In recent years, indium-tin-oxide (ITO) has been proven to be a promising candidate for optical studies, especially due to its real-epsilon-near-zero (ENZ) properties. In this work, ITO thin films were fabricated on fused silica by magnetron sputtering methods using direct current (DC) with/without post annealing at 250 °C and high frequency (HF) alternating current with/without substrate heating under different argon pressures. Crystallographic structures and optical properties were obtained via X-ray diffraction (XRD) and spectroscopic ellipsometry (SE). ITO films have a polycrystalline cubic structure. Complex dielectric constants for ITO thin films were extracted from model fit spectroscopic ellipsometry data measured for UV–Vis to IR spectral ranges. Tauc-Lorentz, Drude and Gaussian oscillator functions were used for modelling the dielectric constants. The free carriers of ITO were obtained by the Drude function. ITO thin films thermally treated at 250 °C show higher values of dielectric constants, free carrier concentrations and mobilities. Thermal treatment shifts the ITO real permittivity peaks in the UV region from 0.34 μm (ITODC) to 0.26 μm (ITODC + T) and from 0.31 μm (ITOHF) to 0.28 μm(ITOHF + T). The post annealing shifts ENZ from 2.4 μm (ITODC) to 1.1 μm(ITODC + T), while the substrate heating shifts the ENZ from 2.1 μm (ITOHF) to 1.7 μm (ITOHF + T).