Influence of ion assistance on the optical properties of MgF2

The optical properties of MgF2 films prepared by evaporation and ion-assisted deposition have been determined from transmittance and near-normal incidence reflectance measurements and also from electronenergy loss spectroscopy (EELS). The results show that oxygen-ion assistance leads to higher extinction coefficientsfor wavelengths <180 nm. Transmission electron microscopy studies show that the crystal grain size of MgF2 films is not strongly affected by oxygen or argon-ion bombardment. The presence of MgO in the films is inferred from RBS measurements and proposed to be the major factor influencing VUV losses. EELS is also demonstrated to be a valuable technique for determination of optical properties from the near-infrared to x-ray regions of the spectrum.