is increased but at different amounts for Si and SiO2 films. The film formed on SiO2 is more easily sputtered than that which forms on Si due to the higher number of C–O bonds in the film formed on SiO2. The difference in the structure of the inhibiting layers formed on Si versus SiO2 enables the selective etching of SiO2 over Si.

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