Investigations of the optical and structural properties of amorphous Ta₂O₅ prepared with ion assist during deposition

The performance of next generation gravitational wave detector is fundamentally limited by the thermal noise of the mirror coatings in the interferometer. Amorphous tantalum pentoxide (Ta₂O₅) is the component in the multilayer coating with the highest mechanical loss. This work investigates the optical and structural properties of tantalum pentoxide aimed at identifying conditions that lead to ultra-high thermodynamic stability. Amorphous tantalum pentoxide single layers were deposited by dual ion beam sputtering. The films were bombarded during growth by an assist Ar ion beam at different conditions. All samples were found to have good optical quality with an absorption loss at 1 µm as low as 7.2 ppm. The tantalum bonding states investigated through x-ray photoelectron spectroscopy (XPS) showed a stoichiometric composition in sample prepared with 100 eV Ar ion bombardment. The use of the Ar assist beam during film growth is shown to favorably affect the films’ mechanical properties.