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

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Abstract
In situ multiple internal reflection Fourier transform infrared spectroscopy and spectroscopicellipsometry are used to study the surfaces of Si and SiO 2 films during etching with CF 4 /H 2 plasmas. At sufficiently low H 2 concentration, a thin fluorocarbon film forms on both Si and SiO 2 surfaces during etching, but Si and SiO 2 removal continues despite the existence of such a layer. The structure of this film depends on the H 2 concentration in the feed gas. Above a critical H 2 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 SiO 2 . 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 SiO 2 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 H 2 concentration is increased but at different amounts for Si and SiO 2 films. The film formed on SiO 2 is more easily sputtered than that which forms on Si due to the higher number of C–O bonds in the film formed on SiO 2 . The difference in the structure of the inhibiting layers formed on Si versus SiO 2 enables the selective etching of SiO 2 over Si.

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