EFFECT OF UV IRRADIATION OF THE FILM DURING PLASMA CHEMICAL VAPOR-DEPOSITION

The effect of radiative energy transfer from the plasma to the bulk of the film during its growth in the plasma chemical vapour deposition (PCVD) process has been evaluated by the calculation of the UV radiation dose absorbed at different depths in the film during its growth. The depth profiles of the UV radiation dose thus determined appeared to be a function of the optical density alphad of the deposited material (alpha is the absorption coefficient of the film and d is the film thickness) and the reflectivity of the substrate used for the film deposition. For low values of alphad (weakly absorbing, thin films) the UV dose profiles are nearly linear, regardless of the substrate's reflectivity, i.e. the UV dose increases almost linearly with the film depth. For high values of alphad (strongly absorbing, thick films) a marked gradient in the UV dose is localized in the surface layer (independently of the substrate reflectivity) and, at high substrate reflectivity, also in the layer adjacent to the substrate. The UV dose profiles imply an inhomogeneous structure in the PCVD films resulting from different cross-link densities. Accordingly, the films are composed of a surface layer of low cross-link density and highly cross-linked, dense bulk material. This implication is strongly supported by a reasonably good correlation found between the calculated UV dose depth profiles and those reported in the literature for the composition and density in the PCVD films.