High precision detection of the change in intermediate range order of amorphous thin films due to annealing

Future gravitational wave detectors, such as Advanced LIGO +, will be limited in sensitivity by thermal noise associated with mechanical loss in the detectors’ amorphous mirror coatings. In order to reduce mechanical loss, we aim to understand the atomic level structures responsible for it and take a directed design approach to suggest improved coatings. Through studies on thin films of amorphous zirconia-doped-tantala (ZrO₂-Ta₂O₅), a potential coating for LIGO mirrors, we present high precision detection of the change in the intermediate range order (IRO) as a function of post-deposition annealing, using grazing incidence x-ray scattering measurements carried out at the Stanford Synchrotron Radiation Lightsource. For the first time, our atomic modeling based on x-ray scattering data is able to capture the changes in IRO. We show that the primary structural units (PSUs), which are disordered oxygen octahedra centered around the metal atoms, remain largely unchanged after annealing. We explain the observed structural changes in terms of the IRO, specifically, the change in metal-oxygen-metal bond angles at the bridging oxygen sites, and the way neighboring PSUs share bridging oxygens. Finally, we discuss correlations between the observed changes in the IRO and mechanical loss.