Effect of H₂ addition on surface reactions during CF₄/H₂ plasma etching of silicon and silicon dioxide films

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.