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Refractive index at infrared wavelengths and dielectric permittivity of pure and fluorinated silicon dioxide from measurements of their thin films deposited on Si

The Fourier-transform infrared (FTIR) spectra of silicon dioxide (SiO2) and fluorinated silicon dioxide (SiOF) deposited on bare Si wafers have been recorded and the interference fringes in the spectra have been used to estimate the refractive index, n, of the SiO2 and SiOF films. It is found from the FTIR spectra that n is reduced by 3.5% on doping with F atoms. The refractive indices and thicknesses of the films were also determined by ellipsometry for comparison. The complex capacitance, C*(ω) = C′ − iC″, of samples from the wafers has been measured in the frequency range 1 Hz–1.8 GHz with broadband dielectric spectroscopy. Two relaxation peaks, one centred at ∼1 kHz and the other in the 10–100 MHz range are observed in both the SiO2 and SiOF coated wafer samples. The measurements of C*(ω) are related to the frequency-dependent complex permittivity, ε*(ω) = ε′ − iε″ of the films with the use of simple equivalent circuit models. ε′ shows a reduction of 16% on fluorine addition. It is suggested that the origin of the high frequency peak is the RC network formed by the wafer substrate and the dielectric film, while the low frequency peak results from charged defects at the Si–SiO2 interface. The dielectric results are compared with the microwave dielectric measurements, made using a different method, on both bare Si wafers and wafers coated with dielectric films, which have been reported recently. The observed dispersion in C′ and peaks in C″ are a result of the layered metal–oxide–silicon structure of the samples.