Determination of the morphology and effective optical constants of non-ideal thin films

Optical techniques are widely used as non-destructive methods for determining the thickness and effective optical constants of vacuum deposited thin films. Implicit inmany such analyses is the use of an ideal morphological model of a thin film. The morphology of vacuum deposited thin films, however, may be far from ideal. The influence of morphological features on normal incidence reflectance and transmittance spectra of thin films is discussed. In particular, the interpretation of the non-ideal surface of a thin film as either a region giving rise to scattering of incident light or, by applying effective medium theory, as a region of graded refractive index will be considered. The results of a comparative study of the normal incidence reflectance and transmittance spectra of a number of sputtered thin films using both surface scattering and surface region of graded refractive index models is presented. It is shown that the latter model is preferred for films with surface features with dimensions of up to a considerable fraction of wavelength. The implications of these normal incidence results for ellipsometric measurements are also discussed.