Effect of preliminary VUV irradiation on the reactivity of alkyl-doped silicon dioxide layers

The effect of plasma treatment on the passivation of the surface of porous layers based on methyl-doped silicon dioxide is considered. A principal difference between the effects of plasma on materials with the pores of different sizes is demonstrated. It is shown that the VUV irradiation passivates the surface of the layers with smaller pores (1.1 nm in radius), while the film with larger pores (5.5 nm) does not get passivated. A model explaining the reasons of this behaviour is proposed on the basis of modelling results. The action of plasma on the film is mainly considered as the UV irradiation (21 eV) causing the rupture of H–C bonds, though the energy of UV radiation is sufficient to break any other bond. It may be stated on the basis of the data obtained in experiments and by modelling that the most probable process is hydrogen atom detachment from the methyl group rather than rupture of Si–O bond because in the latter case the resulting non-volatile fragments would recombine; the excess energy would get distributed over the lattice. Rupture of H atom from the methyl group causes the appearance of a positive charge on the silicon atom and an increase in dπ–pπ overlapping, which in turn leads to a decrease in the reactivity and to layer passivation.