Determination of refractive index and thickness of YbF₃ thin films deposited at different bias voltages of APS ion source from spectrophotometric methods

Ytterbium fluoride (YbF₃) single thin films were prepared on sapphire and monocrystalline silicon substrates through conventional thermal evaporation and ion beam-assisted deposition (IAD), at bias voltages ranging from 50 to 160 V of the Leybold advanced plasma source (APS). By using the Cauchy dispersion model, the refractive index and thickness of the YbF₃ thin films were obtained by fitting the 400–2500 nm transmittance of the monolayer YbF₃ thin films on the sapphire substrate. At the same time, the refractive index and thickness of the YbF₃ thin films on the monocrystalline silicon substrates were also measured using the VASE ellipsometer at wavelength from 400 to 2200 nm. The results showed that the refractive index deviation of the YbF₃ thin films between the fitted values by the transmittance spectra and the measured values by the VASE ellipsometer was <0.02 and the relative deviation of the thickness was <1%. Furthermore, the refractive index of the YbF₃ thin films increased with increasing APS bias voltage. The conventional YbF₃ thin films and the IAD thin films deposited at low bias voltage revealed a negative inhomogeneity, and a higher bias voltage is beneficial for improving the homogeneity of YbF₃ thin films.