Infrared analysis of thin films

Keywords: silicon oxide, FT-IR spectrometer, nanofilms, absorption spectra, transmission spectra, “ion-plasma” method.

We studied infrared transmission and absorption spectra, refractive index, layer thickness, angle of incidence, average interference fringes and standard deviations of thin films obtained by ion-plasma method. Qualitative analysis is designed to check the refractive index of thin films, the thickness of thin films.

We used the chamber of the magnetron device (Epos-PVD-Desk-Pro) to create silicon oxide using the Ion-Plasma method in high vacuum using a molecular turbopump (Pfeiffer Vacuum) [1]. The layer thickness, refractive index, absorption and transmission spectra of nanofilms formed on the silicon surface were measured. The experimental results were obtained on FT-IR spectrometers (IRTracer-100, Bruker-Alpha-II), which require high accuracy.

The spectra obtained on the Bruker — Alpha-II infrared spectrophotometer were used for processing and analysis in the “OPUS” multifunctional software.

Fig. 1. Transmission and absorption spectra of silicon film obtained using the Bruker Alpha-II infrared spectrophotometer

Figure 2 shows the transmission and absorption spectra of the SiO ₂ /Si(111) film in the range 400÷4000 cm ^-1 , with water vapor (H ₂ O) or carbon dioxide (CO ₂ ) in the spectra of molecules undergoing the following adjustments to reduce the effects: addition, smoothing, zero correction of the baseline, normalization, filtration and ATR correction [2].

When silicon is oxidized (naturally, in the atmosphere of air or under the influence of high temperatures), Si-OH hydroxyl groups are formed on its surface, and siox groups are formed in the near-surface layer. The presence of hydroxyl groups leads to adsorption of atmospheric moisture and corresponding changes in the spectra of samples [3].

Fig. 2. Smothing analysis of the transmission and absorption spectrum of SiO ₂ /Si(111)

Figure 2 shows the infrared absorption and transmission spectra of the SiO ₂ film formed on the silicon surface. In the absorption spectrum, the peak was observed in the region of 769.60 cm ^-1 . These lines correspond, respectively, to antisymmetric stretching oscillations of Si-O-Si groups. The peak of the transmission spectrum in the region of 644.22 cm ^-1 corresponds to the “fingerprint» region of the pure silicon spectrum. Silicon dioxide layers have three absorption zones: a low-frequency band of 418.55 cm ^-1 , a weak band of 771.53 cm ^-1 and an intense broadband band with a maximum of 644.22 cm ^-1 . These lines relate to the oscillations of the pendulum, symmetric stretching and antisymmetric stretching of Si-O-Si groups, respectively [4]. Depending on the brittleness of the oxide, the final strip can have a half width from ≈95 cm ^-1 to ≈ 140 cm ^-1 for dense oxide. While studies of silicon oxides have shown that SiO _x (x=1÷2) is formed during deposition and annealing, with a decrease in x , the maximum boundary of the n band (SiOSi) shifts to the region of lower wave numbers (915 cm ^-1 at x=1, 980 cm ^-1 at x=2). The frequency, on the contrary, increases from 780 to 835 cm ^-1 ; the frequency of oscillation of the pendulum increases with increasing x.

Table 1

In the case of thin films, infrared interference spectra carry information about the anisotropy of the material and make it possible to determine the refractive index and rotation of molecules located in the IR region of the spectrum [5]. Knowing the angle of incidence and refractive index in the “calculating film density” function in the data processing section of the IRTracer-100 spectrophotometer, it is possible to measure the thickness of the film, the average number of interference fields and the standard deviation. Table 2 below shows the measured parameters of films of different thicknesses obtained in the magnetron device.

Table 2

For silicon oxide and pure silicon, “average interference edges” were measured. The depth of penetration of infrared light into the crystal was measured. The measured data show that the electrical and optical properties of various nanofilms obtained by “ion-plasma” and “ion-sputtering” methods play an important role in illumination.

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