In ellipsometry, the interesting case of light incident from a dense medium such as an inert liquid onto a film over an absorbing substrate has been considered theoretically for the two cases (i) when the angle of incidence ϕ0 is less than the critical angle ϕc and (ii) when ϕ0 > ϕc. Detailed calculations indicate that the sensitivity with which the thickness of the film can be measured is nearly the same, whatever the first medium, for case (i) but that in case (ii) the sensitivity is very poor. Further, by combining the measurements of the ellipticity parameters with first air and then a liquid as the immersion medium, it is possible to evaluate independently by a self-consistency procedure both the thickness and the refractive index of the film. Measurements of oxide films on a substrate of silicon indicate that the refractive index is 1.484±0.004 in the thickness range 80–300 Å for λ 5461 Å and not 1.460 as has been assumed by previous workers.
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