Fourier transform infrared (FTIR) spectroscopy is a simple, non-destructive measurement technique and it is widely used for material studies. The infrared absorption occurs at the frequencies which correlate to the vibrations of chemical bonds from within a molecule. Through the absorption peaks in the spectrum, the composition and the bonding configuration of the material can be determined. Usually, the incident light is perpendicular to the substrate surface. However, for thin film materials, the off-plane vibration cannot be detected by the usual setup, as shown in Figure 1.
To detect the off-plane vibration modes, oblique incident light is required. This was firstly reported by Berreman  and consequently is called the Berreman effect. He showed that the transverse optical (TO) phonon vibrational mode of a LiF thin film could be detected by p- and s-polarized light with an incident angle at 30°, but the longitudinal optical (LO) phonon vibrational modes could only be detected by p-polarized light. The reason for the different behaviour of s– and p-polarised light is due to the nature that the vibration of s-polarised light is still parallel to the substrate surface at an oblique incident angle. The difference between the two polarities is explained in figure (b). With oblique incidence, the vibration of s-polarized light is still perpendicular to the LO vibration in the dielectric thin film. Vice versa, the p-polarized light is sensitive to the LO vibrational mode, and the higher angle of incidence results in higher absorption of LO peaks.
We use a 25 nm thick Al2O3 film on the c-Si wafer as an example. The typical IR absorption bands of Al2O3 are in the range of 400-900 cm-1 which are assigned to the transverse optical (TO) modes of Al-O-Al vibrations , and a weak signal of SiOx TO modes could be observed at ~1050 cm-1 . Here, we focus on the longitudinal optical (LO) mode of Al-O and Si-O measured by the oblique angle of incidence. When the incident angle is lower than 30º, slight interference fringes were visible due to the interfering multiple reflections within the silicon wafer . These interference fringes were eliminated by using an incident angle of 74º which is Brewster’s angle for c-Si . The p-polarized light has no reflection in the silicon interfaces at Brewster’s angle. At the angle of 10º, the weak and broad SiOx TO mode could be observed, but this peak overlapped with the Al2O3 LO peak at a higher angle of incidence. While the signal of SiOx LO was enhanced, the LO modes of both Al2O3 and SiOx could be observed at ~950 and 1236 cm-1, respectively, and the intensity of the peak increased at a higher angle of incidence due to the nature of the angular dependence of LO peaks. This angular behaviour was also observed in titanium oxide (TiO2) films . It should be noted that the SiOx film is only 1-2 nm thick in this case. Hence, Brewster’s angle FTIR is remarkably sensitive to the critical SiOx interface layer. The other advantage of the LO and TO peak of SiOx peaks is to determine the atomic density of the SiOx layer . More details about this work can be found in our publication: C-Y. Lee et al., AIP Adv., vol. 8, no. 7, p. 075204, 2018.