X-ray photoelectron spectroscopy of ion beam sputter deposited SiO2, TiO2, and Ta2O5

Abstract

X-ray photoelectron spectroscopy (XPS) was used to examine the atomic bonding in thin oxide films used in reflection enhancing optical coatings. SiO2 was deposited by ion beam sputtering of quartz; TiO2 and Ta2O5 were deposited by ion beam sputtering of metal targets. In each case, an increased oxygen background pressure was used to achieve the desired stoichiometry. All three types of films are amorphous as measured by TEM and by x-ray diffraction and have an optical absorption below 100 parts per million for half-wave thicknesses at 1.06 μm. Depth profiles have been performed on each of the films and some layered structures. The XPS data for each film as deposited shows a single bonding state and no evidence of suboxides above approximately a 5% level. The O 1s line in SiO2 is broader (2.5 eV) than in evaporated SiO2, and is not altered by the ion bombardment of the depth profiling process. The O ls line broadens, however, by up to 50% at a SiO2/TiO2 interface. Sputter-deposited TiO2 and Ta2O5 show a relatively narrow (1.7 eV) O 1s line. Neither the composition nor the bonding in SiO2 is affected by sputter etching with Ar+ at 3 keV. In TiO2, however, oxygen is preferentially sputtered and suboxides observed in the XPS spectrum. With Ta2O5, oxygen is also diminished, but only Ta2O5 and metallic Ta are seen in the XPS spectra.