Optical Effects in IR Spectroscopy: Thickness-Dependent Positions of Absorbance Lines in Spectra of Thin Films

Optical effects give rise to thickness-dependent positions of absorbance lines (v₀) in the normal-incidence transmission spectra of thin films. Two reasons can be distinguished. Simple Fresnel refraction on the boundaries of the film shifts the line toward higher wavenumbers, with the maximum increment at small film thicknesses (d). The second effect is due to multiple reflectance. The interference fringes cause the oscillations of the absorbance line around the mean value, which is defined by the effect of the boundaries. Both effects are more pronounced for broad lines. Whereas for the strong bands the first effect dominates, for the weaker ones both can be seen. The effective dielectric constant of the medium increases the first effect and diminishes the second one. Theoretically predicted v_o (d) curves compare well with absorption line positions in silicon nitride and silicon oxide films deposited on silicon wafers.