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

Abstract

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 vo ( d) curves compare well with absorption line positions in silicon nitride and silicon oxide films deposited on silicon wafers.

Mass and bond density measurements for PECVD a-SiCx:H thin films using Fourier transform-infrared spectroscopy

“…For the former, substrates that are effectively IR transparent are typically utilized. This is the case for Si whose strongest phonon absorption band at 610 cm − 1 has an extinction coefficient of only 1.2 × 10 − 3 [64]. For external and internal reflections, two approaches have been developed to partially account for these effects.…”

Fourier transform infrared spectroscopy investigation of chemical bonding in low-k a-SiC:H thin films

“…In the following results and discussion sections, the position and relative changes in position of these absorption bands will be utilized to infer differences and changes in the chemical structure of the a-SiC:H thin films. We note, however, that changes in the positions of IR absorption bands may arise due to both optical dispersion and intrinsic stress related effects that can cause the position of these bands in some extreme cases to shift by as much as 50 cm − 1 [51][52][53]. The former effect arises due to changes in film thickness or refractive index affecting the amount of IR transmission/reflection that occurs at the film/substrate interface.…”

Characterization of silicon oxynitride thin films by infrared reflection absorption spectroscopy

“…Dependency of the TO Si-O vibrational band position with the film thickness has been previously observed in the case of various silicon oxynitride transmission spectra. 33 For silicon oxide, Gunde 35 and Alexandrov 34 had calculated infrared transmission spectra as a function of thickness and compared them to experimental spectra. They showed that the TO Si-O band shifts to lower frequencies as thickness decreases.…”