New Methods of IR Spectroscopic Investigation of Amorphous Insulating Films

The infrared (IR) spectroscopy of very thin insulating films on semiconductors has some perspective for the non-destructive analysis of corresponding microelectronic systems. Therefore, the derivation of IR optical equations for insulating films (on semiconductors) of decreasing thickness is of increasing importance. On the basis of the FRESNEL theory we derive relations for the determination of the reflection (R), transmission (T), and attenuated total reflection (ATR) of such films from their dielectric functions taking into account oblique incidence of polarized light. In this way we find a theoretical explanation for the detection and identification of transverse and longitudinal optical phonons by means of oblique incidence of IR light, realized within different experimental modes (R, T, ART). Furthermore, this framework leads to a theoretical basis for the ATR spectroscopy of very thin films as well as to a very effective method of determination of the IR optical (dielectric) functions of the film from corresponding experimental spectra, wich is based on the inversion of the optical relations mentioned above. Finally, it will be shown that the theory developed is in quantitative agreement with corresponding measurements performed for SiO₂ and Al₂O₃ on silicon.